research report 184 - health and safety executive · this report and the work it describes were...

HSE Health & Safety

Executive

Field studies of the effectiveness of concrete repairs

Phase 3 Report: Inspection of sites, sampling and testing at selected repair sites

Prepared by Mott MacDonald Ltd for the Health and Safety Executive 2003

RESEARCH REPORT 184

HSE Health & Safety

Executive

Field studies of the effectiveness of concrete repairs

Phase 3 Report: Inspection of sites, sampling and testing at selected repair sites

N J R Baldwin BSc MSc FGS MIM CEng

Mott MacDonald Ltd St Anne House

Wellesley Road Croydon CR9 2UL

This report presents the findings of the project Field Studies of the Effectiveness of Concrete Repairs. The projects was undertaken in four phases and is presented in five reports.

HSE has published the following four reports:

Phase 1: Desk study and literature review.

Phase 3: Inspection of sites, sampling and testing , at selected repair sites.

Phase 3a: An investigation of the performance of repairs and cathodic protection (CP) systems at the Dartford West Tunnel, (DWT).

Phase 4: Analysis of the effectiveness of concrete repairs and project findings.

Phase 2 of the project details the selection of study locations and the procedures for investigating and recording the repair sites. This phase is summarised in the Mott MacDonald report R1093 ‘Phase 2 Report Site Planning’.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2003

First published 2003

ISBN 0 7176 2791 8

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

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ACKNOWLEDGEMENTS The authors acknowledge the following for their co-operation and contributions without whose support the project could not be executed:

Health and Safety Executive

Highways Agency

(Nuclear) Industry Management Committee

Institution of Civil Engineers Research and Development Enabling Fund

MM Group Research and Development Fund

Concrete Repairs Ltd

Llewellyn Stonecare Ltd

Geomaterials Research Services Ltd

Building Research Establishment

Weber-Broutin SBD

BNFL Magnox Generation

TRL Limited

Sprayed Concrete Association

Concrete Repair Association

British Energy Generation UK Ltd

Parsons Brinckerhoff Infrastructure Ltd

WA Fairhurst & Partners

Skanska UK Civil Engineering

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CONTENTS

SUMMARY vii

Chapters and Appendices

1 INTRODUCTION 6

1.1 BACKGROUND 6

1.2 OBJECTIVES 6

1.3 SCOPE OF PHASE 3 – SITE VISITS 6

2 FINDINGS: SITE INVESTIGATION DATA 8

2.1 FORMAT 8

2.2 SITE PROCEDURES AND TESTING 8

2.3 SITE INVESTIGATION RECORDS 102.3.1 Site 01 102.3.2 Site 02 142.3.3 Site 03 182.3.4 Site 04A 232.3.5 Site 04B 292.3.6 Site 05 352.3.7 Site 06 432.3.8 Site 07 512.3.9 Site 08 582.3.10 Site 09 612.3.11 Site 10 642.3.12 Site 11 682.3.13 Site 12 722.3.14 Site 13 742.3.15 Site 14 782.3.16 Site 15 812.3.17 Site 16 852.3.18 Site 17 872.3.19 Site 18 892.3.20 Site 19 922.3.21 Site 20 962.3.22 Site 21 992.3.23 Site 22 1022.3.24 Site 23 1042.3.25 Site 24 1062.3.26 Site 25 1112.3.27 Site 26 114

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2.3.28 Site 27 1202.3.29 Site 28 1262.3.30 Site 29 1302.3.31 Site 30 1342.3.32 Site 31 1382.3.33 Site 32 1432.3.34 Site 34 1482.3.35 Site 35 1552.3.36 Site 36 1612.3.37 Site 38 1682.3.38 Site 39 1732.3.39 Site 41 1802.3.40 Site 42A 1872.3.41 Site 42B 1912.3.42 Site 43 1952.3.43 Site 44 2042.3.44 Site 45 2112.3.45 Site 46 2202.3.46 Site 47 225

3 BIBLIOGRAPHY 230

4 GLOSSARY 232

List of abbreviations 232

Definitions 232

5 REFERENCES 234

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SUMMARY

This report has been written by Mott MacDonald to present the findings of the Phase 3 of the project Field Studies of the Effectiveness of Concrete Repairs. This phase of the project was carried out under contract to the Health and Safety Executive.

Phase 2 of the project, planning of the locations and the procedures for investigating and recording the repair sites, is summarised in Mott MacDonald report R1093 ‘Phase 2 Report Site Planning’1. The approach outlined in that report was adopted for the field study phase. This report contains the records of the investigations and testing of repairs carried out at the sites visited in Phase 3.

In total, 46 sites were visited. At certain sites, there was more than one type, generation or condition of repair, and in total 65 locations were examined.

The sites were located in southern, western and northern England and the midlands. They included coastal, estuarine, river and inland locations. Elements included beams, columns, slabs and walls from structures including bridges, tunnels, power stations and other reinforced concrete frame buildings. At 48 locations the repairs were investigated with intrusive sampling, and 60 core samples were examined in the laboratory. At 13 locations the repairs were examined by visual inspection and hammer surveying. At 3 locations visual inspection was supplemented with non-destructive testing (NDT).

The Phase 3 report presents the facts from repair records and from the site investigations of sites with repaired reinforced concrete structures. A draft of the report was presented in October 2001 for review by the Expert Group. Feedback was incorporated into the later stages of the Phase 3 site work and has been instrumental in guiding the discussion and conclusions for the project presented in the final Phase 4 report2.

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1 INTRODUCTION

1.1 BACKGROUND

Mott MacDonald Ltd (MM) was commissioned by the Health and Safety Executive (HSE) in June 2000 to carry out a research study entitled ‘Field Studies of Effectiveness of Concrete Repairs’. The project follows on from the project completed under the Nuclear Safety Research (Industry Management Committee) project CE/GNSR/5020 by Sheffield University (Reference3).

The scope and objectives have been developed between the HSE, MM and other organisations whose interests are represented in an Expert Group associated with the project. Funding has also been received from the Highways Agency (HA) and the Institution of Civil Engineers (ICE) Research and Development Enabling Fund. The project receives substantial additional support from collaborating organisation and individuals, as well as the co-operation of owners of repaired structures.

The project is divided into four main phases with two additional sub-phases relating to work carried out at Dartford River Crossing (DRC). The first stage involved desk study and literature review (Phase 1). Repair sites were selected by the project team and reviewed by the project’s corresponding Expert Group (Phase 2 and 2a). Inspection of sites, sampling and destructive and non-destructive testing, at selected repair sites, has been carried out (Phases 3 and 3a). The final stage involves analysis of data relating to the effectiveness of concrete repairs, and dissemination of the project findings (Phase 4). Phases 2a and 3a related to investigation of the repairs and cathodic protection (CP) systems as DRC.

This report presents the findings of Phase 3, the scope of which is shown in Section 1.3.

1.2 OBJECTIVES

The aim of this project is to evaluate the effectiveness of the range of concrete repair systems as applied in practice, in order to improve practices for restoring and maintaining the serviceability and structural integrity of operational structures and so achieve higher standards of structural safety and reliability and better whole-life structural management.

Phase 3 includes the evaluation of repairs applied to reinforced concrete structures in the UK and this report presents the findings of the site investigations carried out during 2000 and 2001. This report was reviewed by the Expert Group in October 2001 and feedback incorporated into subsequent Phase 3 work to achieve the best possible results from the project and to extract meaningful conclusions.

1.3 SCOPE OF PHASE 3 – SITE VISITS

Phase 3 has involved the following:

1. Site visits, sampling and laboratory testing. In general, more than one repair has been tested at each site. The performance to date of the repairs has been assessed through a combination of:

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· Visual inspection.

· Non-destructive testing e.g. hammer tapping, half-cell potentials, cover meter survey, carbonation depth.

· Destructive testing (where permitted by the structure owner) has included pull-off resistance, extraction of a core for petrographic examination and measurement of carbonation and chloride ingress.

2. Where available, existing proposals for monitoring and maintaining the repair have been reviewed. The measured performance has been compared with the intended performance and comment made on any differences. Records of the condition of repairs have been created for future reference.

3. Interpretation of site work and laboratory testing data.

4. Deliverables consist of this draft report for Phase 3, including a data sheet in standard format for each site.

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2 FINDINGS: SITE INVESTIGATION DATA

2.1 FORMAT

The details for each repair site are presented in the following tables. This includes information derived from desk study and the largely factual records from the site investigation. There is also an element of interpretation and discussion intended to reflect the content to be included in the Phase 4 report2.

The sites have been described in appropriate detail in relation to age, natural and service environment, repair history and condition. However, they remain anonymous for the purposes of Expert Group review and publication.

Where details are incomplete or missing, this data has been pursued and not found or has not been made available by the repairer or structure owner. In several cases the information has been disposed of or is missing from archives.

2.2 SITE PROCEDURES AND TESTING

The procedures followed were those described in the Phase 2 report. The following is a summary of the general approach.

Upon arrival at the site, a site-specific risk assessment was completed and the site personnel made aware of the scope of the work and potential hazards. In general, the site team consisted of one Mott MacDonald engineer and one or two sub-contractors to provide the access, drilling and reinstatement services. In several cases, special access was arranged to sites, and a representative of the structure owner or operator was also present. The structure was then visually inspected and repair sites selected, before carrying out a cover survey to locate the reinforcement, and then core sampling. Cores were taken with the intention of recovering the interface between repair material and substrate intact, examined, labelled and wrapped in cling-film. After examination of the core and core hole, a connection was made to exposed steel and a half-cell survey carried out over the adjacent surfaces, using a silversilver chloride probe. Where the perimeters of repairs were visible, these were recorded against the half-cell readings. Where possible, three pull-off test locations would then be prepared, by coring beyond the repair material and into the substrate, and attachment of dollies to the unbroken core.

The sample holes were then repaired with proprietary high-build repair systems and the site cleared and vacated.

In general, more than one sample location was selected at each site. An attempt was made to sample as many repairs as practicable, using the following parameters:

· contrasting condition i.e. representative of most repairs at the site, and deteriorated and undeteriorated repairs,

· contrasting appearance i.e. apparently different materials, phases, ages, and

· contrasting locations and elements e.g. vertical, overhead, different exposure conditions, etc..

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The detailed descriptions of the repair and substrate materials, the interface between them, and the appearance of cracks and reinforcement originates in part from the visual inspection of the core samples and core holes but relies heavily on the petrographic examinations carried out by GMRS. The petrographic reports (see Section 0) are available for further review and are retained by Mott MacDonald and HSE.

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2.3 SITE INVESTIGATION RECORDS

2.3.1 Site 01

Site investigation record. Site no: 01

Date of inspection/testing: 22 Aug 00/23 Oct 00 Contractor details: Works, access and drilling contractor

Type of structure PCC road deck Location: SE England

Constructed 1963 Repaired: <1995

General visual condition of STRUCTURE.

The road deck has been subject to continuous deterioration resulting from ingress of water and road deicing salts. Traffic impact loading has resulted in delamination of the top mat reinforcement elsewhere in the structure. The structure was replaced in 1999.

General visual condition of REPAIR.

The repair is formed around a road deck manhole. A full visual inspection including hammer tapping was carried out from the deck underside. A black coating – assumed to be bituminous – has been applied to the concrete surrounding the manhole.

Repair details:

Photo of repaired structure: Manhole 4W Element Manhole opening in Type: precast concrete road

deck unit.

Repair Flowable concrete Material(s):

Coatings/ Bituminous coating render: around internal faces

of manhole opening with a 300mm wide apron on soffit.

Condition:

Several cracks, of 0.1-0.2mm width were observed in the repair. No areas of delamination or spalling were observed. Hammer tapping indicates sound concrete.

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Tests conducted:

Test Results

Covermeter No covermeter survey was undertaken. Site inspections indicate 25mm diameter bars at various depths and a minimum cover of 24mm to the upper surface and 42mm to the soffit.

Samples:

Visual inspection of drilled holes/cores/breakout:

Full depth proprietary repair concrete reforming part of the original concrete slab, with a fine repair mortar applied locally to the upper surface.

Photo of Sample(s): Description (supplemented by petrography)

Sample 01.1: Full depth flowable concrete repair material, with 50mm bituminous road surfacing. The reinforcing steel is in good condition.

Sample 01.2: This sample consists of two layers of repair, with 220mm flowable concrete repair material forming the bulk of the deck and a thin, 30mm layer of cementitious repair at the upper surface. This is overlain with 50mm of bituminous road surfacing. The reinforcing steel is mostly in good condition, but there is some corrosion around the bottom steel where water ingress has occurred.

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Laboratory results and desk study

History of repair from desk study:

Petrography

The main repair material is in a single layer and consists of crushed limestone aggregate reaching a maximum size of approximately 10mm diameter, in a paste composed of portland cement and PFA, with numerous granules of graphite. The paste is very uniform in appearance, and contains moderately abundant residual unhydrated cement grains, a small amount of coarsely crystalline portlandite, and a low and patchy porosity. The void content increases slightly towards the upper end of the core. The voids are empty. No evidence has been found for deterioration of the binder as a result of moisture penetration.

There are low levels of microcracking, with sporadic microcracks around some of the coarse aggregate particles close to the upper surface, and microcracks normal to the soffit close to the lower end of the core.

The inner (soffit) end of the core has a rough, broken surface and is coated with a bituminous layer. The upper surface has a smooth, formed surface onto which the bituminous road surfacing was applied.

The extent of carbonation is very low at both ends of the core. At the soffit, carbonation penetrates to 1.5mm depth. Carbonation also penetrates to 10mm depth along a microcrack from the external surface to the upper reinforcement. The paste around the bars closest to the upper and lower reinforcement is uncarbonated. There are traces of corrosion products on the lowest bars whilst the uppermost bars have very few traces of corrosion.

Source and extent of information

Discussions with engineering team involved in the repairs and examination of contract drawings.

Summary of method and materials

Regular inspections of the soffit and wearing surface identified particular problems with durability of the deck at the manholes, where saline water collected and penetrated. The problem was aggravated where the air flow in the ventilated invert caused salty water to be spread over large areas. The client appointed a consulting engineer to prepare a specification for the works and advise during the contract. A concrete repair contractor undertook the sampling investigation, survey and repair works. The engineer supervised the repairs. The repairs were executed during temporary lane possessions, the duration of which were minimised. The deck and the proposed repair activities were assessed by a structural engineer and the deck was propped as necessary. This involved provision of a temporary bridge grade steel plate wearing surface so the repairs were not loaded. The work was done in nightly possessions without closure during day.

The extent of repair was determined by the engineer based on the visual inspection and chloride content data. The extent of break out was determined on site after exposure of the reinforcement and examination of the condition of the bar. The perimeter was sawn and break out was by high pressure water jetting to 250mm beyond any corroded bar, removing the full thickness of the deck and producing a roughly vertical perimeter. The manhole was then boxed out in temporary formwork, the concrete surfaces confirmed to be damp but not wet, and a proprietary flowable repair concrete used to reform the deck slab.

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This material was a preblended cementitious mix, with water added at site to form a pourable or pumpable concrete with rapid strength gain and high ultimate strength complying with the Department of Transport Model Specification BD27/86 ‘replacement of concrete for sides and soffits of beams and crossheads’. The material contained a blend of RHPC, PFA, microsilica and shrinkage-compensating admixtures.

The engineer also extended the system with a 10mm crushed limestone aggregate because of the potentially large volume of fine, highly cementitious material to be applied and the potential thermal gain and cracking that might result. The water content and stone content of the mix was modified in subsequent repairs depending on the repair size and fresh properties required.

The manufacturers datasheet recommended a minimum 7-day curing period. Class F2 formwork were used for full depth repair locations. The formwork was removed after 5 days and a bituminous wearing surface applied to the upper surface within 48 hours. The lower surface was lightly abraded in preparation to receive a bituminous coating.

Interpretation: desk study verses site findings

The microcracks close to the top of the core may have resulted from mechanical damage from road traffic. The microcracks at the lower surface are more likely to have resulted from drying shrinkage.

Although several fine cracks were noted in the soffit, there was no evidence of significant seepage through the repair or at its perimeter, and no evidence of corrosion of the embedded bars or those adjacent to the repair.

The consulting engineer believed the success of the project was in no small way related to having a thorough understanding of the deterioration mechanisms at work and making reasonable estimate of the level and extent of chloride contamination beyond the obviously damaged areas.

Site specific observations and conclusions:

Removal and replacement of all contaminated concrete close to the source of chloride ingress can be effective where a high performance proprietary material is used and a coating is applied to the external surfaces.

Water jetting can provide a good vertical substrate onto which flowable concrete can form a durable bond.

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2.3.2 Site 02


Date of inspection/testing: 28 Jun 00/31 Jul 00 Contractor details: Drilling contractor

Type of structure PCC road deck Location: SE England

Constructed 1963 Repaired: 1992/4


The road deck has been subject to continuous deterioration resulting from ingress of water and road deicing salts. Traffic impact loading has resulted in delamination of the top mat reinforcement elsewhere in the structure. The structure was replaced in 1999.


The repair is formed around a road deck manhole. A full visual inspection including hammer tapping was carried out from the deck underside. A black coating – assumed to be bituminous – has been applied to the concrete surrounding the manhole.

Repair details:

Photo of repaired structure: Manhole 8E – photograph Element Manhole opening in taken during demolition of precast concrete deck unit. Type: precast concrete road Note green coated reinforcement in foreground of photo. deck unit.

Repair Sprayed concrete Material(s):

Coatings/ Bituminous membrane render: around internal faces of

manhole opening with a 300mm wide apron on soffit.

Condition:

Repair appears to be in good condition. 2 No. small areas (<0.1m2) of incipient spalling/voids identified adjacent to manhole during sounding survey.

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Photo of structure during repair showing high pressure water jetting of the soffit of the deck from the invert of the tunnel. Note the clearly marked areas planned for breakout and the temporary propping of the slab.

Tests Conducted:

Test Results

Covermeter No covermeter survey was undertaken. Site inspection of the removed deck segment indicated minimum cover was typically 25mm. Minimum cover in the core samples from the repair location was 36-41mm from the soffit and 10-40mm from the upper surface.

Samples:


Grey repair mortar in lower parts of core applied to original light coloured concrete with flint coarse aggregate with bituminous surfacing at the upper end of core.


Partial and full-depth repair of deck. At core location voids are apparent around the reinforcing steel. No significant corrosion of reinforcing steel is occurring.

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Laboratory Results and Desk Study

Petrography

The sprayed concrete consists of crushed particles of recrystallised limestone graded from 5mm diameter to dust size, within a cementitious binder. This is based on portland cement with GGBS and fibres with some cement grains resembling calcium aluminate cement. The paste had a generally very high and patchy porosity (in contrast to many other areas of sprayed concrete repair in the tunnel with very low porosity) with variable and low levels of microcracking. The concrete contains a variable and locally high voidage with some concentration of voids at the reinforcement and the interface with the substrate. These voids can be large and interconnected, with the largest voids in excess of 20mm in maximum dimension.

The sprayed concrete contains a vertical crack from the lower surface through the full thickness of the repair, and where it makes contact with the concrete substrate runs along the interface. The crack intersects the surface of the lower reinforcement and carbonation has penetrated along the crack to this depth. However, no evidence was found for corrosion of the reinforcement.

The concrete substrate consists of a flint gravel and siliceous sand within a highly porous portland cement binder.



Discussions with engineering team involved in the repairs and examination of construction issue contract drawings retrieved from consulting engineer’s archive.


Regular inspections of the soffit and wearing surface identified particular problems with durability of the deck at the manholes, where saline water collected and penetrated. The problem was aggravated where the air flow in the ventilated invert caused salty water to be spread over large areas. The client appointed a consulting engineer to prepare a specification for the works and advise during the contract. A concrete repair contractor undertook the sampling investigation, survey and repair works. The engineer supervised the repairs. The repairs were executed during temporary lane possessions, the duration of which were minimised. The deck and the proposed repair activities were assessed by a structural engineer and the deck was propped as necessary. This involved provision of a temporary bridge grade steel plate wearing surface so the repairs were not loaded. The work was done in nightly possessions without closure during day. The extent of repair was determined by the engineer based on the visual inspection and chloride content data. Areas with a chloride content greater than 0.3% by weight were marked. The extent of additional break out was determined on site after exposure of the reinforcement and examination of the condition of the bar. The perimeter was sawn and break out was by high pressure water jetting to 250mm beyond any corroded bar, removing all damaged or contaminated concrete, but not always the full thickness of the deck. Reinforcement was cleaned by waterjetting to SA2.5 standard and inspected by the engineer. Where additional reinforcement was required, this was epoxy-coated and tied to the original bars. The break out was typically first carried out from the top surface, and a second phase executed from below, where additional parts of the saltcontaminated soffit were removed.

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The concrete surfaces were confirmed to be damp but not wet, and a proprietary sprayed repair concrete used to reform the deck slab. In repairs to the full depth of the slab, this was also carried out in two phases, spraying from above onto formwork, 3 days later from below into the excavations in the soffit. In some repairs, a 15mm downstand was incorporated into the newly formed soffit to form a drip to prevent saline water tracking across the soffit. The material was a dry sprayed concrete with high build, rapid strength gain, and low shrinkage characteristics, containing polymers, microsilica and superplasticisers. The material complied with the Department of Transport bridge engineering directives. The manufacturers datasheet recommended curing in accordance with good concrete practice.

A waterproofing membrane was then applied to the deck, vertical faces of the manhole, and the soffit, to minimise future ingress of chlorides around the manhole.

Class F2 formwork was used for full depth repair locations and unformed finishes (class U6) used for most sprayed concrete locations.


Break out and reinstatement with sprayed concrete formed part of a series of repairs to the slab. Further repair and installation of a CP system occurred in the adjacent slabs after sprayed repair to the manhole. The repair material is slightly unusual in that it has a high porosity, whilst most similar repairs of this type have a very low porosity. The most likely explanation for this porosity is an increased water content, which could result from overdosing of water at the nozzle during the dry-spray process.

Despite the contract being supervised by the consulting engineer, and great care taken to create dense, high quality repairs with no cavities, significant voidage occurred in various locations within the sprayed concrete. This was most notable at the interface with the substrate and behind the reinforcement bars, where continuous application and compaction are most difficult to achieve. However, there is no evidence that these cavities have compromised the durability of the repair or allowed corrosion of the embedded reinforcement.


Removal and replacement of contaminated concrete close to the source of chloride ingress can be effective where a high performance proprietary material is used and a coating is applied to the external surfaces.

Voids can occur in sprayed concrete at the reinforcing steel and the substrate interface.

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2.3.3 Site 03


Date of inspection/testing: 28/2/01 Contractor details: Drilling contractor (low scaffold required)

Type of structure Hospital Location: SE England

Constructed 1930s Repaired: 1991


A small proportion of the area that had been repaired and coated showed evidence of deterioration. However, the majority of the structure remained in visually good condition. This is an inland location with external elevations facing a busy access road, with some exposure to road salt.


The boundaries of repaired areas were not identifiable by visual inspection or hammer tapping, largely due to of the presence of the coating system. Photographs taken during the repair contract were used to reference features and locations which enabled samples to be taken.

Repair details:

Photo of repaired structure (location 1): Element Reinforced concrete type: wall

Repair Hand applied material(s): proprietary shrinkage

compensated polymer modified cementitious mortar

Coatings/ Elastic coating render: system

Condition:

Coating intact, with no evidence of cracking or deterioration within the repair; perimeter of repair not visible. Hammer tapping indicates sound concrete.

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Photo of repaired structure (location 2): Element Type: R/C canopy

Repair Hand applied Material(s): proprietary

shrinkage compensated polymer modified cementitious mortar

Coatings/ Elastic coating render: system

Condition: extensive blistering of the soffit coating, but no loss or penetrations in it. No visible evidence of deterioration within the repair; perimeter of repair not visible. Hammer tapping indicates sound concrete.

Tests Conducted:

Test Results

Half-cell Location 1 values from –72 to –169mV with more negative values towards ground potential (SSC) level, indicating low to uncertain probability of corrosion and probable increase in

moisture content towards ground.

Location 2 values from –34 to –172mV with values gradually becoming more negative towards the edge beam, indicating low to uncertain probability of corrosion.

Covermeter Location 1: indicates minimum cover of 12mm to 23mm, with the majority less than 20mm.

Pull-off test Location 1: all tests fail within the concrete substrate beyond the repair, at 0.5, 0.5 and 0.8 N/mm2.

Samples:


Grey repair mortar with sand grains typically <1mm diameter applied to original light coloured concrete with flint coarse aggregate.

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Location 1

Repair, in 2 layers, to 70mm thickness, with voids at repair/substrate contact and on the underside of the reinforcement bar, fine voids between the repair layers and a fine crack from the external surface to the reinforcement bar at 17mm depth. Carbonation in repair to up to 2mm, and along the crack to 11mm. The reinforcement bar is patchily coated with a green membrane and is not corroded.

Location 2

Repair, in 2 layers, to 57mm thickness, with voids at repair/substrate contact and fine voids and traces of carbonation between the repair layers. Carbonation in repair is negligible. The reinforcement bar is patchily coated with a green membrane within the repair, and shows traces of corrosion in the adjacent substrate. In the core hole a degree of fragmentation was noted in the repair around the reinforcement bar, possibly related to the coring procedure, or to the occurrence of voids around the bar.


Petrography

The repair material in both locations is similar, and composed of a fine to medium siliceous sand within a binder containing portland cement, PFA cenospheres and clots of microsilica. The material has a low porosity and extremely low incidence of micro-cracking, and shows no evidence of deterioration.

The interface between the repair and the concrete substrate is irregular, with exposed pieces of coarse aggregate, and contains low levels of microcracking in both substrate and repair. However, the abundance and size of voids at the interface could reduce bond strength. The interface between the repair and substrate is suggestive of a water-jetted substrate, in that the surface is irregular and contains exposed, unbroken pieces of aggregate with little evidence of cracking.

Sample 2 has traces of ettringite in the voids in the repair material.

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Contact with operators maintenance department and anecdotal evidence provided. Method statement, specification, correspondence and photographs relating to contract provided by repair contractor. The client appointed a consulting engineer to prepare a specification for the works and advise during the contract. A concrete repair contractor undertook the investigation, survey and repair works.


All were to be surfaces cleaned by high pressure water jetting to expose the concrete. Surfaces were to be surveyed visually with additional hammer tapping, covermeter, carbonation and chloride testing. Areas for repair were agreed with the client and marked on the structure.

Defective areas were removed by high pressure water cutting to the greater of a minimum depth of 25mm or to sound concrete. Corroded bars were to be exposed for the full perimeter; in sound concrete these were not required to be exposed fully. Steel was to be cleaned by water jetting and then primed with a non-shrink solvent-free thixotropic epoxy resin or similar approved material applied by brush or spray. The edges of the repair were to be saw cut to >10mm to avoid feather edging.

Replacement was by using a proprietary shrinkage compensated polymer modified cementitious material with compressive strength of 30N/mm at 28 days: a repair concrete, with shuttering, for depths >50mm, and mortar, applied by hand/trowel to existing profiles for depths <50mm. A primer may be used. The materials to be cured for a minimum of 4 days and not applied during inclement weather.

Apply levelling coat of plasto-elastic acrylic polymer based dispersion with pore filling and crack bridging properties (>1mm proprietary levelling material).

Apply protective elastic coating system capable of accommodating seasonal movement of cracks up to 0.3mm wide, with minimum carbon dioxide diffusion of 100m equivalent of air and a maximum water vapour diffusion resistance of 4m of air. (1 coat of primer and 2 coats of elastic paint to 400um.)

‘The contractor shall ensure and guarantee that the repair system will perform their functions as protective and decorative surface coatings, with acceptable colour fastness for a period of at least 10 years.’ The Engineer also specified modifications to the window head and eaves details to improve water shedding.


The original extensive deterioration resulted from carbonation-induced corrosion and was largely treated in 1991 in widespread repairs intended to produce a high quality durable and aesthetic finish. Further deterioration caused by corrosion not identified in 1991 was noted in 1993, probably resulting from deterioration at active corrosion sites not identified during the original 1991 survey. It was apparent in 2001 that there were some locations where corrosion was occurring and repairs would be needed. However, due to the paint system, it was not possible to determine if these areas related to failure of repaired or non-repaired areas. The paint system appeared generally in good condition.

The extensive contract photographs available confirm that water jetting was employed, and that a large number of quite sizeable patches were broken out, and the reinforcement treated with a green coating. It is not clear if the edges of the repairs were saw-cut; many photos seem to confirm that this was not the case, and Core 2 seems to have a feathered rather than vertical edge.

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Part of the structure was not repaired or painted in 1991 and now shows extensive evidence of cracking and spalling resulting from reinforcement corrosion. In comparison, the structure that was repaired is in far superior condition.

The occurrence of cracking and spalling in some locations indicates that carbonation-induced corrosion has not been fully prevented, and that carbon di-oxide, oxygen and moisture have penetrated the concrete to some extent since the repair and coating. However, aside from these relatively minor exceptions, the repair and coating strategy appears to have been effective, with no significant maintenance demand in the last 10 years. Parts of the coating system are deteriorating, which perhaps is not unexpected.

The specification and method statement may not have been followed exactly. There is no evidence that the perimeter of each repair was saw cut, and the ‘fairing coat’ seems to have been replaced with fairly thick outer layer of repair mortar. Both repairs were deeper than 50mm, where repair concrete might have been used. The quality of the coating to the reinforcement does not appear to be high; the petrographic examination identifies patches in the coating.


Water jetting provides a good substrate for a patch repair to bond to.

Coatings applied to the reinforcing bars should be checked before application of the bulk repairmaterial.

Hand application may not provide a good contact at the interface, where voids may be present.

Voids may be generated at repair layer interfaces.

Cracking may occur from the external surface to the embedded reinforcement bars; use of shrinkagecompensated materials may not prevent this.

Coating is likely to minimise general carbonation from the surface, but may not be effective inpreventing gas diffusion along cracks.

Carbonation can be expected to occur to a considerably enhanced depth along the surfaces of crackscompared to at the external surface.

Comprehensive repair and coating can be effective in controlling maintenance and aesthetic qualities inthe medium to long term in structures suffering widespread carbonation-induced corrosion.

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2.3.4 Site 04A

Site investigation record. Site no: 04A

Date of inspection/testing: 25/2/01 Contractor details: Coring contractor, low scaffold required

Type of structure Car Park Location: London region

Constructed 1972 Repaired: 1994


Mostly in good condition, with repairs evident to unpainted soffits, but with some cracking and spalling to the running surface of the concrete slabs (where not surfaced with bituminous layer). The parapet walls are painted and mostly in good condition, and the outline of some repairs can be made out.

Generally good condition, but with numerous minor defects such as small spalls over bars with low cover in the parapet walls, cracking at previously infilled service pockets in the walls, and cracking in the soffits of the waffle slabs.


The soffit repairs appear mostly sound but small number contain fine cracks. The repairs in the parapet walls are obvious in some locations due to the occurrence of a network of cracks at the surface, reminiscent of shrinkage cracking or crazing.

Repairs to parapet walls are overpainted but the perimeters can often be made out. Mostly these repairs appear sound but some contain shrinkage-like cracks and a small number have incipient or open spalling through ongoing corrosion of reinforcement.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C slab (soffit)

Repair proprietary Material(s): modified high

build repair mortar

Coatings/ none render:

Condition:

The repair was an elongated strip along the length of a reinforcing bar. To increase cover, the repair was finished 5-10mm proud of the soffit. The repair appeared mostly sound but contained a crack of 0.15mm width with one localised patch of corrosion products on the soffit close to the core location. Hammer tapping indicates sound concrete.

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Photo of repaired structure (location 2): Element Type: R/C parapet wall

Repair proprietary Material(s): modified repair

mortar

Coatings/ anti-carbonation render: paint

Condition:

Location and perimeter of repair is identifiable due to the occurrence of a network of fine shrinkage cracks in the wall. There is no other evidence of deterioration of the repair or coating. Hammer tapping indicates sound concrete.

Tests Conducted:

Test Results

Half-cell Location 1 values from +14 to –162mV with sharp increase in negative potentials potential (SSC) towards the strip of repair where the values are more negative than –140mV,

indicating uncertain probability of corrosion, and possible increase in moisture content around the crack in the repair.

Location 2 values from –50 to –175mV with gradual increase towards deck and highest values around the connection to the steel. The results indicate low to uncertain probability of corrosion, and possible increase in moisture content towards the deck and around the core location.

Covermeter Location 1: confirms minimum cover in soffit at 10-20mm and slightly higher in built-up repair.

Location 2: confirms grid of bars with minimum cover 10-15mm

Pull-off Not carried out at this location

Samples:



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Location 1

Repair, in 2 layers, to 39mm thickness, (layers approximately 10 and 16mm), containing a vertical fine crack from the soffit to the interface with the substrate. The repair has debonded from the concrete substrate and the interface is coated with corrosion products. There is no obvious coating to the reinforcement bar, which has corroded.

There are locally abundant voids between the repair layers.

Location 2

Repair, in 2 layers, to 47mm thickness, (layers approximately4 and 40mm), containing a fine crack from the external surface to the reinforcement bar. There is no obvious coating to the reinforcement bar, which has traces of corrosion product. There is patchy high porosity and locally abundant voidage at the interface between repair layers.


Petrography

Repair material from Locations 1 and 2 are similar and consist of a small quantity of siliceous fine, rounded sand in a matrix of dominated by PFA cenospheres with lesser amounts of portland cement. There is obvious layering in both samples. The fine cracks in both samples are normal to the external surfaces and intersect the reinforcement within the repairs, and are considered to result from drying shrinkage.

At Location 1, the reinforcement bar is mainly within the repair but close to and locally at the interface between the repair material, suggesting there was little over-break behind the bar (contract specified minimum 12mm). The depth of carbonation in the bulk repair material is moderately low (6mm), but there is localised carbonation along the crack to the full depth of repair (32mm). There is also evidence of minor deterioration of the cement paste, caused by moisture penetration along the crack.

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At Location 2 there are locally abundant voids around the reinforcement bar in the repair, with minor corrosion of the bar surface. A crack passes from the surface to the bar. The depth of carbonation in the bulk repair material is very low (1mm), but there is very minor and localised carbonation, and traces of secondary calcium carbonate, along the crack up to 34mm depth. The corrosion of the bar is considered to relate to minor ingress of oxygen and moisture along the crack. There is patchy high porosity and locally abundant voidage at the interface between repair layers.

The paste surrounding the bar in each sample appears to be patchily carbonated. The cause for this is not known. Both samples show very low levels of microcracking and low porosity in the repair material. The interface between repair and substrate is very irregular and sharply defined. Close to the contact with the repair there are locally abundant microcracks in some of the coarse aggregate particles, which tends to be parallel or sub parallel with the interface, and be within 2-5mm with the interface. These are considered to relate to mechanical damage during break-out and could represent a potential plane of weakness.

Chloride Content

Chloride content was determined on bulk samples of repair and substrate in both locations, and results expressed as %by weight of cement assuming 14% OPC by weight of concrete.

Location 1, 0-40mm (Repair): 1.21

Location 1, 40-70mm (Deck): 0.50, indicating ingress of chloride, probably through the deck and out of the crack.


Location 2, 40-70mm (Wall): 0.07, indicating no apparent chloride ingress along crack in wall.


Meetings with the client’s principal project engineer. Contractors method statement, noise and safety assessments, COSHH details. Drawings of car park, repair locations, clients records of repair contract and correspondence, detailed description of repair system, data sheets, a complete refurbishment contract document, repair measurement sheets, investigation and test results.



Initial site investigations were carried out in 1991, and determined that the chloride content to the decks was consistent and relatively low. The client carried out a comprehensive visual survey of the car park, finding that the parapet walls of the car park had suffered spalling due to carbonation-induced corrosion at areas of low concrete cover, and isolated defects to the deck soffits. Various repair options were considered, including cathodic protection (CP), and the strategy adopted involved repair to spalled areas and areas of low cover, with coating of the parapet walls to control future low cover problems. The object of the repair contract was to restore the car park to original condition and appearance; there appeared to have been considerable structural reserve and therefore the principal concern was aesthetic. The owners engineer indicated that future maintenance of the car park may not involve patch repair, and that controlled degradation and coating might be more appropriate.

The repair contractor carried out a hammer survey. The client supervised the contract full time.

26


The contractor proposed a proprietary repair system incorporating a number of related materials. The client had specified the system should hold BBA Agrement certification. Note that an elastomeric coating was recommended in the repair system literature, over the repairs, but a normal anticarbonation coat was selected for all surfaces.

The methods of repair was as follows:

Surfaces to be cleaned by water jetting. Edges of repair to be saw cut with angle grinder. Repairs areas were broken out with pneumatic breakers. Bars cleaned by abrasive blasting and treated with a highly alkaline proprietary primer. Repairs filled with a dense polymer-modified mortar with 50N/mm2

ultimate strength (proprietary repair mortar in layers of 20mm maximum on vertical surfaces and 12mm overhead, or with lightweight polymer-modified mortar with 30N/mm2 ultimate strength (proprietary lightweight mortar applied to a maximum of 60mm on vertical surfaces and 30mm overhead. Completed areas to be primed with a sealercoat and 2 coats of proprietary coating material.

It is not clear if the bonding bridge (slurry bond coat) originally selected was used, nor whether a fairing coat was applied.


The repair material appears to be the lightweight version of the mortar, and was applied in layer thicknesses within those specified.

The original deterioration resulted from carbonation-induced corrosion and was largely treated in 1994 by local patch repairs intended to produce durable and aesthetic finish. In 2001, there were very few locations where corrosion was identified, and the paint system appeared to be in good condition.

The occurrence of cracking at Location 1, and a small number of other locations in the soffit repairs, probably indicates ongoing chloride-induced corrosion probably related to seepage through the deck of salty water carried into the structure by cars. This deterioration is likely to be ongoing and lead to local failure of some repairs.

There is no evidence to suggest that the widespread carbonation-induced corrosion in the parapet walls has not been effectively treated. However, carbon di-oxide, oxygen and moisture can penetrate the repairs along the cracks that intersect the surface, and although not currently of great significance, are expected to be the performance-controlling factor.

The overall repair and coating strategy appears to have been effective, with no significant maintenance demand in the last 7 years.

The specification and method statement may not have been followed exactly. It is not clear if the reinforcement was coated, or a bonding coat used.


Shrinkage cracks in the repair can pass from the surface to the embedded reinforcing bars, and can bridge the paint system.

Carbonation occurs preferentially along cracks and can affect the repair material to the full depth and at the reinforcement.

Pneumatic breakers may cause microcracking in the substrate.

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There is some evidence of carbonation of the paste of the repair at the reinforcement; the reason for this is not clear.

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2.3.5 Site 04B

Site investigation record. Site no: 04B


Type of structure Car Park Location: London region



Generally good condition, but with numerous minor defects such as small spalls over bars with low cover in the parapet walls, cracking at previously infilled service pockets in the walls, and cracking in the soffits of the waffle slabs.


Repairs to parapet walls are overpainted but the perimeters can often be made out. Mostly these repairs appear sound but some contain shrinkage-like cracks and a small number have incipient or open spalling through ongoing corrosion of reinforcement.

Repair details:



mortar


Condition:

Location and perimeter of repair is identifiable due to the occurrence of a network of fine shrinkage cracks in the wall. There is no other evidence of deterioration of the repair or coating. Hammer tapping indicates sound concrete.

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Repair cementitious Material(s): mortar


Condition

Perimeter of repair is visible and sounds hollow during hammer tapping. Cracking and incipient spalling at the edge of the repair suggests deterioration is ongoing.

Tests Conducted:

Test Results

Half-cell Location 1 values from -1 to –104mV with most locations less negative than – potential (SSC) 100mV, indicating low probability of corrosion. The highest value, at the

connection, may reflect wetting during coring.

Location 2 values from +22 to –146mV with sharp increase in negative potential at the core sample location, confirming ongoing corrosion of the bar, but with most locations less negative than –100mV, indicating low probability of corrosion.

Covermeter Location 1: confirms minimum cover in wall at 10-20mm.

Location 2: indicates bars at depth close to the corroding area and shallow bars with minimum cover of approximately 15mm in the parapet wall.

Pull-off tests Location 1: two tests have a failure plain passing from the interface between concrete and repair, but also passing up to 15mm within the concrete substrate beyond the repair (at 0.1 and 0.9 N/mm2) and one failed in a flat interface between the layers of repair material at 10mm depth (at 0.6 N/mm2).

Samples:



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Location 1

Repair, in 2 layers, to 47mm thickness, (layers approximately10 and 35mm), containing a fine crack from the soffit to the interface with the substrate. There is no obvious coating to the reinforcement bar, which has only traces of corrosion product on the surface, and is at 16mm depth.

There is patchy high porosity and locally abundant voids between the repair layers.

Location 2

Mortar infilling a cavity (possibly a service duct or lighting cavity) with cracking in the mortar and concrete substrate resulting from corrosion of reinforcement at 100mm depth.

The repair has debonded from the substrate which contains intense and abundant cracking.

The mortar is almost fully carbonated, and the concrete substrate carbonated to a depth of 6mm.


Petrography

The repair materials from Locations 1 and 2 are different. At Location 1, the mortar consists of siliceous fine, rounded sand in a matrix of portland cement, and is similar to the material at Site 04A, but without the PFA cenospheres. There is obvious layering. The reinforcement is intersected by three fine cracks, one of which is normal to, and intersects the external surface. The cracks appear to result from drying shrinkage. There is one reinforcement bar within the repair but in place close to the interface with the substrate material, suggesting either there was little over-break behind the bar or the sample was from the edge of a repair area. There are locally abundant voids close to the bar. The depth of carbonation in the bulk repair material is very low (1mm), but there is very minor and localised carbonation along the crack up to 22mm depth i.e. deeper than the reinforcement. The traces of corrosion on the bar are considered to relate to minor ingress of oxygen and moisture along the crack. The level of microcracking and of porosity is low.

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The interface between repair and substrate is very irregular and sharply defined. Close to the contact with the repair there are locally abundant microcracks in some of the coarse aggregate particles, which tends to be parallel or sub parallel with the interface, and be within 2-5mm with the interface. These are considered to relate to mechanical damage during break-out and could represent a potential plane of weakness.

At Location 2, the repair material is a medium to coarse siliceous sand in a matrix of portland cement, forming a single layer up to 45mm thick. This is mostly carbonated and has a patchy moderate to high level of porosity, suggesting a relatively high original water/cement ratio. The concrete substrate close to the mortar contains abundant surface-parallel cracks similar to those noted for Location 1.



This structure is related to Site 004A and was repaired under the same contract.

Meetings with the owners principal project engineer. Contractors method statement, noise and safety assessments, COSHH details. Drawings of car park, repair locations, clients records of repair contract and correspondence, detailed description of repair system, data sheets, a complete refurbishment contract document, repair measurement sheets, site investigation data.


Initial site investigations were carried out in 1991, and determined that the chloride content to the decks was mostly relatively low, but there were some locations with high concentrations of chloride, mostly in the outer 25mm. The client carried out a comprehensive visual survey of the car park, finding that the parapet walls of the car park had suffered spalling due to carbonation-induced corrosion at areas of low concrete cover, and isolated defects to the deck soffits. Various repair options were considered, including CP, and the strategy adopted involved repair to spalled areas and areas of low cover, with coating of the parapet walls to control future low cover problems. The object of the repair contract was to restore the car park to original condition and appearance; there appeared to have been considerable structural reserve and therefore the principal concern was aesthetic.

The repair contractor carried out a hammer survey. The client supervised the contract full time. The method was as follows:

The contractor proposed a proprietary repair system incorporating a number of related materials. The client had specified the system should hold BBA Agrement certification. Note that an elastomeric coating was recommended over the repairs, in the system literature, but a normal anti-carbonation coat was selected for all surfaces.

Surfaces to be cleaned by water jetting. Edges of repair to be saw cut with angle grinder. Repairs areas were broken out with pneumatic breakers. Bars cleaned by abrasive blasting and treated with a highly alkaline proprietary primer. Repairs filled with a dense polymer-modified mortar with 50N/mm2

ultimate strength in layers of 20mm maximum build on vertical surfaces and 12mm overhead. Completed areas to be primed with a sealercoat and 2 coats of proprietary anti-carbonation paint.

It is not clear if the bonding bridge (slurry bond coat) originally selected was used, nor whether a fairing coat was applied.

It is not likely that Location 2 was included in this repair contract, and was most likely treated at an earlier phase of refurbishment for which details have not been received.

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The repair material at Location 1 appears to be a polymer modified cementitious repair mortar. It may have been applied in layer thicknesses greater than those recommended by the manufacturer of the repair system.

The original deterioration resulted from carbonation-induced corrosion and was largely treated in 1994 by local patch repairs intended to produce durable and aesthetic finish. In 2001, there were some locations where corrosion was identified, but it was not clear whether these areas were within original concrete, the mortar-filled cavities, or the 1994 patch repairs. However, in one location (see detail photo below) it was confirmed that the 1994 repair had spalled, probably due to ongoing corrosion of a relatively shallow bar. The paint system appeared to be in relatively good condition.

Carbonation-induced corrosion in the parapet walls appears to be occurring in some patches. The anticarbonation paint may therefore not be fully effective. There is also slight evidence that carbon dioxide, oxygen and moisture can penetrate the repairs along the cracks that intersect the surface, and although not currently of great significance, this is expected to be a performance-controlling factor.

The overall repair and coating strategy appears to have been moderately effective; there now appears to be a low maintenance demand.

The specification and method statement may not have been followed exactly. It is not clear if reinforcement bar was coated, or a bonding coat used.

View of core taken from repair site, containing typical surface cracking.

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Location 3

Detailed view of open spall in parapet wall, revealing the junction between the fine-grained repair mortar and the coarser concrete, with an exposed, corroded vertical bar. The visual evidence suggests the repair at this location failed to prevent ongoing corrosion, and it is interesting to note the lack of cover to the bar (minimum of 14mm), and the proximity of the exposed bar to the original concrete. It is possible that either shrinkage cracking in the mortar, or at the repair/concrete interface, allowed corrosion.


Shrinkage cracks in the repair pass from the surface to the reinforcement bar, and pass through the paint system.

Carbonation occurs at a very low rate in the proprietary repair material, with notably less penetration in the standard material compared with that in the high-build/light weight material.

Carbonation may penetrate along the surfaces of cracks to considerable additional depths.

The recommendations of the repair system (to use an elastomeric coating) might have prevented or reduced the penetration of carbonation along cracks in the underlying repair materials.

Pneumatic breakers may cause microcracking in the substrate, which may influence the pull-off resistance.

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2.3.6 Site 05


Date of inspection/testing: 4/12/01 Contractor details: Coring contractor (scaffold tower required)

Type of structure Major road viaduct Location: Southern England

Constructed 1938/40 and Repaired: 1993-4 1956/60


Extensively repaired structure in variable condition with some incipient and open spalling and localised rust staining in the reinforced concrete beams. Significant deterioration noted to concrete and repairs surrounding bearings. Extensive evidence of seepage of water and road salts through the deck.


Many repairs present and likely to represent more than one repair phase. There are numerous extensive sprayed concrete repairs to the beams and which are all visible as the underside of the bridge is not painted. Repairs often contain crazing but appear sound. A small but significant proportion have deteriorated most notably at the bearings where chunks have delaminated and fallen to the ground.

Repair details:

Photo of repaired structure (illustrative of location 1): Element Type: transverse beam

Repair proprietary Material(s): sprayed repair

micro-concrete


Condition:

The repairs were over an extensive area of the beam, with patches of reinforcement corrosion towards the base, and crazing in some locations. A hammer survey confirmed the repair and surrounding concrete was sound.

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Photo of repaired structure (illustrative of location 2): Element Type: transverse beam beneath bearing to cross-beam


micro-concrete


Condition:

The repair was in the vertical face of the beam and had spalled and delaminated. A hammer survey confirmed the repair and immediately surrounding concrete was unsound and was removed to mitigate future falling hazard.

Tests Conducted:

Test Results

Half-cell potential (SSC)

Location 1 values range from -95 to –331mV with a marked increase in negative potential towards the base of the beam face and around the patches of corrosion product indicating high probability of corrosion.

Location 2 values range from +42 to +109mV in the original concrete surrounding the spalled repair indicating low probability of corrosion. The exposed reinforcement is clean and uncorroded.

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Covermeter Location 1: confirms minimum cover to face of beam varies from 20 to 50mm.

Location 2: confirms minimum cover to face of beam is approximately 50mm for the vertical bars in the concrete and in repair, and deeper for the horizontal bars.

Samples:


The repair material in Cores 1 and 2 (Location 1) was identical and came from the same repair. The repair material in Core 3 (Location 2) was similar. All consisted of a dense dark grey sprayed material with light grey limestone particles up to 2mm in diameter and a dark grey binder with many small voids less than 1mm in diameter.


Location 1, Core 1

This core was taken from an extensive (2m x 2m) repair in an area with surface crazing. No reinforcement was found. The edges of the repair were not sampled and were not distinct due to overspray/surface coating.

Location 1, Core 2

This was taken from an area of sprayed concrete 1.5m from Core 1, with rust stains at the flattish but irregular, unpainted external surface. The substrate includes a horizontal interface (a construction joint) and a crack, whilst the second crack passes mostly through the repair. The cracks are coated with corrosion products.

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Location 3

Repair area has mostly delaminated; the core was taken at the edge of this area and intersects the plane of delamination which passes through both repair material and substrate concrete. The perimeter of the repair has sawn edges. The exposed reinforcing bars were fully encapsulated within the repair and despite the fracture, are completely uncorroded.


Petrography

The samples all consist of sprayed concrete with an obvious layered structure over a concrete substrate. The sprayed concrete in each sample is similar and is composed of crushed, angular particles of limestone of 3mm maximum nominal size in a portland cement paste with brown resinous material coating or filling the voids. The paste is generally of low porosity with abundant particles of unhydrated cement. There is typically some alternation with lighter coloured layers containing slightly less unhydrated cement and slightly higher porosity. The voids are irregular in shape and sometimes are concentrated in layers. The external surface at each location is slightly rough with a thin laitance.

The concrete substrate at the three locations is similar, and consists of a flint coarse aggregate and siliceous sand within a highly porous portland cement binder. The original water/cement ratio appears to have been high, at 0.57 to 0.6. Traces of ettringite occur in some voids, indicating some penetration of moisture and a low level of recrystalisation of the hydrates.

In each sample there is patchy carbonation to up to 3 to 10mm depth in the cement paste of the concrete substrate close to the contact with the repair. There is very little penetration of carbonation (<2mm) in the sprayed concrete repair material but there is localised carbonation to depths of 31 to 24mm along cracks in the repairs at Locations 1 and 2 respectively.

Location 1, Core 1

This core, in one piece, was taken from a large area of sprayed concrete repair in good condition, with slight surface crazing or fine cracking orientated normal to the external surface and penetrating to at least 27mm depth. The sample consists of sprayed concrete which is firmly attached to the concrete substrate. The core has a flatish but slightly irregular, but not rough, external surface, indicating the sprayed concrete has been finished. It is unpainted. The sample contains no reinforcement.

The substrate concrete is at a depth of 60-75mm and is rough and irregular with particles of coarse and fine aggregate exposed, and appears to have been broken out mechanically. There are traces of microcracking mainly within the cement paste, close to the interface with the repair. The substrate concrete contains flint coarse aggregate, siliceous sand and light buff paste with high excess voidage.

The repair material contains a moderate to high level of microcracking.

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Location 1, Core 2

The core is in four pieces, divided by two cracks (sub-parallel with the external surface) and a joint (normal to the external surface), although there is no parting between the repair and substrate. Fine cracking occurs in the repair material, including one normal to the external surface which passes to a depth of 40mm, along which corrosion products have leached to the external surface. There is one main crack, sub-parallel with the substrate, which has a very flat fracture surface, and which appears to have split aggregate particles in both the repair material and substrate concrete. This is coated with black to brown corrosion products.

A corroded reinforcing bar was intersected at 65mm depth. The sample consists of sprayed concrete over the concrete substrate at a longitudinal joint, on the upper side of which is a mortar consisting of siliceous sand within a porous portland cement binder. Corrosion products are found in the mortar, on the crack surfaces and at the external surface. The sample contains a 2mm diameter piece of wire within the mortar substrate.

The repair material contains a moderate level of microcracking. Close to the substrate there is a zone of locally intense microcracking passing from the repair into the substrate. Within the repair material, the cracks contain gypsum and thaumasite, whilst in the carbonated paste in the concrete there is only gypsum. High levels of chloride were also associated with the cracks. These observations are consistent with a very localised but severe sulphate attack and penetration of moisture containing chlorides. The most likely explanation for this is seepage of water from the carriageway, down through the beam forming the substrate, and out through cracks into the repair at the surface. Close to the interface with the sprayed concrete, the paste in the substrate mortar is stained with corrosion products. Microcracks within the paste are also filled with these products.

Location 2, Core 1

This core is in two main pieces and was taken from a delaminating repair area beneath a bearing. The external surface is flat but slightly irregular and unpainted. The sprayed concrete repair has detached from the substrate and removed some substrate concrete with it, indicating the bond between repair and substrate is very strong. There is no evidence for the corrosion of the exposed steel. The delamination plane is very clean and smooth and appears to have resulted from vibration or impact related to the bearing above. The sample contains cracks which intersect the reinforcement, but there is no evidence for corrosion. The reinforcement is at 52mm depth.

The surface of the substrate is rough and irregular with particles of coarse and fine aggregate exposed. There are traces of microcracking mainly within the cement paste, close to the repair interface. The repair material contains a low level of microcracking.


Meetings with engineers responsible for maintenance of the viaduct and examination of the archived records. Extensive records were found of investigations into condition of the structure at various ages, and references are made to apparent condition of repairs of various ages. The contract document, including records of repair location, from the 1975 repairs, was found. A copy of the original 1993 tender document, including the conditions of contract, specification and bills of quantities, was found in the archived records. A final report dated 1994, from consulting engineers involved in the repairs was also recovered, and this provides a further record of the specification, a record of repair method and site condition, a list of the products and suppliers used in the repair, and some photographs of the contract during execution. For the 1993/4 repair contract, the contractors repair specification and extensive photographs taken during the contract were supplied by the repairs Contractor.

39



The 1975 contract required the location and determination of defective areas of concrete, stripping back to sound concrete, cleaning and protecting of reinforcement, and application of sprayed concrete. At this time, extensive deterioration was apparent throughout the structure. The specification was the Ministry of Transport ‘Specification for Road and Bridge Works’ 1969, Series 1619, Concrete Repairs, with additional requirements for the materials used to comply with British Standards. The specification required preparation of the substrate to 5mm behind the outer layer of reinforcement, coating of the bars with a rust inhibitor, and reinstatement with sprayed concrete achieving a strength of 30 –40N/mm2, with a lightweight anti-crack mesh reinforcement at >25mm cover in repairs >50mm deep. After curing, a silicone-based water repellent was to be sprayed onto the repairs.

Investigations were carried out between 1975 and 1978 into apparent lack of adhesion between the sprayed concrete and the substrate. In areas where the original concrete had been cut back to sound concrete the bond was good. In areas of ‘run out’ or ‘over spray’, where sprayed concrete was applied to the original concrete surface, the bond was poor. It was concluded that the bulk of the repair was sound and protecting the reinforcement as intended, and therefore no further should be taken other than checking condition during routine inspections.

A principal inspection in 1985 identified widespread deterioration and a further special investigations were commissioned in 1986 and 1987. These identified severe corrosion of reinforcement and spalling. Areas of spalling were also identified on the faces of the pier bearer beams under the deck (tie) beams. The deck beams sit directly on the bearers without bearings and the damage appears to relate to movement of the deck beams (this observation remains valid in 2001). Testing found carbonation at between 5 and 40mm depth, cover depth typically of 35-45mm, and identified a small proportion of test locations where half-cell surveying indicated uncertain and high probability of corrosion.

Chloride contents were locally high at 25-50mm depth and some mean values for elements were above those associated with initiation of corrosion, particularly under the longitudinal and transverse joints in the deck. The half-cell investigation was repeated as some high positive results were thought to be anomalous. However, further positive values were found over extensive areas. A negative shift of up to 250mV was achieved when the surface 1-3mm of concrete was removed by abrasion, and it was concluded that care must be exercised in interpreting the half-cell values in dry or well protected areas.

Subsequent testing and inspection in 1992 reconfirmed poor visual condition, very high localised chloride contamination at joints, and generally low levels of carbonation. The results did not identify a clear difference in performance between the two different sections of the structure (c1939 and c1959). Minimum cover was of variable depth, and typically 15 to 30mm. A detailed visual condition survey in 1993 found moderate and severe corrosion, crazing, spalling and delamination in the 1975 gunnite repairs.

The area examined in this research appears to have been repaired in 1975 and in 1993.


The original structure was built in the late 1930’s with a new section added in the late 1950’s to carry additional lanes. Extensive repairs were carried out in 1975 using sprayed concrete, with less extensive repair episodes apparently in 1974, 1976 and 1979. Regular and special inspections were carried out which identified, by the early 1990’s, areas damaged by water ingress, particularly where previous repairs had failed. Special inspection reports confirm the locations of defects and locally high chloride contents in the concrete. Defects (spalling and delamination of earlier repairs) were to the bearer beams, tie beams and cantilevered lengths of deck soffit between the bearer beams. The greatest number of defects were on the older part of the structure.

40


The specification used for the contract was the Specification for Highway Works , 1991, with 12 additional clauses concerning:

Cleaning of concrete surfaces (to remove contaminants)

Sprayed concrete (required to be pre-bagged polymer modified dry spray material with cement content >400Kg/m3

Removal of concrete (requiring saw cutting to 15mm and break-out to 25mm beyond the rear face of the bars, reported to have been by hand held mechanical tools)

Preparation of reinforcement (reported to have been by dry air blast cleaning and then primed with 2 coats of primer)

Preparation of concrete (reported to have been by dry air blast cleaning to remove debris)

Procedure trials (a trial was carried out and incorporated into the permanent works)

Repair with sprayed concrete (reported to have been in accordance with the manufactures instructions)

Surface finish to repair (reported to have been by wooden floats to a dense, smooth, uniform surface)

Curing of repaired areas (reported to have been by sheltering under tarpaulins and plastic sheeting)

Transporting and placing sprayed concrete (reported to have been using equipment and method to produce a dense and homogenous covering without sagging or slumping)

Quality control testing (reported that two trial repairs were carried out and achieved the required properties)

Protective coating (reported to have been sprayed onto surfaces in a layer 2-3mm thick) to be applied in areas where there is less than 35mm cover, comprising a mortar of cement, aggregate, fibrous fillers and copolymer dispersion, to protect the surface from de-icing salts and carbonation.

The Engineer reported carbonation depths of up to 58mm and break out to depths of over 100mm, a total of 1836 defective areas with an average repair depth of 69mm.

The contractors specification was as follows:

The locations for repair were marked by the operator. These were broken out using pneumatic breakers and the edges were saw cut. Exposed reinforcement was inspected and replaced where severely corroded. Some areas of concrete were water jetted at 3000 psi and the reinforcement was cleaned by jetting. A reinforcement primer was applied in 2 coats to the clean steel. The primer was a cement modified epoxy resin based anti-corrosive primer and bonding bridge to be applied in a minimum thickness of 1mm. The size of individual patches varied from small spalled areas to large delaminated patches of gunite up to 2-3 square metres. The 1993/4 repair work used a dry sprayed concrete repair system containing polymers, microsilica and superplasticisers complying with the Department of Transport bridge engineering directives. In addition, the operator marked areas of low reinforcement cover where a thin (2-3mm) cementitious coating was to be applied.

41



The records available for this structure are amongst the most comprehensive found in this research.

The final report written by the Engineer after the 1994 repairs provides the single most comprehensive guide to repairs carried out in 1993/4.

It is assumed that the majority of repairs evident in the structure were carried out in the 1993/4 repair episode, but there may be some remaining 1975 repairs. It is possible that the visual appearance of the repairs at different ages is similar.

The 1975 repairs showed extensive evidence of deterioration after 10 years, and were mostly replaced after 18 years.

The form of the surface of the substrate is consistent with break-out by mechanical methods (as reported by the engineer) .

The coarse carbonation noted in the substrate may in part post-date the repair (Location 1, Core 2). The evidence for this is limited, and carbonation could have occurred at the time between break-out and reinstatement. Post-repair carbonation would indicate some limited ingress of CO2, probably along cracks or at the interface between repair and substrate.

The fine cracking in Location 1 and the moderate to high levels of microcracking within the sprayed concrete repair appear to relate to shrinkage.


The crazing and fine cracking at Location 1, Core 1 penetrates to 27mm which is beyond the depth of cover for the shallowest bars. This cracking could be a performance limiting feature.

At Location 1, Core 2, there is a crack through the repair which is coated with corrosion products. The repair is otherwise dense and of high quality, and it is possible that the cracking results from corrosion products from within the deeper substrate concrete migrating along fine cracks or microcracks in the repair and causing it to split.

At Location 1, Core 2, there is microcracking at the interface between repair and substrate containing sulphate and thaumasite phases resulting from localised sulphate attack. This has probably resulted from seepage of water from the road through the concrete and repair.

At Location 2, there is delamination of the repair and attached substrate concrete without corrosion of the embedded steel. This indicates that the bond between the repair and concrete must be very high, and also that there is a mechanical cause (vibration or impact) for the delamination, which may not have been adequately accommodated in the design. This form of deterioration was noted in 1985 before the 1994 repairs were commissioned.

Treatment of the symptoms, and failure to treat the cause of deterioration, may have limited the durability of repairs and necessitated repeat repair episodes.

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2.3.7 Site 06


Date of inspection/testing: 19/4/01 Contractor details: Coring contractor

Type of structure Road bridge Location: Midlands

Constructed c1960 Repaired: 1993


Generally good condition but with uncoated repairs visible and some very localised and minor spalling resulting from corrosion of the steel with low cover.


The bridge includes several types of repair. Small pockets of repair occur on the abutment walls and soffits of the pre-stressed standard bridge beams, and show no evidence of deterioration (Location 3). Larger repairs to the parapet wall/abutment connection contain a network of cracks but are apparently undeteriorated (Location 2). There is one areas of repair at the wingwall below the parapet wall where the repair is cracked and has white efflorescence indicative of water seeping through the concrete deck. There are also repaired vertical cracks in the parapet wall (Location 1) which do not appear to have opened.

Repair details:


Repair low viscosity Material(s): epoxy resin


Condition:

There were approximately 30 crack repairs in the two parapet walls. All appear undeteriorated and are bounded by concrete that appears sound. Hammer surveying identified no delaminated areas.

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Photo of repaired structure (location 2): Element Type: R/C wall/abutment

Repair proprietary Material(s): microspray

concrete

Coatings/render:

Thin cementitious render used to obscure repair /substrate interface at surface

Condition

The repair contained a grid of very fine cracks at the surface that corresponded closely with the location of the vertical and horizontal reinforcement. Hammer surveying confirmed the substrate was sound.

Photo of repaired structure (location 3): Element Type:

Repair Material(s):

Coatings/ render:

Condition

abutment wall

proprietary styrene butadiene mortar

none

The repair was a discrete patch showing no evidence of deterioration and no deterioration was found in the surrounding concrete.

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Tests Conducted:

Test Results

Half-cell Location 1 values from vary from -38 to +158mV indicating low probability of potential (SSC) corrosion. There is a general increase in positive potential up the wall (i.e. away

from the damp deck).

Location 2 values range in the repaired area from –10 to –316mV but with most values less negative than –200mV, indicating uncertain probability of corrosion. In the original concrete the values range from –76 to +75mV, and are mostly positive, indicating low probability of corrosion.

Location 3 values from vary from -25mV to -283, and are mostly in the range – 30mV to –180, indicating low to uncertain probability of corrosion. There is a general increase in negative potential towards the base of the wall (i.e. toward the ground).

Covermeter Location 1: confirms minimum cover in wall is consistently 40-45mm.

Location 2: confirms minimum cover in wall is consistently 39-45mm.

Location 3: indicates bars have consistently deep cover in excess of 50mm

Pull-off no suitable locations found

Samples:


All three locations show different materials in contact with a light brown concrete containing a mixed gravel aggregate.


Location 1

Core through near-vertical crack in parapet wall. The upper figure shows the core after wetting and some drying; the crack is highlighted by the continuous damp patch through the concrete. The crack is clearly visible through the core in hand specimen. The arrow on the lower figure indicates ‘upwards’ and the crack has been infilled with a plug of material at the surface.

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Location 2

This core was taken close to Location 1 at the parapet wall/abutment connection at the edge of the deep repair. At the external surface the repair has a vertical junction with the adjacent concrete which is steeply inclined into the substrate so that the depth of repair increases rapidly. The repair was greater than 200mm in depth and may have formed the full depth of the wall at this point. The repair consisted of a micro-concrete containing light grey/brown/pink aggregate particles (<10mm) within a grey cementitious matrix. The lower figure shows the external surface with a faint fine crack visible passing above a horizontal reinforcing bar.

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Location 3

Core taken at the edge of a relatively small patch repair, and including the interface between the dark coloured repair mortar and the substrate concrete. The perimeter of the repair appears to have been saw-cut. The repair is to approximately 40mm depth and the reinforcement is at 50mm depth and fully incorporated within the substrate, which is apparently undeteriorated. It is not clear why this repair was carried out.


Petrography

Location 1

The sample contains a vertical crack that is in most places truncated at the surface by a narrow groove filled with a light grey resin mortar, but also sometimes reaches the surface. The thin section shows the crack passing to the external surface through the concrete close to the resin channel. The resin has been ground flat with the adjacent concrete surface, and infills a channel 5mm deep and 5mm wide. The channel has a smooth, curved base and slightly rough sides and resemble sawn surfaces. There is little evidence for the development of microcracking of the concrete and the resin/substrate interface appears sound. The material consists of siliceous sand of 0.3mm maximum nominal size in an isotropic resin matrix of very low porosity containing no microcracks.

The concrete adjacent to the crack is carbonated to a general depth of 0.5mm and maximum depth of 5mm along the crack. There is evidence of leaching and recrystallisation of the cement hydrates, with the formation of secondary ettringite in voids and microcracks close to the main crack, caused by moisture movement.

Location 2

The repair has a layered structure indicative of a spray-applied material. Voids are concentrated at the reinforcement. The external surface of the repair is coated with very thin layer of medium to dark grey cementitious mortar (measuring 0.5-1mm in one location and 1-4mm in another), containing a fine sand of siliceous and calcareous lithologies. There is a fine crack, orientated perpendicular to the external surface, and passing from it to one of the embedded reinforcement bars, and to a maximum depth of 60mm. The sprayed material contains crushed, angular particles of recrystallised limestone (<3mm diameter) within a dark grey binder based on portland cement, and containing an abundance of unhydrated cement grains. The voids are largely empty, but there traces of ettringite in those close to the crack passing from the surface.

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The concrete substrate has a rough, broken surface with a sawn, vertical margin at the surface. There are fine cracks and microcracks in the substrate running approximately parallel to, and sometimes truncated by the contact with the repair, in keeping with the substrate having been prepared by mechanical means. The repair has remained firmly attached to the substrate.

The reinforcement, at 40mm and 69mm depth, was coated with a thin layer of fine resin mortar. The bars show no evidence of corrosion. The repair material is carbonated to depths of up to 4.5mm. Carbonation has penetrated the repair along the surfaces of the crack to a depth of 21mm, and into the concrete substrate up to 1mm from the contact with the repair.

Location 3

This location has a dark brown patch repair composed of a coarse rounded sand within a binder containing abundant unhydrated cement grains, of very low porosity. There is no fine cracking and very low levels of microcracking within the repair. The repair material has negligible carbonation from the external surface and traces of localised carbonation around entrapped voids. There is no evidence of significant deterioration of the repair material.

The edges of the repair are sawn and the substrate at the base of the repair is broken with fine cracks and microcracks in the substrate, in keeping with the substrate having been prepared by mechanical means. The concrete is carbonated to up to 34mm depth from the external surface, but there are only traces of carbonation in the concrete surfaces covered by the repair material.



Conversations with the county council engineer responsible for maintenance of the bridge. The refurbishment contract was overseen by the local borough council who retained scant records of the repair. No as-built drawings were recorded. However, the repair contractor was confirmed and a set of annotated drawings and the repair specification from the repair scheme was provided. The repair contractor has confirmed that detailed records of the contract will be made available.


The repair specification required the contractor to submit a more detailed methodology for both investigation of the defects to determine depth of repair and for repair activities including preparation, treatment, application, materials and surface finishes. The extent of repair was as per marked on contract drawings. All defective and carbonated concrete was to be removed and all corroded steelwork exposed for examination by the resident engineer, and cleaned by blast cleaning or similar, then coated with 2 coats of proprietary 2-component, sulphate and zinc-free, silicate clinker/epoxy resin steel primer. Excavations were to have square cut sides without feather edging and to be cleaned to remove dust with compressed air. Repair materials were to be polymer modified cementitious repair mortars based on an approved polymer type of styrene acrylic or equal equivalent and approved by the BBA. The full repair system was to be certified by the BBA. The bonding bridge was to be a proprietary material or similar with minimum bond strength of 2-3N/mm2. The repair mortar was to be a proprietary material with minimum bond strength of 2-3N/mm2 and minimum compressive strength of 45-55N/mm2.

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The abutment joints (location 2) were to be broken out and reformed with proprietary gunite or similar, in a single application up to 150mm thickness, and with minimum bond strength of 2-3N/mm2 and minimum compressive strength of 50-60N/mm2. The material was to be a single component, cementbased, polymer-modified, pre-bagged product manufactured to British Standards and containing silica fume and high range water reducing agents formulated for machine application using the dry process without set accelerators. Finish was to be by trowel.

Crack injection was to be to surfaces that were clean, sound and free from loose material and contaminants, using proprietary crack injection material, or similar, in accordance with the manufacturers recommendations and with minimum material properties such as viscosity (1800cps @5°, 340 cps @20°), specific gravity (1.08Kg/l), E modulus (3.9 x 10-4 Kg/cm2 ), compressive strength (90N/mm2), tensile strength (45N/mm2 ), flexural strength (85N/mm2 ) and shrinkage (negligible).

Details of the materials and processes used were confirmed by the repair contractor, as follows (March 1993):

Crack repair at Location 1 was by exposing crack with grinder, fixing injection nipples, crack injection with low viscosity epoxy resin, remove nipples and face up crack with styrene butadiene mortar. daily activity report sheets showed nipples were fixed.

Break-out at locations 2 and 3 was by pneumatic breaker (CP9-type) after saw-cutting of the perimeters. Repair at Location 2 involved visual and hammer testing to define extent of repair, disc cutting of perimeter to 25mm, break out of concrete by hand held pneumatic breaker, test for carbonation with phenolphthalein, clean reinforcement and apply rust inhibitor, followed by spray applied repair material to existing profiles. Material was a proprietary sprayed repair mortar ; a cement based, polymer-modified one-component material with silica fume and high-range water-deducing admixtures and 60-70N/mm2 strength at 28 days. The steel was coated with a proprietary anticorrosion primer to be applied to c0.5mm thickness, and day sheets show was applied in 2 coats.

Repair at Location 3 involved visual and hammer testing to define extent of repair, disc cutting of perimeter, break out of concrete by hand held pneumatic breaker, test for carbonation with phenolphthalein, clean reinforcement and apply rust inhibitor, brush apply bonding bridge followed by hand applied styrene butadiene mortar to finish flush with existing, and finally cure with damp hessian and polythene.


Location 1: the effectiveness of the crack repair is limited by the resin channel repair not always being in the area of the crack. The petrographic examination of the thin section shows the main crack to be empty and it does not appear that the crack was successfully treated by crack injection. The paste close to the crack has deteriorated through the penetration of moisture, but it is not clear if this occurred before or after crack repair, or both. There has been some small penetration of carbonation along the crack (5mm). It seems that this crack repair has not been fully effective as the sealing channel misses the crack in some places and the crack itself was not sealed. The locations remain apparently unaffected by corrosion of embedded reinforcement. It is not clear whether an alternative method of treatment was agreed.

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Location 2: the petrography confirms the repair was broken out by CP9 or similar mechanical means, a reinforcement primer was used, and sprayed repair material applied. This has subsequently cracked, and this would be significant if the reinforcement was not protected from the crack by what appears to be a dense and intact primer. The crack passes through the thin finishing coat applied over the surface of the main repair. This repair is effective because it is a system i.e. there is more than one line of defence. It might not be considered aesthetically effective, and it might be less effective without the reinforcement primer.

Location 3: the petrography confirms sawn edges and mechanical break out with repair mortar reinstatement. A bonding bridge from substrate to repair was not detected. The repair appears to be effective but the cause of repair remains unclear.


Resin injection of fine cracks may not be effective.

Great care is required to trace the occurrence of cracks at the concrete surface and further attention required to create a sawn channel intersecting the crack.

Sawn channels in patch repairs make good surfaces against which to repair and cause little deteriorationin the substrate.

Pneumatic breakers appear to result in significant cracking in the substrate.

Fine cracking may develop within sprayed repairs and pass direct to the embedded bars, and bridge thincementitious surface layers.

Carbonation can penetrate the repairs along the surface of the crack to considerable depths.

Microcracks within the repair material do not penetrate the reinforcement primer.

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2.3.8 Site 07



Type of structure Aircraft hangar Location: Southern England



One structure repaired and overpainted, in apparently excellent condition. One structure (investigated) with partly completed repairs and not overpainted. The hangar has numerous cracks and spalls over corroded reinforcement bars. There were clearly more than one generation of repairs, and some old repairs had cracked and spalled.

General visual condition of REPAIRS.

Repairs have been made to the reinforced concrete columns and infill walls and are easy to identify. On one elevation, there are large areas of open spalling not treated in the repair contract. The repairs appear sound but many contain shrinkage-like cracks. One shows minor rust staining at one of the cracks.

Repair details:

Photo of repaired structure (location 1): Element type: R/C wall

Repair proprietary material(s): modified repair

mortar

Coatings/ local light render: coloured paint

Condition:

Area contains several generations of repair; the darker, lower repair was carried out in 1995 and showed traces of reinforcement corrosion at the surface, but otherwise a hammer survey indicated the repair and immediately surrounding concrete were sound.

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Photo of repaired structure (location 2): Element Type: R/C wall/column


mortar (high build)


Condition

Repair is not coated and contains crazing at the surface with traces of white deposits at the cracks. Hammer survey reveals there are localised hollow areas (100mm x 200mm) within the repair, but the surrounding substrate is sound.

Tests Conducted:

Test Results

Half-cell Location 1 values are mostly from -50 to –100mV with an increase in negative potential (SSC) potential (to –176mV) in the lowest 0.5m of wall close to ground level, indicating

low to uncertain probability of corrosion. The potentials do not increase in areas of open spalling. The values probably reflect the dry nature of the substrate and the increase in moisture content towards ground level.

Location 2 values are mostly from +3 to –80mV indicating low probability of corrosion, but with an increase in negative potential (to –138mV) close to ground level, and to –195mV at the connection, indicating uncertain probability of corrosion and possible increase in moisture content towards the ground and around the core location.

Covermeter Location 1: Cover is highly variable, with open spalls with exposed reinforcement allowing cover to be measured directly at the perimeter as 0-10mm minimum. Minimum over in repairs measured as 0-5mm by covermeter. In other parts of the wall, minimum cover found to be 30mm.

Location 2: cover to horizontal bars measured at between 30 and 40mm minimum cover; main vertical bars are deeper.

Pull-off Location 1: all tests have failed within the concrete 5-30mm beyond the repair at 2.4, 0.9, and 0.9 N/mm2 . One failure plane contained the impression of a slightly corroded smooth reinforcing bar.

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Samples:


Grey repair mortar applied to original light coloured concrete with limestone aggregate.


Location 1

The repair is formed by a single main layer up to 28mm thick and contains 2 reinforcing bars at very low cover.

Location 2

The repair is in two layers with fine cracks visible at the surface (see lower figure) passing to the reinforcing bars which occur at the base of the repair.

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Petrography

The substrate concrete in both locations consists of a calcareous aggregate in a highly porous portland cement paste with a highly irregular interface surface. The concrete has a low level of microcracking, but at Location 2 there are microcracks associated with the fine cracks close to the reinforcement. There is also some increase in microcracking within 0.5mm of the interface with the repair, possibly resulting from break-out.

The repair at Location 1 is a dark grey mortar containing a rounded, medium siliceous sand, in a binder with abundant particles of unhydrated cement and abundant PFA, with polypropylene fibres. The material is in one main layer of 6-28mm thickness with a thin layer (<1mm thick) at the interface with the substrate which contains very little aggregate. This interface is very rough and irregular and the contact between the two materials is sound, with little microcracking and few voids. There are two transverse reinforcing bars within the repair, close to the surface, with 0-1mm cover and 4-7mm cover respectively. Though carbonation penetrates the repair material only to a depth of 0.2mm in general, around these bars there is carbonation to a depth of up to 14mm. These bars show traces of corrosion. The paste is generally of low porosity, contains few microcracks and some empty voids.

The external surface was painted with a light grey paint.

At Location 2 the repair is a light grey mortar containing some particles of unhydrated cement and PFA cenospheres, with polypropylene fibres, in two layers totalling 40-48mm thickness. The sand is similar in composition to that at Location 1 but there are small differences in apparent grading. The outer layer is 4-11mm thick, with a very thin, brushed surface layer of 1mm thickness. The interface between the two layers is marked by locally abundant very fine voids. There are fine cracks passing from the surface to the base of the repair, intersecting the reinforcement. White deposits occur around the cracks at the external surface. There are two transverse reinforcing bars which are partly encapsulated within the repair material and partly at the interface between repair and substrate. These bars show traces of corrosion. Carbonation extends to a general depth of 12mm into the repair mortar, and penetrates to 18mm maximum depth along the cracks.

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There is also patchy carbonation of the concrete substrate at the interface with the repair and around the reinforcement at this depth. The paste varies in porosity, but is mostly low with some locally highly porous paste close to clusters of voids. The voids contain abundant ettringite-like phases. There are cracks around the reinforcement and at the interface between repair and substrate, which are separated in this sample. This may have resulted from the coring.



Meetings with the owner’s site manager, maintenance manager, and consulting engineer. Condition survey reports from 1989 and 1994 (condition in 1989 confirmed the hangers had widespread incipient and open spalling, and carbonation depth to be generally greater than the reinforcement depth). The site manager confirmed there had been earlier phases of repair, carried out by the previous operators (MoD), for which there were no records available.

Safety assessments, method statements, drawings of the structures, defect locations, bills of quantities, clients records of repair contract and correspondence were supplied.

The supplier of the repair materials suggest the mortar used was a slightly different (fibre reinforced) system to that recorded in the specification. They report the system was more likely to be pre-bagged product and fibre reinforced. The contract was executed at the time at which these materials were becoming more commonly used than that reported in the contract information (an unreinforced 2-part system).


The client commissioned their consulting engineers to carry out a detailed survey of the structures, marking all defects on plans and elevations of the buildings. The client’s quantity surveyors then prepared the bills of quantities. The deterioration was widespread and attributed to carbonation-induced corrosion throughout the structures. The repair contractor took 10 dust samples from columns, walls, and trusses for chloride content analysis and found low levels of chloride present (mostly <0.1% by weight of cement). Results of carbonation testing at this time were not found.

The extent of repairs was such that the client could not fund a comprehensive repair of all the structures. Therefore the work was prioritised with the most prestigious of the structures (structure A) repaired in full and then coated with a high quality coating. Repairs were then carried out to the most critical structural elements of the less prestigious structure (structure B), with the remainder of the budget used to repair the less critical infill walls. This structure was not coated and indeed there remain some completely unrepaired areas. The consulting engineer reports ‘the external areas (for repair) were restricted to those which were absolutely essential for the stability of the columns. Areas adjacent to columns and in the intermediate panels which did not affect the general stability were all intentionally omitted’. The present investigation was concentrated on the repairs to Structure B. The repair contractor carried out a hammer survey to confirm the extent of repairs at each location. The client’s engineer supervised the contract part-time and provided written progress reports and snagging lists.

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The repairs involved break-out by water-jetting and/or manual break-out (there are references to both) and reinstatement with proprietary repair system materials (proprietary primer to the reinforcement, proprietary acrylic co-polymer bonding bridge as concrete primer and proprietary repair mortar or high build repair mortar). The mortar was a polymer modified cementitious, fibre modified, thixotropic, two pack material, with low-shrinkage characteristics. The manufacturer advises the mortar be applied in layers >4mm but <20mm per layer to a maximum 70mm on horizontal surfaces, (no maximum thicknesses given for vertical applications), and in layers >5mm but <40mm for the high build version. The specification required the mortar to be finished flush with surrounding areas. The compressive strength of the normal material was approximately 50N/mm2 and that of the high-build material approximately 22N/mm2. The edges of the repairs were to be sawn, and the exposed reinforcement cleaned by grit blasting. The measurement of the work was carried out in accordance with the Method of Measurement for Concrete Repair issued by the Concrete Repair Association.

Additional work to Structure A involved application of a general purpose levelling mortar and an anticarbonation, elastic membrane with textured finish.

The work to Structure B differed slightly to that in Structure A, as there was a greater occurrence of repair to the external elevations and the areas and depth of break-out were generally greater. There was also resin grouting of cracks, but these were not found.

There was a significant overspend (1.47 times the contract value) on Structure B resulting from a misunderstanding in the designation of repairs related to certain details and the greater depth of breakout required. The contract had been awarded to the lowest bidder, with most experience of the site, but with re-measurement on completion. In respect of the overspend, the consulting engineer stated ‘there has been a failure in our understanding of the way the log sheets were being interpreted….For closure control in the future we feel that it is essential in repair work of this nature:-

- to include a large contingency sum.

- for all log sheets and site instructions to be more directly related to sums in the Bills of Quantities

- to reduce the time lag between work being executed and the date when the quantities are measured’’.

A year after the issue of the certificate of practical completion, some repairs in Structure B were noted to contain surface crazing. The repair contractor stated these appeared to be related to surface shrinkage, and carried out remedial work by applying a proprietary levelling coat (appropriate for damp proof thin film renders) to cover the surface. The engineers assistant also noted that at least one crack had appeared in unrepaired concrete close to a repair shortly after repair completion.


The original deterioration resulted from carbonation-induced corrosion and was largely treated in 1995 by extensive patch repairs and overcoating with a high quality elastomeric coating on Structure A and less extensive repair with no overcoating on Structure B. The coated structure was inspected and the repair locations were not identifiable; no evidence of ongoing deterioration was found, indicating these repairs were effective to date.

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The uncoated repairs to Structure B contained some shrinkage-like cracks, and in rare locations there was evidence of corrosion of bars very close to the surface. There were also unrepaired locations with open spalling dating from many years ago. It could be argued that the repair scheme to this structure has been of very limited effectiveness, as significant areas remain in poor condition, largely resulting from insufficient funds. The completed repairs may also not be effective where the reinstated cover is very low. Carbonation has penetrated the repair material along the cracking from the surface, and the cracks appear to have allowed the movement of moisture through the material. On the other hand, the intention was always only to repair the structurally significant elements, and not all apparent damage.

The repair at Location 1 appears to be the standard repair material whilst that at Location 2 appears to be the less dense high-build version. A 1mm-thick bonding layer appears to be present in Location 1 and a 1mm-thick fairing/levelling coat at Location 2.

The ettringite in the voids at Location 2 suggest some penetration and movement of moisture through the paste of the repair. The corrosion of the reinforcement of this sample is likely to result from carbonation and moisture penetration. The crack visible at the surface is likely to relate to drying shrinkage. The white deposits at the surface are in keeping with moisture movement along these cracks.


Shrinkage cracks in the repair pass from the surface to the bar.

Carbonation occurs in the repair materials along the cracks to considerable additional depth.

Break-out causes microcracking in the substrate.

Lightweight or high build mortars may be less resistant to carbonation than dense repair mixtures.

Reinforcement, if close to the surface, can act as a pathway for carbonation.

Carbonation appears to occur in patches at the interface between the repair and substrate, and around bars embedded in the repair or at the interface. This suggests exposure to carbon dioxide either at the time of repair or to a very small amount of it subsequent to the repair. If the latter, this would suggest the bar surfaces and repair interfaces are acting as a preferred route for gaseous migration.

The specification may result in poor details – e.g. to finish the surface of the repair flush with the surrounding concrete, despite it being known there was low cover.

The repair material may be changed without an obvious record being kept.

As structure B remains in use, despite some areas of open spalling, it could be concluded that targeted repairs can extend the service life if full funds for refurbishment are not available.

Application of a high performance coating to the external surface would be likely to improve the

durability of the repairs and unrepaired structure.

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2.3.9 Site 08


Date of inspection/testing: 21/02/01 Contractor details: No contractor.

Type of structure Coastal power Location: SE England station

Constructed 1963 Repaired: <1995, 1997


Structure regularly inspected, assessed and maintained. Numerous defects of minor structural significance related to corrosion of bars with low cover.


One area has been repaired and coated whilst a second area has been only repaired.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C wall

Repair patch repair Material(s): mortar


Condition:

The repairs (<1995) remain intact whilst numerous adjacent areas of concrete with low cover have continued to crack and spall.

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Repair proprietary patch Material(s): repair mortar

Coatings/ proprietary render: cosmetic coating

Condition:

Repairs (1997) were carried out to locations with low cover; the outlines of the repairs can be identified beneath the coating. One repair had a small hollow-sounding area but was uncracked.

Desk Study



Conversations with operators engineer and repair contractor.


An inspection by the operators engineer in 1994 identified the locations were suffering cracking and spalling as a result of corrosion of reinforcement with cover low cover (typically <10mm). The locations were reviewed in a further inspection phase in 2000/1.

The time of and details of repair at location 1 was not confirmed. The repairs appear to consist of sand/cement mortar, with straight, apparently saw-cut perimeters, and contain no obvious cracks. The locations of repair are obvious as there is no coating to the surface.

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At location 2, repairs were executed under a large refurbishment contract intended to provide aesthetic, durable repairs and return the structure to a safe, serviceable condition. The edges of the repairs were sawn, and break-out carried out with mechanical breakers. The exposed reinforcement was cleaned by wire brushing and/or needle gun, and primed with a single coat of a proprietary two-pack styrenebutadiene rubber/cementitious material. This was also used as a bonding coat for the substrate. The repair material was applied wet on wet and was a polymer modified cementitious, fibre modified, thixotropic, two pack material, with low-shrinkage characteristics. The manufacturer advises the mortar be applied in layers >4mm but <20mm per layer to a maximum 70mm on horizontal surfaces, and in layers >5mm but <40mm for the high build version. The specification required the reinforcement be reinstated with a minimum of 20mm cover where this could be provided within the depth of the repair the mortar was finished flush with surrounding areas. The compressive strength of the normal material was approximately 50N/mm2 and that of the high-build material approximately 22N/mm2. In some locations with numerous repairs to the concrete or and/or fixings, a coating was applied to the external surface.


The repairs to at Location 2 appear to have been carried out as per the specification and in keeping with the general requirement to provide a durable repair with good aesthetic properties. At Location 1, nearby, repairs were not carried out. The reason for this was not clear and could in part have related to cost and programme. The resulting level of maintenance is unequal between the two locations.


Repair of areas with low cover may be effective in the immediate vicinity of repair but other areas with low cover may then deteriorate. The performance of the repairs and unrepaired areas could be improved by application of a high performance coating.

Implementing two different maintenance strategies in different locations of the same structure may result in differences in condition and repair demand.

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2.3.10 Site 09


Date of inspection/testing: 22/02/01 Contractor details: No contractor.




Structure regularly inspected, assessed and maintained. Some general deterioration (cracking and spalling) apparent to the external surfaces mostly related to low cover to reinforcement.


Repairs to the external surfaces are mostly in excellent condition.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C wall, columns

Repair hand placed Material(s): proprietary repair

mortar


Condition:

Repairs have been built up so they are 20mm proud of the external surface.

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Location 1: detail showing fine cracking in the concrete substrate above the repair which is continuous through the repair.

Desk Study



Conversations with operators engineer and repair contractor. Review of inspection records.


An inspection by the operators engineer in 1994 identified the locations were suffering cracking and spalling as a result of corrosion of the reinforcement links with low cover (typically 5mm). Subsequent inspection in 1997 confirmed the deterioration was ongoing. The defects were treated in a large repair contract shortly thereafter. The repairs were identified in a further inspection phase in 2000/1

The works were intended to provide aesthetic, durable repairs and return the structure to a safe, serviceable condition. The edges of the repairs were sawn, and break-out carried out with mechanical breakers. The exposed reinforcement was cleaned by wire brushing and/or needle gun, and primed with a single coat of a proprietary two-pack styrene-butadiene rubber/cementitious material. This was also used as a bonding coat for the substrate.. The repair material was a hand applied, polymer modified cementitious, fibre modified, thixotropic, two pack material, with low-shrinkage characteristics.

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The manufacturer advises the mortar be applied in layers >4mm but <20mm per layer to a maximum 70mm on horizontal surfaces, and in layers >5mm but <40mm for the high build version. The specification required the reinforcement be reinstated with a minimum of 20mm cover and where this could not readily be achieved the mortar was finished proud of the surrounding areas and profiled using wooden forms. The compressive strength of the normal material was approximately 50N/mm2 and that of the high-build material approximately 22N/mm2. In some locations with numerous repairs to the concrete or and/or fixings, a coating was applied to the external surface.


No evidence was found for deterioration of the repair other than the fine cracking. It was not clear if this crack reflected through the repair from the substrate or related to continued corrosion of reinforcement.


Patch repairs can be formed proud of the existing surface to provide increased cover to shallow bars.

Fine cracks in the substrate may reflect through the repair material.

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2.3.11 Site 10


Date of 31/10/00 Contractor Accompanied by employee. inspection/testing: details:

No contractor.

Type of structure Coastal power station – Location: SE England building frame



Structure regularly inspected, assessed and maintained. Numerous defects of minor structural significance related to structural (differential settlement) and/or thermal movement. Some general deterioration (cracking and spalling) apparent to the external surfaces mostly related to low cover to reinforcement.


The repair at this location involved making good of damage to the reinforced concrete frame and application of a impressed current cathodic protection (ICCP) system, with carbon anode paint, to the external building elements of a reactor building (beams and columns). These appear largely in good condition with the exception of some minor debonding of the surface paint layer.

Repair details:

Photo of repaired structure (location 1): Element Pre-cast and insitu RC Type: concrete building frame

Repair proprietary anode paint Material(s): CP system

Coatings/ paint systems render:

Condition:

Mostly good visual condition with no evidence of cracking or spalling. Paint system shows some evidence of local minor damage (impact, abrasion) but with some evidence also of damage immediately over the primary distribution ribbons.

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Location 1.

Localised impact/abrasion damage to the paint system at the corner of a column.

Location 1.

Damage to the paint system at the distribution anode strip.

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Desk Study



Discussion with operators engineer and examination of inspection and monitoring records.

Inspection and investigation by the operator identified that the structure was exposed to a risk of chloride-induced corrosion through a combination of locally low cover to the reinforcement and exposure to atmospheric chlorides. The exposure environment is aggressive due to the proximity to the sea and exposed location resulting in an abundance of salt spray, wind and rain. The beams were originally pre-cast, and the columns insitu concrete. Some pre-cast elements of the building frame also contained calcium chloride as an accelerator. Patch repairs had been carried out previously.

The operator required a low-maintenance solution to preserve the condition of the structure and a CP system was selected to protect the relatively large exposed surface areas. The requirement was for a system that was intended to operate for the limited remaining intended life of the structure, and an anode paint system with limited life of approximately 10 years was acceptable. This was also to act as a permanent trial for consideration in future repairs.


A consultant was appointed to design the system and a contractor installed the system and has monitored its subsequent performance. One advantage offered by the system was remote monitoring. On-site inspections are carried out annually and reports on the monitoring of the system are presented quarterly.

The surfaces of the structural elements over a whole elevation were treated in 1995/1996 using two different techniques (discreet anode and anode paint) within one system. The system was subdivided into 10 zones. Nine consisted of a conductive coating anode with carbon fibre tape primary anode over-coated with a decorative colour coded top coat. A further zone contained discrete probe anodes with a decorative colour-coded top coat over the concrete surface. The anode paint system was recognised as having a limited life and re-coating was accepted as a necessary part of the design for prolonged operation.

After 5 years the CP system continued to perform well. The 10 zones were initially run at relatively high outputs to achieve the operating criteria of >100mV potential decay in 4 hours. The carbon fibre primary anode had become locally discoloured (blackened), due to operation in a wet environment, but was operating correctly. There was one location with damage to the primary anode and various locations with debonding and abrasion of the anode paint. The decorative top coat was more extensively disbonded. At one location, the trunking had been cut to allow installation of cables and supports, and this may have damaged the distribution system. The transformer/rectifiers remained in good condition but some maintenance and overhaul were recommended. There had been some previous replacement to components of the control system. The options for extending the life of the system were evaluated, and two options presented. To ensure a further 5 years operation, some replacement of the decorative top coat and conductive anode coating was recommended. To ensure a further 10 years operation, replacement of the primary anode, conductive anode coating and decorative top coat and was recommended, or to change the anode system to a discrete anode system.


The system is considered by the operator to be a successful medium to long-term solution.

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In order for paint systems to remain in perfect condition, routine maintenance and ‘touching up’, must be carried out.

Anode paint CP systems can provide ongoing protection to building frames exposed to severe weather conditions.

The CP system has required close monitoring and regular maintenance to remain fully operational, and can be damaged by accidents and intentional works to the structure.

Deterioration of the paint system over the fabric/woven distribution ribbons occurred within 4 years. It is not clear if this is caused by wicking of water or the result of high current flow.

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2.3.12 Site 11


Date of inspection/testing: 31/10/00 Contractor Accompanied by employee. details: No contractor.

Type of structure Coastal power station Location: SE England – steam tunnels

Constructed 1963 Repaired: 1993 to 1999


This structure is a tunnel, below ground level, linking the reactor building to the turbine hall. It is regularly inspected, assessed and repaired. However, the extreme service environment (operating temperatures in excess of 60°C) limits the effectiveness of repairs and repair and re-repair are periodically required. Defects occur in the soffit of the tunnel. These appear to relate to the ingress of water and/or salty water at joints and through cracks, and possibly at the interface between repair and substrate materials. The damage originally related to corrosion of reinforcement.


There are several phases of repair including conventional high build concrete repairs and flowable repair concrete in the soffit of the steam tunnel. Many of the repairs contain fine cracks and there is abundant evidence of ingress of water and salts through the soffit in the form of water marks, staining and efflorescence.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C tunnel soffit

Repair proprietary hand Material(s): applied and

flowable repairs


Condition:

Generally sound but with clear evidence of cracking and seepage.

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Location 1.

The soffit at this location, repaired with superfluid concrete, has a network of fine cracks where abundant seepage has occurred. The mesh has been installed to retain spalling pieces and reduce hazards to personnel and to plant until effective repairs could be completed at the following statutory reactor outage.

Location 2.

The perimeter of the repair is marked by a slightly darker zone. A transverse joint passes through this repair. Brown deposits have seeped from the joint.

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Desk Study



Discussions with engineer involved in inspection, assessment and repair of structure, including guided visit to site. Examination of drawings and documents related to condition and repair of the structure, including repair specification and report by the repair contractor confirming the materials used and presenting the findings of the investigations and break-outs during the repair contract.


Detailed inspection by the operator, in 1992 and 1993, identified areas of spalling, cracking and salt crystallisation, with a concentration of defects at the transverse expansion joints, longitudinal cracks initiating at these joints, and other transverse cracks remote from the joints. The cause of deterioration was found to be corrosion of reinforcement due to ongoing seepage of saline water through the tunnel soffit, related to operation of the plant. The source of the seepage could not be satisfactorily remedied and repairs were not expected to be permanent. The tunnel is exposed to high operating temperatures (>60°C) which were also considered likely to affect the longevity of the repairs. Cracks in the repairs were thermally induced and had caused expansion joints and waterbars to deteriorate. The operator selected a strategy of joint and crack sealing, patch repair and further investigation during outages to ‘hold’ the condition and allow further assessment of the problem, letting a series of repair contracts in the early to mid nineties.

The contractor was required to carry out a covermeter survey and sampling of the concrete for determination of chloride content, and carry out a hammer survey and agree with the client’s engineer the extent of any repair. Photographs were required to be taken by the contractor of the repair areas after fully breaking back, and a report submitted of the extent of corrosion and depth of cover found. A series of typical cracks were also to be identified and cut out to determine the condition of the reinforcement at depth. The findings from this activity were to be incorporated into the repair scheme.

All cutting out was to avoid feather edging by disc cutting to a minimum depth of 20mm. Break out was by light electric powered tools and hand tools to expose all reinforcement 50mm beyond the corroded length. It was not necessary to cut behind bars embedded in sound alkaline concrete checked for chloride content. A structural engineer was to be consulted prior to break out to ensure than structural integrity was not impaired, and where severe corrosion of reinforcement was discovered during break-out. The surface finish was subject to inspection and approval by the engineer. The reinforcement was to be prepared by abrasive grit blasting to SA2½.

Where additional reinforcement was required, bars were to be tied and lapped to existing undamaged reinforcement in accordance with BS8110 and where required resin bonded into holes drilled in the substrate.

Repairs were not to be applied to areas with running water; proprietary water stopping materials were to be used to seal the substrate.

Two types of repair processes were specified. Polymer modified cementitious mortar was specified for patch repairs, comprising reinforcement primer (applied in two layers, the second being blinded with sand), a bonding bridge applied to pre-dampened surfaces, and a pre-batched lightweight mortar applied ‘wet-on-wet’ (to depths of 25-50mm) using a placing technique.

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The system selected held BBA certification. Superfluid microconcrete was specified for large repair areas (e.g. Location 1) and required priming of the reinforcement, pre-soaking of the substrate, construction of watertight formwork and placing of the repair material with bleed pipes arranged to ensure expulsion of air and surplus water. The material was a high performance, non-shrink, free flowing fine grade pumpable concrete for filling large voids or around congested reinforcement without segregation. Cube samples were to be prepared for compressive strength testing of the repair material. In addition, cracks were required to be sealed by resin injection of low viscosity, fast curing, solvent free, two component epoxy resin. For cracks 0.5mm wide and larger where water was flowing freely, a thixotropic epoxy resin was to be used with a proprietary water stop. The existing expansion joint sealant was also to be replaced with high performance proprietary filler and sealant.

Repairs have subsequently been carried out. These have involved proprietary cementitious repair concretes applied to substantial areas of the floor in the tunnels and proprietary grouting/crack injection materials intended to reduce or prevent the ingress of water into the structure. These repairs were also acknowledged to be ‘holding’ repairs; because of the external pressure head and difficult operating conditions it is generally accepted that the only feasible option to provide a truly waterproof structure is to reconstruct.

Discrete anode trials at Site 10 were originally to be considered for use at Site 11 but this was never fully realised within the repair strategy.


The approach to repair was based on a good understanding of the causes of deterioration and the repair contract provided additional information on the extent of contamination and deterioration. The objective of the repair was to provide as durable a solution as possible (removing chloride contamination and attempting to prevent water ingress) whilst not being able to remove the source of saline water. The use of discrete anodes as a long term remedial measure was not adopted by the contractor awarded the subsequent repair contracts.

Some repairs have been subsequently re-repaired and are not deemed to be effective in the long term. Repairs have cracked and delaminated through a combination of ongoing reinforcement corrosion and the effects of high operating temperatures and thermal cycling. Steel mesh has been fastened to the soffit in some locations to minimise spalling hazards as a temporary measure between 2-yearly maintenance outages.

The location is regularly inspected and the condition monitored.


It may not always be possible to remedy the cause of deterioration (e.g. an external source of saline water).

Where conventional patch repair is necessary, the repairs may not be effective in the long term in severe environments.

Failure can occur through exposure to high temperatures and thermal cycling and where ingress of moisture and salts is not prevented.

Acknowledging performance limitations in the repairs, and implementing regular monitoring and rerepair, is an acceptable maintenance strategy.

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2.3.13 Site 12


Date of inspection/testing: 31/10/00 Contractor details: Accompanied by employee.

No contractor.




Structure regularly inspected, assessed and maintained. Numerous defects of minor structural significance related to exposure to salts and water.


Repair is in excellent condition.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C slab

No Photo available Repair proprietary hand Material(s): applied mortar +

corrosion inhibitor


Condition:

Good condition with no evidence of deterioration, cracking or seepage. Soffit is unpainted and dry, but obscured by numerous reinstalled services and cables.

Desk Study



Discussions with operators engineer.

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Inspection and investigation of the suspended floors in the turbine hall building identified areas of reinforcement corrosion caused by seepage of saline water through the slabs. The seepage was relatively localised and the operator deemed patch repair, combined with minimising exposure to water, to be suitable in the short to medium term. The soffit was repaired using patch repairs (generally <0.5m x 0.5m in area) incorporating a corrosion inhibitor.

Repairs currently appear effective but future migration of moisture is a possibility (from vessels/pipes above).


The repairs appeared effective after 5 years. There was no evidence of seepage through the slab, and very little evidence of a source of water, suggesting seepage had been treated or working practices altered to minimise exposure of the slab to moisture.


Local patch repairs incorporating corrosion inhibitors can be effective where external sources of water are removed.

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2.3.14 Site 13



No contractor.


Constructed 1963 Repaired: 1980-1990


Structure routinely inspected, assessed and maintained. Structure is suffering reinforcement corrosion to the reinforced concrete frame, and most notably the seaward facing columns.


The building was repaired in 1980 and 1985, and further treated in 1990 with most parts receiving some repair and painting, and one end of the building receiving additional protective repair and coating.

Repair details:

Photo of repaired structure (location 1): Element Pre-cast, Type: columns

Repair repair mortar Material(s):

Coatings/ paint system render:

Condition:

Building frame remains substantially intact but there is extensive evidence of corrosion of embedded steel within the slender columns.

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Location 1.

This view shows part of a seafacing column with a shallow spall over a bar with low cover, and vertical cracks toward the corners of the column, over the main bars. Rust staining is evident from some of these cracks.

Location 1.

This detailed view shows a crack in the column, with associated rust staining caused by the corrosion of the embedded reinforcement.

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Desk Study



Discussions with engineer involved in inspection, assessment and repair of structure, including guided visit to site. Examination of records of condition and repair of the structure, including condition assessment reports, testing reports, repair proposals and specifications, and correspondence relating to performance of each repair episode.


Repairs to cracked and spalled areas of the structure were carried out in 1980-1981, on the assumption that the corrosion was due entirely to lack of cover (typically 10-30mm). In 1984 further deterioration had been detected to the columns, and spalling presented a safety hazard to personnel below. A preliminary investigation found high levels of chloride in the concrete, originating from either suspected calcium chloride used in construction, or use of sea dredged aggregates, or penetration of airbourne chlorides, and concluded that the deterioration noted since 1981 related to chloride-induced corrosion. The presence of hairline cracks was also identified as being a potential contributory factor in deterioration. The operator recommended the most economical solution was to carry out patch repairs every 3 years with visual inspection every year and removal of dangerous spalling pieces.

A visual and hammer survey in 1985 recorded the locations of all apparent defects including spalls, cracks and rust stains to the building frame. Carbonation depth was found to be typically less than 5mm, but chloride determinations confirmed high levels of chloride (>0.4% by weight of cement) at depths of up to 75mm. A large number of cracks and spalls were found and attributed to corrosion of the reinforcement. The survey also found that all the repairs carried out in 1980 were in good condition and the structure was coated with a filled acrylic waterbound emulsion. The 1985 report recommended repair of the damaged locations with dense waterproof repair concrete and coating with a suitable waterproofing.

The proposed repair method involved removing defective concrete and cutting behind the reinforcement where necessary, cleaning corroded reinforcement (by wire brushing) and applying an anti-corrosion treatment, and reinstatement with waterproof concrete applied in multiple coats to the original profiles. The surface was to be sealed with two coats of a proprietary, exterior grade, pigmented acrylic emulsion with coalescing solvent and dispersing agent.

Photographs taken during the repair contract show the break-outs had feathered rather than sawn edges and that concrete was typically broken out behind the main bars.

The 1985 report also recommended ongoing inspection of the building at no less than 5-yearly intervals. The operator adopted inspections on a 2-yearly basis.

In 1990, one (newer) part of the building was further repaired and re-coated. This was part of the lower storey at one end of the structure. No records of these works were found. Further concrete repair was executed in 1997 to one portion of the structure which was designated as housing Safety Significant plant. The condition of this part appeared good.

76



The repair strategy in 1980/81 was only effective locally and unrepaired areas rapidly deteriorated. This led to an investigation that changed the perceived cause of deterioration and affected subsequent repair strategies. A thorough investigation into the cause of deterioration in 1980 would have been appropriate.

Repairs and coating carried out in 1980 and 1985 appeared ineffective in 2001 as there is extensive evidence of corrosion-related deterioration in the external elevations. The causes could not be confirmed but probably relate to ongoing corrosion of reinforcement resulting from the high levels of chloride, and possible incipient anode formation adjacent to repaired locations. The external coating may have substantially reduced the corrosion rate but after cracking recurred corrosion rate is likely to have substantially accelerated.

The cleaning of reinforcement by hand tools may be significant.

The environment is harsh, with high levels of exposure to airbourne salts, spray and rain.


Investigation into the cause of deterioration is essential in providing an effective repair and maintenance strategy.

Elements containing residual chlorides can continue to deteriorate after repair, even when coatings are applied to the external surface.

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2.3.15 Site 14



No contractor.

Type of structure Estuarine power Location: W England station

Constructed 1964-5 Repaired: 1992


Structure regularly inspected, assessed and maintained. No evidence of deterioration was found.


Repair is in good condition and shows no evidence of cracking or delaminating from the substrate.

Repair details:

Photo of repaired structure (location 1): Element Type: Internal R/C high load support beam

Repair proprietary Material(s): flowable concrete


Condition:

This view shows the cracking and crack-monitoring hardware at the beam face prior to repair.

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Desk Study



Discussions with engineer involved in inspection, assessment and repair of structure, including guided visit to site. Examination of data sheets for the materials used in the repair episode.

The problem arose where cracking developed in an element subject to high cyclic loading. The crack was monitored and cyclic movement was recorded, and the operator suspected debonding and yielding of the reinforcement. This was revised after core sampling confirmed that the concrete was understrength and that the embedded bars were not positioned as intended in the design. It was concluded that bars had been displaced during construction, resulting in the portion in tension under load being minimally reinforced, and the element required modification. The area was closed to operations and repaired under close supervision by the operators engineer.


The repair was approximately 0.3m deep, 7m long, and 1.1m wide, at the top of the beam.

The affected portions of the beam were broken out mechanically. The reinforcement was replaced with additional bars as necessary. The substrate was prepared, inspected by the engineer, and saturated with ponded water, then dried to a surface-dry condition. The concrete was reinstated using a single pour of proprietary high strength, rapid strength gain, shrinkage compensated, flowing concrete. This material contained RHPC, PFA, microsilica, and unreactive 5mm aggregate and had a reported bond strength of >2.5 N/mm2 and 28-day compressive strength of 65 N/mm2, at 20°C. The material was cured by ponding of water followed by the application of multiple coats of a proprietary spray-applied membrane.

The repair was tested under load at 4 days and found to meet the required criteria.


The repair was the result of structural distress caused by localised poor quality construction which was identified in the statutory maintenance inspection regime.

The repair appears to have been effective; it is inspected biennually and monitored continuously under high cyclic loads. There is no evidence of crushing, cracking or debonding. The operational environment includes elevated temperatures and low humidity.

The concrete repair acts compositely as a structural load path. Concrete repairs Concrete repairs were part of the investigation and restoration work. The repair was not to address concrete degradation.


Thorough investigation identified the cause of deterioration and helped define the repair strategy.

Close supervision provided a high quality repair in a critical area.

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Only microcracking (<0.05mm wide) was found in this material, despite the relatively large surface area to volume ratio of the repair, the presence of abundant reinforcement and the high temperature/low humidity environment. This may relate to the site supervision and thorough curing insisted upon and in part executed by the engineer.

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2.3.16 Site 15



No contractor.


Constructed 1964-5 Repaired: 1994/5


Structure regularly inspected, assessed and maintained. Localised defects caused by intermittent exposure to salt and water from vessels/pipes resulting in corrosion of the reinforcement.


Repairs have not prevented ingress of salty water and contain fine cracks and associated white efflorescence and rust staining.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C slab

Repair full depth material Material(s):


Condition:

Soffit is wet or damp and extensively crazed and/or cracked with white deposits and stalactites around the cracks.

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Location 1.

Photo of repaired structure (location 2): Element Type: R/C floor slab

Repair full depth repair Material(s):


Condition:

Damp patches, rust stains and cracking in the soffit above cable clusters. This view shows an extensively cracked repair in the soffit, with white deposits associated with the cracks and the perimeter of the repair.

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Desk Study



Discussions with engineer involved in inspection, assessment and repair of structure, including guided visit to site. Examination of drawings, photographs and documents related to condition and repair of the structure, including repair specification. Further intrusive investigation of the repair at Location 1, by the operator, was scheduled for March 2002.

Inspections were carried out between 1990 and 1993 which showed progressive deterioration (cracking and spalling) of the concrete floor slab at several locations, all related to corrosion of embedded reinforcement. This was believed to relate to discharge of large volumes of salty water during maintenance of the plant. Defective areas were commonly associated with pipe and service penetrations. In one location spalling occurred around a pre-existing repair. An investigation in 1993 identified consistently high to very high levels of chloride within the slab, (>0.4% to >3% by weight of cement) and showed a decrease in chloride content with depth. The depth of cover to the reinforcement was typically 30mm. Visual inspection identified very restricted access and working conditions for repair in some locations.

When first designed, the repair strategy was to be accompanied by a revised method of operation of the plant, in advance of the repairs, which would eliminate saline water discharge in future, and the repairs were intended to provide a long-term solution. However, it became apparent that the discharge would continue and the consequence for repairs was noted and they were then considered to be ‘holding’ repairs. Deterioration was noted as little as 2 years after completion, and after 5 years showed extensive evidence of corrosion.


The operator assessed the apparent extent and severity of defects, the likely extent of break-out, and the requirements for temporary support/propping of the slab during the break-out and repair. Structural assessments were carried out to confirm fitness for purpose. A repair specification was drawn up by the operators engineer based on similar defects in reinforced concrete in a similar environment at another structure.

The specification required the repair contractor to provide all necessary protection to the pipework and services and to carry out a hammer survey and dust sampling for chloride testing of the concrete and agree with the client’s engineer the extent of each repair. A covermeter survey was carried out to determine bar depth prior to preparing repair edges.

All breaking out was to avoid feather edging by disc cutting to a minimum depth of 20mm. Break out was by light electric powered tools and hand tools to expose all reinforcement 50mm beyond the corroded length. It was not necessary to cut behind bars embedded in sound alkaline concrete checked for chloride content, but for corroded bars the concrete was to be cut at least 50mm behind the bar. A structural engineer was to be consulted prior to break out to ensure than structural integrity was not impaired, and where severe corrosion of reinforcement was discovered during break-out. The prepared substrate was to be sound, unfractured, and free from friable or damaged concrete. The reinforcement was to be prepared by abrasive grit blasting to SA2½. The surface finish of both prepared concrete and reinforcement was subject to inspection and approval by the engineer.

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Repairs were not to be applied to areas with running water; proprietary water stopping materials were to be used to seal the substrate. Polymer modified cementitious high-build mortar was specified for patch repairs, comprising reinforcement primer (applied in two layers), a bonding bridge applied to predampened surfaces, and a pre-batched lightweight mortar applied ‘wet-on-wet’ (to depths of 25-50mm) using a placing technique. Curing was to be applied to protect against extreme conditions, and no work was to be carried out at temperatures <3ºC and >30ºC. The system selected held BBA certification.


The repair was partially effective but the ongoing, occasional discharge and ponding of saline water provided a renewed source of water and chlorides to the repaired and unrepaired parts of the slab. The repairs have developed crazing in the top surface and cracking and efflorescence visible in the soffit. Further repairs may now be required.


Repair with high quality proprietary systems may have a limited service life where the original concrete retains some chloride contamination and an external source of saline water remains.

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2.3.17 Site 16


Date of inspection/testing: 30/07/01 Contractor details: Employee carried out survey in confined space.




Structure inspected and maintained when station outage allows.


Repair remains in good condition.

Repair details:

Photo of repaired structure (location 1): Element Type: water culvert roof soffit

Repair proprietary high-build Material(s): mortar


Condition:

Repair in good condition.

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Desk Study



Discussions with engineer involved in inspection, assessment and repair of structure. Inspection of the intake tunnel revealed spalling concrete in the soffit and corrosion of the exposed reinforcement bars. Repair was carried out during an outage in 1998. The tunnel is subject to hydraulic head, high water velocity and turbulence. The cause of the original damage is not clearly understood but was associated with cyclic negative pressure during operation and exacerbated by chloride-induced corrosion. The cover to the reinforcement was found to be as specified at >50mm.


The repairs were carried out by the operators staff during an outage. The affected zones were broken out using light mechanical breakers and the exposed reinforcement grit blasted to SA2½ and a cement slurry priming coat applied. A high build mortar was then applied to the soffit. A hammer survey revealed that the first attempt at reinstatement had delaminated or slumped from the soffit and this layer was removed in full. Subsequent application was in thin layers with hammer surveying to confirm adhesion.

Re-inspection in 2001, after 3 years continual operation life confirmed that repair has remained satisfactory over this period. Hammer test confirms no further delamination at present in aggressive environment.


The repair work was designed and supervised and subsequently reinspected by the operators engineer.


Careful review and inspection of repairs is necessary to identify early failure resulting from loss of adhesion or workmanship issues.

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2.3.18 Site 17



No contractor.

Type of structure Estuarine Location: W England power station



Structure regularly inspected, assessed and maintained. No significant defects in this area.


Repairs are crazed at the surface but otherwise appear in good condition.

Repair details:


Repair Proprietary repair Material(s): mortar and

corrosion inhibitor


Condition:

No evidence of further reinforcement corrosion was found.

Desk Study



Discussions with engineer involved in the inspection, assessment and repair of the structure. Inspection of the structure indicated defects were due to low cover to reinforcement and carbonation induced reinforcement corrosion. The wall is at considerable height and is exposed to the prevailing wind and weather conditions.

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Pre-repair investigation revealed carbonation penetrating from the external surface to between 10 and 25mm. Cover to the reinforcement had been specified at >30mm but localised areas have 5 to 10mm cover at the architectural rib features. The mean chloride content of the concrete was reported as 0.13% by weight of cement. Limited areas with corroding reinforcement were identified.


All defects were identified through visual inspection by rope access where necessary. Defect locations were shown on contract drawings and identified in the bill of quantities. Break outs were prepared with sawn edges and the exposed reinforcement prepared by grit blasting to SA2.5. The substrate was presoaked and a bonding bridge applied.

The defective areas were reinstated using a proprietary two component mix comprising a preblended cementitious mortar mix containing RHPC, fine aggregates and additives and acrylic latex. The material has low-shrinkage characteristics and complies with the Department of Transport standard BD27/86. Curing was in accordance with the manufacturers recommendations.

The repairs and unrepaired areas were then treated with a proprietary migrating corrosion inhibitor, applied in repeated coats to the external surface (2 coats of 5% solution followed by eight further coats of 10% solution). The inhibitor is designed to migrate by liquid and vapour diffusion and form a protective layer at the reinforcement, reducing the effects of anodic corrosion even where the concrete is contaminated with chlorides. Cores samples were taken to confirm the penetration of the inhibitor.


Although there is crazing at the surface of the repairs, no evidence was found for deterioration.


The strategy adopted involved patch repair and application of a corrosion inhibitor to repaired and unrepaired faces. The effectiveness was previously unproven and requires monitoring for long term performance.

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2.3.19 Site 18


Date of inspection/testing: 19/10/01 Contractor details: MM team only

Type of structure Tunnel Location: NW England

Constructed 1930s Repaired: Early 1990’s


The majority of the areas of the structure were covered in soft, white needle-like salts. This branch of the tunnel is unused and has been since 1972.


The repair concrete was found to spall away easily to reveal the parent concrete. Hammer tapping indicated sound, parent concrete beneath the repair.

Repair details:

Photo of repaired structure (location 1): Element Column footings Type: in fresh air duct.

Repair Proprietary Material(s): cementitious

render.

Coatings/ None. render:

Comments:

The concrete was covered in soft, white, needle-like sulphate crystals. The repair concrete broke away easily to reveal relatively sound, parent concrete.

There is a small volume of ground water entering the invert and running adjacent to the column footings. Otherwise the environment is dry and sheltered.

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Surface scaling of the concrete lead to concerns about sulphate attack of the column footings. Earlier attempts to provide protection by the application of a bituminous paint had not been successful and so it was decided to apply a dense protective cementitious render.

At the time of the inspection large areas of the render were found to be delaminated with large volumes of white crystals beneath. The original concrete appeared to be relatively unaffected.


Qualitative analysis of the white crystalline material shows it to be predominantly calcium sulphate (gypsum) with a smaller proportion of calcium carbonate (calcite). This is consistent with the deposit having originated from sulphate-rich ground water that has been in contact with Portland cement. The quantities of soluble sulphate present are more than sufficient to initiate sulphate attack in susceptible cementitious substrates.



Conversations with client’s engineers involved in the investigation and repair of the structure using direct labour.

Few formal records of the work exist as it was considered routine maintenance at the time of the works.


The sulphate problem appears to have been known for some time, possibly since construction. During normal operation, rainwater run-off would tend to dilute the groundwater and help rinse away sulphate rich deposits. It is only since the tunnel was closed to traffic that the problem became sufficiently acute to require further action.

Initial attempts to protect the concrete employed bitumastic paint, but this soon became detached as salts started to form beneath the coating. The purpose of the current repairs was to apply a dense, impermeable layer to the column footings through which the sulphates would not be able to travel. This was achieved by applying a relatively thin (typically less than 10mm thick) proprietary cementitious render.

The render was hand applied and cured under polyethylene in accordance with the manufacturers recommendations. The material is hard and inflexible by modern standards and the sulphates have been able to move through the parent concrete and recrystallised at the interface with the render, developing sufficient pressure to detach the render from the parent.

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Neither the parent concrete nor the render appear to have been significantly attacked by the sulphate. There is some slight scaling of the original concrete but this may, in part, be the result of surface preparation prior to the application of the render.


The sulphate attack on the concrete is thought to occur due to the ingress of sulphate-containing ground water from the sump area. There is a lack of water movement in this branch of the tunnel due to its low usage. This generally stagnant environment has lead to evaporation of the ground water, leading to very high sulphate concentrations.

Given the relatively permeable nature of the parent concrete, it is likely that there were problems in adequately removing dissolved sulphates from the substrate prior to applying the render. High levels of sulphate in the parent concrete, coupled with a plentiful supply in the surrounding environment and the relatively low permeability of the render, has apparently resulted in sufficient pressure being developed to cause delamination at the interface.

Site specific observations and conclusions

The main damage has occurred to the render intended to protect the column footings. The parent concrete has been relatively untouched.

In this situation the sulphates are originating from the groundwater and so the source cannot easily be removed. Alternative solutions to preventing the build-up of sulphate-rich deposits could include improving drainage along the tunnel invert, periodic washing down of the invert and column footings or augmenting the existing flow with additional sulphate-free water. By employing such an approach there should be little or no need to use a coating or render and many of the problems encountered with these repairs would be overcome.

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2.3.20 Site 19


Date of inspection/testing: 5/10/01 Contractor details: MM staff only




Adequate, with localised cracking/spalling defects commensurate with age, nature of structure and maintenance regime.


Generally sound with some isolated delamination and corrosion, and some fine drying shrinkage cracks. The majority of the structure remained in adequate condition. The coating was generally found to be sound, although one area was debonding due to moisture ingress.

Repair details:

Photo of repaired structure (location 1): Element Type: Soffit of deck adjacent to pier.

Repair Proprietary Material(s): sprayed concrete.

Coatings/ Waterproofing. render: Anti-carbonation

coating system.

Condition:

Although the bulk of the repair was sound there were some areas of delamination found with associated corrosion of reinforcement.

In addition an isolated number of small repairs were failing (see photo).

Also there was some coating failures in areas of moisture ingress, typically at construction joints (see photo).

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Photo of structure during repair:

Tests Conducted:

Test Results and Comments

Dual Half Cell Potentials relatively stable at 50 – 100mV vs Silver/Silver Chloride. This technique involves the use two half-cells of the same type, one held stationary whilst the second is scanned across the area of interest.

Hammer survey Some isolated delaminations were detected.

Samples:


Corrosion of steel found when spalling cover concrete removed.

Desk Study



Conversations with consulting engineers involved in the investigation of the structure, preparing the specification and supervising the repair.

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Forced ventilation under the soffit of the deck had resulted in very high carbonation rates for the concrete. This ultimately resulted in general corrosion of reinforcement. Repair areas were broken out by hydro-demolition, and reinstated using sprayed concrete, with limited access for supply and removal of material. An anti-carbonation coating was applied over the soffit.

The sprayed concrete was applied directly onto the prepared substrate with no bonding primers to either the steel or the concrete surfaces. The depth of reinstatement was typically in the range 25–100mm.

Subject to subsequent waterproofing of the deck, the repairs were intended to have a minimum of a 25year life to first maintenance. Given the absence of UV radiation and other weathering factors it is believed that the coating should be capable of similar service.

The need for extensive repairs had been recognised for a period of time but was subject to budget and programming considerations. The extent of the repairs was identified by a detailed investigation by the consulting engineer following identification of potential defects during routine inspections. The soffit of the deck was known to have widespread and deep carbonation allowing corrosion of the embedded reinforcement. This was in a Hennebique configuration and corrosion of the reinforcement resulted in widespread delamination of the cover. The owner appointed a consulting engineer to design/oversee the repairs and a specialist contractor executed the repairs. The defective areas were identified (by hammer tapping, visual inspection, carbonation depth measurement) and broken out by water jetting. The exposed steel was also cleaned by water jetting and the concrete was broken out behind the bars to a minimum depth of 15 mm. The concrete was reinstated using a proprietary pre-bagged dry-spray concrete containing polymers, microsilica and superplasticisers complying with the Department of Transport bridge engineering directives. The surface was finished with a dry float with care taken not to overwork the surface. A coating was spray applied over the repairs and extended to cover the whole of the soffit where it acted as an anti-carbonation coating.


The bulk of the repairs are sound, and the half cell potential survey taken in this area showed a low risk of corrosion. However, a number of areas have been identified where corrosion and subsequent delamination has taken place. It is probable that these areas had carbonated sufficiently to initiate corrosion, but insufficient corrosion had taken place to cause delamination at the time of repair. The ongoing corrosion had resulted in delamination occurring after the coating had been applied.

A number of small repairs were found to be delaminating. Typically these repairs had been feather edged. Edges are normally saw cut to prevent this.

The coating had failed in areas where moisture ingress was present. This would be expected.


Correct identification of all areas to be repaired at the time of a contract can be difficult, due to the variable nature of concrete and the variable nature of the deterioration processes. The main deterioration mechanism in this case is carbonation of the concrete resulting in corrosion. In any survey it is probable that statistically rare phenomenon will be missed, e.g. localised excessive carbonation. If even small areas are not apparent then some isolated areas of ongoing corrosion can take place.

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Correct preparation of repair areas is vital to achieve an adequate repair. Small volume repairs with feathered edges did not appear to be as durable as larger repairs with a sawn perimeter.

Identification and control of moisture is important in achieving a durable coating system.

Where repairs are intended to be effective in the long-term, there is a need to address causes as well as symptoms of deterioration and to subsequently monitor the performance of the repair strategy.

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2.3.21 Site 20






Adequate, with localised cracking/spalling defects commensurate with age, nature of structure and maintenance regime.


Adequate, no visual evidence of deterioration.

Repair details:

Photo of repaired structure (location 1): Element Type: Inside of piers 369-370 in fresh

Left hand side of main tunnel walking towards sump near air duct entrance.

Repair proprietary dry Material(s): spray gunite

Coatings/ Waterproofing render: Anti-carbonation

coating system

Condition:

No visible evidence of deterioration within the repair. Perimeter of repair was visible. Hammer tapping indicates sound concrete. No corrosion products staining surface.

Tests Conducted:


Dual half cell No connection could be made to the steel so differential readings were measured potential between two silver/silver chloride half cells. This was performed over the area of

repair. Occasional ‘hot spot’ areas with potentials between -200 and -300mV were discovered, indicating local potential for corrosion.

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Interview with Engineer on Project.


Forced ventilation under the soffit of the deck had resulted in very high carbonation rates for the concrete. This ultimately resulted in general corrosion of reinforcement. Repair areas were broken out by hydro-demolition, and reinstated using sprayed concrete, with limited access for supply and removal of material. An anti-carbonation coating was applied over the soffit.

The sprayed concrete was applied directly onto the prepared substrate with no bonding primers to either the steel or the concrete surfaces. The depth of reinstatement was typically in the range 25–100mm.

Subject to subsequent waterproofing of the deck, the repairs were intended to have a minimum of a 25year life to first maintenance. Given the absence of UV radiation and other weathering factors it is believed that the coating should be capable of similar service.

The need for extensive repairs had been recognised for a period of time but was subject to budget and programming considerations. The extent of the repairs was identified by a detailed investigation by the consulting engineer following identification of potential defects during routine inspections. The soffit of the deck was known to have widespread and deep carbonation allowing corrosion of the embedded reinforcement. This was in a Hennebique configuration and corrosion of the reinforcement resulted in widespread delamination of the cover. The owner appointed a consulting engineer to design/oversee the repairs and a specialist contractor executed the repairs. The defective areas were identified (by hammer tapping, visual inspection, carbonation depth measurement) and broken out by water jetting. The exposed steel was also cleaned by water jetting and the concrete was broken out behind the bars to a minimum depth of 15 mm. The concrete was reinstated using a proprietary pre-bagged dry-spray concrete containing polymers, microsilica and superplasticisers complying with the Department of Transport bridge engineering directives. The surface was finished with a dry float with care taken not to overwork the surface. A coating was spray applied over the repairs and extended to cover the whole of the soffit where it acted as an anti-carbonation coating.

During the contract, one area was found to be contaminated with chlorides resulting from ingress from saline drainage water. This discovery required the area to be rapidly incorporated into the works, with a slightly modified repair procedure. Waterjetting was used to remove as much concrete as possible in contaminated areas and to clean the embedded reinforcement. This was then inspected and break-out extended where condition of bar indicated more extensive corrosion occurred subject to approval by a structural engineer. A system of greased access holes was incorporated in the reinstatement to enable post-repair half-cell connection. The repairs were subsequently inspected on a regular basis, initially quarterly but latterly annually.


While the repair appears visually intact, the half-cell survey suggests that corrosion is initiating around the edged of the repair, as would be expected as a consequence of both residual chlorides in the substrate and ongoing chloride contamination from the saline drainage water.

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Where it is not possible to carry out the ‘ideal’ repair patch repair techniques can be used as holding repairs, but care must be taken, and it should be recognised that the repairs are of a temporary nature. It is anticipated that a more permanent form of repair will eventually be required in this area. The installation of a small impressed current cathodic protection system is considered the most appropriate approach.

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2.3.22 Site 21



Type of structure Bridge widening Location: NW England



Precast beams sound, but some rust staining and delamination of links in areas of low cover.


Repairs generally sound.

Repair details:

Photo of repaired structure (location 1): Element Type: Deflection pocket on precast beam.

Repair In-situ reinforced Material(s): concrete and

precast concrete beams.

Coatings/ Deflection pocket render: originally made

good with mortar containing calcium chloride accelerator.

Condition:

Chloride contaminated concrete inducing corrosion of reinforcement.

Tests Conducted:


Local breakout In one of the areas broken out the repair material came away easily and some underlying corrosion was found. This was probably one of the original factory repairs containing chloride based accelerators.

The deflection pocket is approximately 30mm below the surface. There was no sign of cracking or staining on the surface, although at the time of the 1999 repairs, it had been the presence of rust staining that first brought the problem to the attention of the consulting engineers and the client.

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The other areas appeared sound and were difficult to break out suggesting adequate repair had taken place in the 1990s.

Samples collected from the area were analysed for chlorides using a combination of pore solution expression and dust analysis.

Dust sampling The chloride content of the concrete was calculated using pore solution expression and dust analysis. The results, expressed as %Cl by mass of cement, assuming a cement content of 14%, are as follows:

Free chloride from pore solution expression 0.207

Total chloride from dust analysis 0.214

A chloride ion content of 0.3% by mass of cement is often associated with the threshold for chloride induced corrosion. These values are below this and corrosion via this mechanism is unlikely. However, traditionally it is considered that a significant proportion of chlorides introduced as an admixture can be chemically bound into the cement matrix. In this case the level of free chloride in the sample was significant suggesting either this is not the case, or over time the chlorides are released into solution. This is known to occur due to carbonation of the concrete, which in combination with the chlorides would result in the corrosion recorded. Testing with phenolphthalein showed the repair mortar to be fully carbonated.

Desk Study



Discussions with engineers associated with investigation after rust staining at the deflection pocket locations initiated a more detailed examination resulting in the identification of chloride admixtures in the repair mortar.


During the manufacture of such pre-cast, pre-stressed beams it is necessary to deflect the tendons at some distance from each end anchorage. Once the concrete has acquired sufficient strength, the deflection system is dismantled leaving deflection plates within the body of the beam within a small pocket in the concrete.

At the time these beams were manufactured it was common to employ a calcium chloride based accelerator in the pocket repair mortar to improve productivity. It was only later that the risk to durability due to corrosion was identified.

Previous knowledge of this type of construction therefore suggested that repairs to deflection pockets might have been carried out with mortar containing chloride-based accelerators during fabrication. Dust testing confirmed this at Site 21 and so the repairs were broken out, since there was a risk that chlorides may migrate from the repair to the tendons in the vicinity.

The pocket was reinstated to ensure the durability of the deflection plate and tendons.

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It appears that while the majority of factory repairs had been identified, one had been missed. This has not caused any deterioration to the structure but enabled the comparison between free and bound chlorides presented above.


The use of Calcium Chloride as an agent to accelerate the set has been strongly discouraged since the 1970’s as it is known to be deleterious and increase the risk of corrosion. However, some areas of concrete that contain chlorides from this source. When subjected to carbonation the chlorides that are chemically combined can release into solution and initiate corrosion. It is likely that carbonation was actually the cause of corrosion at this location but it was probably exacerbated by the presence of chlorides. Elsewhere the pockets have been cleaned out and repaired with a chloride-free proprietary repair mortar and so the problem is unlikely to recur in further places on these particular beams.

There is a need to monitor the performance of repairs and review repair strategies when necessary.

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2.3.23 Site 22



Type of structure Bridge Location: NW England



Away from repaired areas the structure shows limited evidence of deterioration.


Coatings have been applied as holding repairs and appear to be performing adequately. There is some minor rust staining, but no significant section loss. A number of the repairs have degraded in UV and turned green.

Repair details:

Photo of repaired structure (location 2): Element Reinforced concrete Type: beams spanning

between two piers.

Repair Polymer modified Material(s): cementitious fairing

coat with inhibitor.

Coatings/ See below render:

Condition:

Evidence of some surface rust but generally the repair has performed as expected. Although UV exposure has discoloured it, the material is still performing adequately.

Tests Conducted:


Half-cell No significant negative potentials found indicating low probability of corrosion. potential

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Desk Study




Chloride contamination of the reinforced concrete resulted in corrosion of steel and hazard to the public associated with delaminated concrete. As an interim measure loose concrete was removed and reinstated with a polymer modified cementitious faring coat containing a corrosion inhibitor. The expected life span was 3 to 5 years. In most deteriorated areas a phased repair system was implemented, but some holding repairs are still present.

The form of holding repair, involving the removal of the loose and delaminated material followed by the application of a cement-based coating, was developed and specified by the consulting engineers in accordance with the clients’ budget and programming requirements.

Exposed steel was prepared by manual wire brushing only and the coating was applied in two layers each approximately 1mm in thickness.

Prior to any holding repairs being carried out the element was inspected by a structural engineer. The repaired areas are subject to regular visual inspection until such time as a full repair is programmed.


Repair has performed satisfactorily and as expected. Discolouration was a known characteristic and helped identify the area in future inspections. It also assists in demonstrating to the public that the problem has been treated. The half-cell survey did not reveal any evidence of corrosion, but care should be used in interpreting half-cell data from reinforced concrete exposed to corrosion inhibitors. Some minor corrosion was visually evident. The concrete was relatively dry and this probably assisted in the performance of the repair.


This repair method was intended as a holding repair with an intended life span of 3 to 5 years and has performed better than expected. It demonstrates there are alternatives to reinstating to line and level using bulk repair materials.

In general where the original problem was due to carbonation coupled with low cover the repair has been wholly effective in preventing further degradation or corrosion. Where chlorides have been present and the bars have been adequately cleaned the repairs have again been effective. Where it was not possible to clean the bars fully then some rust staining has reappeared.

If water jetting or wet blasting were used instead of wire brushing it may be possible to prevent such partial breakdown of the system.

Where holding repairs are carried out there remains a need for monitoring of condition particularly where there is a risk to the public.

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2.3.24 Site 23



Type of structure RC Pier Location: NW England



Typical condition for structure of this age and construction type, some areas of delaminating spalling concrete in unrepaired areas, with underlying corrosion of reinforcement.

General visual condition.

No significant defects, some minor hollow sounding areas.

Repair details:

Photo of repaired structure (location 1): Element Reinforced concrete Type: with mesh and

overlay system on a pier.

Repair Proprietary gunite Material(s): repair material,

anode mesh.

Coatings/ Proprietary gunite render: overlay.

Condition:

Generally sound with some minor delaminated areas.

Desk Study



Conversations with consulting engineers involved in the investigation of the structure, preparing the specification and supervising the repair and with monitoring the CP system since installation.

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Detailed surveys of the worst affected structures found the concrete to be heavily contaminated with deicing salts through leakage of run off through joints.

Due to the extent of the chloride contamination and associated reinforcement corrosion, only reconstruction or cathodic protection were considered viable long-term solutions. Cathodic protection was selected as it avoided extensive propping and traffic management and allowed the structure to be repaired without disruption to normal operations. Repairs have been prioritised over a number of years to include further piers and beams.


The repairs in 1995 were carried out due to chloride induced corrosion of the reinforcement. A cathodic protection system consisting of a mesh and overlay was applied to the reinforcement after cleaning and repairing the concrete employing hydodemolition and dry sprayed gunite. On completion of the works a number of hollow sounding areas were recorded, which on further investigation were not delaminated but did contain large concentrations of cables for the system.

The monitoring system installed at the time is DOS based, with a front end piece of software that was written for the contractor. Some of the hardware was not year 2000 compatible. This was identified at the time but the problem was difficult to rectify. A number of additional systems have been installed, but compatibility is not possible due to the requirement to competitive tender the systems and time period over installation. As a result there is one system that runs in DOS, one that runs in Windows 3.1 and one that runs in Windows 95. The most recent system has just completed its maintenance period and runs in Windows 2000.

The system on the pier shown is running at around 10 to 20% of its design output. The system is monitored using graphite electrodes and silver/silver chloride electrodes, there have been a number of failures of the monitoring probes as evidenced by unstable or erratic readings but there are sufficient remaining electrodes to enable normal operation of the system. No electrode type has performed significantly better or worse.

The CP system has been effective in controlling degradation since 1994 and very low levels of deterioration are predicted for the future. The combination of gunnite repairs and CP therefore appear to be effective in the long term.


When inspecting repairs including CP systems, hollow-sounding locations may represent cable locations.

CP systems should be monitored regularly.

Consideration must be given to how the system will be upgraded or expanded in the future.

Phased introduction of CP systems can be a successful management strategy for structures withchloride contamination.

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2.3.25 Site 24



Type of structure Depot building Location: Southern England

Constructed 1950’s Repaired: 1979


Very poor condition with numerous cracks and open spalls over bars in beams and columns. Building originally used for MoD food production, then as borough council vehicle depot, and demolished in 2001 shortly after inspection.


Close inspection of spalled areas reveals that repairs are also deteriorating. The damage to the structure is so widespread that it is not possible to determine if deterioration occurs preferentially within or without the repairs.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C edge beam to slab

Repair proprietary epoxy Material(s): resin mortar

Coatings/ sand textured render: paint

Condition:

The repair was at the bottom corner of an edge beam close to ground level where the vertical face returns beneath the building. The repair was in a discontinuous strip over the horizontal reinforcing bar, and was built up in 2 layers. This area was sound but adjacent areas of concrete and repair had failed with open and incipient spalling.

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Repair proprietary epoxy Material(s): resin mortar

Coatings/ sand textured render: paint

Condition

The repair was at the bottom corner of an edge beam close to ground level. The repair was in a discontinuous strip over the horizontal reinforcing bar. This area was sound but adjacent areas of concrete and repair had failed with open and incipient spalling.

Tests Conducted:

Test Results

Half-cell Location 1 values range from -69 to –192mV. potential (SSC)

Location 2 values range from –86 to –250mV. In both areas, most areas have potentials more negative than –130mV, indicating uncertain probability of corrosion, and possibly reflecting the dry condition of the edge beam. The most negative result were at the connections to the steel, possibly due to wetting during coring.

Covermeter The main bar (1 inch diameter) to the bottom corner of the edge beam has the shallowest cover, but this was controlled by numerous spacers which were identified in the spalled and spalling areas. The cover was generally between 1.25 and 1.5 inches (31-37mm) with some locations having 1 inch (25mm) cover. The links (0.5 inch diameter) tended to be shallower than the main bars, resulting in widespread relatively low cover (c12-15mm). Both links and the main bar had deteriorated and both had been repaired in 1979.

Pull-off Location 2: two samples with failure within the adhesive (at 0.3 and 0.4 N/mm2), two samples with failure within the repair material (0.3 and 2.1N/mm2), and one failing at the interface between repair and substrate ( 0.6 N/mm2).

Samples:


Light to medium grey repair mortar with fine, granular texture applied to original cream/buff coloured concrete with limestone aggregate.

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Location 1

Core and lump sample with repair material in two layers from the bottom corner of the edge beam. The vertical face is smooth and the under-surface highly irregular. Both surfaces are coated with a thin layer of resinous material. The samples include section of corroded bar and the impressions of bars at the interface with the substrate concrete and within the repair material, both of which are coated with corrosion products.

Location 2

Core and lump sample with repair material in a single layer. The core sample examined contains two vertical bars at the interface between the repair and substrate, at approximately 25mm depth. The outer surface is coated with a resinlike paint material that contains a little mineral dust, which is poorly attached to the repair and has peeled off in places.


Petrography

The substrate concrete at both locations consists of a natural limestone gravel with siliceous sand within a substantially carbonated portland cement paste. At Location 2 there is some uncarbonated paste at depth, containing porous hydrates with coarse grained portlandite. Close to the repair, the paste is carbonated and contains fine cracks to microcracks filled with carbonation product.

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The repair at Location 1 was in two layers in total approximately 50mm thick. The repair contained a rounded, medium siliceous sand and carbonate dust within a substantially carbonated portland cement binder. Full carbonation extends to 15mm depth, but the deeper parts of the repair appear also to be partly carbonated. There are numerous empty small voids in the repair. The material is highly porous. Where the repair meets the reinforcement there is a fracture surface, coated with corrosion products. Carbonation occurs along the fracture surface and on the surface of the corrosion products. The repair material generally makes close contact with the substrate, with few voids, but there are some microcracks passing from the interface into the paste and aggregate of the substrate concrete, and microcracks also passing from the repair into the aggregate in the substrate.

The repair at Location 2 was in a single layer of 5 to 25mm thickness. The repair consists of medium siliceous sand with carbonate dust in a portland cement binder. The binder is strongly carbonated to 10mm depth and is highly porous and contains numerous areas with low cementitious content. The voids contain some crystals of portlandite. No microcracks were found within the repair material. The interface at the base of the repair runs through the broken aggregate of the concrete substrate. There are occasional small voids and small cracks approximately parallel with the interface and extending up to 3mm in length into the substrate. Reinforcement corrosion products occur at the interface with the reinforcement, in the paste close to it, and along some of the small cracks.



Conversations with district council engineers responsible for maintenance of building and engineer responsible for the repair contract in 1979. No documentation from the time of the repair was available.


The structure was inspected by the owner’s engineers in 1978, and found to have extensive low cover (commonly as low as 5mm) and deep carbonation. There was at that time considerable evidence of deterioration through corrosion of the embedded reinforcement. The engineer identified and marked on the structure, by visual inspection and hammer survey, the areas for repair. A reference drawing was used in the repair contract. The engineer recalls there were ‘hundreds of locations, one every 2-3 feet’.

The areas were broken out with hand-held pneumatic breakers, without saw cut edges, and in some areas the break out was extended behind the reinforcement. The reinforcement was treated by sand blasting. The reinforcement and prepared substrate may have been treated with a primer, and the repairs were then re-formed with an ‘epoxy resin mortar’. The application method may have been by spraying and finished by trowel. All exterior surfaces were then painted with a sand textured masonry paint. There was no particular service life specified at the time of repair; the objective was to provide a safe working environment with an appropriate appearance.


It was noted on site that in many areas the repair had been superficial, covering the bar but not extending beyond the bar into the substrate concrete. There remained some areas of sound repair, unaffected by the deterioration in adjacent concrete. There were also repairs that were delaminating due to corrosion in adjacent areas or failed repair or new corrosion.

The repair material did not appear to contain an epoxy resin, although there was a resinous material at the external surface. The material was not of high quality and despite extensive works being carried out, this has not been effective in the long term.

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Care should be taken to identify and break out carbonated substrate concrete prior to repair.

Repairs to substantially carbonated concrete will not be effective in the long term without further measures to control ongoing carbonation or the ingress of water and atmospheric gasses. The resin or paint coating at this location was not effective in doing so.

The concrete substrate at the interface with the repair tends to be damaged, with some carbonation either remaining or developed in the outer part of the concrete, and some cracking of the substrate, possibly related to the break-out/preparation. The performance of the repair and unrepaired parts might have improved by application of a high quality surface coating.

The repair material at Location 1 was almost fully carbonated and would not be expected to provide any protection to the embedded reinforcement.

Repair materials must be carbonation resistant for long-term performance.

Early identification of deterioration in a structure and review of performance of repairs can inform future management strategy and prolong the life of the structure.

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2.3.26 Site 25



Type of structure Bridge pier Location: NW England



The pier is subject to extensive ASR cracking, particularly in the soffit of the cross-head. Improvement in drainage has limited the extent of further deterioration.


The repair was to a core hole, the core having been acquired for petrographic analysis. The repair appeared in sound condition.

Repair details:

Photo of repaired structure (location 1): Element Type: Repaired core

Repair Simple sand/cement Material(s): repair mortar


Comments:

The core was taken through the interface between the repaired area and parent concrete to monitor the effects of alkali migration from repair mortar into adjacent concrete. Petrographic examination was carried out to check for ASR activity at the interface between the repair and substrate.

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Photo of sample Description (supplemented by petrography)

Core 25/2

This sample was taken at the junction between repair concrete and parent concrete. The parent concrete is identical to that described for Core 25/1. The repair concrete contains a crushed recrystallised limestone aggregate up to 10mm in diameter, and a siliceous sand within a dark mottled brown portland cement binder with very low porosity and very low levels of microcracking. There is a surface layer of mortar over the repair material, consisting of a siliceous medium sand in a carbonated portland cement paste.


Core 25/1

This sample was composed entirely of parent concrete and was taken close to the repair location. The concrete contains a partly crushed siliceous natural gravel coarse aggregate and siliceous sand within a portland cement binder which appears to be of SRPC. The concrete is coherent and robust but shows occasional traces of damp hygroscopic gel patches surrounding some particles of chert coarse aggregate. No evidence was found for cracking resulting from alkali-aggregate reaction. The paste contains a moderate to high level of microcracking orientated radially around the surfaces of fine aggregate particles. Near the surface, some of the cracks contain crystals of calcium carbonate and at greater depths some contain portlandite and traces of ettringite.

Core 25/2

The parent concrete is typically carbonated to depths of up to 3mm from the external surface, and to 15mm along microcracks. The thin mortar layer is patchily carbonated throughout, to 15mm depth, but the underlying repair material is uncarbonated.

Traces of gel were found at the interface between repair and substrate concrete at approximately 65mm depth from the external surface. The traces probably originate from the original concrete. No evidence was found for gel or cracking caused by alkali-aggregate reaction away from the joint surface in either the repair or substrate materials.

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The core was originally obtained for petrographic analysis as part of the detailed investigation of the structure by the consulting engineer on behalf of the client. The core was reinstated using proprietary repair mortar. At the time of the repair, no attempt was made to either limit the alkali content of the mortar or isolate the repair from the original substrate with an epoxy bonding bridge. Review at a later date identified the possibility of high cement alkalis in the repair material generating alkali-silica reaction in the substrate, resulting in a potential cracking and spalling hazard.


Trace quantities of gel were found at the interface between repair and substrate, and originate from the original concrete. No evidence of ASR products was found away from this interface to suggest deterioration of the original parent concrete or repair mortar.


Using repair materials containing a high alkali concrete may initiate some ASR in substrate concretes containing potentially reactive aggregate. However if the area is kept dry it is unlikely that this will be a problem.

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2.3.27 Site 26



Type of structure Road bridge over Location: Southern England river



Generally good condition with a small number of defects (cracking to occasional repair, local spalls over low cover to bars in original concrete) and one location (abutment) showing more significant deterioration of repairs.


Repairs generally in good condition. The repairs at the abutment are located beneath a weeping joint in the deck and have deteriorated. There are numerous relatively small patch repairs to the beams and to a lesser extent the columns. These are all visible as the underside of the bridge is not painted. The side walls have also clearly been repaired but are painted and higher above the ground.

There are at least 2 slightly different repair types; one is a grey cementitious repair mortar and the other lighter and almost white. They could be the same product. Very few repairs contain cracks or any other form of deterioration.

Repair details:

Photo of repaired structure (location 1): Element Type: internal longitudinal beam

Repair proprietary Material(s): cementitious

repair mortar


Condition:

The repairs were close to the bottom of the beam and appeared to extend from the bottom reinforcement up the vertical links. A hammer survey confirmed the repair and surrounding concrete was sound.

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Photo of repaired structure (location 2): Element Type: transverse beam

Repair resin repair mortar Material(s):


Condition:

The repair was in the vertical face of the beam in a shallow, narrow zone, apparently over a reinforcement bar. Towards the top of the repair there was incipient spalling and cracking related to minor corrosion of the bar. A hammer survey confirmed the surrounding concrete was sound.

Photo of repaired structure (location 3): Element Type: Abutment

Repair cementitious Material(s): repair mortar


Condition

The repair was to the abutment wall below the deck and had delaminated from the substrate over a substantial area, with some open spalling revealing underlying corroded bars. A hammer survey of the abutment indicated adjacent areas of concrete were mostly sound.

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Tests Conducted:

Test Results

Half-cell Location 1 values range from +45 to –148mV, with some gradual increase in potential (SSC) negative potential toward the base of the beam, indicating low probability of

corrosion. However, there is a very localised increase to values of –192 to –245mV immediately adjacent to the connection, which probably relates to wetting of the substrate during coring.

Location 2 values are typically from +55 to –4mV in the undamaged concrete beam but sharply increase in negative potential to between –38 and –330mV in the vicinity of the vertical repair and cracked concrete over the link. The observations confirm the likelihood of active corrosion at the damaged repair with low probability of corrosion away from it.

Location 3 values range from -400 to –537mV, with consistently high negative potentials indicating high probability of corrosion. However, these values are likely to reflect the saturated nature of the abutment due to seepage from the deck joint above.

Covermeter Location 1: confirms minimum cover to face of beam at 8-15mm in concrete and in repair (nominal 10mm cover in 30mm deep repair).

Location 2: confirms minimum cover to face of beam is between 5 and 34mm for the vertical bars in the concrete and in repair.

Location 3: confirms minimum cover to face of abutment is in excess of 30mm in the repair and typically greater than 40mm in the surrounding concrete.

Pull-off Location 3: two samples failed at the interface between the repair and the substrate (both <0.6 N/mm2).

Samples:


The repair mortar was found to be different at each of the three locations. At Location 1, this was a dense, dark grey mortar containing PFA. At Location 2 the light grey mortar contains siliceous sand within a resinous binder, and at Location 3 the mortar is dark grey and based on sand within a cementitious binder.

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Location 1

Repair area appears sound and has sawn edges and an uncorroded reinforcing bar fully encapsulated within the repair (at 10mm depth in a 38-50mm thick repair). The material appears dense and contains polypropylene fibres, and has an irregular surface with the substrate. One core from this location was recovered intact whilst a second broke at the interface between repair and substrate.

Location 2

Repair is restricted to a narrow strip over a corroding vertical bar which has caused re-cracking of the concrete and cracking in the repair. The bar is at 8mm depth, at the base of the repair, which does not encapsulate the bar. The margins of the repair do not appear to have been saw cut.

Location 3

Repair area has mostly delaminated but the core was taken from an apparently sound section. The main repair appears to have sawn edges, but with a thin surface zone which appears to feather out in adjacent unrepaired areas. The area contains corroded reinforcing bars that are not encapsulated within the repair (bars at 25mm depth at the interface between the repair and substrate).

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Petrography

The substrate concrete at all locations contained a flint aggregate and siliceous sand within a matrix of highly porous cement paste. At location 3 there was some evidence of deterioration through leaching associated with the movement of water through the paste, and some fine cracks in the substrate that are infilled with calcium carbonate.

At location 1, the substrate includes breaks through the coarse aggregate onto which the repair has been applied. The interface is characterised by an extremely irregular parting with numerous voids in the repair material, and in places the binder of the repair appears on the concrete side of the parting, forming a very thin layer. The concrete is carbonated to up to 0.5mm, and contains a very low level of microcracking. The repair consists of a single layer of mortar composed of coarse siliceous sand in a portland cement matrix with abundant PFA, and clusters of polypropylene fibres orientated roughly parallel with the outer surface. The paste is generally dense but has more porous patches associated with the larger sand grains and voids. The voids are irregular in shape and lined with carbonate material. There is a low level of microcracking which occurs within the paste and through the aggregate. Carbonation typically extends from 0.5 to 1.5mm from the outer surface.

At location 2, the repair is a single layer of not more than 8mm thick and covers a corroded reinforcing bar at the base of the repair. A crack runs parallel to the external surface from the bar, with a parting between the bar and the repair. The repair material is of a uniformly graded, rounded fine siliceous sand within a resinous matrix containing a small proportion of empty voids. The substrate concrete is carbonated and there is a parting between the concrete and the repair, which is partly infilled with corrosion product. The corrosion product also occurs within the paste of the substrate concrete and within microcracks which are sub-parallel with the interface.

At location 3, the repair thickness varies from 1mm to 25mm, and intersects the reinforcement at 25mm depth. A parting occurs between much of the area of the repair material and the substrate, and there are several surface-parallel cracks within both the substrate and repair. The interface is extremely irregular, being mostly surface parallel at 2mm and 25mm nominal depth, with an apparent ‘step’ between the two. This is suggestive of a main break-out with sawn edges and slight over-filling of the cavity beyond the original surface profile. The repair is a medium grey colour and is based on a coarse, rounded to irregular siliceous sand within a matrix of portland cement paste. The paste contains an abundance of residual cement grains within porous hydrates and some voids filled with calcium hydroxide. These features are indicative of a low original water/cement ratio and possible use of a plasticiser. The repair is carbonated mostly to a depth less than 1mm, with traces of carbonation present in it along the interface with the substrate, and up to a depth of 2mm in the substrate concrete.



Conversations and meetings with the county council engineer responsible for maintenance and repair contract of the bridge. Records of the site state that in 1943 there were some locations where the reinforcement was showing, and that in 1972 the abutment had spalled in several areas on the face. Repairs were executed in 1974 but records are not available. Details of the 1995 repair contract have been discussed with the council, and with the contractor’s engineer, and documentation relating to the repair material has been obtained from the manufacturer.

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The bridge is regularly inspected by the County Council and was repaired in an earlier phase (1974). The repair at Location 1 was probably part of the 1995 repair phase, whilst that at Location 3 is believed to relate to the 1974 repairs. The deteriorating repair at Location 2 is of unconfirmed origin.

The repair material (Location 1) was a two component acrylic repair mortar, consisting of a pre-blended cementitious mortar and a re-measured acrylic latex, complying with Department of Transport Standard BD27/86. The mix contains RHPC with a total cementitious content of 590 Kg/m3, fine aggregates, and additives to provide workability and low-shrink characteristics. The mortar, if applied according to the materials suppliers recommendations, would be applied to a damp substrate with a sawn perimeter, with a tacky bond coat composed of portland cement and acrylic latex. The mortar should have been applied in layers not exceeding 15 to 20mm thickness and provided at least 12mm cover to reinforcement. Recommended curing was by curing membrane or 7-day close contact with polythene sheeting.


The repair at Location 1 appears to have been applied in a single layer up to 50mm thick. This is in excess of the manufacturers recommended maximum layer thickness for overhead repairs (20mm). The parting between repair and substrate may be evidence of slumping in the plastic state.

This structure is something of an anomaly. The substrate concrete is highly porous and has had areas of very low cover since construction some 70-80 years ago. Even now there are locally exposed bars and significant areas of low cover, yet there is little evidence of corrosion. One possible explanation is that carbonation has penetrated to the steel in various places but the undersides of the bridge are too dry to support a significant rate of corrosion. Another is that the cementitious hydrates surrounding slowly corroding bars may be sufficiently porous to accommodate substantial quantities of corrosion product before generating cracks in the cover i.e. deterioration is occurring but there is little evidence for it.


At Location 1, in apparently sound repair, there is a parting, or crack, at the interface between repair materials and the substrate. This might have developed prior to final setting of the repair material.

The substrate shows evidence of post-repair carbonation along the parting, though the extent of this may be very limited. However, this carbonation could be a performance limiting factor where restored cover remains relatively low.

At Location 2, the application of resinous material to the steel has not prevented ongoing corrosion. It seems most likely that the bar remained substantially within a carbonated substrate and continued to corrode after repair, resulting in the cracking.

Repair to the cover zone only (Location 3), without encapsulation of the bar, does not appear to be as effective as break-out behind the reinforcement bar and full encapsulation in new material.

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2.3.28 Site 27



Type of structure Car Park Location: Midlands



Generally good condition with surfaces uncoated and repairs visible in soffit Kaiser beams, deck edge beams and columns. No evidence of cracking or spalling was noted.


Most repairs in good visual condition. A number of repairs at the edge beams contain cracks and white efflorescence indicative of water passing through the deck and repair.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C edge/wall beam

Repair proprietary mortar Material(s):


Condition:

The repair was one of several patches to a beam close to (0.2-0.4m) floor level. The repairs all appeared to be undeteriorated and hammer surveying identified no delaminated areas.

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Repair proprietary mortar Material(s):


Condition

The repair was in an edge beam to the off-set deck slab, at the footing below the steel barrier. The areas below the barriers had been repaired in many locations and these characteristically contained cracks and white efflorescence. Despite the cracking, the repair and adjacent areas of concrete were sound under hammer surveying.

Tests Conducted:

Test Results

Half-cell Location 1 values range from -4 to –186mV, with most values between –10 and – potential (SSC) 130mV, suggesting low to uncertain probability of corrosion.

Location 2 values range from –4 to –291mV, with most values between –100 and – 200mV suggesting low to uncertain probability of corrosion. The highest negative potentials (more negative than -200mV) were concentrated on and around the cracked repair; the greatest negative values are indicative of uncertain to high probability of corrosion.

Covermeter Location 1: confirms minimum cover in vertical bars in beam at 10-15mm within both original concrete and in repair.

Location 2: confirms the vertical bars have the lowest cover of typically 40-50mm with most bars at greater than 50mm depth.

Pull-off Location 1: all three samples failed at the interface between the repair and the substrate (0.3, 0.3 and 0.5 N/mm2).

Samples:


Medium grey fine grained repair mortar applied to original cream coloured concrete with siliceous aggregate.

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Location 1

Core taken at a vertical 8mm square bar with cover of 6-9mm. The bar is embedded within the repair in one wall of the core and in the substrate concrete on the opposite side. The bar was undeteriorated with the exception of some slight rusting, and did not appear to carry a coating.

Location 2

Core from the edge of the roughly square area of repair, incorporating the junction between repair and substrate concrete. It is this interface that is associated with the heavy surface deposits (see lower figure).

The repair depth is substantial (70mm) and is in two layers.

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Petrography

At Location 1 the repair was up to 40mm deep and was in two layers. A square reinforcement bar occurs at 6mm depth, partially within the repair and partially in the substrate concrete. The bar has traces of carbonated cement paste attached to it, is slightly corroded with the corrosion products locally staining the repair material. Along the interface between substrate and repair there is fragmented concrete dust forming a locally porous zone at the interface.

The repair material is similar at both locations and consists of PFA cenospheres blended with portland cement and siliceous fine sand. The sand grains are typically sub-rounded and less than 1mm in diameter and of dense lithologies. The paste is generally mid to dark grey and contains spherical, isotropic, layered particles resembling clots of undispersed microsilica. The repairs contain numerous irregular voids of mostly less than 1mm dimension, but with occasional elongate voids of 3mm maximum dimension. These longer voids may be concentrated at the reinforcement and the base of the repair, and are sometimes connected by fine cracks. The material is mostly of low porosity with some locally porous zones at the concrete substrate, possibly resulting from leaching by moisture moving along the interface.

The edges to the repairs are saw-cut perpendicular to the external surface to a depth of 7-10mm, and there appears to be no microcracking associated with these surfaces. The base of the repairs are rough and broken, with some fine cracking and microcracking within the aggregate and paste close to the interface with the repair material. The edges of both repairs are smoothed over the surface of the concrete to depths of between 1 and 3mm.

In both locations the repair contains two layers and at the interface between them there is a patchy zone of mortar with very light grey to white paste, containing no PFA cenospheres and of higher porosity. The external surfaces are smooth, trowelled surfaces. Carbonation generally penetrates the repair material to less than 0.1mm depth but at Location 2 there is patchy carbonation to 20mm depth around some entrapped voids.

The substrate concrete is similar in both samples and consists of a siliceous gravel coarse aggregate and siliceous sand within a moderately to highly porous portland cement binder. In both locations patchy carbonation occurs within the substrate to a depth of 1mm from the repair interface. Carbonation penetrates the concrete substrate to a maximum depth of 7mm from the external surface. The level of microcracking at both locations is low, but increases sharply close to the contact with the repairs.

At Location 2 the repair was up to 70mm in depth, in two layers, with the outer layer approximately 20mm thick and the inner layer approximately 50mm thick. The external surface is coated with abundant white deposits of calcium carbonate and some very fine cracking is visible on the surface of the repair beneath. The voids close to the surface contain a locally abundant crystals resembling calcium carbonate. There are two uncorroded 12mm bars at 80mm depth.

SEM/EDXA Analysis

The white deposits on the surface at Location 2 were analysed by energy dispersive X-ray analysis and found to be dominated by calcium with traces of silica, aluminium, iron, sodium and potassium, with almost no detectable chloride. The analysis is in keeping with the deposits originating from leaching and re-precipitation of phases in the cement paste of the concrete and/or repair.

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Chloride Content

Chloride content was determined on bulk samples of repair and substrate in both locations, and results expressed as % by weight of cement assuming 14% OPC by weight of concrete and 30% OPC by weight of repair.


Location 2, 0-75mm (concrete near repair): 0.51

Location 2, 75-95mm (concrete): 0.17.

The results suggest seepage of chloride along the cracks in the repair material and/or along the interface between repair and substrate.



Conversations with client and with consulting engineers involved in the investigation of the structure, preparing the specification and supervising the repairs. The repairs in 1998 resulted from a thorough investigation and structural integrity review, and were intended to provide an additional operating life of 5 years. This appears to have been achieved, and a subsequent survey was commissioned in 2002/3 to evaluate the requirements for a further 5-year extension of service life.


A full structural analysis and condition survey of the car park was carried out by a consulting engineer. Options for repair and refurbishment were identified. A repair specification and tender documentation were prepared. The locations and approximate areas of repairs were shown on contract drawings but the repairer was responsible for confirming the actual locations and dimensions on site. A repair contractor was awarded the contract and overseen by a Resident Engineer.

Repair methods allowable included flowable repair concrete and trowel applied repair mortar. The materials were to be proprietary polymer modified shrinkage compensated cementitious materials with cementitious content of between 400 and 550 Kg/m3 and total chloride ion content not exceeding 0.1% of the mass of cementitious material. The perimeter of the repair was to be cut perpendicular to the face to a depth of not less than 15mm or to within 10mm of the reinforcement, and care taken to prevent over-break beyond the line of the cut. Removal was by light handheld pneumatic/electric percussive tools/breakers, to form a sound substrate of uniform depth. Where half the diameter of the reinforcement was exposed and the substrate was suitable for repair, excavation was stopped. Where more than half the diameter was exposed, the depth of repair was increased to at least 25mm beyond the bar. Bars were to be exposed beyond the areas of surface corrosion and replaced where significant loss of section was found. Exposed reinforcement was cleaned by needle gunning, grit blasting or other method to remove all scale and loose rust to SP10 quality in accordance with ISO 8501. Before application of any repair material, the surface was to be adequately prepared and inspected by the Resident Engineer.

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Reinstatement was to be using proprietary materials supplied by a manufacturer holding either a current BSI Certificate of Registration or operates quality assurance procedures of a similar standard, and mixed and applied strictly in accordance with the manufacturers instructions, and within appropriate temperature conditions. Immediately prior to reinstatement all dust, debris and loose material was to be removed and the areas wetted down. For trowel applied repairs, the final build up layer was to be less than 10mm and levelled off and profiled to give a finish consistent with the surrounding surfaces.

The repairs were to be cured and protected from drying out in accordance with the manufacturers instructions.

Trial repairs were to be tested and demonstrated to achieve a repair free from excess voidage with good bond to the substrate, reinforcement and repair layers, a compressive strength exceeding 90% of the minimum strength of 30N/mm2 at 7 days, and pull-off strength exceeding 1 N/mm2.


The repair material contains abundant PFA and may be lightweight or high-build repair material.

The reinforcement bar at Location 1 appears to have traces of the original concrete on its surface, suggesting this was not fully removed during break out of the repair. There also appears to have been a small amount of post-repair corrosion.

The white deposits at Location 2 probably resulted from migration of water along the interface between the repair and the concrete substrate as well as through the occasional fine cracks within the repair. Although the surface deposits contain very little chloride, there are elevated levels of chloride in the repair material and concrete close to the repair interface, suggesting salt-laden water, draining from cars during winter de-icing episodes, has drained through the deck. Although the levels of chloride at the depth of the steel is currently low, seepage along the interface between repair and concrete would be expected to contribute to the future development of reinforcement corrosion.


Finishing repairs flush with the surrounding surface can result in low-cover in the repair, though this may not be a significant performance affecting factor in repairs designed for 5-year life.

The interface between repair and substrate may provide a preferred pathway for drainage of water through the deck resulting in efflorescence at the surface. This could further reduce the quality and bond at the interface between the repair and substrate.

Trowel application of repair material may give rise to the formation of elongate voids close to the substrate and other fixed features such as reinforcement, which may then be joined by cracks.

The minimal depth of cover reinstated (at Location 1), in combination with traces of carbonated paste and evidence of moisture penetration into the repair material contribute to a risk of development of corrosion in the long term.

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2.3.29 Site 28



Type of structure Highway bridge Location: Southern England



Good condition with surfaces coated with thick layers of cementitious render, obscuring both repairs and original concrete. No evidence of cracking or spalling of the substrate was noted, but there was surface crazing and loss of material in the render.


Repairs not exposed at surface but found to be in good visual condition where samples were taken. No evidence was found for deterioration within the repairs. The corners to the piers show patches of corrosion and some rust staining relating to deterioration of the beading embedded in the surface mortar layers.

Repair details: (Repaired under same contract as Sites 029 and 030)

Photo of repaired structure (location 1): Element R/C column Type:

Repair site batched sprayed Material(s): concrete

Coatings/ 2 layers of cementitious render: render

Condition:

The repairs were to parts of the original reinforced concrete portal structure contaminated with chlorides and varied in extent and depth. The location of repairs was identified using as-repaired drawings.

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Tests Conducted:

Test Results

Half-cell Location 1 values range from –57 to –387mV, with most values between –65 and – potential (SSC) 200mV suggesting low to uncertain probability of corrosion. The potentials become

more negative towards the pavement and most notably in the lowest 0.5m. This is probably related to an increase in moisture content.

Covermeter Location 1: identifies a regular grid approximately 200mm square. The half-cell connection to the reinforcement confirmed a cover depth of 75mm. The covermeter readings consistently under-estimated the cover, which appears to be consistently deep (>70mm).

Pull-off Location 1: all three samples failed at the interface between the surface mortar and the sprayed repair material at <0.3N/mm2.

Samples:


The cores contain 2 surface layers of grey/buff cementitious render in total 15 to 25mm thick over a dark grey steel fibre reinforced sprayed concrete repair of 95 to 115mm thickness. The substrate is a light coloured concrete containing pink granitic aggregate.


Location 1

Core taken at a pier from the pavement in a deeply repaired area. The repair material shows mostly slight evidence of layering but contains an irregular planar feature at 75-100mm depth that is a discontinuity between layers of repair or a slump feature. The contact with the substrate concrete is largely sound but some elongate voids up to 10mm in maximum dimension are present.

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Petrography

The structure of the sample is similar to that at Site 29 and 30, and consists of a concrete substrate with broken surface coated with layers of a steel-fibre reinforced sprayed concrete repair material, coated with layers of mortar. The core sample from Site 28 has broken into 2 sections along a crack that passes mainly through the outer few millimetres of the concrete substrate, very close to the contact with the sprayed concrete repair.

The repair material consists of a siliceous coarse sand, with angular particles of up to 5mm diameter, within a binder of portland cement containing abundant residual cement grains, of low porosity and with a low level of microcracking. The material has a layered structure and contains numerous irregular and elongate, often crack-like voids that run sub-parallel with the external surface, distributed randomly throughout the thickness of the repair.

There are also small voids up to 1mm in diameter randomly distributed throughout the repair and locally concentrated in layers. These occasionally contain traces of fine crystalline deposits. There is no evidence of deterioration of the embedded steel fibres. The external surface is very rough and irregular. There are only traces of carbonation at this surface.

The substrate concrete consists of granitic coarse aggregate and siliceous sand within a porous binder of portland cement, containing a moderate level of microcracking. Some very fine cracking, and locally abundant microcracking, occurs in the paste and aggregate of the concrete within a few millimetres of the contact with the repair, typically running parallel with the contact. Some of the cracks contain traces of fine crystalline material. There is minor, patchy carbonation in the surface of the concrete in contact with the repair, the majority of which occurs within about 1mm of the contact. The voids within the concrete at this location were mostly empty.

The surface of the repair is coated with two layers of mortar amounting to a total thickness of 1520mm. The mortar is moderately porous and consists of a siliceous sand within a light brownish cementitious binder which is carbonated throughout.

Chloride content

The chloride content of the substrate concrete near the repair, at 120mm-140mm depth, was found to be 0.17% by weight of cement, assuming a 14% cement content.



Conversations with consulting engineers involved in the investigation of the structure, preparing the specification and supervising the repairs.


The consulting engineer who supervised the works reported that the repairs were part of a major road upgrade scheme and were necessary due to chloride contamination of the existing concrete piers and consequent severe corrosion of the mild steel reinforcement. The objective was to provide long term durability to the existing parts of the structure, though some of the engineering team involved believed it would have been simpler and more cost effective to replace the damaged elements.

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The Engineer determined the areas for break out based on chloride content of the concrete (a guidance value of >0.04% by weight of concrete was provided), and pulse-echo investigations to determine the extent of delaminations. All necessary measures to maintain stability, structural integrity and continuity of load transference in the areas undergoing repair were to be ensured and the work programmed accordingly. This involved a limit to the extent of break out in any member, and repair in alternate bays with overlapping edges where larger repairs were required.

Break out was extended a minimum of 150mm beyond the limit of defective concrete and 100mm beyond the corroded steel. The edges were saw cut to a minimum of 10mm depth. Bars were removed and replaced where there was greater than 10% loss of section. The original bars were at nominal 40mm cover, and the break-out was to a minimum of 25mm behind corroded bars. The break-outs were by high pressure water jetting, also used to clean the steel, and no bonding bridge was used.

The repair material was a site batched sprayed concrete of 40N/mm2 characteristic strength, containing a 10mm aggregate and 25mm stainless steel fibres, and applied in dense and homogenous layers of 50mm nominal thickness with each coat washed down prior to application of the next. The final coat had a rough cast finished. Wire or timber guides were used for profiles and minimum finished cover of 40mm was provided. The material was to be protected from weather, temperature and drying out for a minimum of 14 days using methods approved by the Engineer but not including use of proprietary curing membranes. The surface render coatings were also site batched, and assistance was provided in the design by the Cement and Concrete Association in producing a low-strength, low shrinkage mix. The original specification called for inclusion of a styrene/butadiene latex additive. The substrate was prepared where necessary by blast cleaning, thoroughly dampened with water, and priming coats and render was applied in layers.

The repair specification was altered on site, in particular the mix design of the sprayed concrete and render coats. The consulting engineer supervised the works full time, and carried out coring and break outs during the works to confirm that the material was applied to produce a dense, high quality repair. The corners to the piers were formed with galvanised steel beading.


The cracking developed in the concrete within a few millimetres of the repair interface is reminiscent of that generated during surface preparation, perhaps by mechanical means.

The separation of the core into 2 sections occurred during coring but may have been along a preexisting plane of weakness related to the method of substrate preparation.

The surface-parallel voids and partings in the repair material may have developed during the placement of the sprayed concrete, perhaps by slumping of the material prior to final set. There is no evidence of cracking after the final set.


Galvanised steel beading has a limited life within the surface mortar. The repair supervisor stated stainless steel might have provided a more durable beading.

The external render appears to have provided protection to the main repair, but is now locally deteriorating.

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2.3.30 Site 29






Good condition with surfaces coated with thick layers of cementitious render, obscuring both repairs and original concrete. No evidence of cracking or spalling was noted. The render contained areas with crazing.


Repairs not exposed at surface but found to be in good visual condition where samples were taken. No evidence was found for deterioration within the repairs. The corners to the piers show patches of corrosion and some rust staining relating to deterioration of the beading embedded in the surface mortar layers.


Photo of repaired structure (location 1): Element Type: R/C column

Repair site batched Material(s): sprayed concrete

Coatings/ 2 layers of render: cementitious

render

Condition:

The repairs were to parts of the original reinforced concrete portal structure contaminated with chlorides and varied in extent and depth. The location of repairs was identified using as-repaired drawings. The deck joint above the pier tested was clearly leaking and the pier had been regularly wetted. The area sampled was in a dry area on the reverse of the wet pier shown opposite.

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Tests Conducted:

Test Results

Half-cell Location 1 values range from –54 to –224mV, with most values between –90 and – potential (SSC) 150mV suggesting an uncertain probability of corrosion. The potentials become

more negative towards the pavement and most notably in the lowest 0.5m. This is probably related to an increase in moisture content.

Covermeter Location 1: identifies a slightly irregular grid with horizontal and vertical bars at between 200mm and 300mm spacings. The half-cell connection to the vertical reinforcement confirmed a cover depth of 75mm. The covermeter readings provided readings of 60-80mm for the vertical steel and 48-58mm for the transverse bars.

Pull-off Location 1: The samples were prepared by removing the surface mortar and attaching the dollies with a thicker than normal layer of adhesive. Five samples subsequently failed within the adhesive at 0.3, 0.3, 0.4, 0.5 and 0.6N/mm2. One sample failed at the interface between the sprayed concrete and substrate at 0.4N/mm2.

Samples:


The cores contain 2 surface layers of grey/buff cementitious render in total 20 to 25mm thick over a dark grey steel fibre reinforced sprayed concrete repair of 75 to 85mm thickness. The substrate is a light coloured concrete containing pink granitic aggregate.


Location 1

Core taken at a pier from the sloping abutment beyond the pavement. The repair material shows only slight evidence of layering, with occasional clusters of voids often orientated parallel to the surface. The contact with the substrate concrete is highly irregular but largely sound with occasional voids.

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Petrography

The structure of the sample is similar to that at Site 28 and 30, and consists of a concrete substrate with broken surface coated with layers of a steel-fibre reinforced sprayed concrete repair material, coated with layers of mortar.

The repair material consists of a siliceous coarse sand, with angular particles of up to 5mm diameter, within a binder of portland cement containing abundant residual cement grains, of low porosity and with a low level of microcracking. The material has a layered structure and contains small voids up to 1mm in diameter randomly distributed throughout the repair and locally concentrated in layers. There is no evidence of deterioration of the embedded steel fibres. The external surface is very rough and irregular. There are only traces of carbonation at this surface, but also traces of patchy carbonation around voids in the repair up to 58mm depth.

The substrate concrete consists of granitic coarse aggregate and siliceous sand within a porous binder of portland cement, containing a moderate level of microcracking. Some very fine cracking, and locally abundant microcracking, occurs in the paste and aggregate of the concrete within a few millimetres of the contact with the repair, typically running parallel with the contact. Some of the cracks contain traces of fine crystalline material. There is minor, patchy carbonation in the surface of the concrete in contact with the repair, the majority of which occurs within about 1mm of the contact. The voids within the concrete contain varied quantities of ettringite-like crystals.


Chloride content

The chloride content has been determined at Site 29, assuming a 14% cement content for the substrate concrete and 30% cement content for the repair material and surface mortar, and found to be as follows:

Mortar, 0-20mm: 0.7% by weight of cement

Sprayed concrete, 20-95mm: 0.9% by weight of cement

Concrete substrate, 95-200: 0.21% by weight of cement.






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The wet chemical analysis for chloride indicates generally high levels of chloride in the sprayed concrete and mortar and much lower levels of chloride in the concrete substrate. It is probable that this reflects the penetration of moisture containing chloride though the mortar and sprayed concrete. This is in keeping with the site observation that the deck joint immediately above the pier allowed seepage of water from the carriageway down the structure.



The external render appears to have provided protection to the main repair, but is now locally deteriorating.

For structures contaminated with chloride, long-term protection can be provided by extensive break-out and reinstatement with appropriate cementitious materials. However, failure to prevent ongoing chloride contamination will eventually compromise the performance.

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2.3.31 Site 30






Good condition with surfaces coated with thick layers of cementitious render, obscuring both repairs and original concrete. No evidence of cracking or spalling was noted.


Repairs were not exposed at the surface but found to be in good visual condition where samples were taken. No evidence was found for deterioration within the repairs.


Photo of repaired structure (location 1): Element Type: R/C column

Repair site batched Material(s): sprayed concrete

Coatings/ 2 layers of render: cementitious

render

Condition:

The repairs were to parts of the original reinforced concrete portal structure contaminated with chlorides and varied in extent and depth. The location of repairs was identified using as-repaired drawings.

Tests Conducted:

Test Results


Location 1 values range from –299 to +35mV, with most values less negative than – 120mV suggesting a low probability of corrosion. The potentials become markedly more negative towards the pavement and most notably in the lowest 0.5m. This is probably related to an increase in moisture content.

Covermeter Location 1: identifies a regular grid with horizontal and vertical bars at 200mm spacings. The half-cell connection to the vertical reinforcement confirmed a cover depth of 100mm. The covermeter readings provided readings of approximately 55mm for the vertical steel and 44-48mm for the transverse bars.

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Pull-off no tests carried out.

Samples:


Cores contain 2 surface layers of grey/buff cementitious render in total 16 to 25mm thick over a dark grey steel fibre reinforced sprayed concrete repair of 70 to 85mm thickness. The substrate is a light coloured concrete containing pink granitic aggregate.


Location 1

Core taken at a pier from the abutment side on the pavement. The repair material shows only slight evidence of layering, with occasional clusters of voids often orientated parallel to the surface. The contact with the substrate concrete is highly irregular but largely sound with occasional voids.


Petrography

The structure of the sample is similar to that at Site 29 and 30, and consists of a concrete substrate with broken surface coated with layers of a steel-fibre reinforced sprayed concrete repair material, coated with layers of mortar.

The repair material consists of a siliceous coarse sand, with angular particles of up to 5mm diameter, within a binder of portland cement containing abundant residual cement grains, of low porosity and with a low level of microcracking. The material has a layered structure and contains small voids up to 1mm in diameter randomly distributed throughout the repair and locally concentrated in layers. There is no evidence of deterioration of the embedded steel fibres. The external surface is very rough and irregular. There are only traces of carbonation at this surface.

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The substrate concrete consists of granitic coarse aggregate and siliceous sand within a porous binder of portland cement, containing a moderate level of microcracking. Some very fine cracking, and locally abundant microcracking, occurs in the paste and aggregate of the concrete within a few millimetres of the contact with the repair, typically running parallel with the contact. Some of the cracks contain traces of fine crystalline material. There is minor, patchy carbonation in the surface of the concrete in contact with the repair, the majority of which occurs within about 1mm of the contact, but there is also some penetration of carbonation along the cracks to a depth of 7mm. The voids within the concrete contain varied quantities of ettringite-like crystals.










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The external render appears to have effectively provided protection to the main repair.

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2.3.32 Site 31


Date of inspection/testing: 30/01/02 Contractor details: Visual inspection only.

No contractor.

Type of structure Car park Location: SE England



Structure refurbished in 1997 and remains in good visual condition. Some cracking noted in concrete deck slabs not coated with wearing surface system. Also localised areas of hollow-sounding concrete identified by hammer tapping, particularly adjacent to joints and close to deck repairs. No evidence was found for deterioration of repairs in the columns, beams and soffits, which are all painted.


Repairs were generally not detectable beneath the surface coating to the ground floor and top floor. Elsewhere the repairs in the deck slabs were clearly identifiable by change in colour and the linear marks remaining from saw-cutting of the perimeters. Some repairs contain fine cracks but all appear intact.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C floor slab

Repair Not confirmed Material(s):


Condition:

Elongate repair in floor slab at top of down-ramp extending into the coated ramp area and to a joint. The repair contains fine transverse cracks and there are local areas of the repair and adjacent concrete which sound hollow during hammer surveying.

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Location 1.

This detailed view shows the slightly darker coloured repair material in the concrete slab. The outline of the repair can be made out where it extends on to the ramp and beneath the grey surface coating. Note the straight sawn perimeter and fine transverse cracks in the repair.

Location 2.

Several concrete repairs to the slab and at both sides of a joint.

Location 2.

This detailed view of the repair at the joint shows incipient spalling developing in the concrete deck adjacent to a small patch repair. Hammer surveying indicates that some areas of both the repair and the concrete deck are hollow and possibly delaminating.

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Location 3.

This view shows a patch repair to a beam, where the depth of cover to the bottom face has been increased by building up the thickness of the repair. The repair is overcoated with a paint system and shows no evidence of deterioration.

Desk Study



Archived project information all retrieved from storage. Records reviewed for the contract specification, health and safety data, and correspondence. Product literature was found for the deck repairs and waterproofing coating, and soffit and column paint systems. Conversations with the engineer’s staff involved in the refurbishment helped identify materials used and information was received from the contractor and from the material supplier. Some remedial work had been carried out between 1991 and 1993. Some of the repairs were exhibiting signs of deterioration in 1996. A report by the operator in 1994 found the concrete suffered from poor construction and inadequate waterproofing, and defects occurred within the decks due to carbonation and chloride-induced reinforcement corrosion. The report recommended waterproofing, repairing and coating the structure and undertaking regular surveys and maintenance.

In 1996, the operator appointed a consulting engineer to prepare a feasibility and repair options report for a refurbishment of the car park. The operator provided pre-existing information (a structural report and investigation reports) for review and use in devising the refurbishment scheme. The engineer was appointed to prepare and review the works specification and tenders. An investigation in 1991 had identified a high chloride content in the floor slabs and other elements of the car park, at depths of 030mm and 30-55mm. Incremental drilling showed a profile of decreasing chloride content with depth, suggesting the chlorides were originating from road salts brought into the car park. The cover was typically between 7 and 50mm, and carbonation 10-20mm depth. The deck showed signs of cracking and incipient spalling resulting from chloride-induced corrosion, and half-cell results suggested increased risk of corrosion at the construction joints. Break-outs confirmed severe corrosion in the deck adjacent to construction joints, and corrosion was also confirmed in the columns.

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Repairs included to floor slabs and ramps, and repairs to soffits and beams (where old repairs were to be cut out and replaced and over painted). The engineer presented a feasibility study report which outlined three options for repair, based on 5, 10 and 15 year predicted performance, with increased levels of protection provided for the more durable options, involving additional sealing and waterproofing of the decks, resealing of construction joints, application of anti-carbonation coats, application of migrating corrosion inhibitors and cathodic protection of construction joints. The operator selected the 5-year repair option, largely due to financial constraints, and understood the implications for future serviceability.


The repairs were executed in 1997 and a surface coating was applied to the ground floor and ramp up to first floor. A coating had been applied to the open top floor in 1992 and had been noted to reduce the amount of water penetrating the structure. The material applied in 1997 was a liquid applied polyurethane based waterproof wearing surface system incorporating sealer, primer, waterproofing membrane and protective membrane and layers of quartz granules.

The general requirements of the contract, for concrete repair, required the contractor to select appropriate materials, and for the material manufacturer to demonstrate the repair procedure in a trial repair. This was also used to train operatives and to provide a benchmark for quality and appearance. The surfaces for sealing were prepared by vacuum abrasion. The proposed sealing system was to have an in-service track record of at least 4 years, and the contractor successfully apply an initial trial area prior to main application, and achieve a minimum pull-off strength of 2N/mm2. The location and extent of repairs to concrete was to be identified by close inspection and investigation on site during the contract and agreed between the contractor and the contract administrator. These areas were to include existing patch repairs, spalled concrete, hollow sounding areas, and any other appropriate areas. The contractor was to propose the materials and methods to be adopted in the different locations and sizes of repair. The perimeter of areas for repair were to be disk cut to 15mm depth or to within 10mm of the reinforcement, then broken out using light mechanical breakers at the edges and surface to form a regular 5mm profile and good bond for repair. Where corroded bar was exposed, or bar exposed beyond half the diameter, break-out was to be continued 25mm beyond the bar. Where the substrate was acceptable and the bar uncorroded and exposed for less than half the bar diameter, repair was to be applied without further breakout. The exposed bars were to be cleaned by grit blasting to achieve SP10 quality to ISP 8501.

Reinstatement was to be to clean and saturated substrates and in accordance with the selected repair system manufacturer’s instructions. Application was to be to the maximum approved thickness, with a final surface layer no less than 10mm if trowel applied, and the surfaces cured for 14 days. The material was to be a pre-backed proprietary polymer modified cementitious, shrinkage compensated material with a minimum cement content of 400Kg/m3 and total chloride ion concentration less than 0.1% by mass of cement. The repair material was to be tested to demonstrate an average strength exceeding 90% of the minimum (30N/mm2) at 7 days, freedom from excess voidage, good bond to reinforcement and interlayer adhesion and an average pull-off strength exceeding 1.5 N/mm2. The requirement for a proprietary material was modified when a more cost effective alternative of equivalent performance was proposed.

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The contractors method statement was as follows:

Areas of the defective concrete to be marked and agreed with the engineer, then the perimeter cut using a disk cutter to achieve a clean, straight edge of 15mm minimum depth. Defective concrete to be broken out using hand-held percussive tools and the reinforcement exposed to its full circumference unless half the circumference is exposed and the remaining concrete is sound. Any corroded reinforcement to be grit blasted and treated with a proprietary reinforcement primer (two coats of a two component, cement and polymer, highly alkaline, low permeability, flexible primer containing corrosion inhibitors). The repair to be primed with a slurry coat (of cement mixed with proprietary polymer admixture) and filled using a heavy duty site-batched mortar containing a polymer admixture within sand/cement mixture with 6mm granite aggregate. This material was to have a composition approximating to 780Kg/m3 sand, 780Kg/m3 6mm aggregate, 525Kg/m3 cement at a water/cement ratio of 0.23 and 63Kg/m3 polymer. The repairs were wet cured using hessian and polythene sheeting.

Repairs to the soffits were prepared as for the decks, with additional use of a proprietary bonding bridge (single coat of copolymer modified cementitious coating), use of the proprietary reinforcement primer, and a proprietary repair mortar. The mortar was a trowelable, low density, shrinkage compensated, fibre reinforced, low permeability, polymer modified cementitious mortar containing microsilica. The soffits were coated with a proprietary bonding primer (a deep penetrating, fast curing, pre-reacted, two component water based epoxy and modified polyamine) and two coats of a proprietary decorative coating (a single component, water based, polymer and resin material with high build, elastomeric, anticarbonation, and breatheable characteristics).


Modifications were made to the original surface preparation, break out and repair specification clauses as the coating and repair materials were selected and approved.

The repairs have sawn perimeters as per the specification. The repairs have remained intact, but hammer surveying indicates some hollowness in some repairs and adjacent concrete. This may result from ongoing deterioration of the deck reinforcement due to the high chloride content. There is one location showing incipient spalling in the original concrete close to a repair. This might be the result of incipient anode formation.


Localised patch repair to salt-contaminated concrete decks can be effective for 5 years.

There are indications that deterioration has continued or recurred at repairs and adjacent concrete which may necessitate further repair in the future. This condition was anticipated by the engineer and accepted by the operator when the repair strategy was devised.

Use of proprietary repair systems and coatings can provide protection to previously damaged locations (beams).

Application of a water-resistant surface coating to the repair, capable of masking the perimeter interfaces, appears to provide a higher level of protection than repair without a coating.

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2.3.33 Site 32


Date of inspection/testing: 12/9/01 Contractor details: Coring contractor and mobile platform

Type of structure Road bridge Location: Midlands

Constructed 1960s Repaired: Approximately 1990


Generally good but with localised defects. The insitu concrete cantilevered abutments contain cracks caused by low levels of alkali-silica reaction, some crazing in repairs and seepage through fine cracks and at the joints, and small spalls associated with low-cover corrosion. The structure was under refurbishment at the time of the inspection.


The repairs were visible in the soffit close to the half joint and some had been broken out during the 2001 repair contract. The repairs included areas that were hollow under hammer surveying, but generally appeared sound. One was crazed at the surface.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C abutment

Repair proprietary repair Material(s): mortar


Condition:

The repairs were to parts of the original reinforced concrete cantilevered beam seats which were contaminated with chlorides in the vicinity of the half joints. The upper figure illustrates the location of coring, the lower shows a detail of the break-out for re-repair, with the repair visible in the lower portion of the view.

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Tests Conducted:

Test Results

Half-cell Soffit half-cell survey potential (SSC)

Half-cell potential readings were taken across the width of the soffit close to the joint, including repaired areas. The potential readings ranged from +6 to –478mV, with consistent increase in negative potential towards the joint edge. Potentials more negative than –280mV occur within 0.75m of the joint edge indicating a high probability of corrosion at the joint.

In-depth half-cell survey

Half-cell potential readings were taken from the in-depth dust sampling locations beneath the half joints at depths from the soffit of up to 450mm. The potential readings ranged from –161 to –256mV indicating the probability of corrosion was uncertain.

Covermeter The reinforcement was found to be congested but the survey indicated bars with cover mostly in excess of 25mm but with rare locations as low as 5mm.

Pull-off No pull-off testing was carried out

Samples:


Grey micro-concrete containing angular white aggregate within a fine matrix that is buff-coloured close to the soffit and grey in the inner portions.

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Location 1

Core taken vertically from the soffit through the repair in the deck, and including the inclined junction between substrate concrete and repair.

The binder in the repair gradually changes from a buff colour to dark grey at 40-60mm depth.


Petrography

The repair material consists of crushed limestone aggregate particles and siliceous sand in a paste containing portland cement and GGBS. The limestone particles are typically between 1 and 5mm in maximum dimension and the siliceous sand is fine with most particles less than 1mm in diameter. The paste has a patchy, moderate porosity and very low levels of microcracking. There are numerous entrapped voids in the repair which are empty, but there is no concentration of voids close to the substrate.

There is patchy carbonation to depths of between 3 and 5mm in the repair and within the adjacent concrete to 6 to 8mm depths.

The concrete substrate is a rough and broken surface with sawn surfaces at the margins of the repair. The substrate concrete consists of a predominantly siliceous natural gravel and sand in a portland cement binder and shows abundant evidence of alkali-aggregate reaction with macrocracking, numerous very fine cracks and damp patches of gel. There is a concentration of fine cracks and microcracks in the concrete closest to the repair interface. These do not pass into the repair. Carbonation has locally penetrated the cracks.

Surface chloride testing

Two locations in the abutment concrete, close to the half joint, were tested with dust samples at depths, from the soffit, of 5-30mm, 30-55mm and 55-80mm and the cement content calculated as 18.4%. The results, as % by weight of cement, were as follows:

5-30mm 30-55mm 55-80mm

Location A 0.24% 0.27% 0.23%

Location B 2.6% 2.55% 1.84%

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In addition, a core sample was used to determine chloride content as follows:

Repair material 0-80mm: 0.01% (by weight of cement assuming 30% cement content)

Concrete at 5-175mm: 0.12% (by weight of cement assuming 14% cement content)

At-depth chloride testing beneath half-joint

Two locations in the concrete were tested with samples at depths from the soffit of 350-390mm, 390420mm and 420-450mm and the cement content calculated as 18.4%. All the results were <0.1% by weight of cement.

Carbonation depth

Three locations were tested and found to be carbonated to depths of 2, 2 and 3mm .



Conversations with council engineers responsible for maintenance of bridge, consulting engineers report on condition of bridge in 2000, and discussion with consulting engineers involved in re-repair of the structure. No documentation was available to confirm repair type and material.


The engineer supervising the re-repairs believed the original half-joint repairs were executed using flowable concrete, with formwork at the soffit and bleed holes drilled through the deck. This would explain the occurrence of cavities that were found during break-out of the old repairs, between the top of the repairs and the broken surface of the substrate. The cavities appeared to have been formed at the time of repair rather than subsequently through deterioration of the reinforcement. This would also be consistent with the ‘hollow’ sounding patches found in otherwise sound repairs.

The reason for re-repair in 2001 was due to the hollow-sounding repair areas and test results indicating ongoing corrosion of the soffit close to the half joint, which provided an unquantified risk of concrete spalling onto the carriageway below. Re-repair was carried out using proprietary sprayed concrete.


The levels of chloride in the slab were generally all lower than the maximum recommended for new reinforced concrete construction. There were very localised and high values at the soffit, probably related to seepage through the deck at or close to the half joint, and ingress from the soffit and/or halfjoint surfaces.

Alkali aggregate reaction (AAR) has occurred within the concrete substrate. No evidence was found for continuation of these cracks into the repair material, or for delamination at the repair/substrate interface. However, AAR would be expected to be ongoing should moisture continue to penetrate the concrete.

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The concentration of cracking in the substrate concrete close to the repair interface is probably related to the method of break out and preparation of that surface, and is suggestive of the use of mechanical breakers.

The repair contains no layering, suggesting it was placed in one episode, and probably as a flowable material. There is no evidence of deterioration of the repair material, which remains firmly attached to the substrate in both core samples.


Use of mechanical breakers in preparing the substrate can result in a concentration of cracks at this interface, but there is no evidence in these samples that this has detrimentally effected the bond between repair and substrate.

Cavities may develop within flowable concrete repairs applied to soffits at the interface between repair and substrate.

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2.3.34 Site 34



Type of structure Elevated Road Location: Birmingham area

Constructed 1960’s Repaired: 1992-3


Variable condition, many areas showing evidence of extensive repairs. The structure is not coated. Some repaired and unrepaired areas have had wire mesh attached over the surface to prevent debris falling from spalling locations. There are some areas of open and incipient spalling resulting from reinforcement corrosion.


Many repairs contain surface crazing but most are soundly attached to the substrate. Some repairs have bars exposed at the surface, and some areas of the substrate have deteriorated close to the repaired areas.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C pier to trestle

Repair proprietary flowable repair Material(s): concrete


Condition:

Repair at corner of pier with ongoing corrosion and open spalling of original concrete substrate immediately below. There is no evidence of deterioration of the repair. Hammer tapping indicates sound concrete.

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Repair proprietary flowable repair Material(s): concrete


Condition

Repair, showing surface crazing, to large areas of the faces of the pier. There are local areas where corroded reinforcement is exposed at the surface and corrosion products have stained the surrounding paste. Note the two bars that have been slightly broken out to make a half-cell connection.

Tests Conducted:

Test Results

Half-cell Location 1 values: within the repair, these range from +45 to -2mV, and within the potential (SSC) concrete from +10 to –85mV, indicating low probability of corrosion, except very

close to the open spall where readings of –162 to –396mV were recorded, indicating high probability of corrosion.

Location 2 values: within the repair, the values are mostly positive, from 0 to +150mV indicating low probability of corrosion. Within the unrepaired concrete, the values within 0.5m of the ground range from +58 to –170mV, indicating low to uncertain probability of corrosion, but above that the values are mostly positive, indicating low probability of corrosion.

Covermeter Location 1: confirms minimum cover to links is 25-35mm.

Location 2: confirms minimum cover to links is 0-10mm on one face and 2030mm on a second.

Cover depth was found to be variable throughout the structure.

Pull-off tests Depth of repair and abundance of reinforcement prevented pull-of testing.

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Samples:


Grey repair mortar with white aggregate typically <4mm diameter applied to concrete substrate with siliceous coarse and fine aggregate.


Location 1

The repair is to a depth in excess of 115mm and has a vertical sawn margin at the external surface, and no evidence of layering. The material is sound and dense, with some voids at the surface, but very low water absorption characteristics observed on site. There is no evidence of deterioration of embedded steel (vertical bars at 25-35mm and horizontal bars at 30-45mm). The unrepaired area immediately below the horizontal sawn repair margin has corroded and spalled, possibly through incipient anode formation.

Location 2

At this location the repair is unpainted, strongly crazed, with some voids at the surface, and is typical of the other repairs in the vicinity. There are bars exposed and corroding at the surface, but no corrosion on a bar fully embedded in the repair material at 45mm depth, although there are numerous voids up to 5mm diameter close to the bar. The repair material occurs in a single layer to a depth of 90-95mm. The material is sound and dense, with very low water absorption noted on site with the exception of at the fine surface cracks.

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The core broke in a surface-parallel plane at a 45mm where the edge of one reinforcement bar was intersected. Broken faces of the repair material have a slight greenish tint indicative of GGBS in the binder.


Petrography

Location 1

The repair material is a medium grey repair mortar containing subangular light grey limestone fragments, less than 4mm in diameter and fine fragments of black material (<0.5mm). Broken faces are darker in colour and have a slight greenish tint indicative of GGBS in the binder. The material has approximately 2% voidage with numerous entrained voids less than 1mm in diameter and some larger entrapped voids. The paste is based on a blend of portland cement and GGBS, with numerous particles resembling graphite, typically 0.06 to 0.25mm in diameter (the black fragments visible in hand specimen). The paste is of low porosity and contains very few microcracks. There is no obvious layering in the repair.

The edges of the repair appear to be sawn to up to 35mm depth in hand specimen. The thin section shows the sawn surface to penetrate to 23mm depth. Thereafter the edge is rough and appears to be broken out and there is good contact all along the interface and around the bars. There is some minor fine cracking and microcracking within 2 to 3mm of the contact with the repair.

The external surface of the repair is flat and smooth and flush with the adjacent original concrete. The concrete is carbonated to depths of between 5 and 11mm, and the repair mortar to depths of 0.5 to 1mm. Carbonation also penetrates along the joint between the two materials to up to 31mm depth, mostly within the paste of the concrete. The thin section indicates there is an narrow open parting between repair and substrate which grades into a microcrack along the joint. However, the repair remains firmly attached to the concrete.

The reinforcing bars pass from concrete into the repair through the vertical junction. The minimum depth of cover in the core sample is 26mm. There are also the impressions of two more horizontal bars at 85mm depth with very low levels of patchy corrosion within the concrete substrate, but none in the repair, suggesting the bars were well cleaned in the repair contract.

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The substrate is a pinkish brown/buff concrete with polymictic siliceous gravel including red sandstone particles, siliceous sand within a buff portland cement paste. The paste contains abundant coarse portlandite and few residual cement grains and has an estimated original water/cement ratio of 0.56 to 0.58. The paste is moderately to highly porous with a low level of microcracking. Voids within the concrete are empty and no evidence of ASR or other forms of deterioration was found.

Location 2

The repair material and substrate are very similar to those at Location 1. At Location 2 the repair forms a layer 87-99mm thick. The external surface of the repair is flat and slightly etched so that some fine particles of aggregate are slightly exposed. The surface crazing forms fine cracks perpendicular to the surface which penetrate to a maximum depth of 50 to 54mm and intersect the surface of the outer reinforcement bars. The core sample is broken along a surface-parallel crack at 54mm depth. This crack was probably caused during core sampling.

The repair mortar is generally carbonated to a depth of 0.2 to 2.0mm but carbonation penetrates the paste adjacent to the fine cracks to depths up to 51mm from the surface. However, carbonation does not appear to penetrate more than 0.1 to 0.2mm from the crack surface, forming a very restricted carbonated zone.

The contact between repair and substrate is approximately parallel with the external surface. The surface of the concrete substrate is rough and uneven. The contact with the substrate appears sound and the repair is firmly attached to the substrate. There is moderately abundant surface-parallel microcracking passing through the paste and aggregate in the outer 14mm of the concrete substrate.

The substrate concrete at both locations contains no evidence for deterioration resulting from moisture penetration.



The detailed repair contract records originally held by the owner are not available. However, a summary history of repairs was available, and discussions with the council engineers involved in repair and maintenance of the structure, and the supplier of the material used in the repair, provided considerable detail.


The structure was constructed in the 1960’s and included an elevated roundabout above a main road, and an additional elevated section of road above a second road. The majority of the area below the elevated sections, including Site 34, form a series of car parks. Patch repairs were first carried out before 1987, possibly in the 1970’s, and little is known about these repairs. It is believed that many of these original repairs failed and were replaced in subsequent major repair episodes. These were carried out in stages, with extensive patch repairs to many elements with a certain area. There were six different phases of concrete repair between 1988 and 1994; the repairs at Site 34 were carried out in 1992/3. There were also four phases of CP installation, starting with a trial system in 1987 and finishing with three major installations between 1992 and 1995. In addition, one further contract was let in 1991 for partial reconstruction of a pier using repair materials.

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The structure is to be partly demolished as part of a road traffic policy; Site 34 is to be entirely removed. The condition of the structures was not a significant factor in this strategy, and indeed the structures could in practice remain in operation, subject to future maintenance.

The requirement for repair was identified by the council engineers after cracking and spalling became apparent in the piers/trestles and deck soffits. An investigation contract was let to a third party site investigation company, which assessed condition by a combination of visual inspection, hammer survey, half-cell surveys, carbonation depth measurement and chloride content profiling. The council engineers identified the limits of areas to be repaired on the basis of existing damaged locations and undamaged areas with high chloride content (>0.4% by weight of cement) and high half-cell values (more negative than -300mV using a copper-copper sulphate probe).

A repair contract was then let to a specialist repair contractor. The specification was based on preexisting repair specifications used in earlier repair phases at the structure and was a council-written specification incorporating Department of Transport model specification BD27/86.

The perimeter of the repaired areas were sawn. The method of break out at Site 34 was by hand-held pneumatic breakers, in contrast to the water jetting used in the majority of other contracts. Break-out was extended to behind the outer reinforcement and typically to a depth of 130mm. Exposed bars were cleaned by blast cleaning to remove corrosion products. In some locations, test drillings were taken after break-out to determine the chloride content, to confirm the remaining substrate concrete was substantially uncontaminated. The repair areas were then enclosed within waterproof shutters, which were then filled with water to saturate the substrate, then drained and filled, by gravity feed or pumping, with pourable microconcrete. This material was a proprietary, shrinkage-compensated pre-blended cementitious mix requiring addition of water at site. The material was to meet the requirements of the Department of Transport Model Specification BD27/86 Part 4, ‘replacement concrete for sides and soffits of beams and crossheads’.

A likely proprietary material would have contained a blend of RHPC and PFA with a content of 500Kg/m3, with additional microsilica, a nominal compressive strength of 50N/mm2 at 28-days, and the aggregate a 5mm maximum sized graded limestone.

The repair material was formed flush with the original concrete surfaces. The forms were struck typically after 3 days, depending on cube strength results, and the repair was deemed complete after application of a curing membrane.


The core sampling indicates the repair concrete forms a single continuous layer to substantial depths in keeping with the reported method of break-out and replacement. At Location 1 and 2 the substrate deeper than the saw-cut edges has a form suggestive of break-out by mechanical means, in keeping with the report that pneumatic breakers were used.

The petrography identifies GGBS and graphite within the repair material suggesting it is a proprietary system complying with BD27/86 Part 3 for ‘decks and vertical surfaces to piers, columns and abutments’, or a material for use in repairs with a CP system, rather than a mix complying with BD27/86 Part 4, ‘replacement concrete for sides and soffits of beams and crossheads.’

At location 2, the repair is compromised by the very low cover and exposed bars at the surface, resulting from the practice of re-casting the concrete to the original profile. This has not actually led to significant deterioration within the repair, only localised and unsightly rusting of bars very close to the surface.

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There is substantial crazing at Location 2 which penetrates the repair concrete to the depth of the steel. The cracking appears to have resulted from drying shrinkage. This common feature ought not be present in a shrinkage-compensated material if cured for a minimum of seven days as recommended by the manufacturer or 14 days as recommended by the BD27/86. The cracking has not caused significant deterioration but might limit the performance of the repair in the long term, by allowing carbonation of the paste around the reinforcement and the ingress of chlorides, water and oxygen to the steel.


Corrosion-related spalling can occur in unrepaired concrete immediately adjacent to the repair. It is not clear whether this resulted from insufficient break-out of contaminated concrete or inadequate exposure and cleaning of reinforcement at the time of repair, or whether this represents incipient anode formation in previously un-corroding steel.

Reinstatement of low cover, with consequential exposed bars at the surface can result from the contract clauses requiring reinstatement to the original profile.

Cracking commonly occurs in this type of repair and could be a long-term durability limiting feature as well as being aesthetically displeasing.

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2.3.35 Site 35




Constructed 1960’s Repaired: 1991-2


Variable condition, many areas showing evidence of extensive repairs. The structure is not coated. Some repaired and unrepaired areas have had wire mesh attached over the surface to prevent debris falling from spalling locations. There are some areas of open and incipient spalling resulting from reinforcement corrosion.


Some repairs contain surface crazing but are soundly attached to the substrate, but there is little evidence of significant deterioration within the repairs in this area.

Repair details:


Repair proprietary Material(s): flowable repair

concrete


Condition:

The surface is unpainted, slightly crazed, with mostly no voids, and appears to have been worked to a smooth finish. The appearance is typical of the repairs in this area and different to those at Site 34.

In this view, the original concrete occurs above the repair material. Part of the vertical interface between repair and substrate is visible on the left of the pier, where there are two new core-hole reinstatements. There is no other evidence of deterioration of the repair. Hammer tapping indicates sound concrete.

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Photo of repaired structure (face also surveyed by half-cell): Element Type: R/C pier to trestle


concrete


Condition

In this view, the slightly crazed surface of the repair material is to the left of the pier and original concrete to the right.

The face of the pier that was sampled is on the left side of the face shown in this view.

Tests Conducted:

Test Results


Location 1 and 2: values range from -78 to +40mV at locations in the original concrete and –38 to +53mV within the repaired area, indicating low probability of corrosion throughout. There was a tendency for more negative values within 0.25m of ground level.

Face shown in second photo: values range from -180 to -60mV at locations in the original concrete, and mostly +60 to +80 in the repair, but with marked change to 0 to -30mV at ground level. These values suggest mostly low probability of corrosion throughout, with uncertain probability of corrosion.

Covermeter Locations 1 and 2: indicates horizontal bars at minimum depth of 35mm and vertical bars at 41mm.


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Alkali content The alkali content was determined on the repair material, in accordance with test BS1881 Part 124, using an assumed density of 2250 Kg/m3.

Na2O (wt%): 0.06

K2O (wt%): 0.21

Na2O equivalent: 4.46 Kg/m3

Samples:


Grey repair concrete original concrete with polymictic aggregate.


Location 1

The repair is to 95-110mm depth in a single layer. The core broke at the interface with the substrate concrete. There is crazing at the surface of the core which is visible in hand specimen penetrating to at least 50mm. The repair material is a light buff/grey deepening to mid grey at depth, with a coarse sand containing similar lithologies to that in the substrate concrete. There are traces of gel-like material in the repair material around the aggregate particles.

Location 2

Location 2 is 0.5m from Location 1 at the vertical edge of the same repair. The repair material has been worked so that a thin layer (0.5mm) overlaps the adjacent concrete. This location has crazing/fine cracking penetrating from the surface visible to approximately 25mm depth.

The repair is greater than 55mm depth in a single layer and the material is identical to that at Location1.

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Petrography

The two cores contain almost identical repair material and substrate concrete. The repairs in the cores both have flat, smooth external surfaces. Fine cracking is visible in the surface at Location 1. This passes through the full thickness of the repair, through the paste and some aggregate particles, suggestive of formation by drying shrinkage. The repair material consists of a siliceous, sub-rounded coarse sand (to 5mm diameter) in a paste composed of a blend of portland cement and GGBS. Some of the sand grains contain cryptocrystalline quartz and are potentially alkali-reactive. There are traces of material resembling gel on the sawn surfaces of the core from Location 1, but no evidence was found in thin section for the occurrence of gel or cracking caused by ASR. The paste has a low to moderate porosity but has high levels of microcracking orientated radially around particles of aggregate.

There is no evidence for layering, and the repairs form continuous layers up to 110mm thick. The material has a low void content with spherical entrained voids typically less than 1mm in diameter and fewer, larger, patchily distributed entrapped voids. There is negligible general carbonation of the repair material (a maximum locally of 3mm), but carbonation penetrates along the longitudinal crack at Location 1 to up to 54mm depth. The voids are mostly empty but there are traces of ettringite in voids close to the crack at Location 1. This may reflect penetration of water into the repair and localised very minor recrystallisation of the binder.

Location 2 contains the contact between repair and substrate at a vertical perimeter of the repair. In hand specimen, the vertical contact with the substrate appears to be sawn to 40-45mm depth and appears sound. In thin section, the depth of the sawn surface appears to be between 18 and 35mm. Below depths of 35mm the substrate concrete is rough and broken and there is localised microcracking within the outer 2mm of the concrete. The surface appears to have been prepared by a mechanical method.

The substrate is a pinkish brown/buff concrete with polymictic siliceous gravel and siliceous sand within a buff portland cement paste. The paste has a structure in keeping with an original water/cement ratio of 0.58. No evidence of ASR was found in this concrete. Carbonation penetrates to a depth of 20mm from the external surface but along the interface between repair and substrate, at Location 2, penetration is to 40mm. No evidence was found for deterioration of the concrete.




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The structure was constructed in the 1960’s and included an elevated roundabout above a main road, and an additional elevated section of road above a second road. The majority of the area below the elevated sections, including Site 35, form a series of car parks. Patch repairs were first carried out before 1987, possibly in the 1970’s, and little is known about these repairs. It is believed that many of these original repairs failed and were replaced in subsequent major repair episodes. These were carried out in stages, with extensive patch repairs to many elements with a certain area. There were six different phases of concrete repair between 1988 and 1994; the repairs at Site 35 were carried out in 1991/2. There were also four phases of CP installation, starting with a trial system in 1987 and finishing with three major installations between 1992 and 1995. In addition, one further contract was let in 1991 for partial reconstruction of a pier using repair materials.




The perimeter of the repaired areas were sawn. The method of break out at Site 34 was reported to be by water jetting. Break-out was extended to behind the outer reinforcement and typically to a depth of 130mm. Exposed bars were cleaned by blast cleaning to remove corrosion products. In some locations, test drillings were taken after break-out to determine the chloride content, to confirm the remaining substrate concrete was substantially uncontaminated. The repair areas were then enclosed within waterproof shutters, which were then filled with water to saturate the substrate, then drained and filled, by gravity feed or pumping, with pourable microconcrete. This material was a proprietary, shrinkage-compensated pre-blended cementitious mix requiring addition of water at site. The material was to meet the requirements of the Department of Transport Model Specification BD27/86 Part 4, ‘replacement concrete for sides and soffits of beams and crossheads’.


A proprietary material suggested to have been used by the repair material supplier, was a pre-packaged, high performance, non-shrink, self-compacting concrete conforming with Department of Transport Standard (BD27/86, Clause 4) and the Specification for Highway Works (Clause 1704.6). The reported compressive strength after 28 days at 20°C was 60N/mm2. The reported alkali content was 3 Kg/m . The material had a target mixed density of 2350 Kg/m3. This material is unlikely to be that examined in thin section, as the binder observed contains GGBS rather than PFA, suggesting the material complied with BD27/86 Part 3, ‘replacement concrete for decks and vertical surfaces to piers, columns and abutments’.

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3



There remains some uncertainty over the exact proprietary material applied in this site. The specification requirements and the actual material composition do not match.

There is limited evidence of a very low level of ASR in the repair material, which is likely to be related to particles of cryptocrystalline chert aggregate. It is unusual for such reaction to occur in a material containing GGBS within the binder. It is also unusual to find 1.5 times the reported maximum alkali content.

The method of substrate preparation was reported to be by water jetting, but the petrography suggests the surfaces were formed by mechanical break out.


Surface crazing at the surface may penetrate well beyond the surface. Fine cracking can also develop in this flowable repair material, to the depth of the substrate. Carbonation can penetrate along these cracks and potentially be a durability limiting feature.

Carbonation can penetrate along the interface between repair and substrate and might also be a durability limiting feature.

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2.3.36 Site 36




Constructed 1960’s Repaired: 1989/90 and possibly earlier


Variable condition, many areas showing evidence of extensive repairs. The structure is not coated. Some repaired and unrepaired areas have had wire mesh attached over the surface to prevent debris falling from spalling locations. There are some areas of cracking and open and incipient spalling resulting from reinforcement corrosion.


Many repairs contain surface crazing but most are soundly attached to the substrate. Some repairs have cracked in a manor reminiscent of that caused by ongoing reinforcement corrosion, and some areas of the substrate have deteriorated close to the repaired areas.

Repair details:


Repair bonding bridge Material(s): and repair mortar


Condition:

Repair mainly to one face containing rectangular crack pattern at surface.

Hammer tapping indicates this face is sound but there is hollowness in the narrow strip of repair in the face to the left.

The repair material at this location appears different to that seen at Location 2 and Sites 34 and 35, and is probably from a phase earlier than the 1989/1990 repair to the other face of the same pier.

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concrete


Condition

The repair at this location passes from one face into a second and contains a near-vertical crack close to the corner. There are traces of corrosion product close to the crack at the at the external surface.

The crack passes perpendicular to the surface and intersects the reinforcing bar. The core was taken at the crack and has broken along it.

Hammer tapping around the cracked area indicate an incipient spall above and to the left of the crack.

Tests Conducted:

Test Results


Location 1: values in the repair range from –25 to –165mV, indicating low to uncertain probability of corrosion.

Location 2: values in the repair range from +50 to –170mV away from the crack, and increase to up to –375mV close to it, suggesting a high probability of corrosion of the vertical bar. In the original concrete below the repair, the values range from – 34 to –290mV, increasing in negative potential at ground level and at the corner beneath the crack in the repair, indicating an uncertain probability of corrosion.

Covermeter Location 1: confirms minimum cover to horizontal bars is 10-20mm, and to vertical bars is 20-35mm. One bar was intersected at 35mm depth during coring.

Location 2: indicates minimum cover to horizontal bars is 30mm and to vertical bars is 40-50mm. One bar was intersected at 49mm depth during coring.


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Samples:


Buff fine grained repair mortar at Location 1 and grey repair mortar with white aggregate particles at Location 2 applied to original light coloured concrete with polymictic gravel coarse aggregate.


Location 1

The repair is a fine grained cementitious material with generally low voidage and voids <0.5mm, which has cracked throughout and at its perimeter. The repair is to a depth of 33-40mm, and appears to consist of two 2 layers, with a surface-parallel interface at approximately 5mm depth.

The core sample includes a vertical surface crack which passes perpendicular to the surface and intersects the substrate between the two bars. The core has broken at this crack.

Location 2

The core has split at the vertical crack and corrosion products coat the surfaces of the crack and outer face of the bar at 50mm depth. The underside of the bar is not corroded.

The core also split along surface parallel cracks at 50mm and 70mm depth.

The repair is a grey repair mortar with light grey/white fragments (<2mm), and is the same as at Site 34. The repair is at 55-115mm depth, and appears to be typically 60mm deep from one face of the column and 90mm deep from the second.

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Petrography

Location 1

The repair material intersects the reinforcement, at 33-35mm, so that the repair sits partly on the bar surface. The bars are mostly encapsulated within the original concrete substrate. There is no evidence of corrosion of the bars.

This core contains cracks within the concrete substrate and the repair material, mostly around the reinforcement bar at 35mm depth. The repair mortar has debonded at the surface of the resinous bonding primer. It is possible that much of the cracking and the debonding noted in the core resulted during the core sampling.

The repair mortar is carbonated to a depth of approximately 15mm. The paste on both sides of the fine crack is also carbonated to 35mm depth, and there is patchy carbonation at the surface of the concrete substrate, to up to 4mm depth.

The mortar contains a fine siliceous sand, with sub-angular particles mostly less than 0.8mm in diameter. The binder is based on portland cement with a low, patchy porosity, few microcracks and numerous entrained voids typically less than 1mm in diameter. The voids typically contain crystals of ettringite.

The surface of the concrete substrate is rough and irregular with particles of coarse and fine aggregate exposed. There is minor surface-parallel microcracking in the outer 1mm of the concrete, mostly within the cement paste. The surface of the concrete and the bars is coated with a black to brown oily or resinous material that smells of antiseptic. In thin section the coat forms a nearly continuous layer of a clear, resin-like material containing angular shards of slag-like material. The repair mortar has debonded from the substrate along this resinous layer.

The concrete substrate is similar at both locations and consists of a siliceous gravel of 20mm maximum nominal size, a medium siliceous sand. The paste is based on portland cement and contains abundant coarsely crystalline portlandite and little unhydrated cement, in keeping with a high water/cement ratio of 0.58 to 0.6. The voids are mostly empty. There is little evidence for deterioration of the paste as a result of moisture penetration.

Location 2

The bar is partly within the concrete substrate and partly within the repair. Corrosion of the bar appears to have occurred resulting in cracking of the repair rather than the substrate concrete. The surfaceparallel cracking close to the reinforcement may also have been generated in the coring process.

The repair material is in a single layer and contains crushed particles of re-crystallised limestone (<3mm diameter) in a binder composed of portland cement, PFA and particles of black carbonaceous material that resembles graphite. The paste has a low porosity and low level of microcracking. The mortar contains spherical entrained voids throughout the paste. These are empty.

There is minor carbonation of the repair material to up to 3mm depth, but the paste on both sides of the crack passing from the external surface is patchily carbonated to 48mm depth. There are corrosion products in the crack and minor corrosion to the bar at the base of the crack at 49mm depth.

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The concrete substrate has a highly irregular, broken surface with exposed particles of aggregate. There are some voids between the repair and substrate, but very few microcracks in the concrete close to the interface. However, there are traces of a material resembling gel within a cracked aggregate particle close to the contact with the repair material.





The structure was constructed in the 1960’s and included an elevated roundabout above a main road, and an additional elevated section of road above a second road. The majority of the area below the elevated sections, including Site 36, form a series of car parks. Patch repairs were first carried out before 1987 (for example at Site 36 Location 1), possibly in the 1970’s, and little is known about these repairs. It is believed that many of these original repairs failed and were replaced in subsequent major repair episodes. These were carried out in stages, with extensive patch repairs to many elements with a certain area. There were six different phases of concrete repair between 1988 and 1994; the repairs at Site 36 Location 2 were carried out in 1989/90. There were also four phases of CP installation, starting with a trial system in 1987 and finishing with three major installations between 1992 and 1995. In addition, one further contract was let in 1991 for partial reconstruction of a pier using repair materials.


The requirement for repair was identified by the council engineers after cracking and spalling became apparent in the piers/trestles and deck soffits. An investigation contract was let to a third party site investigation company, which assessed condition by a combination of visual inspection, hammer survey, half-cell surveys, carbonation depth measurement and chloride content profiling. The council engineers identified the limits of areas to be repaired on the basis of existing damaged locations and undamaged areas with high chloride content (>0.4% by weight of cement) and high half-cell values (more negative than –300mV using a copper-copper sulphate probe).


The perimeter of the repaired areas were sawn. The method of break out at Site 36, Location 1, was probably by hand-held pneumatic breakers.

At Site 36, Location 2, the method of break out was by water jetting. Break-out was extended to behind the outer reinforcement and typically to a depth of 130mm. Exposed bars were cleaned by blast cleaning to remove corrosion products. In some locations, test drillings were taken after break-out to determine the chloride content, to confirm the remaining substrate concrete was substantially uncontaminated. The repair areas were then enclosed within waterproof shutters, which were then filled with water to saturate the substrate, then drained and filled, by gravity feed or pumping, with pourable microconcrete.

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This material was a proprietary, shrinkage-compensated pre-blended cementitious mix requiring addition of water at site. The material was to meet the requirements of the Department of Transport Model Specification BD27/86 Part 4, ‘replacement concrete for sides and soffits of beams and crossheads’.




At Location 1, the thick coating to the reinforcement had prevented corrosion of the bar despite cracking from the surface to it. The crazing in the repair, and delamination from the corner, indicated the repair was reaching the end of its life. However, it remained functional.

The micro-cracking in the outer 1mm of the concrete substrate is probably related to the mechanical preparation of the substrate, and is in contrast to the very low level of cracking observed at Location 2 where the surface was prepared by water jetting. No evidence was found that delamination had occurred at Location 1 because of the micro-cracks.

The repair mortar contains ettringite within the voids, indicative of penetration of moisture into the repair.

The petrography shows no sign of deterioration of the concrete through moisture penetration, suggesting at depth the pier had remained substantially dry in its sheltered location. However, some moisture must have been present to drive deterioration to result in the two generations of repair observed.

The repair material at Location 2 is a proprietary flowable material containing PFA in the binder suggesting it complied with the specification and is the material identified by the manufacturer. A small element of doubt remains over the presence of graphite which might be associated with a repair material to be used with a CP system.

At Location 2, the bar is partly embedded in the repair, and has continued to corrode. The performance may have been improved if the bar was fully encapsulated in the repair, by further break-out, and suggests that the original extent of repair was not large enough.

No evidence was found for ASR within the substrate concrete, with the exception of the traces of gel found close to the repair interface. This gel may have been generated by alkali migration from the repair, but is clearly at an insignificant level.


Surface crazing may penetrate the full depth of the repair in the form of fine cracks.

Carbonation may penetrate along the cracks to the depth of the concrete substrate and embedded reinforcement.

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The resin-like coating over the concrete substrate in Site 36 Location 1 appears to have provided protection to the underlying reinforcement despite fine cracks leading to this interface. However, the resinous coating may have provided a plane of weakness along which the repair mortar can delaminate, as noted toward the corner of the pier.

Care must be taken to protect the reinforcement from future corrosion where the perimeter of the repair intersects the bar.

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2.3.37 Site 38


Date of inspection/testing: 25/2/01 Contractor details: Coring contractor. Scaffold required.


Constructed 1960’s Repaired: 1993/4


Variable condition, many areas showing evidence of extensive repairs which are raised proud of the original surface. The structure is painted with a standard masonry paint. Some repaired and unrepaired areas have had wire mesh attached over the surface to prevent debris falling from spalling locations. There are some areas of open and incipient spalling resulting from reinforcement corrosion. There is also abundant evidence of seepage through the road deck above, down the face of the beams and columns.


Most repairs appear in good condition and are soundly attached to the substrate. On close inspection, some areas have faint surface crazing beneath the paint, and in rare locations there is open spalling in the repairs.

Repair details:

Photo of repaired structure (location 1): Element R/C pier to Type: trestle


concrete

Coatings/ masonry paint render:

Condition:

The repair has been built up by 20-30mm above the original concrete surface and painted with a white paint, and is typical of the repairs in this area.

The repair was in good condition but areas of spalling concrete occurred close to the repair.

Hammer tapping indicates sound concrete.

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Tests Conducted:

Test Results

Half-cell Location 1: values range from -23 to –145mV within the repaired area, with most potential (SSC) locations less negative than –130mV, indicating low probability of corrosion.

Outside of the repair, in the original concrete, values range from -80 to –245mV with sharp increase in negative potential close to patches of corrosion product at the surface, indicating low to uncertain probability of corrosion. The latter may reflect the generally dry condition of the beam as there are active sites of corrosion visible.

Covermeter Location 1: confirms minimum cover in unrepaired areas at 15mm and within repairs indicates bars at approximately 40mm.


Samples:


Grey repair mortar with white aggregate particles applied to buff concrete with polymictic siliceous gravel coarse aggregate.


Location 1

The surface is painted and this is coated with a layer of calcite deposited by water seeping through the deck above. No crazing was visible at the surface, but fine cracks are visible in the repair, running perpendicular to the external surface, to 70mm and 100mm depths.

The repair material appears similar to that at Site 34 and Site 36 Core 2 and Site 37.

The repair is to 145mm depth in a single layer. The contact with the substrate is good, and the substrate appears to have been prepared by waterjetting. There is no other evidence of deterioration of the repair.

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Petrography

The external surface of the repair is smooth and flat, and coated with a layer of white paint, 0.1 – 0.3mm thick, consisting of crushed particles of limestone, white pigment, and spherical voids within an organic matrix. The paint is coated with coarse crystals of calcium carbonate.

The repair material is up to 150mm thick and shows no obvious signs of layering. The material consists of particles of crushed re-crystallised limestone (<4mm diameter) within a binder based on portland cement with moderately abundant particles resembling graphite. GGBS was also found but in relatively low abundance. There are abundant entrained air voids which are mostly empty. The depth of carbonation in the repair was less than 1mm except where along the crack from the external surface, along which there is carbonation in a narrow zone to some 67mm depth. The repair material is of low to moderate porosity but contains a high level of micro-cracking, with cracks radiating from aggregate particles indicative of drying shrinkage cracks.

The fine crack passes through the paste and particles of the limestone within the repair.

The interface between the substrate concrete and the repair appears sound. The substrate concrete contains a siliceous coarse and fine aggregate within a portland cement paste. No evidence was found for deterioration of the concrete.





The structure was constructed in the 1960’s and included an elevated roundabout above a main road, and an additional elevated section of road above a second road. The elevated sections at Site 38 forms part of the elevated road. Patch repairs were first carried out before 1987, possibly in the 1970’s, and little is known about these repairs. It is believed that many of these original repairs failed and were replaced in subsequent major repair episodes. These were carried out in stages, with extensive patch repairs to many elements with a certain area. There were six different phases of concrete repair between 1988 and 1994; the repairs at Site 38 were carried out in 1993/4. There were also four phases of CP installation, starting with a trial system in 1987 and finishing with three major installations between 1992 and 1995. In addition, one further contract was let in 1991 for partial reconstruction of a pier using repair materials.


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The perimeter of the repaired areas were sawn. The method of break out at Site 38 was by water jetting. Break-out was extended to behind the outer reinforcement and typically to a depth of 130mm. Exposed bars were cleaned by blast cleaning to remove corrosion products. In some locations, test drillings were taken after break-out to determine the chloride content, to confirm the remaining substrate concrete was substantially uncontaminated. The repair areas were then enclosed within waterproof shutters, which were then filled with water to saturate the substrate, then drained and filled, by gravity feed or pumping, with pourable microconcrete. This material was a proprietary, shrinkagecompensated pre-blended cementitious mix requiring addition of water at site. The material was to meet the requirements of the Department of Transport Model Specification BD27/86 Part 4, ‘replacement concrete for sides and soffits of beams and crossheads’.


The repair material was finished proud of the original concrete surfaces. The forms were struck typically after 3 days, depending on cube strength results, and the repair was deemed complete after application of a curing membrane.


The petrography identifies GGBS and graphite within the repair material suggesting it is a proprietary system complying with BD27/86 Part 3 for ‘decks and vertical surfaces to piers, columns and abutments’, or a material for use in repairs with a CP system, rather than a mix complying with BD27/86 Part 4,’replacement concrete for sides and soffits of beams and crossheads.’

This repair appears sound with the exception of the surface cracking. However, the adjacent concrete, both below the repair close to the bottom of the beam, and to the side of the repair, shows evidence of corrosion of bars at 15mm depth with spalling of cover concrete. This may relate to development of incipient anodes, or more simply through the ingress of chlorides from the surface through the limited cover zone. If the latter, it could have been prevented by extending the built-up area further beyond the limits of the repair to help protect other low-cover areas. It was also noted at the site that where the face of the beam had been repaired to the bottom, and extended across the soffit, there was much less likelihood of ongoing deterioration close to the aris compared to locations where the repairs stopped short of the aris. This is demonstrated to some extent in the photograph of the site.

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The enhancement of cover can provide additional protection to embedded reinforcement particularly in areas of low cover.

Flowable repair materials may develop cracks perpendicular to the surface, along which carbonation can penetrate.

Painting of the surface may assist shedding of water from it and bridge crazing or fine cracks in the surface.

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2.3.38 Site 39


Date of inspection/testing: 25/2/01 Contractor details: Coring contractor, scaffold required




Mostly good condition; some areas showing evidence of repairs, and structure is coated with anode paint as part of the operational CP system. There is no evidence of ongoing reinforcement corrosion within the protected areas, but there is extensive evidence of corrosion of fragments of tie wire embedded in the soffit of the beams. Outside of the protected areas there are locations with cracking and spalling that sometimes have mesh applied to the surface.


The outlines of some repairs can be identified; others are hidden by the paint system. The repairs appear to be in good condition. A CP system is applied to the surface of the beams and columns and consists of metal wire primary distributors embedded within a wide woven strip coated with an anode paint and overcoated with a second paint layer.

Repair details:

Photo of repaired structure: Element Type: R/C pier to trestle


concrete and CP system

Coatings/ anode paint render: system

Condition:

There is no other evidence of major deterioration of the repairs. However, there is evidence of seepage of water through the deck and disruption of the paint system, and corrosion of tie wire fragments in the beam soffit. Hammer tapping indicates sound concrete.

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Photo of repaired structure: Element Type: R/C pier to trestle


concrete and CP system

Coatings/ anode paint render: system

Condition

This view shows the locations of the three core samples in the face of the beam. The lower distribution wire is visible running to a junction box 1.3m distant. Seepage of water and salts is evident below the guttering.

Core 39.1 is from the bottom right core hole, 39.2 from the bottom left hole, and 39.3 from the top left hole.

Tests Conducted:

Test Results

Covermeter Location 1: confirms minimum cover for vertical bars at 35-45mm.

Pull-off tests Depth and uncertain geometry of repair prevented pull-of testing.

Samples:


Three core samples taken from a beam supporting the deck of the elevated road, protected by an anode paint CP system. One sample includes a portion of a patch repair and two consist entirely of the original concrete.

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This view shows the cores and the external painted surfaces with the primary distribution wires in Cores 39.1 and 39.2.

Core Reference: 39.1

This sample was taken close to the bottom of a beam at the lower of two primary distribution wires separated by a vertical distance of 0.8m. The surface is coated with a light coloured paint over a black anode paint system, and includes a section of the distribution wire embedded within a woven fabric ribbon. The thin section reveals a further resinous layer beneath the black layer. The outline of a repair to the lower face of the beam and the beam soffit is visible under the surface coatings.

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This sample was taken 0.55m horizontal distance from Core 39.1, close to the bottom of a beam, and at the lower of two primary distribution wires. The surface of the sample includes a section of the same distribution wire and ribbon found in Core 39.1, and the anode paint system.

The core consists entirely of the original concrete, composed of siliceous polymictic gravel aggregate and sand within a portland cement binder. No evidence of deterioration, including ASR, was found.


This sample was taken at 0.4m vertical distance from Core 39.2, mid way up the face of the beam, equidistant between the two primary distribution wires. The surface of the sample is coated with the anode paint system.

The core consists entirely of the original concrete, composed of siliceous polymictic gravel aggregate and sand within a portland cement binder. No evidence of deterioration, including ASR, was found.

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Petrography and SEM/EDXA


The core sample at this location contains a repair concrete in one layer of 51 to 185mm depth. A 0.5-inch diameter vertical reinforcing bar at 48mm depth is almost fully encapsulated within the repair, but a small amount of its circumference is in contact with the original concrete substrate. Crazing is not visible at the external surface, but fine cracks are visible in the walls of the core passing to >55mm depth, to the steel bar and substrate. The thin section confirms the presence of fine cracking and microcracking around the reinforcement and some cracks pass through the repair material from the external surface to the bar. These appear to have resulted from drying shrinkage. Carbonation typically penetrates to less than 2mm from the external surface but has penetrated along the fine crack to 57mm, to the bar. There is no further evidence of deterioration of the repair or bar.

The aggregate in the repair material consists of crushed particles of recrystalised limestone up to 3mm in diameter. The paste is a blend of portland cement and GGBS, with numerous graphite particles. The paste has very low levels of microcracking and has a low general level of porosity. The voids throughout the repair are empty.

The surface of the substrate is rough and broken but the repair is firmly attached to it. The features of the surface are in keeping with preparation by water jetting. The substrate concrete consists of a siliceous gravel and siliceous sand in a portland cement paste and shows no macroscopic evidence of deterioration.

The thin section shows the presence of crystals of calcium carbonate at the base of the anode paint layers and on outer surface of the repair concrete. There is also a patchy porosity to the paste just below the external coatings.

Chemical analyses generally show uniformly low levels of sodium and potassium but enhanced levels of chlorine at the surface and sulphate at 2-3mm depth. However, close to the surface of the fine crack there is a more marked change in ionic distribution with depletion of sodium and potassium, and increase in chlorine and sulphate in the outer 2mm. At the depth of the bar there is also a strong increase in potassium and slight rise in sodium, accompanied by a strong reduction in sulphate and slight reduction in chlorine.


The concrete contains a 0.5inch diameter vertical bar at 40mm depth, and shows no corrosion of the steel.

Chemical analyses generally show uniform levels of sodium and potassium but enhanced levels of sulphate at 2-4mm depth. Chlorine levels are moderately high and generally greater than 0.3% and show a gradual increase to a peak of 1% at 14mm.


The concrete contains a 0.5inch diameter vertical bar at 45mm depth (the same bar as at Core 39.2), and shows no corrosion of the steel.

Chemical analyses show uniform levels of sodium, potassium, chlorine and sulphate below 15mm depth but with strong peaks for sulphate at 0-12mm depth, a strong potassium peak at 5-10mm depth, and a less well defined peak for sodium at 5-10m depth.

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Discussions with the City Council engineering staff involved in the maintenance of the structure and involved in the repair contract. Reports on the installation and performance of the systems provided by the contractor responsible for monitoring the systems.

The commissioning report, in November 1993, concluded the two systems installed were fully functional although there was some interference in some signal cables on interruption of current.

The Phase VI system review report in January 1996 identified that over extended periods of dry weather, the circuit resistance of the CP zones to the crossbeams increased. During the much wetter weather late in 1996, the circuit resistance dramatically fell. This was interpreted as resulting from ingress of rainwater through the deck and into the top surfaces of the crossbeams. It was concluded that the CP systems were particularly susceptible to variances in circuit resistance with seasons.

In May 1996, the performance monitoring report showed that the maximum anode/concrete current density of 20mA/m2 had not been exceeded for any anode zone; typical anode/concrete current density was 10mA/m2

The systems were operating at the maximum dc output voltage.

Each anode zone was individually assessed against draft European Standard CEN clause 8.6. In many zones, decay criterion for the embedded electrodes were not met after 24 hours, but the 25 hour OFF readings were less negative than –150mV and thus the zones were considered to be adequately protected. At several electrodes the readings were too unstable to collect useful data and at one the instant off criterion was not met as the reading was more negative than –1100mV.

The visual inspection identified that some areas required repair to areas of paint delamination and that metallic fixings in the anode zones could corrode and cause deterioration of the anode system. It was recommended that non-metallic fixings be used.

It was also recommended that the transformer-rectifier output LED’s be re-fitted, the location of each zone be properly identified recorded, and the future annual monitoring involve collection of 24-hour potential decay data.

In November 2000, the visual inspection identified numerous areas of defective coating, due to water damage from leaks in the bridge deck and drainage system. The primary anode wires had also been locally pulled from the columns, though this did not appear to detrimentally affect operation of the system. The operating current was adjusted in 9 zones; increased by 10 or 20% in five zones and reduced by 10 to 30% in four zones. Similar adjustments were made in April and October 2001.

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Areas showing evidence of deterioration were repaired using a proprietary flowable patch repair system prior to installation of the CP system. There was an abundance of tie wire, originating from the original construction, embedded in the soffit of the beam. The wire had corroded and stained the soffit, resulting in local minor spalling, and was also largely removed by hammer and bolster. This has resulted in a ‘pock-marked’ appearance to the lower surface of the beam. However, since application of the paint system, many sections of wire, not removed prior to painting, have continued to corrode, and stain the lower surface of the beam.

The anode system was a conductive paint coating, fed by metal wires embedded in carbon fibre ribbon. The coating included primer and two coats of the conductive paint. Monitoring included use of embedded silver/silver chloride reference electrodes. The cable to reference electrode joint was a crimped connection protected by two layers of heat shrink sleeve insulation. The system was controlled by several separate transformer rectifiers, which were situated in a room adjacent to the car park.

The CP system relies on a series of primary distribution wires (10mm2 cross section, with 2.5 mm2

cross-section signal wires) which run to each element within plastic sheaths connected with junction boxes and thereafter on the surfaces of each element, embedded within a wide ribbon band overpainted with the anode paint system. The ribbons remain firmly embedded within the beams, which are at 3 to 6m height. However, the ribbons also pass vertically down each the face of each column to close to ground level. Some of these ribbons have become detached from the substrate, either through vehicle collision (the area beneath the elevated road is a car park) and/or through vandalism. Once partly detached, it is simple to further remove the ribbon system.

The majority of the elements protected by the CP system appeared to be in good condition. However, there were areas where seepage of water and road salts through the road deck had clearly resulted in deterioration of the anode paint system. However, no evidence was found for deterioration of the underlying concrete substrate.


Repair has been carried out prior to application of the CP system but bars were not necessarily fully broken out. The flowable repair material is similar to that used in other repair contracts at different locations at the structure. Shrinkage cracking occurs from the external surface to the embedded steel.


Cracking can occur in the repair material, from the surface to the substrate, intersecting the embedded reinforcement. Carbonation can penetrate along the crack.

Application of a CP system with robust external coatings can provide a high level of protection to a deteriorated structure where ingress of moisture may still occur.

Testing shows there are microstructural changes at the interface between repair and surface coatings but where the concrete remains dry there is little unambiguous evidence of change in ionic distributions in repair and substrate concrete. More notable effects occur close to the surfaces of a crack. This may be related to an increased moisture content facilitating ionic movement. The concentration of sulphate just below the carbonated surface zone is likely to have resulted from migration of ions during the carbonation process.

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2.3.39 Site 41


Date of inspection/testing: 25/2/01 Contractor details: Coring contractor, scaffold required


Constructed 1960’s Repaired: 1992/3


Mostly good condition; some areas show evidence of repairs, and the structure is coated with anode paint as part of the operational CP system. There is no evidence of ongoing reinforcement corrosion, but there is extensive evidence of corrosion of fragments of tie wire embedded in the beam soffits.


The outlines of some repairs can be identified; others are hidden by the paint system. The repairs appear to be in good condition. A CP system is applied to the surface of the beams and columns and consists of metal ribbon primary distributors embedded within a wide woven strip coated with an anode paint and overcoated with a second paint layer.

Repair details:

Photo of repaired structure (location 1): Element R/C column Type:

Repair proprietary flowable Material(s): repair concrete and CP

system

Coatings/ anode paint CP system render:

Condition:

This location was selected as there appeared to be a repair over part of the surface of the column, allowing samples to be taken from repaired areas and unrepaired original concrete at the primary distribution ribbon and remote from it.

The repair was detectable as the surface had been built up by approximately 20mm above that of the original concrete.

There is no other evidence of deterioration of the repair. Hammer tapping indicates sound concrete.

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The view shows some areas of white deposits at the top of the column and along the beams, from seepage through the deck. There is also an area painted black on the beam to the left, which is additional anode paint.

Tests Conducted:

Test Results

Covermeter Core samples indicate minimum cover is 30mm.


Samples:


Samples all have an external coating. The original substrate concrete has a siliceous coarse and fine aggregate in a portland cement binder and in two locations there is a fine grained grey repair mortar applied over the concrete.


Core 41.1 was within the unrepaired original concrete at 0.3m horizontal distance from the primary distribution ribbon and Core 41.2.

Core 41.2 was within the unrepaired original concrete at the primary distribution ribbon.

Core was 41.3 was taken within the repaired part of the column at 0.3m horizontal distance from the primary distribution ribbon and Core 41.4.

Core 41.4 was taken within the repaired part of the column at the primary distribution ribbon.

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Traces of corrosion occur on the paste at the impression of a 1.25inch diameter vertical reinforcing bar at 30mm depth, as illustrated in the photograph opposite. The concrete shows traces of ASR.

Carbonation penetrates the concrete to a depth of 6-8mm.


Traces of corrosion occur on the paste at the impression of a 1.25inch diameter vertical reinforcing bar at 40mm depth. The concrete shows no traces of ASR.

Carbonation penetrates the concrete to a depth of 6-8mm.


The repair is a fine grained medium grey render, in 2 main layers, (2540mm and 2-25mm), with a third 2mm-thick surface finish of similar. The method of breakout unknown; the repair material is similar to that found in Site 36 Core 1.

Traces of corrosion occur on the on paste at a 1.25inch diameter vertical reinforcing bar at 36mm depth and the impression of a bar at 114mm depth.

The substrate concrete shows no traces of ASR.

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The repair is a fine grained medium grey render, in a single main layer of 15mm thickness and a 2mm surface finish layer. The material appears to have been applied direct to the surface of the concrete and may have been intended to enhance cover or protect areas close to deeper repair (e.g. at Core 41.3).

Traces of corrosion occur on the paste at a 1.25inch diameter vertical reinforcing bar at 37mm and the impression of a bar at 43mm depth.

The concrete shows no traces of ASR.


Petrography and SEM/EDXA

The four samples all contain a similar concrete with siliceous natural gravel and siliceous sand in a porous portland cement paste. Samples 41.3 and 41.4 have broken surfaces covered with layers of repair mortar. The repair mortar consists of a blend of PFA cenospheres and fine siliceous sand in a portland cement paste.

All samples are coated with a continuous layer of paint with an inner, black carbonaceous layer (0.05-0.15mm) covered by an outer white layer (0.15-0.3mm) of very low porosity containing a very fine crushed quartz filler and white pigment. Each sample contains reinforcement which shows very minor corrosion.


Chemical analyses show increased levels of chlorine and sulphate at the surface (0-1mm) and a substantial peak for sulphate at 12-17mm depth. There are also higher levels of sodium and potassium, and low levels of chlorine, at 1-12mm depth within the carbonated zone. At the depth of the bar there is a an increase in potassium and reduction in sulphate.


The chemical analyses reveal similar trends as for Core 41.1, and show increased levels of chlorine and sulphate at the surface (0-2mm) and a substantial peak for sulphate at 12-17mm depth. There are also higher levels of sodium and potassium, and low levels of chlorine and sulphate, at 2-14mm depth within the carbonated zone. At the depth of the bar there is a an increase in potassium and possible reduction in sulphate.

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The repair material is in 3 distinct layers of 20-39mm total thickness. Carbonation penetrates to 1mm depth in the repair material. There is also patchy carbonation of the concrete substrate to depths of up to 8mm from the repair interface. The aggregate in the repair mortar is a predominately quartz sand with sub-rounded particles less than 1mm in diameter. The paste has a high, slightly patchy porosity, low levels of microcracking, and empty voids. Irregular voids are locally abundant at the base of the repair mortar. No evidence was found for significant deterioration of the repair material.

There are locally abundant surface parallel cracks in the aggregate and paste of the concrete substrate within 2mm of the contact with the repair, and this surface has features associated with preparation by mechanical means.

The chemical analyses on a traverse through the repair material show increased levels of chlorine and sulphate and depleted sodium and potassium at the surface (0-1mm). There are then elevated sodium and sodium values at 1-3mm and at roughly 10 and 25mm depths. The chlorine values remain consistently low but there are peaks for sulphate at 10, 17 and 27mm. At the depth of the bar there is a an increase in potassium and potassium, and an atypical increase in sulphate. A second, surfaceparallel traverse revealed relatively consistent values except for a sharp increase in potassium at the bar.


The repair material is in 2 layers of 15-17mm total thickness. Carbonation penetrates the concrete to a depth of 6-8mm.

The traverse from the surface of the core passes through the repair material (0-18mm) then the original concrete (18-40mm), intersecting the bar at 36mm. The chemical analyses are relatively complex but show a pattern. There are increased levels of chlorine at the surface (0-2mm) and relatively consistent and low values throughout the remainder of the repair material. The chlorine value is consistently low in the outer carbonated paste of the substrate concrete and increases in deeper uncarbonated paste. The sulphate values are low immediately at the surface (0-1mm) and then consistently higher in the repair material before peaking sharply at 15mm very close to the concrete substrate. In the concrete, sulphate is depleted in the carbonated zone (17-25mm), peaks sharply at 27mm and is depleted at 35-37mm depth at the bar.

The potassium values within the repair peak at 0.5mm and are high at 7mm and 13mm. In the substrate, there is a consistently high potassium content in the carbonated zone (17-25mm) and low values at greater depths with marked increases at the bar (34-37mm). The sodium values within the repair increase at 7mm and decrease at 15mm, and are elevated in the carbonated zone of the concrete substrate, and at the bar at 35-37mm.


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Discussions with the City Council engineering staff involved in the maintenance of the structure and involved in the repair contract. Reports on the installation and performance of the systems provided by the contractor responsible for monitoring the systems.

The commissioning report, in November 1993, concluded the two systems installed were fully functional although there was some interference in some signal cables on interruption of current.

The Phase VI system review report in January 1996 identified that over extended periods of dry weather, the circuit resistance of the CP zones to the crossbeams increased. During the much wetter weather late in 1996, the circuit resistance dramatically fell. This was interpreted as resulting from ingress of rainwater through the deck and into the top surfaces of the crossbeams. It was concluded that the CP systems were particularly susceptible to variances in circuit resistance with seasons.

In May 1996, the performance monitoring report showed that the maximum anode/concrete current density of 20mA/m2 had not been exceeded for any anode zone; typical anode/concrete current density was 10mA/m2

The systems were operating at the maximum dc output voltage.

Each anode zone was individually assessed against draft European Standard CEN clause 8.6. In many zones, decay criterion for the embedded electrodes were not met after 24 hours, but the 25 hour OFF readings were less negative than –150mV and thus the zones were considered to be adequately protected. At several electrodes the readings were too unstable to collect useful data and at one the instant off criterion was not met as the reading was more negative than –1100mV.

The visual inspection identified that some areas required repair to areas of paint delamination and that metallic fixings in the anode zones could corrode and cause deterioration of the anode system. It was recommended that non-metallic fixings be used.

It was also recommended that the transformer-rectifier output LED’s be re-fitted, the location of each zone be properly identified recorded, and the future annual monitoring involve collection of 24-hour potential decay data.

In November 2000, the visual inspection identified numerous areas of defective coating, due to water damage from leaks in the bridge deck and drainage system. The primary anode wires had also been locally pulled from the columns, though this did not appear to detrimentally affect operation of the system. The operating current was adjusted in 9 zones; increased by 10 or 20% in five zones and reduced by 10 to 30% in four zones. Similar adjustments were made in April and October 2001.

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Areas showing evidence of deterioration were repaired using a proprietary flowable patch repair system prior to installation of the CP system. There was an abundance of tie wire, originating from the original construction, embedded in the soffit of the beam. The wire had corroded and stained the soffit, resulting in local minor spalling, and was also largely removed by hammer and bolster. This has resulted in a ‘pock-marked’ appearance to the lower surface of the beam. However, since application of the paint system, many sections of wire, not removed prior to painting, have continued to corrode, and stain the lower surface of the beam.

The anode system was a conductive paint coating, fed by mixed metal oxide coated titanium ribbon. The coating included primer and two coats of the conductive paint. Monitoring included use of embedded silver/silver chloride reference electrodes. The cable to reference electrode joint was a crimped connection protected by two layers of heat shrink sleeve insulation. The system was controlled by several separate transformer rectifiers, which were situated in a room adjacent to the car park.

The CP system relies on a series of primary distribution wires (10mm2 cross section, with 2.5 mm2

cross-section signal wires) running to each element within plastic sheaths connected with junction boxes and thereafter as 10mm-wide metal ribbons on the surfaces of each element, embedded within the anode paint system. The ribbons appear firmly embedded within the beams, which are at 5 to 7m height. However, the ribbons also pass vertically down each the face of each column to close to ground level. Some of these ribbons have become detached from the substrate, either through vehicle collision (the area beneath the elevated road is a car park) and/or through vandalism. Once partly detached, it is simple to further remove the ribbon further.

The majority of the elements protected by the CP system appeared to be in good condition. However, there were areas where seepage of water and road salts through the road deck had clearly resulted in deterioration of the anode paint system. However, no evidence was found for deterioration of the underlying concrete substrate.


The repair material is different to that commonly used in other contracts at many other parts of the structure. It was applied over substantial portions of one face of the column forming a layer 20mm proud of the surface. The anode paint and the primary distributor ribbons appeared to be in good condition, though in other parts of the site paint had been damaged by ingress of water, and by abrasion by vehicles, and the distributors had been pulled from the surface of the columns (probably vandalism).


Application of a CP system with robust external coatings can provide a high level of protection to a deteriorated structure where ingress of moisture may still occur.

The hardware of CP systems may be damaged (by vandalism) in public places.

Testing shows there is evidence of change in ionic distributions in repair and substrate concretes relating to carbonation and the operation of the CP system. Carbonation appears to have resulted in depleted chloride and sulphate, and elevated alkali contents in the carbonated zone, with concentration of sulphate just below the carbonated zone. CP has resulted in some increase in alkalis and reduction in sulphate at the bar. The trends occur in both repair and substrate concrete.

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2.3.40 Site 42A



Type of structure Parapet wall in Location: Midlands Zoo



This structure was in apparently pristine condition with no evidence of deterioration evident. Note this is a Listed structure.


The location of repairs was not obvious due to the presence of a coating system and possibly a fairing coat. The location of repairs was estimated through comparison with photographs taken at the time of the repair, showing areas of break out. There was no evidence to suggest the repairs had deteriorated.

Repair details:



mortar

Coatings/ anti-carbonation render: and masonry

paints

Condition:

The repaired element has been recently painted and appears to be in good condition, with no evidence of cracking or spalling throughout. The location of repairs is not evident, and the sample location was estimated from a photograph of the contract showing areas of break out around vertical bars, and by location of the bars with a covermeter. Hammer tapping indicates sound concrete.

Tests Conducted:

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Test Results responsible

Half-cell Values range from -98 to –310mV, with most locations less negative than –280mV, potential (SSC) indicating low to uncertain probability of corrosion. The highest values were close

to the connection and may reflect wetting during coring.

Covermeter Confirms minimum cover to vertical bars in wall at 11-22mm and minimum cover to horizontal bars at 16-25mm.

Pull-off tests The requirement to keep intrusive testing to a minimum prevented pull-off testing.

Samples:


Grey repair mortar with sand grains up to 4mm diameter applied to original light coloured concrete with polymictic gravel aggregate.


Location 1

The repair material is dense, dark grey and contains a polymictic coarse rounded sand with particles to 5mm diameter. The contact with the substrate is good. No evidence was found for deterioration of this repair.

There are several layers of paint at the external surface; a grey base coat, then white layer, then most recent cream coloured paint. The two inner coats may be the anti-carbonation system.

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Petrography

The core consists of repair concrete and substrate concrete. The repair is to at least to 30mm depth, and in the lower portions of the sample fully surrounds the vertical bar at 12mm depth, whilst at the top of the sample the break out appears to gradually reduce in depth to the surface, so that the bar is partly covered by the repair and partly within the original concrete. There are traces of corrosion on the upper surface of the reinforcement bar at 12mm depth. This occurs mostly where the bar is within the original concrete, and there is very little evidence of corrosion of the bar within the repair material. An impression of a second bar was noted at 90mm depth, which showed slight traces of corrosion, indicating the bar at 12mm depth may have been thoroughly cleaned during the repair contract.

The external surface is smooth and consists of an outer layer of paint (0.15 to 0.4mm thick) and an inner cementitious layer containing fibre reinforcement. The repair material is carbonated to a depth of 2-3mm.

The repair contains no macrocracking and consists of a single layer. The repair mortar is composed of a coarse siliceous sand, with particles up to 4mm in diameter, within a matrix based on portland cement. The matrix contains abundant unhydrated cement grains (estimated to account for 7% by volume of the binder) and has very patchy moderate to high porosity hydrates. The voids within the paste are mostly very small (<1mm diameter) and are empty. Traces of gel were found close to a particle of chert within the repair material, but no evidence was found for significant alkali-silica reaction.

The surface of the substrate concrete is rough and broken, with pieces of the coarse and fine aggregate exposed. There is a patchy resin-like layer containing quartz dust on the surface of the substrate, typically 0.1 to 0.3mm thick. The original concrete is carbonated to depths of between 12mm and 26mm. The paste surrounding the bar at 12mm depth is carbonated.

The substrate concrete is based on portland cement with a high level of porosity. The voids are mostly empty although there are traces of ettringite in some. No evidence was found for significant deterioration through moisture penetration. The coarse and fine aggregate contain siliceous lithologies that are potentially reactive with alkalis. No evidence of ASR was found in the substrate. There are traces of fine cracking in the concrete substrate, mostly restricted to within 4mm of the contact with the repair. There is also abundant microcracking in this zone. There is also some longitudinal fine cracking in the concrete substrate associated with the embedded slightly corroded reinforcement.



No records were available from the owners. However, there are some details available in product literature where the structure is identified. This has been used in combination with records from the materials supplier of the materials used on site to deduce the method and materials.

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The structure was constructed in in-situ concrete before 1937. Deterioration of the structure through carbonation-induced corrosion had become apparent with extensive rust staining and areas of cracking and spalling in the 1980’s. The repair specification was produced by the planning and architecture department of the council responsible for the structure. The specification was based on options presented within the proprietary system literature, selected as it had BBA certification and a track record.

The repair project features in relevant literature which includes photographs of the substrate being prepared by hand-held mechanical breaker. The break-outs are mostly restricted to the vicinity of vertical and horizontal bars, forming isolated repairs of less than 0.3 x 0.3m area, and linear patches several metres in length. The substrate was tested with phenolphthalein to control depth of break out, which was generally to behind the outer bars. The reinforcement exposed was cleaned by grit blasting. Reinstatement was with a proprietary repair system. A reinforcement primer was applied in two layers, the second being blinded with sand. A bonding bridge was then applied to pre-dampened surfaces and the pre-batched mortar applied (to depths of 10-25mm or 25-50mm dependent on material). A pigmented solvent-free ethylene copolymer based elastic paint system was applied to further enhance durability. The structure had been re-painted in 2001 with masonry paint shortly before investigation was carried out.


There appears to be a bonding bridge between substrate and repair mortar. There is also a separate coating to the reinforcement, and external paint system reported to be an ‘anti-carbonation’ system. This external layer is apparently undamaged and appears to afford the substrate a significant degree of protection.

The irregularity and fracturing in the substrate is in keeping with break-out through mechanical means such as hand-held breakers as noted in photographs of the repair works.

The substrate concrete is carbonated to up to 26mm depth, and for example around the bar close to the repair. This suggests the paste was carbonated at the time of repair but the bar was not actively corroding, and possibly protected by the adjacent corroding area. It is also possible that there has been a small amount of new corrosion since repair.

There is no evidence of saw cutting at the edges of the repair.


Care should be taken when identifying all carbonated substrate materials.

Repair of carbonated structures can be successful where the bar is not fully exposed, the substrate remains locally carbonated, and the repairs have feathered edges, where a high quality surface coating is maintained.

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2.3.41 Site 42B



Type of structure Zoo building Location: Midlands



This structure showed some incipient and open spalling in the substrate concrete and close to some repaired areas. The majority of the exposed surfaces appeared undeteriorated and had been recently painted.


Repairs were mostly not detectable due to the presence of a thick coating system and a render layer and possibly a fairing coat. The spalling noted in the substrate appeared to relate to corrosion of reinforcement with low cover within the original concrete rather than deterioration within the repairs. However, at least at one location, this had occurred very close to a repair and had affected its effectiveness.

Repair details:



mortar


Condition:

At this site the sample was located close to the bottom of a beam, where the soffit was spalling and a corroding bar was exposed. The faint outline of a repair was visible under the painted soffit face of the beam.

191


Tests Conducted:

Test Results

Half-cell The half-cell failed to provide normal readings when connected to the steel exposed potential (SSC) in the coring operation, though a reason for this could not be found. The potential

values ranged from +30 to -124mV indicating low probability of corrosion, even at areas close to the corroding bar exposed in the soffit.

Covermeter The spalling confirmed cover as low as 5mm, and the covermeter indicated minimum cover ranging from 5 to 30mm in the soffit and much deeper, at 25 to 40mm, in the vertical face.

Pull-off tests The requirement to keep intrusive testing to a minimum prevented pull-off testing.

Samples:


Grey repair mortar with sand grains typically <5mm diameter applied to original light coloured concrete with polymictic gravel coarse aggregate.


Location 1

At this location the repair consists of a repair mortar similar to that at Site 42A, forming a layer up to 80mm deep at the lower aris of the beam, with a layer of fine grained render of 12-15mm thickness on the vertical face only. The concrete and repair are coated with layers of coatings as described for Site 42A. The core broke at the steel the and interface with the substrate.

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Petrography

A reinforcement bar was encountered at 45mm depth. This was coated with a green epoxy-like bond coat, and there were also patches of this green coating at the repair/substrate interface.

The inner main repair contains a high voidage and has a crack-like feature propogating from the edge of the substrate which is possibly a slump feature. There is no other evidence of deterioration of the repair. Hammer tapping indicates sound concrete.

The repair consists of two distinct repair layers. The layers are firmly attached and the contact between them is sharply defined. The thick inner layer contains a coarse siliceous sand within a binder based on portland cement containing coarsely crystalline portlandite, moderate amounts of unhydrated cement and a moderate level of porosity, with numerous clots of undispersed cement measuring up to 5mm in diameter. The void content is generally low but there are locally abundant voids at the surfaces of the cement clots. The voids are empty. There is a low level of microcracking. The material in the inner layer is similar to that in the repair at Site 42A.

The outer layer is much thinner (<15mm) and contains a fine siliceous sand (particles mostly <0.4mm diameter) within a portland cement paste with moderate amounts of unhydrated cement and moderate to high patchy porosity. The void content is low and voids are small (<1mm diameter), uniformly distributed and empty. The layer has a low level of microcracking. There is no evidence of ASR in the repair layers.

The sample contains macrocracking, mostly within the substrate surrounding the reinforcement. There is also some fine cracking in the inner repair layer. The surface of the substrate is rough and broken There is an abundance of microcracks in the substrate within 4mm of the contact with the repair. The substrate concrete is similar to that at Site 42A and is based on portland cement with a high level of porosity. The voids are mostly empty although there are traces of ettringite in some. No evidence was found for ASR or for significant deterioration through moisture penetration.

The sample contains a reinforcing bar at 43mm depth from the vertical face of the beam. The bar is contained within the repair mortar and is coated with a green coating. There is no evidence to suggest deterioration of the bar within the sample.

The external surface is smooth and coated with a layer of white paint (0.08 to 0.33mm thick). There is minor carbonation of the repair to up to 3mm depth and some patchy carbonation of the concrete substrate to 6mm depth from the contact with the repair.



No records were available from the owners. However, there are some details available in product literature where the structure is identified. This has been used in combination with records from the materials supplier of the materials used on site to deduce the method and materials.

193



The structure was constructed in in-situ concrete before 1937. Deterioration of the structure through carbonation-induced corrosion had become apparent with extensive rust staining and areas of cracking and spalling in the 1980’s. The repair specification was produced by the planning and architecture department of the council responsible for the structure. The specification was based on options presented within the proprietary system literature, selected as it had BBA certification and a track record.

The repair project features in relevant literature which includes photographs of the substrate being prepared by hand-held mechanical breaker. The break-outs are mostly restricted to the vicinity of vertical and horizontal bars, forming isolated repairs of less than 0.3 x 0.3m area, and linear patches several metres in length. The substrate was tested with phenolphthalein to control depth of break out, which was generally to behind the outer bars. The reinforcement exposed was cleaned by grit blasting. Reinstatement was using a proprietary repair system. A reinforcement primer was applied in two layers, the second being blinded with sand. A bonding bridge was then applied to pre-dampened surfaces and the pre-batched mortar applied (to depths of 10-25mm or 25-50mm dependent on material). A pigmented solvent-free ethylene copolymer based elastic paint system was applied to further enhance durability. The structure had been re-painted in 2001 with masonry paint shortly before investigation was carried out.


The irregularity and fracturing in the substrate is in keeping with break-out through mechanical means such as hand-held breakers.

The clots of undispersed cement in the inner layer could result from insufficient mixing or from the material having become damp prior to use. There is no evidence to suggest this has resulted in significant reduction in performance of the material. The location was selected where a corroding bar was exposed in the soffit of the beam.

The inner layer (up to 75mm thick) is probably the main repair mortar which ought to have been applied in layers up to 50mm thick. The outer repair mortar is likely to be a repair mortar or a levelling mortar.

The green coating is likely to be the specified reinforcing primer. No evidence was found for a bonding bridge layer. The external coating is likely to be that specified.


The bar had very low cover (5-10mm) which had spalled. This occurred partially within the repair which extended along the soffit of the beam and up the vertical face. The cause of the re-spalling is likely to be a combination of:-

-corrosion ongoing within the unrepaired substrate at the bar at shallow depth, and

-carbonation penetrating the low cover along feathered edges of the repair/substrate interface.

The above could be treated by extending the area for break-out (to include additional areas of low cover) or enhancing the depth of reinstated cover. The latter may not be acceptable in a listed structure, so the preferred method would be to ensure the repair area is sufficiently extensive and the bar sufficiently well protected to minimise potential for ongoing corrosion i.e. quality and extent of cover.

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2.3.42 Site 43



Type of structure Car park Location: West of England

Constructed late 1960’s Repaired: 1992/3


Variable condition with extensive defects to upper surface of slabs and some cracking and local spalling to columns. There are various locations where additional permanent supports have been installed at the columns. Some repairs have deteriorated. The structure was extensively refurbished in 2002/3.


Repairs are in variable condition, with many to the columns in apparently good condition, but some showing signs of cracking. There are numerous relatively small and linear patch repairs to the soffits of slabs, some of which are deteriorating. The repairs are all visible as the car park was not painted at the time of the inspection. Repairs are also present in the surfaces of the deck, including linear crack repairs continuous over many metres.

Repair details:

Photo of repaired structure (location 1, Cores 1 and 2): Element Type: Column



Condition:

The repairs were to part of the face shown in this figure and over the full face to the left of that shown (shown in figures below). A hammer survey confirmed the repair and surrounding concrete was sound. There was no evidence of deterioration to this face.

195


Location 1, Core 3.

Condition:

On the second face investigated there was surface crazing (picked out in places by the damp fine cracks) but no other evidence of deterioration.

The lower figure shows a detail of a core hole showing the horizontal epoxy-coated bars. Note a number of these are coated with corrosion products. Crazing at the external surface is also visible and is shown to penetrate as fine cracking to the depth of the steel.

A void is visible between the top bar and the one below. The void extended to form a cavity behind the bars. The extent could not be determined.

196


Photo of repaired structure (location 2): Element Type: suspended slab

Repair resin crack repair Material(s):


Condition:

This repair was to a continuous crack in the deck, spanning between columns. The crack is visible in the soffit below. There was no evidence to suggest the crack had re-opened since repair.

Photo of repaired structure (location 3): Element Type: Suspended slab

Repair cementitious Material(s): repair mortar


Condition

This was a small patch repair to the deck. The perimeter of the repair was clearly visible and there was no evidence of cracking or other deterioration.

A hammer survey of the abutment indicated adjacent areas of concrete were sound.

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Tests Conducted:

Test Results

Half-cell Location 1 (Cores 1 and 2): values within the original concrete and repair were potential (SSC) mostly from +61 to –107mV, with some gradual increase in negative potential

toward the base of the column, indicating low probability of corrosion. However, there was a very localised strong increase in negative potential to –245 to –360 in the vicinity of the core holes which probably relates to wetting of the substrate during coring. However, corrosion was noted to a bar at Core 2.

Location 1 (Core 3): values are typically from +30 to –215mV indicating a low to uncertain probability of corrosion. However, close to Core 3 values range between –190 and –359mV, reflecting an uncertain to high probability of corrosion.

The above observations suggest there is normally a low probability of active corrosion in the repaired column but when connections are made to exposed steel, the conditions are right for active corrosion. This is most likely to reflect the additional presence of oxygen and water at the steel. The presence of slightly corroded reinforcement is unlikely to be restricted only to those areas exposed, and hence corrosion must have occurred in the past, and now stopped, occurs intermittently, or occurs continuously but at a very low rate.

Half-cell surveys were not carried out in the deck slabs. There was ample evidence of corrosion-induced spalling and delamination to the decks throughout the car park, indicative of chloride-induced corrosion.

Covermeter Location 1 (cores 1 to 3): Visual inspection and covermeter surveys confirm there is a variable spacing and depth of cover to the reinforcement with local congestion and minimum cover of 15-20mm in the repairs. Examination of open spalling at other locations in the car park suggests minimum cover in the original concrete ranges from 10 to 25mm in the columns and 5 to 20mm in the slabs.

Samples:


Core holes at Location 1 show a layered structure to the repair with congested reinforcement at 2040mm depth. The perimeter of the repair can not be precisely located as the repair material has been spread over the surface of the adjacent concrete. Locations 2 and 3 are in the deck and no reinforcement or evidence of reinforcement corrosion was found.

198



Location 1

Two cores (Cores 1 and 2) were taken adjacent to one another in an attempt to avoid all contact with the reinforcement. Core 1 was recovered in a single piece from a column with additional support steelwork. The external surface is flat and unpainted. The repair material is in 2 layers forming a total thickness of up to 50mm. The outer layer is slightly lighter and is fine grained, mid grey and 2-14mm thick. The inner layer is 10-40mm thick, dark grey and fine grained. A single reinforcement bar, coated with green epoxy, was encountered at 40mm depth. No evidence was found of deterioration. A second bar was encountered in Core 2, at 20mm depth. This was also coated with green epoxy where embedded within the repair material, but showed traces of corrosion where it was embedded within the original concrete.

The edges of the repair are sawn to 12mm depth and the repair makes sound contact with the substrate.

Core 3 was recovered in 3 pieces, broken at the interface between two repair layers, and at fine cracks that occur within the repaired face. The repair is in two layers, with an outer layer 10-16mm thick. The thickness of the inner layer was not determined, as coring ceased where 5 reinforcing bars were encountered. The bars were at between 18mm and 24mm depth and were all coated with green epoxy and embedded within the repair material. However, a cavity was present immediately behind the bars, three of which were corroding.

199


Location 2 (Core 4)

This core was recovered in a single piece from the car park deck. The core was taken through a repaired linear ‘channel’ with saw-cut edges, 15mm wide and 5-10mm deep, and filled with translucent material (upper figure).

A fine crack is visible extending below the base of the channel (lower figure) to the to base of the core at 65mm depth. No reinforcement was encountered.

Location 3 (Core 4)

This core was recovered from the deck in a single piece, at the interface between repair and original slab concrete. The repair has sawn, vertical edges to 25mm depth, and then more or less vertical, broken surfaces. The repair is a single layer. The contact between repair and substrate is good.

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Petrography

Location 1, Cores 1 – 3

The surface is smooth, slightly etched and uncracked at Cores 1 and 2. The repair material covers the original surface of the concrete to a depth of 1mm. The surface is smooth and crazed at Core 3.

Cores 1, 2 and 3 all contain a similar structure with two layers of repair mortar. Both layers consist of PFA cenospheres and a fine siliceous aggregate in a portland cement paste. The inner layer also contains abundant irregular clots resembling undispersed microsilica, and the binder is typically of very low porosity whilst that in the outer layer is of moderate to high porosity. The outer layers of the repair are typically extensively carbonated (to up to 23mm depth in Core 1 and 12mm in Core 3) whilst the inner layer is predominantly not carbonated. The outer layer contains abundant irregular voids whilst the inner layer contains a very low voidage. The voids are randomly distributed and are predominantly empty.

The substrate concrete in each location consists of crushed particles of recrystallised limestone coarse aggregate and a sand composed of siliceous and calcareous lithologies within a light coloured highly porous portland cement binder. No evidence was found for deterioration of the paste as a result of moisture penetration or leaching. The perimeter of the repairs have sawn edges (to 11mm depth in Core 1). Below this, the concrete substrate has a rough broken surface with traces of microcracking within 3mm of the repair. These features are consistent with preparation by water jetting.

In Core 3 a surface parallel crack runs within the outer layer of mortar close to the interface with the inner layer. There are also cracks orientated normal to, and passing from, the external surface to the bars at 19-23mm depth, and the paste adjacent to the cracks is carbonated. The core sample contains the impressions of the bars, and these are partly coated in the green coating and minor amounts of corrosion product. Traces of carbonation have been identified at the bottom of the crack where it intersects the reinforcement.

Location 2 (Core 4): This sample is largely composed of concrete similar to that described for Cores 1 and 2 at Location 1. A vertical crack passes through the core and at the surface has been repaired with a resin material infilling a channel with sawn vertical edges to 10mm depth, approximately 15mm wide. The channel is completely infilled at the sample location, but was partially filled in other parts of the deck where the material appeared to have flowed away. No evidence was found for penetration by the resin in to the cracks at the base of the channel. There was also no evidence of cracking within the resin or between the resin and the substrate, and the resin effectively seals the crack at the surface. The resin has a negligible porosity. Carbonation penetrates to a typical depth of 28mm from the external surface, and along the crack through the full length of the core (>70mm).

Location 3 (Core 5): This sample from the deck contains the vertical interface between the repair and the concrete deck. The concrete substrate is similar to that described for Cores 1 and 2 at Location 1. The repair mortar forms a single layer >61mm thick, and consists of PFA cenospheres and a fine siliceous aggregate in a portland cement paste with traces of clots resembling undispersed microsilica. The binder is typically of very low porosity and contains a low voidage with voids typically less than 1mm in diameter. A very fine crack passes from the external surface, through the repair material, very close to the contact with the concrete substrate. The repair material is carbonated to depths of 1-3mm and the concrete typically to 30mm from the external surface. However, there is minor carbonation along the crack to depths of 33mm.

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The substrate concrete is as described for Location 1. The perimeter of the repair was sawn to a depth of 44mm and the repair is trowelled across the external surface at the edge of the repair. Below 44mm, the concrete has a rough broken surface with localised microcracking within 3mm of the surface.



Conversations with the council staff involved with current maintenance and repair of the car park. Contact with previous leaseholder involved with earlier repair activities. Contact with consulting engineer retained during earlier lease and repair activities. Contact with manufacturers of repair materials likely to have been used during repair.

Records exist from the original construction, and confirm that calcium chloride admixtures were used to accelerate construction.


Several parties were involved in operation, assessment and repair of the car park when the repairs in this study were executed. The leaseholder acted as employer for the repair contracts whilst the owner maintained some decision making roles and provided some engineering input. A consulting engineer provided specialist investigation services and presented repair options. A contractor undertook the initial works prior to a specialist repair contractor completing further repairs.

Condition surveys undertaken in the late 1980’s identified a need to maintain the structure and an options study presented the possible repair options. Subsequent inspections by the consulting engineer identified cracking in some columns, indicative of compression failure, and further intrusive investigations identified high chloride content within the columns and severe corrosion of the reinforcement, which was not always apparent at the surface. Structural analysis found the columns to be under-strength and a contract was let to replace many of the affected columns. This was achieved by propping, then demolition and replacement of the columns, using flat jacks to bring load back into them. There remained numerous other areas of deterioration throughout the structure. A specialist repair contractor was appointed and the owner assumed further responsibilities in surveying and managing the repairs. At this time, in the early 1990’s, areas for repair were identified and repairs carried out by the specialist contractor, including the repairs within this study. The consulting engineer carried out some rope access inspections and assessment of risk related to deterioration, but generally had no further involvement in the repair activities.

After the owner resumed the lease, regular inspections were carried out but no repairs were executed. A further evaluation of the structure was carried out by a third party and a repair strategy presented. This was used as the basis for the repair element of a refurbishment contract executed in 2002/3.


The crazing/fine cracking in Core 3 is probably related to drying shrinkage. This cracking intersects the reinforcement. Carbonation has penetrated along the crack to the repair material. Corrosion products occur on the surfaces of the bars indicating a small amount of post-repair corrosion.

The concrete of the columns and deck appear very similar in composition.

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- surface crazing may form fine cracks which penetrate to the depth of the steel

- congested bars may prevent effective placement of repair material and create voids

- corrosion may occur within repairs on bars protected by a coating

- corrosion may occur in bars in original concrete immediately adjacent to repaired areas (though this did not appear to be currently causing damage)

- half-cell investigations of repairs require careful interpretation and cross-reference with visual inspection data

- cracks can be effectively sealed by filling a channel at the slab surface (i.e. without crack injection)

- patch repairs can reinstate small areas of a deck but may not be effective unless part of a comprehensive system of maintenance and repair.

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2.3.43 Site 44



Type of structure Pedestrian Location: Southern/Western England underpass



Generally good engineering condition with few defects (rare cracking in original concrete and in repairs).


Repairs to parapet base fixings were generally in good condition. The majority of repairs are to the retaining walls where the pillars for the pedestrian fence are fixed. The repairs are mostly sound but the fairing coat extending from the repair over the adjacent substrate is extensively delaminating.

Repair details:

Photo of repaired structure (location 1): Element Type: retaining wall


Coatings/ proprietary fairing render: coat

Condition:

Each repair was readily identifiable at the base of the fence pillar. The light coloured rectangular area appears to represent the original extent of the fairing coat, which has delaminated from the perimeter.

A hammer survey confirmed the repairs and surrounding concrete were sound.

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Photo of repaired structure (location 2): Element retaining wall Type:


Coatings/ proprietary fairing render: coat

Condition:

A longitudinal crack passed through the top of the retaining wall, beneath the coping, and through the repair. The crack was continuous for many metres, passing through the concrete and individual repairs and could be a construction joint in the underlying concrete reflected through the repair. The coping around the fence pillar was also damaged.

The repair contained no evidence of deterioration other than the longitudinal crack. A hammer survey confirmed the surrounding concrete was sound.

Photo of repaired structure (location 3): Element Type: stairs

Repair resin repair mortar Material(s):


Condition

The perimeter of each repair is clearly visible.

Some repairs sound hollow during hammer surveying and have partially delaminated from the substrate.

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Tests Conducted:

Test Results

Half-cell Location 1: values range from -144 to –257mV, with some gradual increase in potential (SSC) negative potential toward the top of the retaining wall, backed by an earth

embankment, indicating uncertain probability of corrosion. There was no significant increase adjacent to the connection, suggesting the retaining wall is saturated throughout; water did penetrate the subway through joints and weep holes.

Location 2: values are typically –150mV in the repair and from –102 to –190mV in the concrete, suggesting a low to uncertain probability of corrosion. There is a strong increase in negative potential at the base of the wall close to the steps, to between –250 and –310mV, probably related to an increase in moisture content. The wall is backed by the pavement and road.

A half-cell survey was not carried out on the steps at Location 3

Covermeter Location 1: indicates minimum cover is in excess of 50mm. Observations on site confirm bar present at 80-100mm depth.

Location 2: indicates minimum cover is in excess of 50mm. Observations on site confirm bar present at 100mm depth.

Samples:


Locations 1 and 2 contain slightly different patch repairs composed of a single main layer of cementitious mortar, but with a thin fairing coat at the external surface. Location 3 is a repair to the granolithic topping to the stairs.


Location 1

This core is in a single piece and contains a vertical contact between repair material and the substrate concrete. The repair has a sawn perimeter from 12-30mm depth, and makes good contact with the substrate.

The repair is in one main layer to up to 95mm depth, with a thin, 2mm thick, 2-layer cementitious surfacing over the repair and adjacent concrete.

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The repair material is dark grey with siliceous sand in binder with uniformly distributed spherical voids typically less than 3mm in diameter.

The substrate concrete comprises limestone coarse and fine aggregate in a cream coloured binder.

Location 2

This core is in three pieces, separated by the near horizontal crack passing through the concrete and repair at the top of the retaining wall. The core contains a vertical contact between the repair material and the substrate concrete, showing a sawn perimeter from 30mm depth, and good contact with the substrate.

Location 3

This core is in three parts, taken vertically into a repaired nosing to steps into the pedestrian subway. The structure is of concrete but with a 20mm granolithic topping. The core has parted at the vertical interface between repair and granolithic topping. It is the topping that has been repaired with a light coloured fine grained granular material.

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Petrography

The concrete substrate at Locations 1, 2 and 3 is similar and consists of a sub-rounded limestone coarse aggregate and a medium limestone sand in a portland cement paste of moderate or low porosity, with low level of microcracking. The original water/cement ratio appears to have been of the order of 0.5. There is little evidence to suggest deterioration of the concrete as a result of moisture penetration.

Location 1

The repair material consists of a coarse, sub-rounded siliceous sand (particles <2mm diameter) in a paste based on portland cement, forming a single layer 24 to 105mm thick. The paste contains abundant particles of unhydrated cement and has a low porosity. There is some very fine cracking at the surface penetrating to some 14mm depth. There is very little carbonation in this sample, with maximum recorded depths of 0.2mm for both repair material and substrate concrete. There are traces of ettringite in some voids, and also traces in microcracks that occur at the base of the repair close to the concrete substrate.

There are two layers forming a coating up to 3mm thickness at the external surface. The lower layer is much thicker than the outer, and consists of an isotropic matrix of very low porosity containing a white pigment and very fine particles of portland cement. The outer layer consists of a portland cement paste containing PFA and a very fine siliceous sand.

The repair material is firmly attached to the substrate. The substrate surface is rough and broken and appears to have been prepared by mechanical means. Microcracks occur in the concrete within 3mm of the repair and in the base of the repair. The latter contain ettringite.

Location 2

The repair is in one main layer to up to 58mm depth, and has a thin, 1.2mm thick, 2-layer surfacing over the repair and adjacent concrete. The two layers are similar in composition to those described for Location1. The main repair material is dark grey with small amounts of a predominantly siliceous sand (particles <1mm diameter) in a very porous portland cement paste containing abundant PFA cenospheres and some clots of undispersed microsilica. The paste contains abundant coarse crystals of secondary calcium carbonate and ettringite occurs in the paste and the voids, suggesting a high level of deterioration resulting from moisture penetration. The material contains a high void content with numerous spherical voids to 3mm in diameter.

Macrocracks pass from the surface, through the surface coatings and into the repair mortar, and continue into the concrete substrate. Carbonation penetrates the full depth of the repair mortar along this crack, and the concrete substrate is carbonated to over 10mm depth from the repair interface at this crack. The repair mortar shows patchy and locally coarse carbonation from the external surface.

The substrate has a sawn perimeter to approximately 30mm then a rough broken surface (indicating preparation by mechanical means) with fine cracking and microcracking close to the interface. The mortar has partially debonded from the substrate, and the plane of failure is mainly across the broken surface of the substrate whilst the repair remains attached to the sawn surface. The substrate concrete contains the impression of a twisted bar at 100mm depth.

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Location 3

The repair is a resin mortar containing a fine siliceous sand (particles <0.5mm diameter) and rare empty voids forming a layer between 21 and 29mm thick. The granolithic topping at this location consists of crushed particles of limestone in a portland cement paste. The resin repair mortar is not carbonated, has a uniform very low porosity and contains a very few microcracks. The substrate concrete and granolithic topping are carbonated to depths of 3 and 4mm respectively, but there are also coarse crystals of calcium carbonate along the joint between the repair and the topping and the joints between the topping and repair and the substrate concrete.

There is a vertical sawn perimeter between repair and granolithic topping and a rough interface between repair and concrete substrate, with little microcracking in the substrate. Both the repair mortar and the topping have debonded from the concrete, possibly due to moisture penetration from the surface along the interfaces between repair and granolithic material and the substrate. The concrete surface appears to have been slightly broken out, probably by water jetting.



Conversations and meetings with council engineers responsible for maintenance and repair of the subway. The council provided records of the repair contract and condition reports and photographs.


The specification for the works was prepared by the council. Repairs at location 1 and 2 were specified as follows:

Break out, prepare and prime in accordance with contract drawing. Form shuttering around repair and reinstate cover to fixings using a proprietary shrinkage compensated, high strength cementitious grout (containing a blend of cements, graded aggregates <2mm diameter, and high flow characteristics without segregation). The substrate was to be clean, sound and well saturated but free of standing water before application.

The repair at Location 3, to damaged stairway nosings, was simply specified as ‘break out and prepare nosings to 25mm beyond damaged edge and to a depth of 25mm. Repair nosing using a proprietary high strength epoxy patching mortar’. The material selected was a two or three component, thixotropic, epoxy resin based, heavy duty, patching mortar with high adhesion and impact and abrasion resistance suitable for use in damp and high humidity environments. The material was to be applied ‘wet on wet’ to a clean, sound surface primed with the mortar.

Repair locations in the walls were also specified. These were not identified and sampled. The repairs were to be carried out with layers applied ‘wet on wet’, applying a proprietary polymer modified repair mortar or high build mortar, using a placing rather than rendering technique to ensure adequate compaction, to restore original line and level. For repairs in excess of 40mm deep, high build materials were to be used, in layers ensuring previous layers are well keyed and hardened, but with further preparation and priming where the materials were over 48 hours old. The proposed material was a twocomponent, fibre reinforced, high strength polymer modified cementitious repair mortar with high sag resistance, high bond strength, low shrinkage, low permeability and waterproofing characteristics.

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The repair specification does not closely match the site observations; the material at Locations 1 and 2 appear to be different and there is no mention of a fairing coat in the repair specification.

Location 1 appears to have an effective repair using a dense portland cement mortar and two surface coatings to provide a dense, uncarbonated repair.

Location 2 appears to have similar surface coatings but different, highly porous bulk repair mortar containing portland cement, PFA and microsilica. Such a combination would normally be expected to produce a high quality, dense, binder of low porosity, suggesting the repair material was improperly mixed with too high a water content. The resulting patch repair is ineffective due to a high degree of carbonation and moisture penetration. The site also is affected by cracking, probably within the concrete substrate, which the repair could not have accommodated.

Location 3 has a specialist repair product which remains undeteriorated but which has delaminated at the perimeter and from the substrate.


Materials correctly mixed and placed can form effective repairs. Surface coatings can provideadditional protection to the repair. Shrinkage cracks may still develop in deep repairs.

Materials incorrectly mixed can be of high porosity and hence low quality/durability.

Repairs are unlikely to be effective if cracks remain in the substrate and are not treated.

Repairs to floors exposed to water may have a life limited by the bond between repair and substrate.

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2.3.44 Site 45



Type of structure Motorway bridge Location: West of England

Constructed 1971 Repaired: 1996, 1999


Generally good condition with evidence of extensive repairs. Some cracking and spalling towards the top of the columns and in the soffits of the beams, very minor defects elsewhere in the columns.


Repairs generally in good condition although many of the repaired columns are crazed (Location 1). Sample holes have also been reinstated in the piers (Location 3). The abutment bearing shelves are extensively repaired and contain rare fine shrinkage cracking. Some cracks in the main repairs have been repaired (Location 2). The repairs are all visible as the bridge is not painted.

The abutments and piers were repaired in separate phases using different materials.

Repair details:

Photo of repaired structure (location 1): Element Type: RC column


material

Coatings/ possibly silane render:

Condition:

The repair extended across one face and into parts of the adjacent faces from ground level to 1 to 1.5m height.

A hammer survey confirmed the repair and surrounding concrete was sound.

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Photo of repaired structure (location 2): Element Type: RC bank seat / bearing shelf


Coatings/ possibly silane render:

Condition:

The abutment bearing shelf was substantially repaired to considerable depth and the new material had cracked. The darker linear repairs are to these cracks.

A hammer survey confirmed the surrounding concrete was sound.

Photo of repaired structure (location 3): Element Type: RC column

Repair repair mortar Material(s):

Coatings/ None render:

Condition

The repair appeared to be a reinstatement of a core sample hole. The repair material is common to a number of other local structures and in rust stains are associated with some of them.

The repair at this location has a semicontinuous parting around the perimeter and does not appear to make continuous sound contact with the substrate.

A hammer survey of the abutment indicated adjacent areas of concrete were mostly sound.

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Tests Conducted: (see also Desk Study)

Test Results

Half-cell Location 1: values in the original concrete increase in negative potential towards potential (SSC) pavement level, from -3 to –230mV, indicating low to uncertain probability of

corrosion. Values in the repair typically range from +35 to –123, indicating a low probability of corrosion, but with an increase in negative potential at pavement level to up to –230mV. The increased negative potentials close to ground level probably relate to enhanced moisture content.

Location 2: values are mostly –50 to –167mV on the face, indicating a low to uncertain probability of corrosion. There is a marked increase in negative potentials at the base (where the face meets a horizontal ‘step-out’) and the top (where the face ‘steps-in’ to form the horizontal shelf). At the base, the values range from –160 to – 220mV, indicating an uncertain probability of corrosion. At the top, and coincidental with a band of new, black paint-like coating, values range from –236 to –282mV, indicating an uncertain to high probability of corrosion. It is not clear if these values are affected by the coating system or perhaps salt deposits on it.

Location 3: values typically range from +1 to –90mV, indicating a low probability of corrosion, with a sharp increase in negative potential at the core hole and connection (to –180mV) and at ground level (to –320mV). These increased negative values are likely to reflect the higher moisture content at the core hole and ground level, although observations at other nearby structures confirm corrosion-related damage is most evident close to ground level and in the splash zone.

Covermeter Location 1: confirms minimum cover to column as 40-50mm.

Location 2: minimum cover to face of abutment indicated as 60mm and bar intersected during coring at 90mm.

Location 3: indicates minimum cover to column is 40mm and bar encountered at 60mm.

Samples:


Grey repair concrete applied to original concrete in Locations 1 and 2 and fine grained sand/cement repair mortar applied to original concrete at Location 3. Location 2 includes a crack in the repair material that is orientated normal to the external surface and has itself been repaired.

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Location 1

This core is in one piece, and was taken from one face of a column with a vertical junction between repair and original concrete. The repair surface is smooth and finished flush with the surrounding concrete, and has surface crazing. In hand specimen, the crazing does not appear to penetrate into the repair material but the thin section reveals microcracks penetrating to at least 22mm. The repair is in a single main layer to 65-78mm depth. The interface with the substrate appears sound, with no voids. The substrate appears to have been water jetted.

The repair material has a dark grey binder with white limestone fragments to 2mm size. The substrate is a concrete with grey limestone coarse and buff limestone fine aggregate and dark binder, with reinforcement at approximately 40mm.

No evidence was found for deterioration of the repair.

214


Location 2

This core consisted of two pieces, to 220mm depth, with the repair/substrate interface at 135150mm depth. The external surface is unpainted, crazed, and contains a diagonal linear feature which is a repair to a crack in the bulk repair material. This consists of a fine mortar to a depth 2023mm. The fine crack continues to penetrate to 90mm. An uncorroded bar, fully incorporated within the repair material, was intersected at 90mm depth.

There is no evidence that the crack has re-penetrated to the surface.

Location 3

This sample consisted of two cores taken to include an earlier repair and the adjacent concrete. The sample was broken along preexisting fractures or openings within the materials or at their interfaces. The repair appears to be a non-proprietary reinstatement to a break-out to the horizontal bar, possibly for half-cell surveying. The reinstatement appears to have slumped down and out of the cavity and does not make continuous contact with the substrate. A 20mm diameter corroded bar occurs at 62mm depth and has stained the repair material.


Petrography

The substrate concrete at the three locations is similar, and consists of crushed particles of recrystallised limestone coarse aggregate and a predominantly calcareous fine aggregate in a portland cement paste. This paste is of low porosity at Location 1 (indicative of a water/cement ratio of 0.5) and high porosity at Location 3 (indicative of a water/cement ratio of 0.55). The substrate at Location 1 and 2 has a rough broken surface with exposed pieces of aggregate but very little microcracking close to the repair interface, indicative of preparation by water-jetting. The repairs are both firmly attached to the substrate.

215


Location 1

The repair has a layered structure and consists of crushed particles of recrystallised limestone (<2mm diameter) in a portland cement paste. The paste has a very low porosity, contains very little microcracking, but has moderately abundant irregular voids lined with brown isotropic material resembling an organic polymer. The external surface is flat and smooth. Fine cracks are visible at the external surface and pass into the repair material to a depth of 10mm. These appear to have resulted from drying shrinkage. Carbonation penetrates typically to a maximum depth of 3mm in the repair material, but penetrates along a fine crack from the external surface to up to 22mm depth. No evidence has been found for deterioration of the paste as a result of moisture ingress.

There is negligible carbonation of the concrete substrate.

Location 2

The external surface is flat and contains the junction between the repaired crack channel and the main repair material. The main repair material consists of crushed particles of recrystallised limestone (<3mm diameter) in a portland cement paste containing PFA and graphite. The paste has a very low porosity and contains very little microcracking. Carbonation penetrates typically to a maximum depth of 2mm in the repair material, but penetrates along the interface between the crack-repair mortar and the bulk repair material to a depth of 14mm. There is very little evidence of carbonation along the main crack beneath the crack repair material. There are some microcracks and voids close to the macrocrack and the surface that contain traces of ettringite.

There is negligible carbonation of the concrete substrate. The interface between bulk repair material and substrate appears sound in this core, with no voids. The substrate appears to have been water jetted.

The main crack that has been repaired appears to have resulted from drying shrinkage. The channel at the surface where the crack has been repaired is filled with a fine mortar consisting of siliceous sand (particles <0.5mm diameter) in a portland cement paste. The paste has a low patchy porosity and contains very little microcracking, although there are sporadic microcracks at the interface between the two repair materials. The edges of the channel are smooth, sawn surfaces and there is very little evidence for the development of microcracking in the bulk repair material.

No evidence has been found for deterioration of the paste in the bulk material or the crack repair as a result of moisture ingress. However, there are traces of ettringite close to the crack and the surface, suggesting there has been localised minor moisture penetration.

Location 3

The repair material is a light brown fine grained granular mortar containing fine siliceous sand (particles <0.5mm diameter) in a portland cement paste with an abundance of voids. The material occurs in a single layer to a maximum depth of 74mm. The external surface is slightly irregular but smooth and the repair material occurs as a thin coating at the edge of the repair on top of the original external surface of the concrete. The discontinuities in this sample commonly occur at the interface between repair material and substrate. The repair material is extensively carbonated, particularly along the cracks and discontinuities, and only the central core remains uncarbonated. The substrate concrete is typically carbonated to 2-7mm depth. The substrate has a rough, broken interface with locally abundant microcracks running parallel with the repair contact, indicative of break out by mechanical methods. There are traces of coarse portlandite and ettringite in the voids indicating penetration of moisture and a low level of deterioration of the hydrates.

216




Discussion with the engineers previously responsible for maintenance and involved in some of the repair works carried out (e.g. Location 2). Review of the technical literature for the materials reported to have been used. Meetings with the engineers currently responsible for maintenance and repair of the bridge. Examined inspection and maintenance records, including special inspection reports detailing progressive deterioration in the abutments and columns. Repairs to the columns were executed in 1999/2000, after an earlier phase of repair (1996). Repairs to the abutments were also carried out in 1999/2000 and the specification was also absent from the archives. Some details are present, including the technical approval documents and drawings. Full records of the repair contract are believed to have been retained by the repair contractor in each case.

The Principal Inspection in 1993 noted cracking and spalling in the columns, and more severe cracking and spalling to the abutments. Seepage of water through the deck, and drainage onto the bearing shelf was also noted. The piers and abutments contained high levels of chloride (testing indicated values commonly greater than 0.3% by weight of cement), combined with cover depths of 30-50mm and carbonation depths typically less than 10mm. The inspection report recommended repairs to the columns and bearing shelf, replacement of the bearings and re-waterproofing.

A special inspection in 1995 confirmed the findings and recommendations of the 1993 report, and found that the condition of the columns and abutments had deteriorated and the chloride content significantly increased. The 1995 inspection found that continuity readings suggested good continuity in the reinforcement and recommended that a cathodic protection system be considered, in a feasibility study, as a long term solution to deterioration. The report noted that repairs were to be carried out in 1995/6.

The Principal Inspection in 1998 noted cracking at patch repairs to the columns, passage of water to the tops of the columns and spray from road traffic over the lower sections of the columns. There was also extensive deterioration to the front faces of the abutment capping beams and to the bearings due to drainage of carriageway water through the structure.

Testing of the columns indicated the following:

- cover was generally acceptable at approximately 50mm

- 15% of half-cell readings indicated a high probability of corrosion

- resistivity readings suggested a high probability of corrosion

- the depth of carbonation in uncracked areas was generally less than the cover depth

- the chloride content was generally higher than 0.3% by weight of cement and suggested a high probability of corrosion

- 5 out of 10 break-outs confirmed pitting corrosion was occurring

both alkali and sulphate levels were higher than expected and at levels that could be associated with deleterious reactions.

217


The consultant presented several options for repair, recommending the adopted strategy of monitoring for further corrosion and staged repairs to the bottoms of the columns only with no propping and minimal disruption to traffic. The repairs were extended slightly below ground level and to approximately 1.5m height. The alternative option, to prop the bridge and repair the tops of the columns and replace the bearings, was almost 1 order of magnitude more expensive and required prolonged lane closures, and was rejected. Desalination was also proposed as an alternative to repair of the columns.


The repairs at Location 1 were carried out after comprehensive investigation and testing of the columns affected by chloride-induced corrosion. Break-out was by hydro-demolition. The repair material was a pre-blended polymer modified cementitious sprayed concrete containing an inert limestone aggregate complying with the Department of Transport specification for repairs to highway structures.

The repairs at Location 2 were part of an extensive refurbishment to the bearing shelf instigated after thorough inspection and testing of the structure identified extensive chloride-induced corrosion in the reinforced concrete element. Much of the existing shelf was broken out to considerable depth by hydro-demolition, and the reinforcement cleaned and replaced where necessary. The element was then reinstated by pumping a proprietary flowable concrete into formwork. This material was a pre-bagged product complying with Highways Agency Specification for Highways Works and Department of Transport specification BD27/86 for a high strength flowing concrete. It contained non-reactive 5mm aggregate with RHPC, PFA, microsilica and shrinkage compensating admixtures. The uppermost portion of the bearing shelf was then reinstated with a second material. This was a proprietary flowable cementitious grout and was designed to be sacrificial as saline run-off water does drain to the bearing shelf and can pond on the upper surfaces. This was a pre-blended material containing portland cements, graded aggregates and admixtures, and was not sampled during the investigation.

The repairs at Location 3 were probably carried out prior to the 1990’s and no records were found concerning them. They are probably of hand mixed sand/cement mortar.


The repair at Locations 1 and 2 closely match the description of the materials reported to have been used in the 2000 repair contract.

The repeated inspections between 1993 and 1998 all confirm progressive seepage, contamination with chloride, and reinforcement corrosion. The reports all recommend repairs and waterproofing be carried out, and acknowledge that delaying these activities, though unlikely to affect structural stability, is likely to have a cost implication. It is of note that 7 years pass between the first identification of the problems and repair being instigated.

The reports also comprehensively state the cause of deterioration as being chloride induced corrosion due to salt spray and saline drainage water. All parts of the structure remain exposed to salt spray, and therefore deterioration might be expected to recur in unrepaired areas and at cracked locations, necessitating future patch repair. Consideration might be given to executing a whole-life cost exercise to identify whether further means of protection might be appropriate.

218



Half-cell surveys require careful interpretation.

Surface crazing may appear to be entirely restricted to the surface laitance, but can penetrate repair materials as fine cracks and microcracks, along which carbonation can penetrate.

Fine cracking may develop in extensive and deep repairs, with cracks passing at least to the depth of the reinforcement.

Reinstatement of break-outs and core holes using sand/cement mixes may not provide long-term protection to embedded steel, even where the original water/cement ratio was low and the binder has a low porosity.

Where an external source of chlorides remains, the structure and the repairs ought to be regularly inspected for signs of deterioration.

Prompt reaction to deterioration, and more importantly the causes, might reduce the severity and extent of damage and reduce repair costs.

219

2.3.45 Site 46






The structure shows abundant evidence of deterioration with salt and rust staining, cracking and spalling apparent to the bearing shelves, deck beams, the tops of the piers, under the bearings, and the pier cross-heads. There is some deterioration of the various repairs.


The piers have all been extensively repaired to approximately 1.5m height. The repairs are finished approximately 50mm proud of the surface of the original concrete piers, producing a ‘jacket’ around the base. The repairs commonly show crazing at the surface and a small number contain distinct fine cracks penetrating from the surface.

There are also repairs at the tops of the piers. These are in variable condition.

The repairs to different elements appear to have been executed at different times.

Repair details:

Photo of repaired structure (location 1): Element Type: R/C Pier (West No. 11)


material

Coatings/ silane render:

Condition:

The repairs were close to the bottom of the beam and appeared to extend from the bottom reinforcement up the vertical links. A hammer survey confirmed the repair and surrounding concrete was sound.

220


This view show an area of the repair surface that is typical of the piers at this location, with a flat, crazed surface.

Tests Conducted:

Test Results

Half-cell Location 1: in the repairs, values range from -10 to –110mV, indicating low potential (SSC) probability of corrosion, with a strong increase in negative potential to –280mV at

ground level reflecting the higher moisture content. Above the repaired jacket, values range from +40 to +140mV, reflecting the low moisture content of the column and indicating low probability of corrosion.

Covermeter Location 1: in the repair, the readings range from 9 to 20mm, and are affected by the fine mesh embedded within the repair material at 25-26mm depth. In the main concrete, above the repaired jacket, minimum cover varied greatly from 28mm to 50mm, and is mostly of the order of 40mm.

Samples:


Grey repair concrete, containing evidence of layering and fine cracking penetrating through it, applied to original concrete.


Location 1

The core was taken in 2 segments; the outer segment is composed entirely of repair material and was terminated at 95mm depth where a reinforcing bar was intersected. Adjustment of the coring position to avoid the bar provided a second section from 95 to 285mm depth, although the edge of a second bar was intersected at 85mm depth. The repair/substrate interface is at 145-165mm. This interface appears sound but there is some concentration of voids in the repair. The substrate appears to have been water jetted.

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Location 1: external surface showing fine crack/crazing at the flat, unpainted surface.


Petrography

The repair has an obvious layered structure resembling that of sprayed concrete. There is a distinct layer interface at between 32 and 40mm with a thin carbonated zone. The full thickness of the sprayed material is 148 to 165mm. The repair consists of angular fragments of recrystallised limestone, up to 3mm in diameter, with lesser amounts of siliceous fragments, within a portland cement paste containing PFA. The distribution of aggregate is very patchy. The paste is predominantly of low to moderate porosity and has low levels of microcracking. The more porous areas often occur as near surface parallel layers. There is also some concentration of voids into layers. Most of the voids are empty, but there are traces of ettringite within some.

The fine crack at the surface of the core appears to penetrate the repair to up to 24mm depth, but in thin section the fine crack has been observed to grade into a microcrack passing to depths in excess of 70mm. The crack appears to have resulted from drying shrinkage. There are traces of ettringite in the crack indicating the penetration of water into the sprayed concrete and a low level of very localised deterioration of the paste.

There is negligible carbonation (1-2mm) at the external surface of the sprayed concrete, but patches of carbonation occur to 26mm depth along the crack intersecting the surface. There is a fine mesh of 3mm diameter wire at 23-26mm depth. There is no evidence of corrosion of the wires.

The substrate is a concrete with flint gravel coarse aggregate and predominantly siliceous sand in a medium to dark grey portland cement binder, and shows no evidence of deterioration. The substrate surface is rough and broken, and appears to have been prepared by water jetting.

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Meetings with the engineers responsible for maintenance and repair of the bridge, and conversations with engineers involved with previous maintenance contracts and the most recent repairs. Examined inspection and maintenance records, including special inspection reports detailing progressive deterioration in the columns, and a report on the condition of the reinforcement exposed during repair. Repairs to the columns were executed in 2000 but the repair specification was not available. Records of the repair contract are believed to have been retained by the repair contractor.

The Principal Inspection in 1994 noted deterioration in the columns and abutments, and seepage of water through the deck at these locations. The report recommended further investigation of the deterioration and replacement of the deck waterproofing and surfacing. Postponement of repairs was identified as carrying a potential cost penalty.

A special inspection in 1998 noted cracking and spalling to the columns, passage of water to the tops of the columns and spray from road traffic over the lower sections of the columns. Testing of the columns indicated a high chloride content in trafficked faces in the tops and bottoms of the columns with highly negative half-cell readings indicating a high probability of corrosion. Limited break-outs confirmed pitting corrosion was occurring at 65% of the locations with up to 80% loss of section. In addition, the alkali and sulphate levels were higher than expected and at levels that could be associated with deleterious reactions. The consultant recommended that the cover concrete be removed, the reinforcement replaced where necessary and the surface reinstated with a repair concrete, and all surfaces then be silane impregnated. Further monitoring and inspection work was also recommended, noting that observations made during the repairs indicated that corrosion tends to be most severe in the corners of the columns and where the concrete has low cover.

The repairs were carried out in six-phases with repair to the lower 1.5m of each column. The break-out was to a minimum of 25mm below the main reinforcement. A 50mm deep haunch was sprayed over the repairs for aesthetic purposes and to ensure adequate cover to the reinforcement and thus infer long term durability. All surfaces below ground level have been ‘blackjacked’.

‘Holding’ patch repairs were carried out to the tops of the columns. The report identifies a future need to prop the bridge and fully repair the tops of the columns and replace the bearings.


The repairs at Location 1 were carried out after comprehensive investigation and testing of the columns affected by chloride-induced corrosion. Break-out was by hydro-demolition. The repair material was a pre-blended polymer modified cementitious sprayed concrete containing an inert limestone aggregate complying with the Department of Transport specification for repairs to highway structures. After spraying, the surfaces were finished, then grit blasted and silane applied. The contract was supervised full time.

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The repair material appears to be the proprietary sprayed concrete reported to have been used. The slightly carbonated interface at 32 to 40mm depth probably represents the original depth of repair and the outer layer represents the additional ‘jacket‘ of material applied to build up the cover beyond the original surface. The fine wire mesh present in the outer layer was probably intended to assist in crack restraint.

The development of carbonation at the interface between layers suggests a time lapse of at least several days. The occurrence of a continuous drying shrinkage crack through both layers suggests it developed after application of the second layer, or that the crack existed in the first layer and was reflected into the outer layer by ongoing shrinkage.

The column bases remain exposed to salt spray.

The ‘holding’ repairs noted may have deteriorated and the column tops and bearings were subject to further inspection, by others, at the time of this survey.


Crazing at the surface may penetrate the repair material. Carbonation can penetrate the cracks to a limited degree.

Inclusion of mesh within the repair may assist in controlling the depth of penetration if not the occurrence of crazing.

Crazing was obvious to the general public and was aesthetically unacceptable to the maintaining authority, and was in contrast to the intended finish; however, the repair was sound at the time of inspection.

The shrinkage cracking intersects the reinforcement at depth and thus forms a pathway along which chlorides from an external source can migrate, potentially compromising the intended ‘long term durability’.

Half cell values require careful interpretation where high negative potentials are found close to ground level, as this may reflect high moisture content rather than active corrosion. However, it can not be surmised that no corrosion was occurring as excavations were not made below pavement level to assess the condition of the foundations nor was reinforcement substantially exposed within 0.5m of the pavement.

Building up of the repair surface provides effective enhancement of cover to the main steel reinforcement.

High cement content in proprietary repair materials may be a factor in the tendency to craze and crack.

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2.3.46 Site 47






Generally good condition with evidence of recent repairs to portal. Minor defects elsewhere, including at previous sample hole reinstatements and some unrepaired fire damage to the surface of one abutment.


Repairs to portals generally in good condition although they are crazed and/or cracked. The surfaces of the repairs have been ‘ragged’ with cementitious slurry or mortar and the crazing is in part concealed. However, fine horizontal cracks are still apparent.

Sample holes have also been reinstated in the abutments; corrosion products have seeped from these and are visible at the external surface indicating deterioration of the steel reinforcement.

Repair details:

Photo of repaired structure (location 1): Element Type: Pre-cast, post tensioned R/C abutment

Repair proprietary dry-Material(s): spray shotcrete

Coatings/ none/cementitious render: ragged surface

Condition:

Repairs were over a considerable area of the abutment walls and at the exposed faces of the portal. The surface of the repairs appeared to have been ground flat and smooth, but this might have been achieved by finishing the material prior to final set. In places the surface has been subsequently ‘ragged’ with a thin cementitious coating. Crazing and fine cracks remained visible.

Note the seepage running down the face of the abutment over the repaired area.

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Location 1.

This detailed view of the exposed face of the portal frame shows a horizontal crack in the flat external surface of the repair material. Note the loss of material where the crack intersects the corner.

Tests Conducted:

Test Results

Half-cell Location 1: the values range from +56 to –86mV, with exclusively positive values potential (SSC) in the unrepaired concrete portal. The observations suggest a low probability of

corrosion.

Covermeter Location 1: the covermeter indicates minimum cover to the horizontal bars at 40mm and 50mm for the vertical bars. One bar was intersected at 50mm depth.

Samples:


Grey repair concrete applied to original concrete with fine cracking penetrating from the surface of the repair to the reinforcement.


Location 1; Core 1

Core 1 was recovered in a single piece. This core was taken from an area of fine surface crazing and the outer 2mm of the core tended to delaminate and fragment after coring.

Two core samples were taken from the same repair. Core 1 was taken in an area of surface crazing and Core 2 at a fine crack.

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Location 1; Core 2

Core 2 was recovered in a single piece. This core was taken at a fine crack and consists entirely of repair material to a depth of 42mm, where the core intersected a reinforcement bar and was broken. The crack clearly penetrates from the surface to the bar at 42mm.


Petrography

Cores 1 and 2 contain an identical repair material.

The repair material has a poorly defined layered structure and consists of white crushed limestone fragments to 3mm size within a dark grey portland cement binder. The binder contains abundant particles of unhydrated cement and is of very low porosity. There are moderate to high levels of microcracks orientated radially around, and sometimes through, the aggregate particles suggesting formation by drying shrinkage. The voids are uniformly distributed, mostly less than 1mm in diameter, and typically empty or lined with traces of brown isotropic material reminiscent of a polymer. There is a patchy high voidage and some large voids at the interface with the substrate.

Carbonation typically penetrates the repair material to depths of less than 0.1mm. In hand specimen, the crazing at the surface of Core 1 does not appear to penetrate the repair. However, in thin section the crazing is noted to penetrate as microcracks, and there is patchy carbonation of the paste to depths of up to 18mm along them. The surface fragmentation was caused during core retrieval and may have occurred at weaknesses caused by the surface crazing. Carbonation also penetrates along the crack in Core 2 to a maximum depth of 21mm. There are traces of carbonation, up to 1mm in depth, in the concrete substrate of Core 1. No evidence was found for deterioration of the repair as a result of moisture penetration.

The substrate concrete has a flint coarse aggregate and siliceous fine aggregate and dark portland cement binder. The binder has a moderate to high, patchy porosity, with low levels of microcracking, and appears to have been mixed with an original water/cement ratio of 0.54. The substrate concrete was intersected at 38-42mm. The interface appears sound and the repair remains firmly attached to the substrate. The substrate is rough, with aggregate particles standing proud, and appears to have been water jetted. There is some localised microcracking in the paste and aggregate within 1mm of the interface. One reinforcement bar was intersected at 60mm depth within the substrate. No evidence was found for deterioration of the concrete as a result of moisture penetration.

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Meetings with the engineers responsible for maintenance and repair of the bridge. Examined inspection and maintenance records, including special inspection reports detailing progressive deterioration in the abutments and columns. Repairs to the abutment were executed in 2001 but the repair specification was not available. Records of the repair contract are believed to have been retained by the repair contractor in each case.

Discussion with the engineers previously responsible for maintenance and involved in the repair works carried out at Location 1. Review of the technical literature for the materials reported to have been used.

General Inspections have been carried out in 1980, 1983, 1987, 1989 and 1994. Principal inspections were carried out in 1985, 1992 and 1993. A materials survey was carried out in 1991. The special inspection report from 1995 states that this bridge was programmed within year 3 of a five year rolling inspection programme from 1993/4 to 1997/8. The report identifies the following:

- vertical cracks in the abutment plinths were noted in 1980

- seepage between sections of the east abutment was noted in 1985

- water running down the abutment faces was noted in 1985 (and continues at present)

- minor seepage between deck beams was noted in 1985, 1987 and 1994

- high chloride and numerically high negative half-cell potentials were noted on one abutment in 1991

- cracks at construction joints on the abutments were noted in 1992

- low cover was noted in 1992 and 1993

- stains at the abutment joints were noted in 1993.

The structure was identified as requiring some remedial works due to corrosion caused by the ingress of de-icing salt laden water.


The repairs at Location 1 were carried out after comprehensive investigation and testing. The abutments were contaminated with chlorides from run-off water seeping through the joints above. Break-out was by hydro-demolition. The repair material was a pre-blended polymer modified cementitious sprayed concrete containing an inert limestone aggregate and complied with the Department of Transport specification for repairs to highway structures. After spraying, the surfaces were finished, then grit blasted and silane applied. The contract was supervised full time.

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The layered repair material examined is in keeping with the proprietary sprayed concrete reported to have been used.

The presence of carbonation in the concrete substrate suggests a short period of exposure between break-out and reinstatement.

The abundance of crazing and cracking suggests strong drying shrinkage characteristics for the material or curing conditions.


Surface crazing may occur in young repairs. This can appear to be restricted to the outer few millimetres, but can penetrate as microcracks along which carbonation can occur.

Fine cracking may occur in young repairs. This cracking can penetrate to the embedded reinforcement.

The repair has reinstated concrete damaged by chloride driven reinforcement corrosion. However, the leaking abutment joint has not been treated and the abutment walls remain wet. Salt-laden water can migrate along the cracks to the bar and potentially induce corrosion that would initiate a future repair cycle.

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3 BIBLIOGRAPHY

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5316/1-2 ‘Report on the petrographic analysis of two concrete Phase 3 site samples from Site 3’ Issued in May 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5316/3-6 ‘Report on the petrographic analysis of four concrete Phase 3 site samples from Sites 004A and 004B’ Issued in June 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5316/7-9 ‘Report on the petrographic analysis of two concrete Phase 3 site samples from Site 007’ Issued in May 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5410 ‘Report on the petrographic analysis of samples of concrete and concrete repair materials ’ Issued in September 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/19 ‘Report on the petrographic analysis of samples of Core 1 from Site 2’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/1-3 ‘Report on the petrographic analysis of samples of Cores 5.1, 5.2 and 5.3 from Site 005’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5446/1-4 ‘Report on the petrographic analysis of four concrete cores from Site 06’ Issued in October 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5446/5-6 ‘Report on the petrographic analysis of two concrete cores from Site 27’ Issued in October 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5446/7-9 ‘Report on the petrographic analysis of three concrete cores from Sites 28, 29 and 30’ Issued in October 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5454 ‘Report on the petrographic analysis of samples from sites 32’ Issued in November 2001.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5500/1-2 ‘Report on the petrographic analysis of cores 34.2 and 34.3 from Site 34’ Issued in January 2002.

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Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5500/1-2 ‘Report on the petrographic analysis of core 38.1 from Site 38’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5500/8-10 ‘Report on the petrographic analysis of cores 39.1, 39.2 and 39.3 from Site 39’ Issued in February 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5500/11-14 ‘Report on the petrographic analysis of cores 41.1, 41.2, 41.3 and 41.4 from Site 41’ Issued in February 2002.


Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/14-18 ‘Report on the petrographic analysis of cores 43.1, 43.2, 43.3 and 43.4 from Site 43’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/11-13 ‘Report on the petrographic analysis of cores 44.1, 44.3 and 44.4 from Site 44’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/4-6 ‘Report on the petrographic analysis of cores 45.1, 45.2 and 45.4 from Site 45’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/7‘Report on the petrographic analysis of cores 46.1 from Site 46’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5507/8-9 ‘Report on the petrographic analysis of cores 47.1, and 47.2 from Site 47’ Issued in January 2002.

Geomaterials Research Services Ltd report for Mott MacDonald on behalf of for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report 5025 ‘Report on the analysis of fifteen concrete cores from Dartford West Tunnel road deck’ Issued in January 2002.

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4 GLOSSARY

List of abbreviations

The following abbreviations are used throughout this document:

ASR Alkali Silica Reaction. BBA British Boards of Agrement. COSH Control of Substances Hazardous to Health. CP Cathodic Protection.

DRC Dartford River Crossing Ltd. DWT Dartford West Tunnel EDXA Energy-Dispersive X-ray Analysis.

GGBS Ground Granulated Blastfurnace Slag

GMRS Geomaterials Research Services Ltd HA Highways Agency.

HSE Health and Safety Executive. ICCP Impressed Current Cathodic Protection. ICE Institution of Civil Engineers. MM Mott MacDonald Ltd. MoD Ministry of Defence. MS Microsilica.

NDT Non-Destructive Testing.PCC Pre-Cast Concrete.

PFA Pulverised Fuel Ash. R/C Reinforced Concrete.

RHPC Rapid Hardening Portland Cement. SEM Scanning Electron Microscope.

SRPC Sulphate-Resisting Portland Cement. SSC Silver-Silver Chloride.

UK United Kingdom.

Definitions

Cathodic Protection A system designed to supply an electrical current to the reinforcement within concrete in order to minimise corrosion.

Half-cell survey A technique used to measure (spatial changes in) electrical potential in steel reinforcement embedded within concrete.

NDT Testing (of concrete) carried out in-situ without intrusive sampling.

Petrography Visual examination of samples (of concrete) in hand specimen and thin section using transmitted light microscopy.

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Pull-off testing Testing which can be carried at in-situ where force is applied perpendicular to the surface to measure the tensile strength of the substrate material or bond strength of layers within it.

SA2 ½ Standard of preparation for steel surfaces prior to coating.

SP10 Standard of preparation for steel surfaces prior to coating.

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5 REFERENCES

1 Mott MacDonald Report for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report R1093 ‘Phase 2 Report: Site planning’ issued in April 2001.

2 Mott MacDonald Report for HSE Research ‘Field Studies of Effectiveness of Concrete Repairs’: Report R1134 ‘Phase 4 Report’ issued in February 2002.

3 Sheffield University Centre for Cement and Concrete, Mott MacDonald Special Services Division, DEW-Pitchmastic PLC (1999), “Concrete repair materials and protective coating systems”, Nuclear repair contract BL/G/31221/S, IMC Reference CE/GNSR/5020.

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Printed and published by the Health and Safety ExecutiveC30 1/98

Printed and published by the Health and Safety Executive C1.10 01/04

ISBN 0-7176-2791-8

RR 184

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research report 184 - health and safety executive · this report and the work it describes were...

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