Design guidance forstrengthening concretestructures using fibrecomposite materials

Second Edition

Concrete Society Technical Report No. 55Second Edition

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Design guidance forstrengthening concretestructures using fibrecomposite materials

Second Edition

Report of a Concrete Society Committee

The Concrete Society

Concrete Society Technical Report No. 55Second Edition

Design guidance for strengthening concrete structures using fibre composite materials

Concrete Society Technical Report No. 55

ISBN 1 904482 14 7

© The Concrete Society 2004First edition published 2000

Further copies and information about membership of The Concrete Society may be obtained from:

The Concrete SocietyRiverside House, 4 Meadows Business ParkStation Approach, BlackwaterCamberley, Surrey GU17 9AB, UKE-mail: enquiries@concrete.org.uk; www.concrete.org.uk

All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, storedin a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced inany form or by any means, without the prior permission of the copyright owner. Enquiries should be addressedto The Concrete Society.

The recommendations contained herein are intended only as a general guide and, before being used in connectionwith any report or specification, they should be reviewed with regard to the full circumstances of such use.Although every care has been taken in the preparation of this Report, no liability for negligence or otherwise canbe accepted by The Concrete Society, the members of its working parties, its servants or agents.

Concrete Society publications are subject to revision from time to time and readers should ensure that they are inpossession of the latest version.

Printed by Cromwell Press, Trowbridge, Wiltshire

Members of the Project Committee viii

List of Figures x

List of Tables x

Executive Summary xi

1 INTRODUCTION 1

2 BACKGROUND 32.1 Principles of strengthening 32.2 Assessment of structures to be strengthened 32.3 Strengthening solutions 42.4 Advantages and disadvantages of fibre composite

strengthening 52.4.1 Advantages 52.4.2 Disadvantages 6

2.5 Design life 72.6 Economics 7

2.6.1 Installation 72.6.2 Whole-life costing 8

2.7 Level of strengthening 8

3 MATERIAL TYPES AND PROPERTIES 93.1 Introduction 93.2 Fibres 9

3.2.1 Types of fibre 93.2.2 Performance of different types of fibre 9

3.3 Fabrics 103.4 Plates 113.5 Rods and strips for near-surface-mounted (NSM)

reinforcement 113.6 Preformed shells for column confinement 113.7 Specials 123.8 Adhesives and laminating resins 123.9 Environmental aspects and Health and Safety 13

3.9.1 Environmental aspects 133.9.2 Health and Safety 13

3.10 Choice of materials for design 133.10.1 Plates versus wet lay-up sheet systems 133.10.2 NSM systems 133.10.3 Specific composite material 143.10.4 Stiffness issues 14

4 REVIEW OF APPLICATIONS 154.1 Introduction 154.2 Buildings 16

4.2.1 Beams and slabs 164.2.2 Columns 174.2.3 Connections 184.2.4 Walls 18

4.3 Bridges 184.3.1 Beams and slabs 184.3.2 Columns 204.3.3 Continuity 21

4.4 Other structures 214.4.1 Towers and chimneys 214.4.2 Tunnels 214.4.3 Marine/coastal structures 214.4.4 Miscellaneous structures 22

5 STRUCTURAL DESIGN OFSTRENGTHENED MEMBERS 23

5.1 Symbols 235.2 Overview of available design guidance 245.3 Basis of design 245.4 Mechanical properties of materials 25

5.4.1 Properties of concrete and steel reinforcement 25

5.4.2 Properties of fibre-reinforced polymers (FRP) 25

5.4.3 Properties of adhesives and laminatingresins 26

5.4.4 Stress–strain curves 265.5 Partial safety factors for loads 265.6 Design values for material properties 26

5.6.1 Introduction 265.6.2 Design strength of steel and concrete 265.6.3 Design elastic modulus of FRP 265.6.4 Design ultimate strain of FRP 275.6.5 Design ultimate strength of FRP 275.6.6 Steel stress 275.6.7 Deflection and cracking 285.6.8 Adhesive 28

CONTENTS

v

vi

5.7 Extreme loadings 285.7.1 Behaviour of structures in fire 285.7.2 Seismic loading 285.7.3 Impact loading 295.7.4 Blast loading 29

5.7.5 Vandalism 29

6 STRENGTHENING MEMBERS INFLEXURE 31

6.1 General 316.2 Moment capacity 31

6.2.1 Introduction 316.2.2 Requirements of the existing section 316.2.3 Preliminary design 326.2.4 Design resistance moment of FRP-

strengthened beam 326.2.5 Example design method 32

6.3 FRP separation failure 336.3.1 Introduction 336.3.2 Bond failure 346.3.3 Design procedure 34

6.4 Flexural strengthening with NSM reinforcement 366.4.1 Introduction 366.4.2 Design basis 376.4.3 Bond behaviour 376.4.4 Modes of failure 376.4.5 NSM separation failure design 386.4.6 Anchorage design 38

6.5 Flexural strengthening plate location 396.6 Thick and multi-layer laminates 396.7 Redistribution 406.8 Serviceability 40

6.8.1 Crack widths 406.8.2 Deflections and material stresses 406.8.3 Fatigue 416.8.4 Stress rupture 416.8.5 Strengthening under non-static live load 41

6.9 Strengthening prestressed structures 42

7 SHEAR STRENGTHENING 477.1 Introduction 477.2 Design procedure 47

7.2.1 Maximum shear capacity 477.2.2 FRP shear strengthening design 48

7.3 Spacing of FRP strips 497.4 Additional axial FRP 49

8 STRENGTHENING AXIALLY LOADEDMEMBERS 53

8.1 Introduction 538.2 Stress–strain model for FRP-confined concrete 538.3 Compression 55

8.3.1 Introduction 558.3.2 Tensile rupture of FRP 558.3.3 Lap joint failure 568.3.4 Shear 568.3.5 Serviceability 57

8.4 Flexure 578.4.1 Introduction 578.4.2 Moment capacity with axial load 588.4.3 Debonding 598.4.4 Anchorage 59

8.5 Ductility 598.6 Strengthening columns with non-circular cross-

section 60

9 EMERGING TECHNOLOGIES 659.1 Emerging strengthening technologies already

used in practice 659.1.1 Prestressing using FRP composites 659.1.2 Unstressed FRP anchorage techniques 679.1.3 Bolted plate anchors 679.1.4 Concrete masonry walls 68

9.2 Emerging strengthening technologies at the research stage 689.2.1 Prestressed NSM bars 689.2.2 NSM bars for shear strengthening 68

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9.2.3 FRP anchor systems 689.2.4 Steel-reinforced polymers 689.2.5 Deep embedded bar for shear

strengthening 699.2.6 Prestressed carbon FRP straps for shear

strengthening 699.2.7 Mechanical fastening techniques 699.2.8 Strengthening for torsion 699.2.9 Life expectancy modelling 69

9.2.10 Inorganic adhesives 69

10 WORKMANSHIP AND INSTALLATION 7110.1 Introduction 7110.2 Evaluation of concrete condition 7110.3 Concrete preparation 72

10.3.1 Concrete surface for plates and fabric 7210.3.2 Slots in concrete surface for NSM

material 7310.4 Material conformity 7310.5 Storage of materials 7310.6 Site conditions 7310.7 Mixing and application of adhesive 74

10.7.1 General 7410.7.2 Application to substrate prior to plate

installation 7410.7.3 Application to FRP plates 7410.7.4 Application to substrate prior to fabric

installation 7510.7.5 Application to FRP fabrics 7510.7.6 Inserting adhesive into slots for NSM

reinforcement 75

10.8 Assembly and visual inspection 7510.8.1 Installation of FRP plates 7510.8.2 Installation of FRP fabrics 7610.8.3 Installation of NSM reinforcement 77

10.9 Control samples 7710.10 Non-destructive tests 7710.11 Application of over-coatings 7710.12 Identification/warning signs 78

10.13 Records 78

11 LONG-TERM INSPECTION, MONITORINGAND MAINTENANCE 79

11.1 Inspection and monitoring regime 7911.2 Frequency of inspections 7911.3 Routine visual inspection 8011.4 Detailed inspection 8011.5 Maintenance 80

REFERENCES 81

APPENDICES 89

A Glossary of terms 89B Systems available in the UK 91C Quality control of materials 95D Specialist suppliers, contractors, consultants,

universities/research organisations and owners 97

INDEX 99

viii

MEMBERS OF THE PROJECT COMMITTEE

Neil Loudon Highways Agency (Chairman)Brian Bell Network RailBob Berry Concrete Repairs Limited (representing the Concrete Repair Association)Peter Brown Oxfordshire County Council (representing the CSS)Lee Canning Mouchel ParkmanJohn Clarke The Concrete Society (Secretary)Antony Darby University of BathSteve Denton Parsons BrinckerhoffJohn Drewett Concrete Repairs Limited (representing the Concrete Repair Association)Edward Donnelly Fyfe Co. LLCNeil Farmer Tony Gee and PartnersAlan Hardie Network RailChris Hughes weber building solutionsTim Ibell University of BathSarah Kaethner Arup R & DJohn Keble weber building solutionsMike Langdon Degussa Construction ChemicalsSam Luke Mouchel ParkmanMick Mahon Toray Europe LtdPeter Milligan Fyfe Co. LLCJim Moriarty London UndergroundSteve Richards Exchem Mining and ConstructionMartin Richardson SikaPaul Russell Degussa Construction ChemicalsJon Shave Parsons BrinckerhoffIan Smith Tony Gee and PartnersSimon Walters Mouchel Parkman

CORRESPONDING MEMBERSMichael Johnston British Energy Generation UK LtdTony McNulty Health & Safety Executive – Nuclear Inspectorate DirectorateWendel Sebastian University of BristolDavid Tann University of Glamorgan

ix

ACKNOWLEDGEMENTS

The work of preparing this Report was funded by the following organisations:

Degussa Construction ChemicalsExchem Mining and ConstructionFyfe Co. LLC and Fyfe Asia Pte.Highways AgencyLondon Underground LtdNetwork RailSikaToray Europeweber building solutions

The Concrete Society is grateful to the following for providing photographs for inclusion in the Report:Concrete Repairs Ltd (Figures 5, 6, 8, 11, 44 and 52)Cornwall County Council (Figures 17, 37, 39, 45 and 50)Halcrow Group Ltd (Figures 3, 40 and 43)Highways Agency (Figure 9)Makers UK Ltd (Figures 4, 13, 38 and 42)Maunsell structural Plastics (Figures 12 and 48)Parsons Brinckerhoff (Figure 15)Sika Ltd (Figures 7, 14, 46 and 49)weber building solutions (Figures 10, 41 and 47)

x

LIST OF FIGURES

Figure 1: Different types of structural strengthening,applied to beams, slabs, walls and columns.

Figure 2: Flow chart of assessment process.Figure 3: Installing fibre composite plates in a culvert. Figure 4: Installing FRP plate, showing the flexibility of

the material. Figure 5: FRP plate installed behind existing services.Figure 6: Overlapped carbon FRP plates on Dudley Port

Bridge, West Midlands. Figure 7: Coil of carbon FRP plate. Figure 8: Cutting carbon FRP plate on site. Figure 9: Shear reinforcement straps.Figure 10: Strengthening around hole cut through slab.Figure 11: Underpass, Great Missenden. Figure 12: Applying carbon fibre sheet to Greenbridge

Subway, Swindon. Figure 13: Strengthening Glade Bridge. Figure 14: Strengthening the top surface of a bridge using

carbon fibre plates. Figure 15: Column wrapping. Figure 16: Machine for wrapping columns. Figure 17: Bible Christian Bridge, Cornwall. Figure 18: Assumed stress–strain curves.Figure 19: Strengthening beams and slabs with FRP.Figure 20: Stress–strain curve for reinforcing steel in the

design of strengthened beams in flexure.Figure 21: Possible failure modes and locations for FRP-

strengthened beam.Figure 22: Variation in FRP separation strain with bonded

length, based on Denton et al.(112).Figure 23: Characteristic bond failure force vs anchorage

length.Figure 24: Shear reinforcement configurations.Figure 25: General notation for shear strengthening.Figure 26: Typical variation in ultimate strain capacity with

bonded length (after Neubauer and Rostasy(111)).Figure 27: Experimental verification of design method

(after Denton et al.(112)).Figure 28: Idealised stress–strain curve for FRP-confined

concrete.Figure 29: Stress–strain model.Figure 30: Laps in columns.Figure 31: Stress–strain behaviour of variably confined

concrete.Figure 32: Recommended stress–strain curves for com-

bined flexure and axial load.Figure 33: Assumed confined region for FRP-wrapped rec-

tangular column.Figure 34: Overlapping parabolas in confined region.Figure 35: Dead-end prestressing system (left) and stress-

ing anchorage (right).Figure 36: Prestressing system used on Lauterbridge,

Gomadingen.Figure 37: Use of pull-out test to determine concrete

strength.

Figure 38: Surface grinding. Figure 39: Filling imperfections with quick-setting repair

mortar. Figure 40: Pull-off specimen after removal from concrete

surface. Figure 41: Mixing adhesive. Figure 42: Application of adhesive to concrete surface.Figure 43: Application of adhesive layer on to fibre com-

posite plate. Figure 44: Cutting fabric. Figure 45: Applying resin using roller. Figure 46: Impregnation of fabric. Figure 47: Installing FRP plates, using a roller to apply

pressure.Figure 48: Wrapping fabric round an arched member. Figure 49: Wrapping fabric round column. Figure 50: Rolling fabric to consolidate layers. Figure 51: Double-lap shear test. Figure 52: Spray application of mortar over-coating. Figure 53: Example of warning printed on carbon fibre

plate.Figure 54: Examples of proposed warning plates fixed to

structure adjacent to strengthened area.

LIST OF TABLES

Table 1: Typical dry fibre properties.Table 2: Examples of strengthening of buildings in the

UK.Table 3: Examples of strengthening of bridges in the UK.Table 4: Limit states relevant to FRP strengthening sys-

tems.Table 5: Partial safety factors for Young’s modulus at the

ultimate limit state.Table 6: Recommended values of additional partial safe-

ty factors, to be applied to manufactured com-posites, based on Clarke(94).

Table 7: Partial safety factor for strain at the ultimatelimit state.

Table 8: Maximum stress ranges as a proportion of thedesign ultimate strength (%).

Table 9: Maximum stress under service loads to avoidstress rupture as a proportion of design strength(%).

Table 10: Reduction in strength of adhesive for given live-load strains at FRP–concrete interface (%).

Table B1: Suppliers of strengthening materials.Table B2: Properties of fibre composite sheet materials.Table B3: Properties of composite plate materials.Table B4: Properties of NSM rods and strips.Table B5: Properties of epoxy adhesives.Table B6: Properties of laminating resins.

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EXECUTIVE SUMMARYFibre composites (or fibre-reinforced polymers, generally known as FRPs) have been used successfully for many years in theaerospace and automotive industries. They are used in construction, for example as structural elements and for cladding. ThisReport does not consider such applications but deals only with a recent development, strengthening concrete structures bybonding fibre composites to the surface.

Suitable fibres are made from carbon, aramids or glass. These may be used in the form of:

• composite plates, made from fibres and epoxy resins which are fixed with epoxies to the soffits of beams and slabs• sheet materials, which are wrapped round columns and similar members• preformed shells, bonded round columns.

Advantages

The principal advantages of using composites over steel plates are their high strength and light weight; typical properties aregiven for commercially available materials. This makes installation simple and quick and eliminates the need for temporarysupport. The materials can be easily cut to length on site. The availability of long lengths and the flexibility of the materialsalso simplify installation because:

• laps and joints are not required• the material can take up irregularities in the shape of the concrete surface and can follow a curved profile• the material can be readily installed behind existing services• overlapping, required when strengthening in two directions, is not a problem because the material is thin.

These various factors in combination lead to a significantly simpler and quicker strengthening process than when using othermethods. This is particularly important for bridges because of the high costs of lane closures and possession times on majorhighways and railway lines. An additional advantage of FRPs over some other types of strengthening is that the weight of thestructure and the dimensions of the member are not significantly increased. The latter may be particularly important forbridges, tunnels and other structures with limited clearance.

Disadvantages

One disadvantage of FRP strengthening is the risk of fire, vandalism or accidental damage. For bridges over roads the risk ofsoffit reinforcement being hit by over-height vehicles should be considered. In general, some form of protection will berequired.

Examples of FRP strengthening

There are many concrete structures around the world that have been externally strengthened with FRP. The Report concen-trates on applications in the UK. The floors of various buildings have been strengthened to carry additional loads and FRPhas been used in structural alterations. Columns have been strengthened in several multi-storey car-parks by wrapping withcarbon fibre sheet.

Several major highway bridges and a large number of small bridges have been strengthened using FRPs to increase their loadcapacity. Most applications have been on soffits but some bridges have had FRP bonded to the upper surface or around thecolumns. Other strengthening applications in the UK include lighthouses and cooling towers. Elsewhere in the world almostevery type of concrete structure, from chimneys to tunnels, has been strengthened.

Design approach

Fibre composites have a straight-line stress–strain response to ultimate with no yielding. Thus elastic methods of analysiswith no redistribution are appropriate. For members in bending, the traditional design assumptions are still valid. However,further checks are required to avoid peeling failure at the ends of the laminate and debonding from the concrete. If failureoccurs, it will be in the outer layer of the concrete; the proposed, conservative, approach is to limit the longitudinal shearstress in the concrete at ultimate to 0.8N/mm2. To minimise the risk of debonding, the strain in the FRP should not exceed0.8% when the applied load is uniformly distributed and 0.6% if combined high shear forces and bending moments are pres-ent. A minimum anchorage length of 500mm is recommended.

xii

FRP strips may be used to strengthen members in shear. The material may be treated as an external stirrup, again using tradi-tional design assumptions but the strain in the FRP should be limited to 0.4%.

Wrapping circular columns with FRP increases the axial load capacity as well as the bending and shear capacities. (Only lim-ited increases are possible with square and rectangular columns.) Approaches are given which relate the enhanced ultimatestress and strain in the concrete to the degree of confinement.

Workmanship and installation

The installation of FRP materials must carried out correctly, to ensure good long-term performance. Detailed guidance isgiven, including the selection of the appropriate material and adhesive, adequate preparation of the concrete surface, applica-tion of the composite and correct curing of the adhesive. It is important that the work is carried out by a suitably qualifiedcontractor with suitably trained staff.

Inspection and maintenance

As strengthening with FRPs is a relatively new technique, regular inspection and maintenance regimes should be set up. Thisis particularly important for buildings which, unlike bridges, are not generally subject to any form of routine inspection.Where practical, additional material should be installed, which can be removed at a later stage for testing. Information on thematerials used, along with information on the actions to be taken in the event of damage to the FRP, should be included in theHealth and Safety File.

Changes and additions in the Second Edition of TR55

Since the publication of the First Edition of TR55 in December 2000 materials and techniques have developed rapidly, alongwith the range of applications. Hence it was thought necessary to produce this Second Edition of TR55. A number of thechanges are matters of detail, brought about by additional research findings and further experience of the use of the materials.However, significant changes or additions have been made in some areas, including:

• modification of the treatment of partial safety factors in the design process; factors are now applied to the FRP strains ratherthan the stresses

• extreme loadings• design of members in shear, to provide a less empirical approach, allowing wider and more confident application of the

technique• column design, to provide a more unified approach for axial and combined axial and flexural strengthening; a more detailed

approach has been developed for the strengthening of rectangular columns• design guidance for the new technique of near-surface-mounted (NSM) reinforcement.

In addition, an overview of emerging technologies, such as the use of prestressed composites, mechanical anchorage systemsand alternatives to the adhesives currently used, has been included.

1

Fibre composites have been successfully used for manyyears in the aerospace and automotive industries. They arealso used in construction, for example, for structuralelements, particularly in aggressive environments such aschemical plants, and for cladding. This Report does notconsider such applications but deals only with a relativelyrecent development, the strengthening of concrete structureswith fibre composite materials bonded on or near thesurface.

There are a number of situations where the load-carryingcapacity of a structure in service may need to be increased,such as change of loading or use, or where the structure hasbeen damaged. In the past, strength would be increased bycasting additional reinforced concrete or dowelling inadditional reinforcement. The technique of strengtheningconcrete structures by bonding steel plates to the surface ofthe tension zone with adhesives and bolts was developed inthe 1960s. Since about the late-1980s the use of fibre-reinforced polymers (generally known as FRPs) in thisapplication has been developing rapidly.

Initial applications were with FRP plates, generallycontaining carbon fibres. They have many advantages oversteel plates in this application and they can be used insituations where it would be impossible or impractical to usesteel: for instance, they can be formed in place intocomplicated shapes. Fibre-reinforced polymers are lighter inweight than steel plates of equivalent strength. This makesinstallation much simpler and quicker and eliminates theneed for temporary support for the plates in mostcircumstances while the adhesive gains strength. Fibre-reinforced polymers can also be easily cut to length on site.

Some types of fibre are also available in the form of fabrics,which can be bonded to the concrete surface. The chiefadvantage of fabrics over plates is that they can be wrappedround curved surfaces, for example around columns orcompletely surrounding the sides and soffits of beams.

A sketch showing a wide range of strengthening applicationsto a hypothetical structure is shown in Figure 1. Clearly notall structures are suitable for strengthening using fibrecomposites. A major limitation will be when the concretestrength is low or where there are ongoing corrosion or otherdurability problems. The amount of strengthening that canbe applied will often be limited by failure being inducedelsewhere in the structure.

1 INTRODUCTION

Figure 1: Different types of structural strengthening, appliedto beams, slabs, walls and columns.

Flexural strengthening can be achieved by bonding pul-truded strips or rods into slots cut in the cover region of theconcrete. This application is termed Near-Surface-Mounted(NSM) reinforcement, and has benefits where the exposedconcrete surface is to be trafficked or otherwise exposed topotential damage. In the UK this technique has been appliedto car park decks, and overseas to jetties and dock-sidestructures that are subjected to loads from the movement ofshipping containers. The technique is also applicable wherethe surface of the concrete is undulating, or if there isexcessive laitance or a thin layer of poor quality concretenear the surface. Installation is more costly than forexternally bonded reinforcement, due to the need to cut theslots and a slightly more complicated method for surfacepreparation. Usually the technique would only be used whereexternally bonded reinforcement is not a good technicalsolution.

An appreciable number of structures in the UK andelsewhere have been strengthened using FRP materials andthe rate at which the technique is being used is increasingrapidly. It is estimated that several hundred structures havebeen strengthened in the UK to date, with the amounts offibre composite material involved ranging from a few metres(or m2) for a small job to several kilometres on a major one.

There was little independent guidance on how the design ofstrengthening works should be carried out until the FirstEdition of Technical Report 55 was published in December2000(1). Subsequently, guidance documents have been

Design guidance for strengthening concrete structures using fibre composite materials

2

published in various countries, including the USA(2) andCanada(3). In addition, the Canadian Standards Associationhas published the first national code for strengthening withFRP(4). Following the publication of TR55, the ConcreteSociety published Technical Report 57, Strengthening

concrete structures using fibre composite materials: accep-

tance, inspection and monitoring(5) in 2003, which coversthe vitally important topics of inspection and maintenance.

Materials and techniques are developing rapidly, as is therange of applications. Hence it was thought necessary toproduce this Second Edition of TR55. A number of thechanges are matters of detail, brought about by additionalresearch findings and further experience of the use of thematerials. However, significant changes or additions havebeen made in some areas, as follows:

• Material and system selection guidance• The treatment of partial safety factors in the design process

has been modified; factors are now applied to the FRPstrains rather than the stresses

• Extreme loadings• A more systematic and comprehensive approach to separa-

tion failure• Design of members in flexure, to give a more rational

approach that is closer to that used for reinforced concrete• Design of members in shear, to provide a less empirical

approach, allowing wider and more confident applicationof the technique

• Column design, to provide a more unified approach foraxial and combined axial and flexural strengthening. Amore detailed approach has been developed for thestrengthening of rectangular columns

• Flow charts for flexure, separation, shear and columndesign

• Design guidance for the new technique of NSM rein-forcement

• Overview of emerging technologies, such as the use ofprestressed composites, mechanical anchorage systemsand alternatives to the adhesives currently used.

The guidance in this Report is not specific to any particulartype of FRP material or any particular strengtheningtechnique. It covers the use both of manufactured compositematerials bonded on or near the concrete surface and compo-sites formed in situ on the surface.

The Report deals mainly with the design of strengthenedmembers, i.e. beams, slabs and columns. Other aspects, suchas currently available materials, appropriate applicationtechniques and current uses, are described. It is intended tocover the principles involved, not the detailed approachesthat are applicable to individual materials and techniques.Further details of material properties and techniques can beobtained from materials suppliers and from specialistdesigners and contractors. The important topics of inspectionand maintenance are covered briefly in this Report; moredetailed coverage is given in Concrete Society TechnicalReport 57(5), which should be read in parallel with thisReport; reference is made to specific parts of TechnicalReport 57 (abbreviated to TR57) where appropriate.

The Report is specifically concerned with strengtheningconcrete structures. Fibre composites have been successfullyused to strengthen metallic(6) and other structures. The basicprinciples of this Report will still be applicable but thedetailed design recommendations will not apply.

To help readers unfamiliar with composites, a glossary ofterms is given in Appendix A.

3

2 BACKGROUND

2.1 PRINCIPLES OF STRENGTHENING

A concrete structure may need strengthening for manyreasons. Examples are:

• To increase live-load capacity, e.g. of a bridge subject toincreased vehicle loads or a building, the use of which isto change from residential to commercial

• To add reinforcement to a member that has been under-designed or wrongly constructed

• To improve seismic resistance, either by providing moreconfinement to increase the strain capacity of the concrete,or by improving continuity between members

• To replace or supplement reinforcement, e.g. damaged byimpact or lost due to corrosion. (This will only be prac-tical if the cause of the damage is identified and treated.)

• To improve continuity, e.g. across joints between precastmembers

• To provide replacement reinforcement following structuralalterations, e.g. around holes cut through floor slabs forlift or stair installation or through walls to accommodatenew services.

In most cases it is only practical to increase the live-loadcapacity of a structure. However, in some situations it may bepossible to relieve dead load, by jacking and propping, priorto the application of the additional reinforcement. In thesecases, the additional reinforcement will play its part in carry-ing the structure’s dead load. Prestressing techniques usingcomposite materials are being developed that will also helpto carry part of the dead load. This approach is discussedalthough not covered in detail in the design sections of thisReport.

Three basic principles underlie the strengthening of concretestructures using fibre composite materials, which are the sameirrespective of the type of structure:

• Increase the bending moment capacity of beams and slabsby adding fibre composite materials to the tensile face

• Increase the shear capacity of beams by adding fibrecomposite materials to the sides in the shear tensile zone

• Increase the axial and shear capacity of columns bywrapping fibre composite materials around the perimeter.

These forms of strengthening can be used to increase theseismic performance of structures and also to enhance theperformance of joints between members.

2.2 ASSESSMENT OF STRUCTURES TOBE STRENGTHENED

The decision to strengthen a structure will come at the end ofwhat may be a prolonged assessment process. This is illus-trated, in outline only, in Figure 2. The process is indepen-dent of structure type and should be based on rigorouscriteria and sound engineering judgement. The assessmentprocess will usually involve some investigation of the condi-tion of the structure or some re-analysis and study of the back-ground issues. Guidance may be obtained from documentssuch as Concrete Society Technical Report 54 Diagnosis of

deterioration in concrete structures(7) and the Institution ofStructural Engineers’ Appraisal of existing structures(8). Inall cases an experienced engineer should be part of the assess-ment team. The process will usually be aimed at providinganswers to some or all of the following questions:

• Has the condition or load-carrying capacity of the struc-ture decreased significantly?

• Has the loading changed significantly?• Is the concrete of adequate quality and strength to make

strengthening a feasible option? This applies to the struc-ture overall and not just to the surface or surfaces towhich the FRP is to be bonded. It is suggested that theminimum concrete strength should be 20N/mm2 with apull-off strength of the surface in the zone to bestrengthened 1.5N/mm2.

• What are the risks to the public, to commerce and to thestructure of taking no action?

• What are the cost implications of strengthening, includingdirect costs, future costs and the cost of disruption whilethe work is carried out?

• What are the cost implications of demolition and rebuild-ing, including direct costs, future costs, costs associatedwith loss of use of the structure and disruption while thework is carried out?

• What is the anticipated future life of the structure in itspresent form?

• Will inspection and maintenance be possible?• How would strengthening works affect local infrastruc-

ture, commerce, safety and the environment?• Are any political issues involved?• What is the age of the structure and is it of historical

importance?• What parties and authorities would be required to approve

the works?• Are there any programming or funding constraints?

Design guidance for strengthening concrete structures using fibre composite materials

4

By addressing these issues, decisions about the appropriate

action for a particular structure can be made. In some cases

strengthening will not be a sensible option, unless remedial

work is carried out first. Examples are structures with signi-

ficant materials problems, such as high chloride content

leading to severe reinforcement corrosion. In general it will

not be appropriate to strengthen a deteriorated structure

unless the cause of the deterioration (e.g. chloride ingress)

has been addressed and, where possible, mitigated.

Once it has been decided that strengthening is a realistic

option and that the structure is suitable for strengthening, the

next step is to identify an appropriate strengthening scheme.

The feasibility study should include consideration of the

points listed above in relation to possible schemes, such

issues as whole-life costs of the various options and careful

assessment of the residual life and strength of the structure.

The risks associated with each option should be assessed

during the feasibility study. This assessment should compare

the possible higher risks associated with new techniques

with little history of long-term performance to those of older,

tried and tested methods. However, the benefits of newer

techniques can outweigh this perceived disadvantage: the

risks associated with premature failure are low if

strengthening is to be provided only for the live-load case.

Figure 2: Flow chart of assessment process.

2.3 STRENGTHENING SOLUTIONS

Strengthening solutions considered in a feasibility study can

range from repair of a damaged structure in order to restore

its original strength to adding elements to increase its capacity.

All solutions are, to a greater or lesser extent, project-specific

but some general approaches are commonly used. Repair typi-

cally involves crack injection and/or breaking out damaged

areas and reinstating with cementitious repair mortars or flow-

ing concrete. As stated above, this approach is used where

the aim is to restore the original strength of a structure.

The most common traditional techniques for strengthening

are as follows:

• Increase the reinforced concrete cross-section. Approval

authorities and owners of structures usually readily

accept this solution as it has a proven track record.

However, loading restrictions are required while the con-

crete cures to an acceptable strength. This restriction may

be critical in some instances – for example where a bridge

closure would lead to unacceptable disruption.

• Add prestressing to relieve dead load. Like increasing the

cross-section, this technique has a proven track record

Background

5

and gains ready acceptance. Loading restrictions may berequired during installation, which may not be accep-table. This technique requires the existing structure to becapable of withstanding high local prestressing forces.

• Use plate bonding to enhance tensile reinforcement of

elements. Steel plate bonding has been widely used andcan be considered to have a proven track record. Designguidance is given in the Highways Agency Advice NoteBA 30/94(9). Disadvantages of the technique are the weightand difficulty of handling the plates, difficulty in cuttingto shape, the need to apply and maintain corrosion protec-tion and to anchor the plates to the concrete section whileavoiding damage to embedded reinforcement. As discus-sed above, access and installation times may be criticalissues in some locations.

• Add material to provide confinement of the concrete in

compression members. This can be achieved by installingin situ reinforced concrete or prefabricated steel collarsor wrapping the element with resin-bonded fibre compo-site material. The use of collars is the most commontechnique where space permits. The technique tends to bereadily accepted as the increase in the cross-section canbe clearly seen. With in situ reinforced concrete collars,loading restrictions on the structure are required whilethe concrete gains strength.

• Shear strengthening. This can be achieved by installingexternal steel straps to beams.

Fibre composite strengthening is seen as a viable alternativeto some of these traditional methods. Fibre composite platebonding is being used widely in place of steel plate bondingbecause of the speed and ease of installation and the easewith which the material can be cut to shape and bent to fitslightly curved surfaces. As the technique is relatively new,a proven long-term track record does not exist and this isseen by some as a disadvantage. However, the basic tech-nique and the adhesives are similar to those used for steelplates, which have been widely and successfully applied.Fibre composites are particularly attractive in locations wherespace does not allow a significant increase in cross-sectionor where the installation time is critical. Columns can bestrengthened by wrapping with fibre composite material, toincrease their axial capacity and their resistance to bendingand shear. Overall the advantages of fibre composites tend tooutweigh the perceived disadvantage of a lack of track recordand the reluctance of some approval authorities and ownersof structures to adopt new materials.

2.4 ADVANTAGES AND DISADVANTAGESOF FIBRE COMPOSITESTRENGTHENING

2.4.1 Advantages

Fibre composite strengthening materials have higher ultimatestrength and lower density than steel. When taken togetherthese two properties lead to fibre composites having astrength–weight ratio significantly higher than steel plate insome cases, though it is generally not possible to use this fully.

The lower weight makes handling and installation signifi-cantly easier than steel. This is particularly important wheninstalling material in cramped locations. Figure 3 showscarbon fibre plates being installed in a culvert with limitedheadroom.

Figure 3: Installing fibre composite plates in a culvert.

Work on soffits of bridges and building floor slabs can oftenbe carried out from man-access platforms rather than fullscaffolding. Steel plate requires heavy lifting gear and mustbe held in place while the adhesive gains strength. Boltsmust be fitted through the steel plate into the parent concreteto support the plate while the adhesive cures and to reducethe effects of peeling at the ends. When applying FRP plateor sheet material pressure is applied to the surface using aroller to remove entrapped air and excess adhesive. It may beleft unsupported. In general, no bolts are required; in fact,the majority of FRP strengthening material is uniaxial (thatis all the fibres are aligned in one direction) and the use ofbolts would seriously weaken the material unless additionalcover plates are bonded on or the plates are designed with aproportion of fibres in the transverse direction. Furthermore,because there is no need to drill into the structure to fix boltsor other mechanical anchors there is no risk of damaging theexisting reinforcement. Fibre composite materials are avail-able in very long lengths while steel plate is generally limitedto 6m. The availability of long lengths and the flexibility ofthe material (see Figure 4) also simplify installation:

• Laps and joints are not required.• Within limits, the material can take up irregularities in

the shape of the concrete surface.• The material can follow a curved profile; steel plate

would have to be prebent to the required radius.• The material can be readily installed behind existing

services (see Figure 5).• Overlapping, required when strengthening in two

directions, is not a problem because the material is thin(see Figure 6), but care must be taken with the applica-tion process in the region of the overlaps.

Design guidance for strengthening concrete structures using fibre composite materials

6

Figure 4: Installing FRP plate, showing the flexibility of thematerial.

Figure 5: FRP plate installed behind existing services.

Figure 6: Overlapped carbon FRP plates on Dudley PortBridge, West Midlands.

The materials – fibres and resins – are durable if correctlyspecified, and require little maintenance. If they are damagedin service, it is relatively simple to repair them, by adding anadditional layer.

The use of fibre composites does not significantly increasethe weight of the structure or the dimensions of the member.The latter may be particularly important for bridges and otherstructures with limited headroom, and for tunnels.

In terms of environmental impact and sustainability, studieshave shown that the energy required to produce FRP materialsis less than that for conventional materials. Because of theirlight weight, the transport of FRP materials has minimalenvironmental impact.

These various factors in combination lead to a significantlysimpler and quicker strengthening process than when usingsteel plate. This is particularly important for bridges becauseof the high costs of lane closures and possession times onmajor highways and railway lines. It has been estimated thatabout 90% of the market for plate strengthening in Switzer-land has been taken by carbon plate systems as a result ofthese factors.

2.4.2 Disadvantages

The main disadvantage of externally strengthening structureswith fibre composite materials is the risk of fire, vandalismor accidental damage, unless the strengthening is protected.A particular concern for bridges over roads is the risk of soffitreinforcement being hit by over-height vehicles (‘bridgebashing’). However, strengthening using plates is generallyprovided to carry additional live load and the ability of theunstrengthened structure to carry its own self-weight is un-impaired (see also Section 2.7). Damage to the plate strength-ening material only reduces the overall factor of safety andis unlikely to lead to collapse. An additional cause of damageis that from following trades, such as drilling through FRP tofix brackets, etc.

As detailed later, workmanship is critical to the success of afibre composite strengthening scheme. Thus a further causefor concern is the difficulty of ensuring that the work iscarried out correctly. In the UK a certification scheme foroperatives and supervisors involved with strengthening hasbeen proposed and is currently being developed by TWI.

Problems with the adhesive layer will not generally bevisible from the surface. Similarly, it is difficult to assess thepresence of voids in wet lay-up systems and thus there isuncertainty as to the properties that have been achieved.

Currently the properties of materials used in FRP strength-ening schemes are not covered by British or InternationalStandards. However, the ‘Classification and Assessment ofComposite Materials Systems for use in the Civil Infrastruc-ture’ project, being led by Oxford Brookes University, isdeveloping a classification scheme for adhesives and lamin-ating resins. Further information may be obtained from theweb site (www.compclass.org). In addition the British Boardof Agrément is developing approval for certain types ofstrengthening materials used in bridges under the HighwaysAuthorities Product Approval Scheme (HAPAS).

Experience of the long-term durability of fibre composites islimited, though some installations have been in service for13 years. This may be a disadvantage for structures for which

Background

7

a very long design life is required (see Section 2.5) but canbe overcome by appropriate monitoring (see Chapter 11) andas detailed in TR57(5).

A perceived disadvantage of using FRP for strengthening isthe relatively high cost of the materials. However, compari-sons should be made on the basis of the complete strength-ening exercise (see Section 2.6) taking into account ‘hidden’costs such as delays and disruptions to the users of thestructure. Installation can require large areas of the concretesurface to be prepared, particularly with fabrics, which canbe labour-intensive.

A disadvantage in the eyes of many clients will be the lackof experience of the techniques and suitably qualified staff tocarry out the work, but this can be overcome by usingsuitably qualified designers and contractors.

2.5 DESIGN LIFE

The Highways Agency document BD 84/02, Strengthening

of concrete bridge supports for vehicle impact using fibre

reinforced polymers(10), uses 30 years for the design life of afibre composite strengthening system. This figure is consi-dered reasonable, based on current experience of the adhe-sives used in steel plate bonding. There is considerableexperience of the use of adhesives in other applications, suchas marine structures, which would suggest a design life of atleast 40 years. Fibre composite structures such as the WestMill Bridge(11) have been designed for significantly longerlives.

Ideally, the design life for the strengthening system shouldbe related to the remaining life of the structure and shouldtake into account the future plans for the structure. In manycases, if a mature structure is to be strengthened, a 30-yearlife for a strengthening system may well be appropriate.However, this may not be the case for structures with longdesign lives, such as bridges and nuclear structures. Here, itmay be necessary to accept a strengthening system with adesign life less than the anticipated remaining life of the struc-ture, on the understanding that the life of the strengtheningsystem will be reassessed at a future date.

Because of the relative lack of long-term experience of theperformance of fibre composite strengthening systems,regular inspection and maintenance regimes should beinstigated (see Chapter 5 of TR57). This is particularlyimportant for buildings, which, unlike bridges, are notgenerally subjected to any form of routine inspection. Wherepractical, additional material should be installed, which canbe removed at a later stage for testing. This approach hasbeen adopted on a number of structures including the BarnesBridge in Manchester and the John Hart Bridge in BritishColumbia (see Section 4.3). It may be possible to incorpo-rate some form of monitoring system in the fibre composite.

2.6 ECONOMICS

2.6.1 Installation

The relative economics of the use of fibre composites andother strengthening systems depend on the circumstances.Many factors are involved, and it is necessary to comparecosts both in the short and long term. The latter may bedifficult to quantify as the life-time behaviour can only beestimated fairly crudely. In many cases the alternative maybe demolition and replacement of the structure, with theconsequent disruption.

Factors such as the cost of access and possession time shouldbe taken into account as they can have a significant influence.High closure costs are often incurred by highway and rail-way works. These will vary significantly depending on arange of factors, including the location, the season and thetime of day. However, they will not take into account thesocial costs of disruption. As an example, upgrading of amajor highway in New York City had to be carried out atnight as there was a requirement for the road to be fully openduring the day. The penalty for failure to reopen the carriage-way in the morning was $30,000 per hour, with a penalty of$20,000 per day for overrun of the complete project(12).

Studies carried out for Railtrack (now Network Rail) haveindicated that strengthening with FRP materials will beapproximately 30% cheaper than the equivalent strengtheningusing steel plate. The use of FRP for column strengtheningon one UK highway bridge halved the cost, as well as short-ening the contract duration and significantly reducing theneed for lane closures.

Loss of revenue can be significant when a structure is under-strength and hence cannot be used to its full capacity. It wasreported that the Trenchard Street car park in Bristol waslosing £1M per year in lost sales prior to strengthening.

In Florida, the beam–column connections in a parking garagewere strengthened by bonding carbon fibre sheet material tothe sides of the beams(13). It was estimated that the adhesivelybonded repair was 35% cheaper than the conventionalmethod, which would have involved dowelling in additionalsteel reinforcement and encasing the joint with additionalconcrete.

In Edmonton, Canada, carbon fibre reinforced polymer com-posite sheet material was applied to the soffits and sides of abridge, to improve its shear resistance(14). The cost wasreported as $70,500 for strengthening the complete bridge. Aconventional external stirrup system was estimated to costsome $100,000. Thus the bonded solution showed approxi-mately 30% saving in costs, due chiefly to the fact that thework was carried out from below the bridge and avoided thetraffic closures that would have been required for theconventional system.

Design guidance for strengthening concrete structures using fibre composite materials

8

Beams of the Maryland Street Bridge in Winnipeg, Canada,were strengthened with vertical and horizontal sheets ofcarbon fibre to increase the shear capacity. It was estimatedthat the cost was about 70% of the conventional approach,which would have involved removing parts of the bridgedeck, installing post-tensioned external shear stirrups andcasting additional concrete round the beams. This compari-son was on the basis of direct costs and did not considerfactors such as traffic delays.

A 30-year-old processing tower in Qatar was strengthenedwith 3500m of carbon FRP plate. Several options were con-sidered but the material was chosen because of the speed ofinstallation. The plant was shut down for 25 days to allow thework to be undertaken, but the contract was actually comple-ted in 20 days. This enabled production to restart earlier thanplanned, which was clearly of great benefit to the operatorsof the plant(15).

Hooks and Cooper(16) give two examples of significant costsavings. The crossheads of a 1950s bridge in New York werestrengthened in flexure and shear using carbon FRP plates, ata cost of $18,000. It was estimated that conventional repairwould have cost $150,000. Concrete box beams in Kentuckywere strengthened at a cost of $105,000, when replacementof the structure would have cost $450,000.

Some economic considerations for particular applications arereported in later chapters. Unfortunately, the information islargely qualitative, but can be used for guidance when investi-gating the economics of a situation.

2.6.2 Whole-life costing

The technique of whole-life costing can play an important partin making decisions on when and how to repair or strengthenconcrete structures. This is recognised in BS EN 1504 Part9(17), which lists among the factors to be considered whenchoosing between repair options:

• The number and cost of repair cycles acceptable duringthe design life of the concrete structure

• The cost and funding of the alternative protection or repairoptions, including future maintenance and access costs.

The whole-life cost of a repair or strengthening solution isthe sum of the initial (installation) cost and the future(maintenance) costs over the remaining life of the structure.To permit meaningful comparisons to be made, future costsare discounted to present day value. To carry out a life-cyclecost analysis requires an understanding of:

• deterioration processes as they relate to the particularstructure or different parts of the structure

• repair and strengthening methods and their durability• costs of repair or strengthening and maintenance activities• indirect costs due to loss of service• the owner’s requirements for the serviceability and service

life of the structure.

In many cases, the basic data to permit reasonable assess-ments of the various elements that make up the whole-lifecost are not available. Nonetheless, it can be appreciated thatstrengthening using fibre composites can be competitive inwhole-life cost comparisons because both installation andmaintenance costs are usually lower than those of competingtechniques and possession times are shorter.

Prolonging the useful life of structures that will still berequired for a long time into the future (e.g. road or railbridges) becomes an attractive proposition in whole-life costterms. This is because, if replacement can be delayed formany years, the cost at present day value is considerablyreduced. For example, if a discount rate of 8% is assumed, acost of £1,000,000 at year 20 has a present day value of only£200,000. It can be more economic, in whole-life cost terms,to strengthen now and replace in 20 years, than to replace now.

One factor which is difficult to take into account in whole-life costing is the time until the structure becomes obsolete.This may happen for physical, economic, functional, techno-logical, social or legal reasons. This uncertainty can lead tothe lowest initial cost option being favoured on the basis thatthere is little to be gained from additional spending now, ifthe structure is unlikely to be required in its present form inten years.

2.7 LEVEL OF STRENGTHENING

A key factor in the choice of strengthening system will be thelevel of strengthening (i.e. the maximum increase in loadcapacity) that can be achieved. Strengthening against onemode of failure (e.g. bending) may increase the probabilityof occurrence of another mode (e.g. shear). This must beconsidered in the design process. In addition, the design mustexplicitly consider the risks associated with any possiblepartial or complete failure of the strengthening, due forexample to fire, vandalism or accidental damage. Because ofthe lack of long-term experience of fibre composite strength-ening, some clients are recommending that the approachshould only be used to increase the factor of safety againstcollapse. In other words, failure of the composite will notlead to the collapse of the structure.

9

3 MATERIAL TYPES AND PROPERTIES

3.1 INTRODUCTION

Fibre composites are formed from high performance fibrescombined with an appropriate resin. Epoxies are generallyused, but some development has been carried out on inorganiccement-based matrices(18). For strengthening applications,the composite may be preformed into plates or panels andbonded to the concrete. The most common example iscomposite plates bonded to the soffits of beams or slabs.Alternatively, the fibres may be combined with the resin insitu as part of the application process, such as in the wrap-ping of columns. The mechanical properties of fibre compo-sites are chiefly controlled by the type, amount, orientationand distribution of fibres in the cross-section. The role of theresin is to transfer stresses to and from the fibres and also toprovide some protection from the environment. This Chapterprovides a general introduction to the fibres and resins usedfor strengthening. For further information on the propertiesand behaviour of composites, the reader should consultstandard textbooks, such as An introduction to composite

materials(19) and Composite materials: engineering and

science(20).

3.2 FIBRES

3.2.1 Types of fibre

The most suitable fibres for strengthening applications areglass, carbon or aramid. (Aramids are better known by thetrade names Kevlar® and Twaron.) Each is a family of fibretypes in general, with individual fibre types within the fami-lies that may vary. Typical values for the properties of fibresare given in Table 1. It should be noted that these values are

for the plain fibres alone, not woven fabrics nor for the resul-ting fibre composites. The strength and modulus for manufac-tured composites will be significantly lower (see Sections3.4 and 3.6). The values in Table 1 should only be taken asindicative; where necessary, actual values should be obtainedfrom the manufacturer. The fibres all have a linear elasticresponse up to ultimate load, with no significant yielding.Details of some available materials are given in Table B1 ofAppendix B.

3.2.2 Performance of different types of fibre

The selection of the type of fibre to use in a particularapplication will depend on many factors – the type of struc-ture, the expected loading, the environmental conditions, andso on. Some information is given in this section; furtheradvice can be obtained from the suppliers of strengtheningmaterials. Throughout, the comments refer to the performanceof the fibre itself; in most situations this will be modified bythe resin or adhesive.

Chemical resistance

Carbon and aramid fibres are resistant to most forms ofchemical attack. Many types of glass fibre, including thewidely used E glass, are attacked by alkalies (pH greater thanabout 11) but not by acids. Alkali-resistant (AR) glass fibresare specially formulated for use in highly alkaline environ-ments and are therefore suitable for strengthening concretestructures. Aramids absorb much more water than either ofthe other two fibres, which can cause problems with theresin–fibre interface. There is some evidence to suggest that,in the presence of salts, fracture of all types of fibre can occurdue to the formation of angular crystals.

FibreTensile strength

(N/mm2)

Modulus of elasticity

(kN/mm2)Elongation (%) Specific density

Carbon: high strength*Carbon: high modulus*Carbon: ultra high modulus†

4300–49002740–54902600–4020

230–240294–329540–640

1.9–2.10.7–1.90.4–0.8

1.81.78–1.811.91–2.12

Aramid: high strength and high modulus‡ 3200–3600 124–130 2.4 1.44

Glass 2400–3500 70–85 3.5–4.7 2.6

Table 1: Typical dry fibre properties.

* Based on polyacryonitrile precursor† Based on pitch precursor‡ Aramids with the same strength but a lower modulus are available but are not used in structural strengthening applications.

Design guidance for strengthening concrete structures using fibre composite materials

10

Resistance to ultraviolet light

Glass and carbon fibres are not affected by ultraviolet light.Aramid fibres change colour under ultraviolet light and thestrength is reduced. However, when embedded in a resinmatrix this degradation only occurs near the outer surfaceand there is little effect on the overall mechanical properties.(Direct exposure to sunlight can embrittle all resins and aprotective paint is normally recommended if direct exposureis likely.)

Electrical conductivity

Aramid and glass fibres are non-conducting and hence aresuitable for use close to power lines, electrified railway linesand communications facilities. As carbon fibres conduct elec-tricity they should be electrically isolated from any steel toprevent the establishment of a galvanic cell. In general theresin will be sufficient for this, but where there is a particularrisk it is recommended that a glass fibre sheet be additionallyincluded as the outermost layer of the FRP strengtheningsystem.

Designers should also be alert to the possibility of carbonfibres within an FRP attracting induced currents when placedclose to an AC electricity supply. While no experimentalwork appears to have been carried out in this area, it is theor-etically possible that induced currents within a carbon FRPcould lead to unacceptable heating of an ambient cure adhe-sive as it has been shown that the conducting properties ofcarbon fibre can be used to pass an electric current to achievea higher adhesive cure temperature.

For UK railway applications it is a requirement that anyconducting material that could become live due to inducedcurrents or short circuits from traction power sources must beelectrically connected to the return conductor. For metallicstructures this is normally achieved by attaching an electricalbond between the return conductor and the structure. Due tothe distributed nature of carbon fibres within the adhesivematrix of a carbon FRP it is virtually impossible to guaranteethat every single fibre can be connected to the return con-ductor by an electrical bond. Hence Network Rail will onlypermit the use of aramid FRP in close proximity to its ACoverhead electrification systems; however, carbon FRP ispermitted where DC electrification systems are present.

Care is needed when handling or cutting carbon FRP close toelectrical equipment due to the risk of short-circuit by air-borne particles (see Section 3.9). In addition, when used closeto power lines etc., steps must be taken to ensure that, in theunlikely event of adhesive failure, the composite does notcome into contact with the electrical source.

Compressive strength

The compressive strengths of carbon and glass fibres are closeto their tensile strengths; that of aramid is significantly lower.

Stiffness

The elastic modulus of carbon fibre is similar to, or signifi-cantly greater than, that of steel. The stiffness of aramid islower and that of glass significantly lower.

Impact resistance

Performance of fibres during impact is highly dependent onthe elastic strain energy generated and absorbed. Fibres com-bining high strength with high elongation (tensile strengthgreater than 3,500N/mm2 and elongation greater than 2%)are most suitable for applications where impact resistance isimportant. Selected grades of carbon, aramid and glass fibrecan meet these requirements.

Fire

Glass fibres retain strength up to their melting point (over1000°C) while carbon fibres oxidise in air above 650°C.Aramid fibres are not normally used above 200°C. None ofthe fibres will support combustion. In composites, the resinbehaviour will dominate performance; most generate toxicsmoke. Several composite systems have coatings that canprovide protection.

3.3 FABRICS

Fabrics are available in two basic forms:

• Sheet material. The fibres are generally in a unidirec-tional arrangement, though biaxial and triaxial arrange-ments are available. They may be on a removable backingsheet or in the form of a woven or stitched cloth.

• Prepreg material. This consists of fibres preimpregnatedwith resin, which is cured once in place, by the applicationof heat or by other means.

The selection of the appropriate form of fabric will depend onthe application.

The properties of the sheet materials depend on the amountand type of fibre used. An additional consideration is thearrangement of the fibres; parallel lay gives unidirectionalproperties while a woven fabric has bidirectional properties.In woven fabrics, perhaps 70% of the fibres are in the ‘strong’direction and 30% in the transverse direction. It should benoted that the kinking of the fibres in the woven materialsignificantly reduces the strength and stiffness. In additionsome fabrics are formed with equal amounts of fibres in twodirections, at ±45° to the longitudinal axis.

The thickness of the material will depend on the type andarrangement of the fibre. Fabrics are available in variouswidths to suit the particular application. The sizes and proper-ties of some available materials are given in Table B2 ofAppendix B.

Material types and properties

11

3.4 PLATES

Unidirectional plates are usually formed by the pultrusionprocess. Fibres, in the form of continuous rovings, are drawnoff in a carefully controlled pattern through a resin bath,which impregnates the fibre bundle. They are then pulledthrough a die, which consolidates the fibre–resin combina-tion and forms the required shape. The die is heated whichsets and cures the resin, allowing the completed composite tobe drawn off by reciprocating clamps or a tension device.The process enables a high proportion of fibres (generallyabout 65%) to be incorporated in the cross-section. Hence, inthe longitudinal direction, relatively high strength and stiff-ness are achieved, approximately 65% of the relevant figuresin Table 1. Because most of, if not all, the fibres are in thelongitudinal direction, the transverse strength will be very low.

Plates formed by pultrusion are 1–4mm thick and aresupplied in a variety of widths, typically between 50 and150mm. The sizes and properties of some available platesare given in Table B3 of Appendix B. (It should be notedthat, while plate properties and dimensions of plates can betailored to suit the particular application, it will generally bemore economical to use stock sizes.) Carbon is the mostwidely used fibre though glass is used in some applications.As pultrusion is a continuous process, very long lengths ofmaterial are available. Thinner material is provided in theform of a coil, with a diameter of about 1m, as shown inFigure 7. It can be easily cut to length on site using a simpleguillotine (see Figure 8).

can be produced, with the width and thickness being tailoredto the particular application. Widths up to 1.25m have beenproduced and thicknesses up to 30mm. Other forms of manu-facture, such as resin infusion, are sometimes used but theseare generally less attractive commercially.

Figure 7: Coil of carbon FRP plate.

Plates can also be produced using the prepreg process, whichis widely used to produce components for the aerospace andautomotive industries. Typically plates have a fibre volumefraction of 55% and can incorporate 10% off-axis fibres(usually glass aligned at an angle of 45° to the longitudinalaxis) to improve the handling strength. Lengths up to 12m

Figure 8: Cutting carbon FRP plate on site.

3.5 RODS AND STRIPS FOR NEAR-SURFACE-MOUNTED (NSM)REINFORCEMENT

As NSM material is installed within the cover region of theconcrete, the diameter of the rod, or maximum possibledimension of the cross-section of the strip, is limited. Mostexperimentation to date has used circular bars of diametersin the range 7–16mm, or rectangular strips of thickness lessthan 2mm. Initial UK applications have used carbon FRP barwith a circular cross-section of less than 10mm diameter.The sizes and properties of some rods and strips are given inTable B4 in Appendix B.

3.6 PREFORMED SHELLS FOR COLUMNCONFINEMENT

Preformed shells have been used to strengthen columns on anumber of structures. (It should be noted that the basicprinciples of strengthening columns given in Chapter 8 areapplicable, but strengthening with shells is a more complexdesign process.) There have been a number of applications inNorth America but only one in the UK to date. For a circularcolumn, the most appropriate manufacturing process isprobably filament winding. Resin-impregnated fibres arewound round a mandrel, in the pattern required to give therequired hoop and longitudinal properties. Once fully cured,the cylindrical shell is removed from the mandrel and cutlongitudinally so that it can be bonded round the column.Alternatively, shells can be formed, by hand lay-up or otherprocesses, on the inside or outside of a suitable mould.

Design guidance for strengthening concrete structures using fibre composite materials

12

In general, the internal diameter of the shell should be closeto that of the external diameter of the column, to keep theincrease in the overall diameter to a minimum. Typically,shells are installed with a clearance of between 50 and 150mmfrom the concrete surface, with the annulus later filled withan expansive grout. This will induce a permanent tensile stressin the composite and compression in the concrete. It will benecessary to check that the stress in the FRP is low enoughto avoid the risk of stress rupture.

The strength and stiffness of the shell in the hoop andvertical directions will depend on the type and proportion offibres in the cross-section and on the method of manufactureof the composite. They will be significantly lower than thevalues in Table 1. The performance of the shell is highlydependent on the efficiency of the connection between thecomponent FRP units.

Because of the cost of fabricating mandrels or moulds, thisapproach is only likely to be cost-effective when a largenumber of identical columns are being strengthened, such asin multi-span bridges or multi-storey buildings.

3.7 SPECIALS

Plates formed into an ‘L’ shape may be used as an externallink to provide shear reinforcement on beams, with the lowerleg of the ‘L’ providing the anchorage for the vertical por-tion(21,22) (see Figure 9). The same type of unit could be usedto provide anchorage at the top of a beam, at the interfacewith the slab or at beam–column connections. There havebeen various applications of this type in Germany andDenmark but only one in the UK.

Figure 9: Shear reinforcement straps.

3.8 ADHESIVES AND LAMINATINGRESINS

General information on adhesives may be found in publica-tions such as Adhesives in civil engineering(23) and A guide to

the structural use of adhesives(24). The adhesives and lamina-ting resins most commonly used with concrete are epoxies

(usually solvent-free, two-pack materials which cure atambient temperature). The properties of some availableepoxy adhesives and laminating resins are given in TablesB5 and B6 in Appendix B. Generally the adhesives should beprocured from the same supplier as the plates or fabrics, toensure that the materials are compatible.

The adhesives that are sometimes considered as alternativesto epoxies have certain drawbacks:

• Polyester adhesives have high curing shrinkage, high co-efficient of thermal expansion, can be subject to alkalinehydrolysis, and are difficult to bond to when hardened.

• Vinyl ester adhesives are subject to curing shrinkage, andthe bond is badly affected by moisture.

• Polyurethane adhesives have high curing shrinkage, canbe affected by moisture and are difficult to bond to.

The selection of the type of epoxy to be used in a particularapplication is governed by various factors, including theenvironment and the required speed of fabrication. Adviceshould be obtained from the adhesive manufacturer. The adhe-sive should be able to withstand a maximum temperature of50°C in service and generally have a glass transition tempe-rature (Tg) between 50 and 65°C. In special circumstances,

such as bonding FRP material to the top surface of a bridgedeck which is to receive hot bituminous surfacing (see Section4.3.1), the adhesive may be heated significantly. This mayrequire an epoxy to be selected with a higher glass transitiontemperature but the adhesive’s performance at lower tempera-tures may then be affected. Advice should be sought fromthe supplier.

Where fire is a significant design consideration, such as intunnels and confined spaces, the adhesive selected should beone that releases a minimum amount of toxic gases. Ownersmay have their own standards for the approval of materials(e.g. Fire safety performance of materials used in the Under-

ground(25)). Advice should be sought from the supplier.

Adhesives are generally specified on the basis that theconcrete surface is maintained in a dry condition during thestrengthening work and is in a normal atmospheric exposuresituation in service. Where the concrete surface cannot bekept dry during the work or where, for example, the surfaceis submerged or sometimes submerged in service, adhesiveswith special properties may be required and specialist adviceshould be obtained from adhesive manufacturers.

Where it is exposed to significant ultraviolet light, protectivepaints, which must be compatible with the adhesive, willgenerally be required to prevent the exposed epoxy resin ina fabric system degrading. Guidance should be sought fromthe supplier of the strengthening system.

For porous surfaces, a priming coat may be required, whichmust be compatible with the adhesive. (It was noted earlierthat bonding to a honeycombed surface is not feasible.) As

Material types and properties

13

indicated in Section 10.2, the quality of the surface should beassessed after priming by pull-off tests; tests have shown thatcorrectly specified primers increase the pull-off strength byabout 10%.

3.9 ENVIRONMENTAL ASPECTS ANDHEALTH AND SAFETY

3.9.1 Environmental aspects

Under CDM Regulations(26), designers in the UK mustconsider all environmental aspects, including the eventualdisposal of the materials used. Aramid, glass and carbon fibresare all non-toxic and inert, and are not considered to behazardous as waste. For landfill disposal, they do not containany substance that could leach out to contaminate the ground-water or the air. The most commonly used adhesive andmatrix materials, when fully cured, are also substantiallyinert at normal ambient temperatures and so are not hazar-dous. However, incineration of matrix and adhesive materialsmay not be an appropriate disposal method unless specialcare is taken. In addition, incineration of carbon materialsmay release fine electrically-conductive particles into the air.

Various approaches are being developed for recycling compo-sites, mainly involving grinding the material to form a fillerin new composites.

3.9.2 Health and Safety

All fibres when encapsulated in cured matrix or adhesivepresent negligible risk to human health in normal use. How-ever, care must be taken when cutting and machining allcomposites, because fine fibre particles may irritate skin,eyes and mucous membranes. In addition, care must betaken when handling resins; suitable protective clothingshould be worn. Reference should be made to the COSHHRegulations(27) and to manufacturer’s data sheets. See alsoSection 10.1.

3.10 CHOICE OF MATERIALS FORDESIGN

3.10.1 Plates versus wet lay-up sheet systems

Presently, in most concrete flexural strengthening projects,the chosen system involves the use of carbon FRP pultrudedplates, bonded to the concrete structure through adhesive,because:

• minor unevenness in the surface can easily be bridged bythe adhesive layer of a plate system

• less surface area of concrete needs to be prepared thanwould be the case if wider, but thinner, wet lay-up sheetswere used

• plates are usually easier to install than sheets• pultruded plates contain more fibres than a wet lay-up

sheet of similar cross-section.

However, there are specific instances where the use of wetlay-up sheets is preferred over the use of plates for flexuralstrengthening, often due to the lowering of longitudinal shearstress in the adhesive layer due to the sheets being thinnerthan plates. In particular, one might consider the use of wetlay-up sheets under the following circumstances:

• high demand on longitudinal shear stress within the adhe-sive layer, particularly in short-span situations

• poor quality substrate material, so that longitudinal shearcapacity is low

• requirement for a special anchorage system, such as thatdescribed in Section 9.1.2

• strengthening around a corner• transportation of discrete plates difficult• shallow structure requiring low levels of strengthening

distributed over a large area.

The working practices of the installer may dictate whether aplate or wet lay-up system is used. From the point of view ofthe environment, plates must be wiped down with a solventprior to installation, whereas sheets require no such chemicalpreparation. On the other hand, the adhesive associated withplates does not drip, whereas wet lay-up adhesive may drip.

While quality control needs to be particularly high duringinstallation of either plate or wet lay-up systems, it is fair tosay that quality control needs to be even higher for wet lay-up in order to minimise unevenness, misalignment, lamina-tion defects, voids and crimping.

In situations where wet lay-up sheets are used to strengthenstructures in shear, as much of the structure should be wrappedas possible, so that the use of sheets is preferred over that ofplates under such circumstances. Such applications mightmean U-wrapping a beam, for instance, rather than merelyadhering sheets to the sides of the beam. Furthermore, insuch circumstances, practicality of detailing means that theU-wrap will lead to sheets aligned vertically, rather thaninclined sheets.

3.10.2 NSM systems

The following situations lend themselves to consideration ofNSM systems for strengthening:

• The strengthened surface of the structure is trafficked orsusceptible to damage.

• A thin layer of poor quality or loose concrete exists onthe surface to be strengthened, but the rest of the sub-strate is of high strength.

• The surface is very uneven.• There is limited headroom (although installing NSM over-

head can be difficult).

Particular care should be taken to prevent damage to existingreinforcing bars when cutting the required slots. It would beunwise to use NSM in a situation where the depth of coverwas low.

Design guidance for strengthening concrete structures using fibre composite materials

14

While NSM has been proven to be practical and of realbenefit in niche applications, its relative cost against the moreconventional plate or sheet systems should be considered,together with the level of NSM experience in the industry,and the availability and quality of trained NSM installers.

3.10.3 Specific composite material

To date, most concrete strengthening applications involvingcomposites have used a carbon system, mainly due to highinstalled stiffness and strength requirements. Furthermore,such carbon systems are usually less expensive than othersystems due to less material being required, the area of sur-face preparation being small and the time of installationbeing short. This is also reflected in the level of worldwideresearch and testing, which focuses heavily on carbon.Therefore, it seems sensible that a carbon strengtheningsystem should be considered initially because of confidenceand knowledge in its use, although various reasons maysway the choice towards other materials instead. Such circum-stances where other materials (aramid or glass) should beconsidered include the following:

• Strengthening against blast: Du Pont has conducted muchresearch into the use of aramid systems for such strength-ening, so that the knowledge base is high.

• Electromagnetically inert material is required, perhapsnear to overhead electrification on railway lines or radio/radar installations.

• Robustness and/or toughness of the material is a particu-larly important design criteria: under such circumstances,aramid might be considered (although a protective layeron carbon can be used, e.g. an abrasion-resistant layer ona car park column).

• Low-level strengthening required, so that relatively low-cost glass could be considered, placed in substantiallythicker layers than the equivalent carbon.

• Wrapping of columns in the hoop direction to enhanceconfinement in the event of seismic actions. Under suchcircumstances, glass could be considered.

3.10.4 Stiffness issues

When using carbon systems, it is usual to use ‘standard’modulus fibres. Such materials are normally adequate for themajority of strengthening schemes. Higher stiffnessmaterials (usually denoted HM for ‘High Modulus’) aresubstantially more expensive than the equivalent standardmodulus materials, so that good reasons for their use areusually required. Such reasons might include the following:

• High strains cannot be induced into the carbon FRP, sothat high stiffness fibres are required.

• The quantity of standard modulus carbon fibre requiredfor a particular stiffness is excessive.

Outside the area of concrete strengthening, it is usual tostrengthen iron and steel structures using very high-moduluscarbon FRP plates due to the otherwise large quantities ofstandard-modulus carbon FRP that would be required.

15

4 REVIEW OF APPLICATIONS

4.1 INTRODUCTION

It is estimated that to date (Summer 2004), approximately150 structures in the UK have been strengthened with FRP.Some examples are given in Tables 2 and 3, for buildingsand bridges, respectively. The tables give details of only

those structures that have been described in published papersor articles, i.e. for which information is in the public domain.Details of other projects may be found in information sheetsproduced by suppliers and specialist consultants. Furtherdetails of the structures in the tables, and others, are given inthe subsequent Sections.

Location Date Details of strengthening Material Published reference

King’s College Hospital, London 1996 Soffit of slab to carryadditional storey

Carbon plate Parker(28), Hollaway andLeeming(29)

Wormsley Library, Oxfordshire 1996 Soffit of slab, followingremoval of load-bearing wall

Carbon plate Gold and Martin(30)

Nestlé Factory, Tutbury, Staffordshire 1997 Soffits of beams Carbon plate Luke(31), Hollaway andLeeming(29), Taylor et

al.(32)

Allders Department Stores, Croydon andPortsmouth

1997 Soffit of slab aroundopenings for new escalators

Carbon plate Gold and Martin(30)

Nuclear power plant 1997 Walls, to resist accidentalloading

Carbon plate Garden(33)

Abertillery Leisure Centre, Gwent, Wales Soffits of beams Carbon plate Luke(34)

Car park, Manchester 2002 Columns, so that two furtherstoreys could be added

Carbon sheet Russell & Lomax(35),Russell & Modi(36)

Car park, Bristol 2002 Top surface of slabs Near-surface-mounted rods

Farmer(37)

Car park, Liverpool 2003 Top surface of slabs Near-surface-mounted rods

Farmer(38)

Table 2: Examples of strengthening of buildings in the UK.

Table 3: Examples of strengthening of bridges in the UK.

Location Date Details of strengthening Material Published reference

Haversham Bridge, Milton Keynes 1996 Top of slab, longitudinally Carbon plate Soudain(39), Anon(40),Luke(34), Taylor et al.(32)

Subways, Jarrow, Tyne and Wear 1996 Soffit of slab Carbon plate Hollaway andLeeming(29)

Underpass A413 Great Missenden, Bucks 1997 Soffit of slab Carbon plate Anon(41)

Devonshire Place Bridge, Skipton, NorthYorkshire

1997 Edge of slab Carbon plate Smith(42), Lane et al.(43),Taylor et al.(32)

Bible Christian Bridge, A30 BodminBypass, Cornwall

1998 Wrapping of columns Carbon sheetAramid sheetGlass sheet

Parker(44)

Greenbridge Subway, Swindon, Wiltshire 1998 Soffit of slab Carbon sheet Anon(45)

Glade Bridge, Leatherhead, Surrey 1998 Soffit of beams Carbon plate Farmer(46)

Design guidance for strengthening concrete structures using fibre composite materials

16

Location Date Details of strengthening Material Published reference

St. Columb Gas Works Bridge, Cornwall 1998 Soffit of slab Carbon plate Anon(47)

Dudley Port Bridge, Dudley, WestMidlands

1998 Soffit of slab Carbon plate Anon(40)

River Gardens Bridge, Hounslow,Middlesex

1999 Soffit of slab Carbon plate Barton(48)

Barnes Bridge, Manchester 1999 Soffit of slab Carbon plate Sadka(49)

Coopersale Lane Bridge, Essex 2000 Wrapping of columns Aramid fabric Denton et al.(50)

A19, Tyneside 2000 Wrapping of columns Glass-reinforcedshell

Kendall(51), Pinzelli(52)

Brockley slip road, M1 2001 Upper surface of cantileverdeck

Carbon plate Luke and Canning(53)

Chilthurst Bridge 2002 Soffit of slab with innovativeanchorage

Carbon fabric Denton(54)

A92 Tay Road Bridge, north approachviaduct

2003 Wrapping of columns Aramid sheet Drewett(55)

Patchway Viaduct, A38 2003 Wrapping of columns Aramid sheet Richardson(56)

Theydon Bois Viaduct 2002 Upper surface of deck atsupports

Carbon plate Luke and Canning(53)

St Michael’s Road Bridge, Liverpool 2003 Soffit of slab Carbon plate Luke and Canning(53)

Table 3: Examples of strengthening of bridges in the UK (continued).

4.2 BUILDINGS

4.2.1 Beams and slabs

Additional load capacity

Carbon FRP plates were bonded to the soffit of the concretetrough slab which formed the roof of Normanby College,part of King’s College Hospital in London, to strengthen itsufficiently to carry an additional floor(28). It was suggestedthat the conventional strengthening approach using steelplates would not have been possible because of the problemsof inserting bolts into the soffits of the thin ribs.

Similarly the main beams supporting the floors in a factoryin Tutbury were strengthened using carbon fibre plates toincrease the flexural capacity by 30% to cater for the installa-tion of new plant and processing equipment(32). The work wascarried out with minimum disruption to the factory operations.

Structural alterations

As part of the refurbishment of Allders Department Store inCroydon, new escalators were required. This necessitatedcutting holes up to 10m by 6m in the 300mm-thick flat slaband strengthening the adjacent slabs. After consideringvarious options, carbon fibre plate bonding was selected as itminimised disruption to the operation of the store. The sameapproach was used at the company’s store in Portsmouth,where new stairwells were constructed(30). Figure 10 showscarbon fibre plates installed around a hole cut through a slab

to allow additional services to pass through. (Note that,although this technique is widely used to strengthen slabslocally, no specific guidance relating to the global perfor-mance of the slab is given in this Report. An example of thetechnique is shown in Figure 10.) In addition there have beensituations where the floor slab has been strengthened withcarbon fibre sheet material rather than plates, such as theBeyer Building, Manchester University, where openings wereformed for new air conditioning ducts.

Figure 10: Strengthening around hole cut through slab.

At Wormsley Library, Oxfordshire, the installation of newservices required the removal of a load-bearing wall. Theconcrete slab above was strengthened with carbon fibre plateto carry the resulting increase in dead and live loading(30).

Review of applications

17

Some of the main beams in a car park in Cleveland, Ohiorequired modification to increase the headroom. This requiredthe removal of as much as 270mm of concrete from thesoffits in some areas and the installation of new prestressingstrands. Carbon fibre NSM rods were installed in the top ofthe slab near the columns to help achieve the requiredmoment capacity(57).

Due to a design error, the simply supported beams of a ware-house in Belgium were under-strength and had insufficientbearing at the intermediate supports. To improve the perfor-mance they were made continuous using a combination ofcarbon fibre sheet and steel plate(58).

Insufficient reinforcement

Carbon FRP plates have been used to strengthen balconyslabs in Germany to overcome problems of deflections causedby insufficient steel reinforcement(59). The same approach hasbeen used in Italy.

The shear capacity of the ends of precast prestressed double-tee beams in a multi-storey car park at Pittsburgh Interna-tional Airport were strengthened using carbon fibre sheet(60).

NSM reinforcement has been used to improve the seismicresistance of a number of concrete shear walls in buildingsin Turkey. The approach has also been used for seismicupgrading in Italy and USA.

Incorrectly located reinforcement

The top reinforcement in the cantilever slabs of a car park inBristol had been depressed, resulting in a cover of up to95mm, significantly reducing the strength. Carbon fibre NSMrods were installed to a depth of about 20mm(37) to reinstatethe strength.

At Yarborough School, Lincoln, carbon fibre strips were usedto strengthen precast stair treads which had been installedthe wrong way up.

Structural damage

In Italy carbon fibre strips have been bonded in twodirections to both faces of a prestressed double curvatureconcrete shell roof structure. The structure had beendamaged, resulting in the loss of some prestress; conven-tional repair techniques were deemed to be not appropriate.Carbon fibre strips were also used to strengthen the mainroof beams of an exhibition building, increasing both theflexural and shear capacity. The ground floor beams of aresidential building, which had been damaged by an earth-quake were repaired with carbon fibre sheets wrapped roundand bonded to the concrete.

Fire damage

A number of prestressed concrete beams in a multi-storeycar park in Orpington were damaged due to a vehicle fire.Following repairs to the concrete, the beams werestrengthened with carbon FRP plates. After strengthening thebeams were load tested and insulation boards fitted toprovide one-hour fire protection. Similar work was under-taken at a retail premises in Portsmouth, using a combinationof carbon FRP plate and wrapping.

Repair

The steel–concrete composite slab of a car park in Chicagosuffered severe corrosion damage, both to the steel deckingand the top continuity steel over the supports, because of de-icing salts. As part of the repair, carbon fibre plates werebonded to the top surface of the slab over the supports. Assome of the existing cracks were about 3mm wide, the plateswere debonded on either side of the support to reduce thepeak stresses(61).

Corrosion induced by de-icing salts had seriously weakenedthe decks of a car park in Liverpool. After making good thedamaged concrete, the slabs were strengthened using NSMcarbon fibre composite rods(38).

4.2.2 Columns

Wrapping a column with fibre composite (glass, carbon oraramid) significantly increases the structural capacity of thecolumn. This is most effective on circular columns, and issignificantly less effective for square or rectangular columns.Much work has been carried out in Japan and the USA withthe aim of developing cost-effective retrofitting to increasethe seismic resistance of columns. A major programme onthe performance of concrete columns enclosed by compo-sites was carried out at Southampton University(62).

Additional load capacity

Aramid fibre sheets were used to strengthen the maincolumns of a seven-level car park in Manchester so that twofurther storeys could be added, providing an additional 300car parking spaces(35,36). The material was chosen inpreference to conventional approaches, such as casting anadditional layer of concrete round the columns, because ofthe speed of installation and the minimal increase in thecolumn dimensions.

Insufficient reinforcement

Newly constructed circular columns for a multi-storeybuilding in Dublin were found to have insufficient links.They were strengthened by wrapping with carbon fibre sheet.This approach caused minimal disruption to the constructionprogramme.

Design guidance for strengthening concrete structures using fibre composite materials

18

Incorrect detailing

During remedial work on a multi-storey car park in WestLondon, it was found that the links in the columns werelocated inside the main bars rather than outside. To rectifythis fault, all 400 columns were wrapped with carbon fibresheet in discrete bands, replicating the links.

Incorrect design

Excessive ground movements and floor loadings led to theshear failure of newly-constructed square columns in thebasement car park of a hotel in Dublin. After repairing theshear failure, the columns were strengthened by wrappingthem with carbon fibre sheet. The approach was found to bequicker than traditional strengthening methods.

Additional seismic capacity

In Canada glass FRP shells have been bonded to the surfaceof damaged columns to improve their load-carrying capacity.In Japan and the USA columns have been strengthenedfollowing earthquake damage by wrapping them with carbonFRP, in the form of either thin strips or sheets. Similarly,columns have been strengthened by wrapping them witharamid fibre tape, bonded to the surface.

4.2.3 Connections

In Florida the beam–column connections in the parkinggarage of the Palm Beach Hilton Hotel have been strength-ened by bonding carbon fibre sheet material to the sides ofthe beams(13). This approach was chosen in preference to theconventional solution of increasing the size of the connectionby dowelling in additional steel reinforcement and encasingthe joint with additional concrete. It was estimated that theadhesively bonded repair was 35% cheaper than the conven-tional method.

4.2.4 Walls

In 1997, pultruded carbon fibre plates were installed for thefirst time in an operating nuclear power station in the UK(33).The plates, of only 1m length, were bonded in several loca-tions across structural cracks in reinforced concrete walls.The objective was to restore the original reinforcementcontribution of the embedded reinforcing bars, which hadyielded due to widening of the cracks. The length of thecomposite plates, and their cross-sectional dimensions, weretailored to suit the substrate material properties andanticipated design loads in the walls.

Trials in the UK and the USA have demonstrated that aramidfibres bonded to the faces of concrete walls can significantlyincrease their blast resistance.

4.3 BRIDGES

4.3.1 Beams and slabs

Additional load capacity

In 1997, a small concrete underpass beneath the A413 at GreatMissenden in Buckinghamshire (Figure 11) was strength-ened with carbon fibre composite plates(40). The alternativewould have been the complete reconstruction of the bridge,with consequent major traffic delays and disruption.

Figure 11: Underpass, Great Missenden.

The late 1960s Greenbridge Subway in Swindon wasstrengthened with carbon fibre fabric to increase its flexuralcapacity to that required for current traffic loadings; Figure12 shows the material being applied. Carbon fibre plateswere used to strengthen the soffit of the River GardensBridge in Hounslow(48) to increase the live-load capacity andallow heavy vehicles into an industrial estate. Two bridges inCrawley were strengthened with carbon fibre plates appliedto the soffits to increase the load capacity.

Figure 12: Applying carbon fibre sheet to GreenbridgeSubway, Swindon.

Carbon fibre plates have been applied to the top surfaces ofseveral bridges to increase the transverse bending capacity,including the Adur Viaduct on the A27 in Sussex and part ofthe M40 in Buckinghamshire.

Review of applications

19

Both the bridge deck and the cross-heads of the A71Williamston Interchange Bridge in West Lothian were foundto be under-strength. They were strengthened with twowidths of carbon FRP, which caused minimal disruption totraffic.

The Glade Bridge (Figure 13) carries an access road over therailway between Leatherhead and Bookham in Surrey. Theprecast concrete slabs were strengthened using carbon fibreplates to upgrade the capacity from 5 to 17 tonnes(47). Thiswas the first bridge in the UK over a railway strengthenedusing carbon fibre plates. As the electrical supply is ‘thirdrail’ there were no concerns about the electrical conductivityof the material (see Section 3.2.2).

Figure 13: Strengthening Glade Bridge.

The soffit of the Parkhouse Bridge, Helhoughton, Norfolkwas strengthened using NSM reinforcement. Two sizes ofcarbon FRP bars were used, namely 16mm and 20mm, bothwith a peel ply finish. The reason for using NSM rather thancarbon FRP plates was because of concern that materialfloating in the river might hit the plates and remove them.This would appear to be one of the first situations in whichNSM has been installed overhead.

Woven carbon fibre mats have been bonded directly to thesoffit of a bridge over the A10 motorway in France tostrengthen it(63). This appears to be the first application ofcarbon fibre mats in Europe.

In Canada, carbon fibre sheets were applied to the soffits andsides of the Clearwater Creek Bridge near Edmonton, Alberta,to improve the shear resistance(14). This is a three-span high-way bridge with a length of about 18m. Four beams of theMaryland Street Bridge in Winnipeg(64) were also strength-ened with vertical and horizontal sheets of carbon fibre toincrease the shear capacity by 36%. The alternative wouldhave been to remove parts of the bridge deck, install post-tensioned external shear stirrups and cast additional concreteround the beams. The work was carried out without inter-rupting the traffic on the bridge and was estimated to costabout 70% of the conventional approach. This comparisonwas based on direct costs and did not consider factors suchas traffic delays.

The ends of 64 beams of the John Hart Bridge in PrinceGeorge, British Columbia(64) were strengthened with diagonalsheets of carbon fibre, to increase the shear capacity by about20%. In addition to the areas that required strengthening,carbon fibre sheet was applied to non-critical locations.These may be removed at a later date to determine the long-term performance.

Repair following damage to the structure

The edge of the slab of the Devonshire Place Bridge inSkipton, Yorkshire was repaired with carbon fibre plate fol-lowing damage to one of the tendons(32,42). The edge beam ofa bridge in Crawley, West Sussex that had been struck by avehicle, was strengthened with carbon fabric.

The soffits of some of the beams of the Ibach Bridge, nearLucerne in Switzerland, were repaired with carbon FRP platesfollowing damage to a prestressing tendon(65). Similarly,repair work to the soffits of beams has been carried out inItaly(66) to repair the damage caused by vehicle impact, thecarbon plates being used to provide some additional shearcapacity as well as increasing the flexural capacity. A beamon Interstate Highway 95 at West Palm Beach, Florida, wasalso strengthened using carbon fibre sheet after it was struckby a truck, causing twisting and longitudinal cracking.

Insufficient reinforcement

A bridge to the north of Wilmington, Delaware, USA haddeveloped longitudinal cracks because of insufficient trans-verse reinforcement in the bottom of the precast box-beams.They were repaired with carbon fibre sheet(67).

Incorrect reinforcement detailing

The top surface of Haversham Bridge in Milton Keynes wasstrengthened using carbon fibre plates to increase the hog-ging capacity. (Figure 14 shows a similar strengthening jobin Switzerland.) The plates were provided because the topsteel had insufficient lap lengths and anchorage for theincreased loading requirements. Carbon fibre plates werechosen in preference to steel plates because of the improveddurability and the absence of the bolts required withsteel(32,34,39). (The composites are protected by the runningsurface during normal operation but there is some concernthat they may be susceptible to damage when the surface isplaned off prior to resurfacing.)

The soffit of the Barnes Bridge, which carries the A34 overthe M60 Manchester Outer Ring Road, was found to haveinadequate laps in the reinforcement during an assessment ofits ability to carry 40-tonne vehicles(49). It was strengthenedwith carbon FRP plate of three different sizes (Figure 15).

Design guidance for strengthening concrete structures using fibre composite materials

20

Figure 14: Strengthening the top surface of a bridge usingcarbon fibre plates.

Figure 15: Column wrapping.

4.3.2 Columns

In this Section, the applications are grouped according to thetype of strengthening material used. The materials aregenerally applied by hand (see Figure 15), though specialistmachines have been developed for large structures. These areclamped around the column and a carrier head revolvesaround the column, laying down a continuous fibre tape undertension. The machine is gradually raised round the column, asthe required thickness of fibre is installed. Figure 16 showsone such machine.

Wrapping with fabrics

The first trial application of FRP for the wrapping ofcolumns was carried out on the Bible Christian Bridge overthe A30 Bodmin Bypass in Cornwall(44,68) (see Figure 17).Three different systems were applied to the 6m-high800mm-diameter columns. The materials were glass, carbonand aramid, in either sheet or ribbon form. The concrete sur-face was first cleaned and repaired, then generally impreg-nated with a thixotropic epoxy resin before the application ofthe first layer of fibre. In each case, several layers of the fabricwere applied vertically, to increase the flexural capacity, aswell as in the hoop direction to increase the shear capacity.

Figure 16: Machine for wrapping columns.

Figure 17: Bible Christian Bridge, Cornwall.

Columns of the approach spans of the A92 Tay Road Bridge,which was constructed in the mid-1960s, were wrapped witharamid fibre sheet to improve their vehicle impact resis-tance(55). Similar work was carried out on the PatchwayViaduct on the A38(56). As part of the installation, addi-tionalbands of aramid were installed above the area to bestrengthened that included deliberate defects. Trials weresubsequently carried out to assess the effectiveness of thermo-graphy in locating the defects.

Carbon sheet for wrapping columns was developed in Japanand has been widely used for strengthening bridges, parti-cularly to improve their seismic resistance(69). The approachhas been approved by the California Department of Transpor-tation since the early 1990s(70).

Review of applications

21

In New York State, the piers of a railway bridge over a majorhighway were wrapped with a water-cured prepreg glassfabric(71). This appears to be the first use of a water-curedmaterial. It is not clear whether the wrapping provided addi-tional strength or was mainly to protect the concrete, whichhad suffered severe damage; the bridge was described as‘structurally sound’.

In Canada, repairs were carried out in August 1996 at Saint-Étienne-de-Bolton, Québec, where nine columns of a bridgeover Highway 10 were repaired, five with glass fibre andfour with carbon fibre, supplied by three different com-panies(72,73). The circular columns are 6m high, with a dia-meter of 760mm. The work was backed up by laboratorystudies, including the behaviour of the wrapping materialsunder wet–dry and freeze–thaw cycles.

In Montreal, one of the main piers of the Champlain Bridgeover the St Lawrence River was wrapped in October 1996. Atotal of nine layers of glass fibre wrap were installed to givea thickness of 10mm, both to strengthen and protect theconcrete from ice damage. The column is 1.37m in diameter.To reduce the problems associated with working over water,the fibre sheets were preimpregnated with resin and wrappedround a roller on land. They were then installed on the pierby simply rolling out while the resin was still wet(73,74). Thistechnique should not be confused with the use of a prepregpart-cured material.

Combined plates and wrapping

The reinforcement in circular columns of a bridge in Polandwas heavily corroded. After repair of the concrete, the areaof longitudinal reinforcement was found to be insufficientand the links needed to be reinstated. The repaired columnswere strengthened longitudinally with carbon FRP plates andthen wrapped with carbon sheet(75).

Expansive grout combined with fabric

As part of the repair of the Leslie Street bridge in Toronto(74),an expansive mortar was cast round a deteriorated column.The repair was wrapped with a plastic sheet, followed by aglass fibre wrap. As the mortar continued to expand it ten-sioned the glass fibre, putting the parent concrete into biaxialcompression.

Preformed shells

Various types of prefabricated glass fibre composite shell arebeing developed in the USA, including the full height ‘Hard-core’ system, as used on the Santa Monica Freeway in LosAngeles and the segmental ‘Clockspring’ system(76).

Preformed shells were used to strengthen columns on theNew Jersey Turnpike, which had heights between 3 and4.5m(77). The shells were installed with a clearance of50–150mm from the concrete surface, which was later filledwith grout. In some locations the lower end of the shell was

below water. Possibly the largest application to date has beenthe Yolo Causeway, west of Sacramento, California, where3000 columns were wrapped with glass fibre reinforcedpreformed shells(78). The first application in the UK was onthe A19 on Tyneside, where 24 columns were strengthenedwith glass FRP shells(51,52). The original columns weretapered, but the shells were of uniform diameter throughout.Thus the thickness of the annulus filled with grout increasedfrom the bottom of the column to the top.

4.3.3 Continuity

There has been limited use of composites to improve thecontinuity of bridges. In 1986 the joints in the KattenbuschBridge in Germany were strengthened by bonding a largenumber of glass fibre reinforced polymer composite platesacross them(65). The plates were 3.2m long, 150mm wide and30mm thick.

4.4 OTHER STRUCTURES

4.4.1 Towers and chimneys

In Japan, deteriorated concrete chimneys have been strength-ened by means of carbon or aramid fibre tapes bonded to thesurface, generally to increase the seismic resistance but alsoto increase the resistance to wind and thermal loading(79).When a former cement plant in San Antonio, Texas wasconverted into a retail and entertainment complex, the chim-neys were wrapped in glass FRP to increase their flexuraland shear capacities and to improve their appearance(80).

Trials are planned for NSM reinforcement strengthening onthe chimneys of a disused power station in London. Inaddition, a cathodic protection system will be installed toprotect the steel reinforcement. Hence aramid FRP rods havebeen selected for the NSM because they are non-conducting.

4.4.2 Tunnels

Carbon fibre sheets have been used in a number of highwayand railway tunnels, to repair cracks in concrete linings andalso to increase the strength. Fukuyama et al.(81) reported thatthere were approximately 25 such applications in Japan in 1996.

A large-diameter water chamber forming part of theFrontenac Hydroelectric Power Plant in Sherbrooke, Quebec,Canada was strengthened in 1998 with glass FRP on both theinside and the outside faces. This was an environment withvery high humidity and the strengthening was made morecomplicated by water seeping through the highly porousconcrete(82).

4.4.3 Marine/coastal structures

Various lighthouses in the North Sea have been strengthenedwith carbon fibre sheet material. The alternative, steel bands,lifted into position using a helicopter, would have been amore expensive option.

Design guidance for strengthening concrete structures using fibre composite materials

22

The deck of the 29-span Langstone Bridge, which connectsHayling Island near Portsmouth to the mainland, wasstrengthened with carbon fibre plates to enable to bridge tocarry 40-tonne vehicles.

Following the construction of a new pier at the Humber SeaTerminal, part of the approach span required strengthening.Carbon fibre plates were bonded to the underside of theapproach ramp(83).

The US Navy is carrying out trials on various compositematerials for strengthening concrete piers(84). Gee(85) reportsthat piles in the tidal zone were wrapped using an epoxyspecially formulated for use underwater and the strengthenedarea then protected with a layer of plastic sheet until the resinhad fully cured. The supporting columns of various piers andother coastal structures in California have been strengthenedby wrapping with carbon fibre. In some cases the strength-ening extended below ground level. At Fort Mason in San

Francisco, 245 submerged reinforced concrete foundationpiers were retrofitted for seismic confinement using acombination of systems.

4.4.4 Miscellaneous structures

Vertical and horizontal bands of aramid FRP were used tostrengthen the cooling towers of West Burton Power Stationin Nottinghamshire(86). Aramid was chosen because of itsabrasion resistance. The turbine support units at TornessPower Station were strengthened using carbon fibre sheet.

In Japan, concrete electricity transmission poles have beenstrengthened using carbon fibre sheet material. In Montreal,Canada, laboratory trials have been carried out on railwaysleepers strengthened with polyester fabric(87). A 30-year-oldprocessing tower in Qatar was strengthened with 3500m ofcarbon FRP plate. Several options were considered but thematerial was chosen because of the speed of installation(15).

23

5 STRUCTURAL DESIGN OFSTRENGTHENED MEMBERS

5.1 SYMBOLS

The following symbols are used in this Report. They arelargely compatible with BS 8110(88). It should be noted thatsome of these symbols may differ from those used in Euro-code 2, EN 1992 Design of concrete structures(89).

Ae effectively confined area of concrete

Af area of FRP

Afa additional longitudinal FRP strengthening area

due to shearAfs area of FRP shear reinforcement

Ag gross cross-sectional area of section

Aol area of overlapping parabolas for rectangular

columnsAsa area of effectively anchored additional

longitudinal tensile steel for shear requirement inBS 5400

AsaF increase in FRP required if Asa steel is ignored

a major dimension of elliptical columnb width of sectionba width of adhesive layer

bbarperim perimeter of NSM FRP bar

bf width of laminate

bnotchperim effective perimeter of NSM notch

c minor dimension of elliptical columnD diameter of circular columnd effective depth of sectiondf effective depth of FRP shear reinforcement

Ec initial modulus of elasticity of concrete

Efd design elastic modulus of FRP

Efk characteristic elastic modulus of FRP

E0 secant modulus of concrete = 0.67fcu/(γmc εc0)E2 slope of linear portion of confined concrete stress–

strain curveEs modulus of elasticity of steel

e eccentricity of load on column = M/N

fat design adhesive tensile strength

fcc confined concrete axial compressive stress

fccd design confined concrete compressive strength

fc0 unconfined concrete compressive strength =

0.67fcu/γmc

fctm tensile strength of concrete = 0.18(fcu)2/3 (ideally

derived from in situ pull-off tests)

fcu characteristic compressive cube strength of

concreteff stress in axial FRP at location of shear force

ffd design tensile strength of FRP

ffk characteristic tensile strength of FRP

ffm mean tensile strength of FRP

fr confinement pressure

fy characteristic tensile strength of steel reinforcement

G dead loadgs shape factor for non-circular columns

h overall depth of memberIcs second moment of area of strengthened concrete

equivalent transformed cracked sectionk confinement effectiveness factorkb bond force factor, defined in Equation 16

lol length of overlapping region of parabolas

lnsm anchorage length provided for NSM bar

lnsm,max anchorage length for NSM bar required to generate

Tnsm,max

lt anchorage length

lt,max maximum anchorage length

M design ultimate momentMadd additional required moment capacity

N ultimate axial load on columnn factor for anchorage of shear strengtheningne number of effective axial reinforcing bars

Q live loadRc corner radius of rectangular column

s standard deviationsf spacing of FRP strips

Tk characteristic bond failure force

Tk,max ultimate bond failure force

Tnsm characteristic anchorage force for NSM

Tnsm,ad characteristic adhesive bond failure force

Tnsm,max maximum NSM anchorage force

tf thickness of FRP laminate

V shear force due to ultimate loadsVadd shear force additional to that present at the time of

strengtheningVc shear resistance of concrete

Vf shear resistance of FRP

VR,max maximum allowable shear resistance of member

Vs shear resistance of steel reinforcement

Vu ultimate shear capacity

Design guidance for strengthening concrete structures using fibre composite materials

24

vmax maximum permissible shear stress

x depth of neutral axis of FRP-strengthened memberz lever arm

αe modular ratio of steel to concrete∀f modular ratio of FRP to concreteß Angle between the principal fibres of the FRP and

a line perpendicular to the longitudinal axis of themember

∆ff change in force over length ∆y of FRP for longitu-dinal shear stress

∆y short length along FRP for longitudinal shearstress

γmA partial safety factor for adhesiveγmc partial safety factor for concreteγE partial safety factor for modulus of elasticity of

FRPγmE design partial safety factor for modulus of

elasticity of FRPγmf design partial safety factor for strength of FRPγmm partial safety factor for manufacture of FRPγms partial safety factor for steelγme design partial safety factor for strain of FRPγε partial safety factor for strain of FRP

εc0 axial strain in unconfined concrete at peak stress

=2.4 x 10–4√(fcu/γmc)εcc confined concrete axial strainεccu confined concrete ultimate axial strainεfd design ultimate strain of FRPεfe effective FRP strainεff final strain of FRP for flexural strengtheningεfk characteristic failure strain of FRPεfse effective strain in the FRP for shear strengtheningεt position of transition region between parabola and

straight line for confined concreteεy yield strain of steel = 0.002φ creep coefficientτ longitudinal shear stress

5.2 OVERVIEW OF AVAILABLE DESIGNGUIDANCE

Since the publication of the First Edition of this TechnicalReport, a number of national and international guidelineshave been introduced dealing specifically with the design ofexternally strengthened concrete structures. In particular theFederation Internationale du Beton (FIB) task group 9.3 havepublished Bulletin 14(90) and the American Concrete Institutehas published ACI 440.2R(2). Other guidelines have beendeveloped by the Japan Society of Civil Engineers(91), the ISISCanada Research Network(3) and by Täljsten in Sweden(92).In the UK other relevant publications are provided by the

Highways Agency. Their design guide BD 84/02(10) providesadvice on strengthening concrete bridge supports using FRP.The Agency is currently drafting further guidance on usingFRPs for strengthening highway structures. The ConstructionIndustry Research and Information Association (CIRIA) haspublished a report on the use of composites in construction(93)

and guidelines on strengthening metallic structures usingFRPs(6). Advice on the design of adhesively bonded joints,for fibre composite materials, is given in the EUROCOMP

design code and handbook(94).

5.3 BASIS OF DESIGN

This part of the Report provides the necessary guidance forengineers to carry out the design of non-prestressed FRPstrengthening systems for concrete structures. It should beread in conjunction with BS 8110(88) and BS 5400: Part 4(95)

as appropriate. It is important, however, to recognise that thebasis of strengthening using FRPs differs from the design ofconventional steel-reinforced concrete structures in a numberof important respects. These include the elastic-brittlebehaviour of FRP materials and the bond behaviour ofexternally applied FRPs.

The strengthening will generally be carried out following adetailed appraisal. When the structure is a bridge, referenceshould be made to the Highways Agency’s BD 44 The assess-

ment of concrete highway bridges and structures(96), whichgives modified forms of the equations in BS 5400 for appraisal.The partial safety factors used are lower than those used inBS 5400, reflecting the reduced level of uncertainty, and theuse of the ‘worst credible’ strength is permitted. For buildings,the Institution of Structural Engineers’ Appraisal of existing

structures(8) also suggests lower partial safety factors, butappropriate equations have not been developed, though theapproaches adopted by BD 44 should be equally applicableto other types of structure. Where appropriate, guidance inthe present document is generally based on BS 8110.

The design of FRP strengthening systems should be based onlimit state principles. The aim of limit state design is theachievement of an acceptable probability that the structurebeing strengthened will perform satisfactorily during itsdesign life. This involves checking that the structure doesnot reach a limit state during its intended life, which mayrender it unfit for use.

Limit states broadly fall into two categories: ultimate andserviceability. Ultimate limit states normally encompassmechanisms that cause partial or complete collapse of thestructure while serviceability limit states correspond to statesthat principally affect the appearance or proper performanceof the structure. Examples of ultimate and serviceabilitylimit states relevant to FRP strengthening systems are givenin Table 4.

Structural design of strengthened members

25

Table 4: Limit states relevant to FRP strengthening systems.

Ultimate Serviceability

Strength DeflectionBending CrackingShear Steel stressCompression FatigueAnchorage/plate separation Creep

Fire Stress ruptureDurability

The design of FRP strengthening systems is mainly con-centrated on the ultimate limit state of strength (see Chapters6 to 8). This includes checks for bending, shear and compres-sion, conditions normally associated with reinforced concretedesign, as well as checks for plate separation that are peculiarto FRP-strengthened structures. Since structural strengtheninginvariably increases the stiffness of flexural members, whichin turn increases the risk of brittle failure, a check on ductilitywill also be necessary (see Section 6.2.4). Since the propor-tional increase in stiffness will be less than the increase instrength, it will also be necessary to check the deflection ofthe strengthened structure against the appropriate limits.

Service loads should not adversely affect the appearance orefficiency of strengthened structures. Generally, FRP-strengthened structures should experience closely spacednarrow cracks provided that good bond exists between theFRP and the concrete substrate. However, where problemsare anticipated the designer should take steps to ensure thatthe design crack widths do not exceed the limits recom-mended in BS 8110 or BS 5400, as appropriate. The steelreinforcement should not yield under the service load,otherwise permanent deformations in the structure will result.Fatigue and stress rupture are taken into account by usinglower design stresses determined in accordance with Section6.8. Much of the testing work that has been carried out hasconfirmed that carbon FRP retains its chemical and physicalproperties when exposed to conditions typical of thoserelevant to concrete construction. However, other materialsare less stable when exposed to moisture or ultravioletradiation, and consideration must therefore be given to theuse of protective coating systems (see also Section 10.11).For buildings, fire should also be included in the above limitstates as it will influence the properties of both the FRP andthe adhesive used to attach the FRP to the concrete (althoughsome fire-rated structural systems are available). This aspectis discussed further in Section 5.7.1.

5.4 MECHANICAL PROPERTIES OFMATERIALS

5.4.1 Properties of concrete and steel reinforcement

The strength of the concrete to be used in the design equationsgiven in Sections 6 to 8 should be the characteristic (28-day)compressive cube strength, fcu. The characteristic tensile

strength of modern mild steel and high-yield steel reinforce-

ment, fy, should be taken as 250 and 460N/mm2, respectively.

Both steel types have a mean modulus of elasticity, Es, of

200kN/mm2. Different steel strengths may be appropriate forhistoric structures.

Where there is sufficient knowledge of the properties of theactual materials in the structure, modified values may be used.BD 44(96) uses the concept of the worst credible strength,both for the steel and the concrete. Where actual values areavailable, modified values for the partial safety factors givenin Section 5.6 may be used.

5.4.2 Properties of fibre-reinforced polymers (FRP)

The mechanical properties of FRP materials depend princi-pally on the type and percentage of fibre used. These aspectsare likely to vary between competing composite products,and since there is currently no agreed standard specificationfor their manufacture, all design must be on the basis of theactual properties obtained from the manufacturer, who shouldsupply either characteristic values or mean values and stan-dard deviations. Only materials manufactured in accordancewith an approved quality control scheme should be used;Appendix C gives guidance on the appropriate level oftesting required.

The normally available mechanical properties of FRP aretensile strength, modulus of elasticity and elongation atfailure. For fabric materials, the mechanical properties maybe measured directly on samples, which may be assumed tobe representative of the material applied to the structure. Theproperties of plates should be determined on representativesamples. For wet lay-up systems, test samples should beprepared under the same conditions as the composite isapplied to the concrete. Fully cured samples may then betested to give an indication of the in situ properties. Themechanical properties of other manufactured composites,such as shells, should be determined by the manufacturerfrom tests on coupons.

For example, the characteristic tensile strength of FRP, ffk, is

related to the mean tensile strength, ffm:

ffk = ffm – 2 s (Equation 1)

where s is the standard deviation. (Note: sufficient samplesmust be tested to ensure that two standard deviations isrealistic, see Appendix C.)

Tables B2 and B3 in Appendix B give typical mechanicalproperties for a range of FRP strengthening systems that arecurrently available. The information is taken from manu-facturers’ data sheets and is thought to be correct at the timeof publication. For design purposes, actual properties mustbe obtained from the manufacturer. As test methods vary, theinformation should detail the basis for the information (e.g.frequency of testing, standard deviation).

Design guidance for strengthening concrete structures using fibre composite materials

26

5.4.3 Properties of adhesives and laminating resins

Tables B5 and B6 in Appendix B give typical properties forepoxy adhesives and laminating resins that are currentlyavailable. (The comments in Section 5.4.2 regarding theinformation on FRP are equally applicable.) It is importantthat the adhesive or laminating resin being used is compatiblewith the laminate or fibre. Ideally, to ensure compatibility,all the components of the system (including any priming ortop coating materials) should be from a single supplier.

5.4.4 Stress–strain curves

The equations developed for the design of FRP strength-ening systems given in Sections 6 to 8 are based on therectangular parabolic stress–strain relationship for concretein compression and the relationship for reinforcing steeldescribed in Section 6.2.4, which gives the reason foradopting this relationship. Alternatively, more exact stress–strain curves, such as those given in Eurocode 2(89), may beused in analysis.

Unlike steel reinforcement, all FRP has a linear elasticresponse to failure, with no or very limited yielding. Wovenfabrics have a degree of non-linearity, but this may beignored for design purposes.

5.5 PARTIAL SAFETY FACTORS FORLOADS

For the ultimate and serviceability limit states, the designloading will normally be obtained by multiplying the charac-teristic dead and imposed loads by appropriate partial safetyfactors taken from BS 8110 for buildings or from BS 5400or BD 37(97) for bridges.

Prior to strengthening, the designer will need to assess theprobable effect of an accidental loss of strengthening effec-tiveness resulting from fire, vandalism or impact. Guidanceon assessing the flexural strength of structures in fire is givenin Section Four of BS 8110: Part 2. See also Section 5.7 ofthis Report.

If the magnitude of the initial strains that exist in the struc-ture need to be estimated in order that they can be excludedfrom the strain in the FRP (see Section 6.2.5, part (c)), thepartial safety factor for dead load and imposed load shouldbe taken as 1.0. The level of imposed load to be used in thecalculations will be very structure-dependent and will be amatter for judgement.

5.6 DESIGN VALUES FOR MATERIALPROPERTIES

5.6.1 Introduction

The characteristic material properties (see Section 5.4) aredivided by appropriate partial safety factors (γmE, γmε, γmm)

from Tables 5 to 7 to give the values to be used in design.

The partial safety factors are intended to take into accountthe uncertainties associated with the material itself and withits use in the structure. Guidance on developing project-specific partial safety factors can be found in the CIRIAreport on FRPs in construction(93). However, in most situa-tions and in the absence of independent field-testing of mate-rial properties as installed, the partial safety factors given inthe following Sections may be used.

The magnitude of the partial safety factors applied to theFRP will depend on the type of fibre and the stage in themanufacturing process at which the test samples are taken(see Section 5.6.3). The partial safety factors are intended totake into account changes in material properties with time. Inthis respect they differ from the factors applied to traditionalconstruction materials such as steel and concrete, whoseproperties are assumed not to change with time.

5.6.2 Design strength of steel and concrete

In general, the design strength of the steel and concreteshould be assessed using appropriate values of the partialsafety factors for concrete, γmc, and for steel reinforcement,γms, given in BS 8110 or BS 5400 as appropriate. When theworst credible strengths are used (see Section 5.4.1), thefactors may be modified in line with the recommendations ofBD 44(96).

5.6.3 Design elastic modulus of FRP

In most practical design situations the limiting factor govern-ing the failure of an FRP-strengthened structure is the strainin the FRP (e.g. anchorage, separation failure, etc.), althoughrarely ultimate strain. It is therefore the stiffness of the FRPthat is of importance. Although durability tests in laboratoryconditions on unloaded glass and carbon FRP compositeshave shown that there is little significant degradation of themodulus of elasticity under long-term (10,000hr) environ-mental exposures such as salt water, high alkalinity, humidityand freeze–thaw(98), the modulus of elasticity of FRP maychange with time under load and may vary according to themethod of manufacture and application. In particular, lack ofstraightness of fibres can significantly affect the stiffness. Inaddition, the accuracy with which the properties are obtainedfrom test samples is dependent upon the method of manu-facture. Therefore, it is necessary to apply partial safetyfactors relating to both material type and method of manu-facture to the modulus of elasticity of FRP in arriving at thedesign strength of structures strengthened with externalreinforcement:

Efd = Ef / γmE (Equation 2)

where

γmE = γE × γmm (Equation 3)

Structural design of strengthened members

27

Recommended partial safety factors for modulus of elas-ticity are given in Table 5 and partial safety factors formethod of manufacture and application can be taken fromTable 6.

Table 5: Partial safety factors for Young’s modulus at the ultimate

limit state.

Material Factor of safety, γE

Carbon FRP 1.1Aramid FRP 1.1AR glass FRP 1.6E glass FRP 1.8

Table 6: Recommended values of additional partial safety factors,

to be applied to manufactured composites, based on Clarke(94).

Type of system (and method of Additional partial

application or manufacture) safety factor, γmm

Plates

Pultruded 1.05Prepreg 1.05Preformed 1.1Sheets or tapes

Machine-controlled application 1.05Vacuum infusion 1.1Wet lay-up 1.2Prefabricated (factory-made) shells

Filament winding 1.05Resin transfer moulding 1.1Hand lay-up 1.2Hand-held spray application 1.5

5.6.4 Design ultimate strain of FRP

It is also possible in some situations that the ultimate strainin the FRP may govern failure of a strengthened structure(e.g. shear strengthening or ultimate strain of confined con-crete) although, typically, other strain limits are reached first.Again, durability tests on unloaded specimens of glass andcarbon composites have demonstrated significant long-termultimate strain reductions, particularly due to exposure tohumidity(98). As for the material partial safety factor formodulus of elasticity, the partial safety factor for ultimatestrain is also related to both material type and route of manu-facture and application. Thus, the design strain is given by:

εfd = εfk / γmε (Equation 4)

where

γmε = γε × γmm (Equation 5)

Recommended partial safety factors for ultimate strain aregiven in Table 7 and partial safety factors for manufacturemethod can, again, be taken from Table 6.

Table 7: Partial safety factor for strain at the ultimate limit state.

Material Partial safety factor, γεCarbon FRP 1.25Aramid FRP 1.35AR glass FRP 1.85E glass FRP 1.95

5.6.5 Design ultimate strength of FRP

In some instances the ultimate tensile strength of the FRP isrequired during the design of a strengthened structure (e.g.ultimate flexural strength, although actual failure is likely tobe due to separation of the FRP from the concrete, which, ingeneral is related to FRP strain). In such cases, the designstrength can be derived from the design modulus of elas-ticity, Efd, and the design strain, εfd:

ffd = Efd εfd (Equation 6)

Hence, given the partial safety factors acting on the modulusof elasticity and the ultimate strain of the FRP, the partialsafety factors acting on the strength of the material are asfollows:

γmf = γmE ×γmε = γE × γε × (γmm)2 (Equation 7)

So the design strength is given by:

ffd = ff / γmf (Equation 8)

It should be noted that the resulting material partial safetyfactor for ultimate strength is equivalent to that proposed inthe First Edition of TR55. Figure 18 shows the relationshipbetween design stress and strain and the partial materialsafety factors.

εfεf

ffd

ffk

Ef

Ef

Stress

Strain

Figure 18: Assumed stress–strain curves.

5.6.6 Steel stress

The designer must check that the steel reinforcement doesnot yield under service loads, otherwise the structure maysustain permanent deformations. Therefore it is recommendedthat the partial safety factors for steel reinforcement beincreased to 1.25 in performing this check. This conditionmay be rather onerous for older structures reinforced with

Design guidance for strengthening concrete structures using fibre composite materials

28

grade 230 steel, which would otherwise be considered suit-able for strengthening. Under these circumstances, designersmay consider increasing the allowable steel stress to 1.0 fy,

provided that other factors, e.g. crack widths and concretequality, do not preclude this approach to strengthening.

5.6.7 Deflection and cracking

The deflections and crack widths in structures being strength-ened should be kept within values specified in current codesand standards, such as BS 8110 or BS 5400. The calculationsshould be based on the partial safety factors contained inthese documents.

5.6.8 Adhesive

In general, the ultimate behaviour of a strengthened sectionwill be governed by the strength of the concrete and not bythe strength of the adhesive, provided the following aresatisfied:

• All the materials used are in accordance with recognisedstandards.

• The material properties are checked on samples made onsite.

• The in-service temperature does not differ significantlyfrom that at which the test samples were made and cured.

• The work is carried out by suitably experienced staff, inaccordance with the advice in Chapter 10.

• Detailed and proven method statements and specifica-tions are used.

• The structure is ‘fail-safe’, i.e. failure of the strength-ening will not lead to failure of the structure.

If any of the above parameters are not satisfied, higher valuesof γmA, the partial safety factor for adhesive, will be required.An approach for determining appropriate partial safety factorsmay be found in A guide to the structural use of adhesives(24).

It should be noted that cyclic strains applied to an adhesiveduring the curing period, for example due to traffic loadingon a bridge under repair, may lead to a change in theproperties of the adhesive. However, it has been suggestedthat these changes are likely to be small, perhaps a 10%reduction in the strength of the fully cured material.

As a general recommendation, the sustained stress in theadhesive should be kept below 25% of the short-term strength,which equates to the recommended minimum material partialsafety factor of 4.0.

5.7 EXTREME LOADINGS

5.7.1 Behaviour of structures in fire

For design of reinforced or prestressed members, fireresistance is generally ensured by the provision of adequatecover to the reinforcement. More detailed analysis can becarried out working from a standard time–temperature curve,

such as that given in BS 476: Part 20(99), for the required fireendurance. A design approach, only for members in flexure,is given in Section Four of BS 8110: Part 2, which givesreduced values for the strengths of the steel and concrete atelevated temperatures. However, as fire is considered as anaccidental load, the partial safety factors on the materials arereduced and, more importantly, the partial safety factors onthe dead and live loads are also reduced. Thus in many cases,the fibre composite strengthening could fail completely with-out risking failure of the structure. This may be illustrated bythe following simple example, which ignores any materialproperty changes due to the elevated temperatures:

1. Consider a floor slab with a dead load G and live load Q2. Original design capacity = 1.4 G + 1.6 Q3. The slab is strengthened to carry an additional 50% live

load4. Hence modified design capacity = 1.4 G + 1.6 (1.5 Q)5. Required design capacity in fire = 1.05 G + 1.0 (1.5 Q)6. In a typical floor slab, G Q, so required design capacity

in fire = 2.55 G7. Capacity of unstrengthened slab = 3.0 G which is greater

than the required capacity in fire.

If failure of the fibre composite strengthening in fire wouldlead to the collapse of the structure, it will obviously benecessary to consider the behaviour of the fibre compositematerials as well as the behaviour of the adhesive. The fibresthemselves are unlikely to be affected by the elevated tempe-rature. However, it is likely that the adhesive will be affectedif the temperature exceeds the adhesive’s glass transitiontemperature, which may be of the order of 50 to 60°C forconventional materials. If the glass transition temperature isreached then the effectiveness of the FRP strengthening willbe reduced.

Specific advice on fire resistance of FRP materials should besought from the manufacturer. If necessary, options for in-creasing the fire resistance of FRPs may include providing alayer of suitable insulating material over the fibre composite.The carbon fibre mats used to strengthen the soffit of a bridgeover the A10 motorway in France were covered with a layerof plaster and mortar for fire protection(100). Unless a rigorousanalysis is undertaken it is sensible to neglect the strength-ening from FRP in fire situations. As shown above, such asituation does not preclude the use of FRP strengthening.

In addition to concerns about the structural behaviour ofstrengthened structures in fire, the emission of smoke andtoxic fumes will be a major consideration, particularly inenclosed situations such as tunnels. Improved performancecan be achieved by the use of appropriate fillers and the useof intumescent coatings or other high-temperature foaminsulation barriers.

5.7.2 Seismic loading

Seismic loading will not be a major loading case for mostUK structures. However, it may be important for strength-ening work in connection with nuclear-related structures.

Structural design of strengthened members

29

This is a highly specialised area of design, which is outsidethe scope of this Technical Report. Reference should bemade to publications by experts in the field, such as Priestleyet al.(101), Seible et al.(102) and Triantafillou(103).

5.7.3 Impact loading

The consequences of structural collapse due to vehicle impacton bridge supports are considerable. In an impact, about 80%of the energy is absorbed by the vehicle crushing, with theremainder being absorbed by the structural element. It isnecessary to ensure that both the flexural strength anddeformability of a column are adequate. The designer shouldensure that the deformability of the strengthened structure isat least as great as it would be for the equivalent conven-tionally designed structure.

It has been shown that there is an increase in concretestrength at very high loading rates. While it is not clearwhether there is such an increase in concrete strength undervehicle impacts, tests commissioned by the Highways Agencyon circular columns wrapped with aramid FRP have shownthat the wrapping is at least as effective under impact loadingas under static loading. Therefore, it would seem that methodsof designing for impact loads based upon the application ofequivalent static loads are reasonable.

Tests on rectangular columns carried out by Suter et al.(104)

have shown the effectiveness of longitudinal aramid FRPfollowed by hoop wrapping in increasing the flexural capa-city and hence the energy-absorbing capacity of columnsunder equivalent static loading. It should be noted that testshave only been carried out using aramid FRP, due to itstoughness. However, this does not mean that other FRP typesare necessarily unsuitable.

5.7.4 Blast loading

There are two fundamental issues for the designer toconsider when blast is involved. The first is strengthening ofthe structure to withstand the blast so that the structure isserviceable immediately following overload. The second isproviding protection to the public by preventing the façade,in particular, from disintegrating into projectile debris.

While both issues are clearly of vital importance to security,public-domain blast test results are notoriously difficult tocome by. Furthermore, much of the research conducted onFRP strengthening of structures against blast has been under-taken on concrete masonry walls under static loading(105).However, some studies of the effects of real blast loading onFRP-strengthening schemes against debris projectiles havebeen carried out(106).

It seems that masonry walls can indeed be strengthened toresist disintegration during blast loading(107). However, anobvious question that remains is what effect the increase instrength might have on an increase in stiffness of the samewall. Such an effect could be problematic in seismiczones(108).

5.7.5 Vandalism

In situations where deliberate vandalism is considered to bea potential problem (primarily in an urban environment)there are a number of possible actions that can be taken.Firstly, the FRP may be physically protected, by providingsome form of barrier that limits accessibility (either to thesurface of the strengthened structure or around the structureas a whole). Secondly a form of strengthening which is inhe-rently resistant to physical attack, such as NSM, may bechosen if possible. Thirdly, frequent inspection or moni-toring should be carried out so that any damage resultingfrom vandalism can be quickly remedied. However, it is notpossible for these measures to prevent damage from a deter-mined attack. It is therefore necessary, as in the case of firedamage, for the unstrengthened structure to be able to satis-factorily carry the unfactored service loads.

Accidental damage may also occur due to contractors drillingthrough FRP or removing protective coatings without reali-sing the structural implications. This can be avoided bymaking contractors aware that the FRP should not be inter-fered with. This may be achieved by application of someform of printed warning, either directly on to the FRP or inclose proximity if the finish of the FRP is important. Furtherguidance and suggested warning signs can be found Section10.12 and in the Concrete Society’s TR57(5).

31

6 STRENGTHENINGMEMBERS IN FLEXURE

6.1 GENERAL

The flexural strength of reinforced concrete beams and slabscan be increased by bonding FRP laminates to the tensionfaces of the members, as shown in Figure 19. For membersstrengthened in flexure the following should be considered:

• maximum moment• risk of peeling failure at the ends of the FRP• risk of debonding of the FRP and the concrete substrate• shear capacity of the section• ductility of the strengthened member• compliance with relevant serviceability limit states, e.g.

cracking, deflection, fatigue, creep-rupture.

Figure 19: Strengthening beams and slabs with FRP.

In addition, this design guidance is dependent on the follo-wing assumptions:

• No slip between the FRP strengthening and the substrate(i.e. plane sections remain plane). This assumption placeslimits on adhesive thickness, adhesive shear modulus andFRP composite in-plane shear rigidity.

• Inter-laminar shear strength of the FRP strengthening isgreater than the adhesive bond shear strength. This shouldbe covered in the specification by specifying the type ofresins that are acceptable, limits on fibre volume fractionand elastic modulus of the FRP strengthening.

• The substrate quality is such that it will not reduce theeffectiveness of the FRP strengthening. Therefore theactual condition must be established and taken intoaccount in design together with likely future deteriora-tion which may be indicated by chloride levels, existingcracks, moisture and half-cell potential of the concretesubstrate. The specification should also outline clearlythe allowable minimum compressive and tensile strengths

of the concrete to ensure a proper bond and long-termdurability. Tests to determine these properties (e.g. pull-off tests) should be outlined in the specification.

• The surface preparation of the concrete substrate is suffi-cient to achieve the required level of bond strengthrequired in the design.

In addition, this design guidance is dependent on assump-tions based on the installation methods and specificationdetailed in Chapter 10.

6.2 MOMENT CAPACITY

6.2.1 Introduction

The section should be designed such that yielding of the steelreinforcement precedes both compressive failure of the con-crete and tensile failure of the FRP.

In cases where the FRP will theoretically reach its designtensile strain before the concrete crushes, failure normallyoccurs due to plate separation rather than plate rupture andthe strain limits for debonding discussed in Section 6.3.3will frequently govern the design. In some cases the concretewill crush before the FRP reaches its design tensile strain(see Section 6.2.4). Provided that the steel strain at failure issufficiently large, however, this should not result in brittlefailure of the strengthened member.

Design ultimate moments should normally be determined bylinear elastic methods. If there is evidence of local yieldingtaking place, the results of an elastic analysis need to beapplied with care. Since members undergoing strengtheningwill usually be steel reinforced, some redistribution of elasticmoments may occur near ultimate. Section 6.7 gives furtherguidance on redistribution.

6.2.2 Requirements of the existing section

The ultimate capacity of the existing section should beassessed by conventional concrete design methods, such asthose in BS 5400-4:1990 clause 5.3.2.1 or BS 8110-1:1997clause 3.4.4.1 as appropriate to the structure being considered.

Particular care should also be taken to apply appropriatematerial parameters and factors for the age of the structure inquestion, in order to reflect likely variability in materials atthe time of their incorporation into the structure. In parti-cular, since many of the structures that require strengtheningwill have been built prior to publication of the 1997 edition

Design guidance for strengthening concrete structures using fibre composite materials

32

of BS 8110, it would seem appropriate to use a partial safetyfactor for steel, γms, of 1.15 rather than the value of 1.05 nowrecommended by the code.

The section should only be considered for strengthening ifthe ultimate capacity of the unstrengthened (existing) sectionis at least as great as the effects arising from the unfactoredloads to be applied. This ensures that even in the event ofremoval of the FRP strengthening by some unforeseen event,catastrophic collapse of the structure is not likely.

6.2.3 Preliminary design

An initial but potentially non-conservative estimate of theFRP requirement for the section can be obtained by assu-ming that the position of the neutral axis remains approxi-mately equal to that of the unstrengthened section. Theapproximate area of FRP required, Af, can therefore be

obtained by dividing the required additional moment capa-city of the beam, Madd, by the product of the steel lever arm,

z , and the design stress in the FRP (given by εfe Efd) as follows:

Af = Madd / εfe Efd z (Equation 9)

whereεfe = the lesser of εfk / γmε (the design ultimate strain of

FRP) and a strain of 0.008. Typically the value of0.008, which would result in separation failure (seeSection 6.3.3) governs.

Efd = design modulus of elasticity of FRP, Efk /γmE

z = steel lever arm

This calculation becomes a less reliable predictor of the FRParea required if the existing section is already heavily rein-forced, or if the section is doubly reinforced. In any event, itis always necessary to proceed with the detailed designmethod rather than rely on this initial estimate.

6.2.4 Design resistance moment of FRP-strengthened

beam

When analysing a cross-section to determine its ultimatemoment of resistance the following assumptions should bemade:

• The strain distribution in the concrete in compression andthe strains in the reinforcement, whether in tension orcompression, are derived from the assumption that planesections remain plane and that no longitudinal slip occursbetween or within the components of the section.

• The stresses in the concrete in compression are derivedfrom the stress–strain curve in either BS 5400-4:1990Figure 1 or BS 8110-1:1997 Figure 2.1, with the strain atthe outermost compression fibre at failure taken as nomore than 0.0035.

• The tensile strength of the concrete is ignored.

• The stresses in the metallic reinforcement are derivedfrom the stress–strain curves in BS 8110-1:1997 (seeFigure 20). This condition is more onerous and appro-priate for longitudinal shear than the stress–strain curvein BS 5400-4, while having negligible effect on ultimatemoment capacity. The material partial safety factor forsteel, γms, should be set to an appropriate value for thedate of construction and with the compressive stresslimited to fy/(γms+fy/2000), as it is in BS 5400-4.

• The strains in the FRP reinforcement take into accountthe strains present in the bonded surface at the time ofapplication of the reinforcement.

• The stresses in the FRP reinforcement are derived fromthe assumption that the FRP has a linear elastic characte-ristic until rupture.

In addition, if the ultimate moment of resistance, calculatedin accordance with this clause, is less than 1.15 times therequired value, the section should be proportioned such thatthe strain at the centroid of the tensile reinforcement is notless than 0.002 + fy/(Esγms).

stress

strain

fy/γms

E=200 kN/mm2

Figure 20: Stress–strain curve for reinforcing steel in thedesign of strengthened beams in flexure.

Allowance should also be made for the requirements ofadditional tensile capacity (to carry the tensile forces arisingfrom the truss analogy for resisting shear) if significant shearand bending moment coincide, as would be the case whenstrengthening a continuous structure over a support.

6.2.5 Example design method

The requirements of Section 6.2.4 can be met by adoptingthe following example design method at any section thatmay be critical:

(a) Calculate the loads to be applied to the structure, deter-mining loads applied before the strengthening (such aselement self-weight) and loads after strengthening (suchas traffic loading in the case of a bridge) separately. Fromthese determine both shear forces and bending momentsat the section considered. To simplify later stages theseload effects should also be determined both unfactoredand with ultimate limit state load factors applied.

(b) Calculate the area of reinforcement necessary to satisfythe Asa requirements of BS 5400-4:1990 clause 5.3.3.2.

(c) Calculate the strain in the section at the position wherethe FRP is to be applied under the unfactored loading

Strengthening members in flexure

33

present at the time the strengthening is to be applied. Incalculating this value, appropriate allowance should bemade for the duration of the loads, since these loads arelikely to be predominantly dead loads and consequentlythe modulus of elasticity adopted should be that for long-term loading. In calculating these strains the followingassumptions should generally be adopted:• Plane sections remain plane.• The reinforcement, whether in tension or compression,

is elastic with a modulus of elasticity of 200kN/mm2.• The concrete in compression is elastic with an appro-

priate modulus of elasticity as discussed above. Thismay be half the value in Table 3 of BS 5400-4:1995.

• The concrete has zero tensile capacity.(d) Assume an initial concrete maximum compressive strain,

which should be less than 0.0035.(e) Assume an initial neutral axis position.(f) Adopting the assumptions described in Section 6.2.4

calculate the forces in the component parts of the cross-section. The strain used to calculate the force in the FRPshould be evaluated by subtracting the initial strain in theconcrete at the position of the FRP at the time ofstrengthening (calculated in step (b)) from the strain atthe position of the FRP from the assumed linear strainprofile (dependent on the assumed neutral axis positionand maximum concrete strain in steps (d) and (e)). Op-tionally, the area of existing steel tensile reinforcementassumed in this calculation should be the provided areaminus the value of Asa required at this section, as calcu-

lated at step (b).(g) Iteratively adjust the assumed neutral axis position and

concrete maximum compressive strain until step (f) resultsin zero net axial force present in the section (i.e. ‘forcebalance’ is achieved) and the moment of these forcesmatches (or exceeds) the required bending moment.

(h) Check the calculated stresses and strains against thefollowing criteria:• The concrete maximum compressive strain should

not exceed 0.0035.• The FRP maximum tensile strain should not exceed

the limit calculated in accordance with Section 6.3.3.• The section exhibits adequate plasticity. In the absence

of more rigorous examination, the strain at the centroidof the tensile reinforcement should be not less than:

(Equation 10)

unless the ultimate moment that can be resisted by thesection is at least 1.15 times the applied ultimatemoments.

• The stress in the FRP is less than the ultimate capacityof the FRP.

(i) If the area Asa was not subtracted at step (f), the area of

FRP calculated shall be increased by an area AsaF, where:

AsaF = V /(2 εff Efd) (Equation 11)

mss

y002.0γE

f+

whereV = the shear force due to ultimate loadsεff = the final strain in the FRP at step (g)Efd = Efk / γmE

If the calculation described does not converge at a solutionthat meets the criteria described, then the quantity of FRP tobe applied may need adjustment and the calculation repeateduntil an adequate design is achieved.

6.3 FRP SEPARATION FAILURE

6.3.1 Introduction

For members strengthened in flexure, failure can occur whenthere is a loss of composite action between the FRP and theconcrete section. Typically failures occur through the develop-ment of a longitudinal failure-plane close to the interfacebetween the FRP and the concrete or at the level of the mainreinforcement. In experimental studies, FRP separation hasbeen found to be the most common failure mechanism. It istherefore essential that it is taken into account in the designof FRP strengthening schemes. Figure 21 illustrates typicalFRP separation failures observed in tests.

Figure 21: Possible failure modes and locations for FRP-strengthened beam.

Despite the importance of FRP separation, it remains a subjectthat stimulates considerable research. A number of differentinitiation mechanisms have been identified and proposals tocategorise them developed, for example see Blaschko et

al.(109) and Teng et al.(110). While the precise mechanisms arestill the subject of some debate, the design approach givenhere has been used in many practical strengthening schemesand is recommended.

It should be noted that the proposed method is largely basedon laboratory and field data obtained from strengtheningschemes using carbon FRP. It may not therefore be directlyapplicable to other composite materials, although the sameprinciples should apply. In some cases the approach may beunduly conservative for composite materials other thancarbon.

Work on steel plate bonding has shown that separationfailures tend to initiate from the ends of the plates. To addressthis, limitations on plate aspect ratio are incorporated in the

Design guidance for strengthening concrete structures using fibre composite materials

34

Highways Agency Advice Note on steel plate bonding, BA30(9), together with requirements for bolting. Early work onFRP separation failures similarly focused on the ends of theplates.

However, for FRP strengthening schemes, experimental evi-dence now shows that separation can also initiate fromflexural cracks in the span, shear cracks or concave irregu-larities in the surface profile, and that all of these cases needto be taken into account in the design. Importantly, researchhas also shown that externally bonded FRP strengtheningcan be highly effective without the need for bolting or theuse of other mechanical fixings.

It has been shown that increasing the area of FRP bonded tothe concrete and reducing the FRP thickness reduce thelikelihood of separation failure modes.

6.3.2 Bond failure

The bond behaviour of externally bonded FRP differs mar-kedly from that of embedded steel reinforcement. Experi-ments have shown that the longitudinal shear stress that canbe transferred between the FRP and the concrete is notindependent of the bonded length, as typically assumed forembedded steel reinforcement. Thus, while it is possible toanchor steel reinforcement by providing an anchorage lengthbeyond which the full strength of the reinforcement can bedeveloped, this is not typically the case for externally bondedFRP. This aspect of the behaviour of externally bonded FRPgreatly influences, and adds complexity to, the design ofstrengthening schemes.

In tests on the anchorage of FRP externally bonded to con-crete, it has been found that beyond a limiting bonded length,of the order of 50–300mm, there is no further increase in theultimate anchorage load-capacity with increased bondedlength. Furthermore, this ultimate anchorage capacity can bevery much less than the ultimate tensile capacity of the FRP.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2tfrp (mm)

for varying anchorage lengths and a range of different FRPplate thicknesses. From this Figure it can be seen that, for allthe cases considered, the maximum force that can be deve-loped in the FRP anchorage is less than 25% of the ultimateFRP capacity.

Experimental studies have, however, shown that the FRPforce that can be developed in the span of strengthened beamscan be very much greater than the FRP anchorage capacity.These findings indicate that, provided there is a gradual build-up of stress outside the anchorage region, it is possible forthe FRP to sustain stresses in excess of the anchorage capacitywithout separation failure occurring. Importantly, it seemsthat this gradual build-up of FRP stress relies on someflexural cracking of the concrete as the ultimate limit state isapproached. Thus, particular care is required in cases wherethe FRP is bonded to concrete that is not expected to crackat the ultimate limit state, for example because of changes insection properties or the presence of prestress.

6.3.3 Design procedure

The design procedure to account for FRP separation failuresfirst requires two structure-dependent conditions to bechecked, namely that failure will not be initiated either by shearcracking or by irregularities in the concrete surface profile.Provided these are satisfied, three further design-specificcriteria must be considered relating to the strain in the FRP,the longitudinal shear stress between the FRP and theconcrete and the stresses developed in the anchorage region.

Shear-crack-induced FRP separation

The presence of shear cracks can lead to a tendency for a stepto develop in the tension face of the member to which theFRP is bonded. This can result in the development of size-able transverse tensile stresses in the adhesive and the surfaceconcrete, leading to the initiation of FRP separation failure.

Such a mode of failure may be disregarded if the maximumapplied shear force can be carried by the concrete alone,neglecting any contribution to the shear capacity provided byshear reinforcement. However, for beams this criterion maywell not be satisfied. Experimental studies(113) have shownthat shear cracking will have initiated at or before 67% of theultimate shear capacity of the section (including all forms ofreinforcement). Therefore should the maximum applied shearforce exceed 67% of the ultimate capacity, it may be presumedthat shear-crack-induced separation failure will occur. Shouldthe maximum applied shear force lie between the concrete-only shear capacity and 67% of full capacity, careful consi-deration should be given to shear crack initiation.

Surface irregularity-induced FRP separation

Concave irregularities in the profile of the surface to whichthe FRP is bonded will lead to the development of tensile peel-ing stresses in the adhesive and surface concrete as the FRPattempts to straighten under load. Such transverse tensilestresses can promote the initiation of FRP separation failure.

Figure 22: Variation in FRP separation strain with bondedlength, based on Denton et al.(112).

Such behaviour is shown in the work of Neubauer andRostasy(111). Based upon their model, the ratio of the strainwhen FRP separation occurs to the ultimate FRP straincapacity is plotted in Figure 22 (based on Denton et al.(112))

E frp = 230GPa

ε frp = 0.015

F ctm = 3MPa

K b = 1

εfrp

db

, m

ax/ ε

frp

u

ε frp = ε frpdb (Neubauer & Rostasy)

ε frp = ε frpu

ε frp = 0.5ε frpu

ε frp = 0.004

Strengthening members in flexure

35

It is usually the case during strengthening works that thesurface to which FRP will be bonded is concavely curved tosome extent. Such unevenness is sometimes relatively local,perhaps due to formwork being flexible during casting, oralternatively it could also be more global, for example whenthe entire soffit of a structure is curved.

Through testing(114,115), it has been found that concave curva-ture can significantly affect the degree of strengtheningachieved. The work of Eshwar et al.(114) suggests that theextent over which the concave curvature exists may affect thesignificance of such concavity, and it seems that the beha-viour of strengthened members is more sensitive to globalthan local curvature.

It is advised therefore that if the soffit of a concrete structureto be strengthened is globally concave, reference should bemade to specialist literature(114,115), and specialist advicesought.

If undulations in the concrete are local with a smoothlyvarying profile, the influence of curvature may be dis-regarded in the design provided over any 1-m length, anyconcavity in the FRP profile, as installed, does not exceed3mm in depth. Fabric-based systems tend to closely followthe profile of the concrete to which they are bonded and it istherefore essential that the Specification for such schemesrequires the concrete surface to which the FRP will bebonded to have only smooth variations in profile with amaximum unevenness of 3mm in 1m. For plate-basedsystems, the FRP tends not to follow the profile of theconcrete so closely and therefore greater unevenness in theconcrete profile, up to 5mm in 1m, may be acceptableprovided the FRP, once installed, has a smooth variation inprofile with a maximum unevenness of 3mm in 1m. In suchcases, the difference in the concrete and FRP profile must betaken up in the adhesive.

Maximum FRP strain

Early work on FRP separation failures sought to establishdesign values of the FRP strain below which separation wouldnot occur. Such design values were typically rather less thanthe ultimate FRP strain capacity. Neubauer and Rostasy(111)

suggest a limit of 5εy (critical for mild steel) or half theultimate plate strain, which for the materials tested was0.0075. Other workers have suggested somewhat lower limits,in the order of 0.006 for sagging moments and 0.004 forhogging moments(116).

Such an approach alone does not fully capture the mechanicsthat underpin the initiation of FRP separation failure. How-ever, it does seem prudent at present to retain a limit on themaximum design FRP strain. When used in conjunction witha limit on the maximum longitudinal shear stress between theFRP and the concrete, UK experience suggests that higherstrain limits than those described above are reasonable.

It is therefore recommended that the strain in the FRP atultimate limit state should nowhere exceed 0.008. This strainlimit will be more critical than the ultimate FRP straincapacity in the substantial majority of design cases,particularly when standard modulus materials are used. Itmay be quite conservative in some cases and may potentiallybe relaxed if specialist advice is sought.

Longitudinal shear stress between FRP and concrete

To ensure that the build-up of stress in the FRP outside theanchorage region is sufficiently gradual, it is recommendthat the longitudinal shear stress between the FRP and theconcrete, determined as described below, should nowhereexceed 0.8N/mm2 at the ultimate limit state. This value ismentioned in the current Highways Agency Advice Note forsteel plate bonding, BA 30(9), and is broadly in line with boththe allowable shear stress values recommended in BS 8110:Part 1 and BS 5400: Part 4. Provided specialist advice issought, it may be reasonable to moderately increase thislimiting longitudinal shear stress in special cases.

For surface-mounted reinforcement, when both the originalsection and the applied FRP are prismatic and do not taperalong their length, assuming the concrete and steelreinforcement to behave linear-elastically, the longitudinalshear stress, τ, can be calculated using the expression:

τ = Vadd∀f Af (h – x) / Ics ba (Equation 12)

whereVadd = difference between the ultimate shear force and the

applied shear force when the strengthening is installed∀f = short-term modular ratio of FRP to concrete

= Efd /Ec

Af = area of FRP plate

x = depth of neutral axis of strengthened sectionIcs = second moment of area of strengthened concrete

equivalent cracked sectionba = width of adhesive layer

h = total depth of the section

The longitudinal shear stress should be checked near to theplate ends, where the shear force acting on the strengthenedportion of the member will be at its greatest. Equation 12may be used in this position provided both the concrete incompression and steel reinforcement are still behavingapproximately elastically. The longitudinal shear stress neednot, however, be checked within lt,max of the end of the plate,

where lt.max is determined in accordance with Equation 15.

Additionally, the longitudinal shear stress must be checkedwhere any changes in section properties occur, at positionswhere there are discontinuities in shear force, such as at theposition of point loads, and at the location along the spanwhere the steel reinforcement stress–strain behaviour goes

Design guidance for strengthening concrete structures using fibre composite materials

36

from elastic to yielding. At this point the longitudinal shearstress can increase dramatically because the steel reinforce-ment no longer contributes to the flexural stiffness of thesection and any increase in bending moment must be carriedby the FRP alone. The position in the span where the steelfirst yields must be found, either by an iterative trial-and-error method or by calculating the strain profile along thelength of the beam. Once the position where the reinforce-ment yields is found, the longitudinal shear stress should befound by calculating the stress ff in the FRP at this position

and at a position a short distance, ∆y, further along thesection in the direction of increasing bending moment. Thedifference in stress in the FRP, ∆ff, along this length can thenbe found and the resulting shear stress can be approximatedusing the equation:

(Equation 13)

This approach to calculation of longitudinal shear stressshould be used wherever the section properties are non-linear.

Anchorage design

In addition to maintaining low longitudinal shear stresses,adequate FRP end anchorage must be provided. Work on endanchorage lengths has been carried out by a number ofauthors. The recommended approach is based upon the modelproposed by Neubauer and Rostasy(111).

Figure 23 illustrates the model. Also as described above, itcan be seen that the characteristic bond failure force, Tk,

increases with increasing anchorage length, lt, but that there

is a threshold anchorage length, lt,max, above which no increase

in the bond failure force is possible.

y

ft

∆∆= f

fτ

Figure 23: Characteristic bond failure force vs anchoragelength.

The maximum ultimate bond force, Tk,max, and the corres-

ponding maximum anchorage length, lt,max, needed to acti-

vate this bond force can be calculated using the followingexpressions:

Tk,max = 0.5 kb bf √ (Efd tf fctm) (N) (Equation 14)

lt,max = 0.7 √ (Efd tf /fctm) (mm) (Equation 15)

wherekb = 1.06.√ [(2 – bf /bw) / (1 + bf /400)] > 1.0

(Equation 16)bf = plate width (mm)

bw = beam width or plate spacing for solid slab (mm)

tf = plate thickness (mm)

Efd = elastic modulus of the plate (N/mm2)

fctm = tensile strength of concrete = 0.18 (fcu)2/3 (N/mm2)

(Equation 17)

(Ideally fctm should be obtained from pull-off tests on the

actual concrete.)

It is recommended that, where the FRP is curtailed in thespan, a minimum anchorage length of 500mm should beprovided. In situations where it is not possible to provide ananchorage length in excess of lt,max, the bond force will be

less than Tk,max and may be calculated using the following

expression:

Tk = (Tk,max lt /lt,max) [2 – lt /lt,max] (N) (Equation 18)

The anchorage design should be undertaken by determiningthe point in the span where the FRP is no longer required.From a sectional analysis, in accordance with the approachdescribed in Section 6.2, the force that will be developed inthe FRP if it is bonded to the member at this point shouldthen be determined. It is important to recognise that, althoughthe FRP may not be required at this point, the fact that it isbonded to the concrete will nevertheless mean that someforce will be developed in it. The resulting FRP force shouldbe checked to ensure that it is less than the ultimateanchorage capacity, Tk, and if so, an acceptable anchorage

design will result from extending the FRP by an anchoragelength beyond this point. If this condition is not satisfiedthen the FRP should be extended further towards the supportor consideration given to using a thinner but wider FRPlaminate. Alternatively, consideration may be given to usingan anchorage device, provided that its capacity has beenproved by testing.

6.4 FLEXURAL STRENGTHENING WITHNSM REINFORCEMENT

6.4.1 Introduction

Flexural strengthening can be achieved by bondingpultruded strips or rods into slots cut in the surface of theconcrete. This application is termed Near-Surface-Mounted(NSM) reinforcement, and has benefits where the exposedconcrete surface is to be trafficked or otherwise exposed topotential damage. The technique is also applicable where thesurface of the concrete is undulating, or if there is excessivelaitance or a thin layer of poor quality concrete near the

–

Strengthening members in flexure

37

surface. The method also results in an increased bond area,which helps to delay the onset of debonding type failure.Installation is more costly than for externally bonded rein-forcement, due to the need to cut the slots and prepare thebond surface. Usually the technique would only be used whereexternally bonded reinforcement is not a good technicalsolution.

6.4.2 Design basis

Other than for FRP curtailment (see Section 6.4.4), the basisfor design of NSM schemes is substantially the same as forsurface-mounted flexural strengthening. Appropriate allow-ance must be made for the fact that the FRP reinforcement islocated within the section, and will therefore be strainedslightly less than the surface of the concrete. Flexural designmethods and limits should therefore be as for surface-mounted reinforcement (see Section 6.2).

Material details and compatibility should be in accordancewith the manufacturer’s recommendations. Complete systemsshould be adopted, since details, such as the preparation ofthe surface after the groove has been cut, depend upon theadhesive used. For example, clean dry surfaces are normallyrequired, but primers may be necessary before certain adhe-sives are applied to the concrete. The type and thickness ofepoxy to be used should be in accordance with the manu-facturer’s recommendations. Since this may also be affectedby the location and orientation of the groove and specificrequirements of the structure (e.g. environmental exposure,service conditions such as elevated temperatures) this shouldbe discussed with the manufacturer early in the designprocess.

6.4.3 Bond behaviour

Curtailment and anchorage of NSM FRP differs from that ofsurface-mounted FRP. The adhesive is a thicker block thanthe thin layer used for surface-mounted strips. The interfacearea between FRP and adhesive is potentially much smallerthan that between adhesive and concrete. Furthermore thepracticalities of surface preparation are likely to result indifferent qualities of preparation on the sides of the slots thanon the bottom of the slot. For these reasons, some of thedebonding criteria used for surface-mounted plates are notapplicable to anchorage of NSM rods.

There are more variable factors for NSM applications thanfor surface-mounted strips and plates. The key factors includebar size and shape, bar material, bar surface preparation(deformed or sandblasted), groove size, shape and surfacepreparation, bonding agent (epoxy or cementitious material),strength of concrete and location of existing bars. The effectsof these factors have been noted experimentally and are stillbeing actively researched. However, few experimental pro-grammes have examined all these variables and how theyinteract.

Most experiments to date have been performed on NSM barswith circular or rectangular cross-section. Circular bar dia-meters range between 7 and 16mm while strips are usuallyrectangular in shape of thickness less than 2mm. Most testson NSM bars have used carbon or glass FRP bars(117,118).Tests have shown that higher average bond stresses areobtained from bars made from carbon than those made fromglass(118) probably due to the higher stiffness of carbon.

To improve the bond capacity, NSM bars have either aprepared surface (by grit-blasting, abrasion or peel ply) or adeformed surface, with ribs similar to deformed steel bars orspiral surface deformations, as a result of the method ofmanufacture. It has been observed that deformed bars performbetter in terms of bond than grit-blasted bars(118).

The size and shape of the groove into which the bar is placedaffect the mode of failure of the anchorage of an NSM bar. Itshould be noted that other factors such as strength andthickness of the adhesive surrounding the bar interact withthe size (depth and width) to determine the mode of failure.

Although cement-based adhesive materials can be used tofill the grooves and surround the bar, experimentation hasshown that this gives lower average bond strengths thanepoxy adhesives, which have higher tensile strengths.Expansive cement-based mortars should be avoided sincethe expansion can introduce cracks and weaken the bond(119).Furthermore, most of the existing experimental results onNSM techniques have been obtained using epoxy adhesives.

Due to the experimental results noted above, and reflectingthe bars currently available in the UK market, guidance ispresented here for a subset of the possible NSM schemes.

6.4.4 Modes of failure

Both pull-out tests and beam tests have indicated that theultimate load carried by NSM bars increases with increasingbond length. As for surface-mounted strips, there is a limit tothis length beyond which any further length increase doesnot enhance the bond strength(117). However, the strengtharising from this limit is a much higher proportion of theFRP capacity than is the case for surface-mounted strips, andis correspondingly less likely to be a limiting design factor.

There are two main modes of failure associated with NSMbars. Failure normally involves debonding, which occurs atthe adhesive–bar interface or concrete–adhesive interface,although in some cases tensile failure of the FRP bars mayoccur before bond failure. The two main failure modes are:

• Adhesive splitting failure: splitting of the adhesive coversurrounding the bar as a result of high tensile stresses thatare initiated at the FRP bar–epoxy adhesive interface

• Concrete splitting failure: splitting of the concretesurrounding the adhesive as a result of concrete at theinterface reaching its tensile strength. This mode normallyoccurs when the adhesive strength is much higher thanthe surrounding concrete.

Design guidance for strengthening concrete structures using fibre composite materials

38

These modes of failure are generally accompanied by pull-out of the NSM bar along the interface where the failure isinitiated. Mixed mode failure involving a combination of thetwo has also been reported(117–120).

Adhesive splitting is predominantly governed by the thick-ness of the epoxy surrounding the NSM bar while concretesplitting is governed by the groove size. By using adhesivesof high tensile strength, adhesive splitting failures, whichform with longitudinal cracking through the adhesive cover,can be minimised(121). It is then possible to increase thegroove dimensions and minimise the induced tensile stressesat the concrete–adhesive interface thus preventing concretesplitting failure.

6.4.5 NSM separation failure design

Shear-crack-induced separation, surface irregularity-inducedseparation and maximum FRP strain should be checked asfor surface-mounted strips (Section 6.3.2). In checking irregu-larity, the profile of the as-installed bars should be con-sidered, since NSM bars may be appropriate for an undulatingsurface if this is corrected by the groove-cutting process.

Outside the anchorage length, the longitudinal shear stressbetween the adhesive and concrete should also be checked asfor surface-mounted strips, except that in applying Equation12 an equivalent value should be substituted for ba,

representing the useful perimeter of the groove. For arectangular groove this would normally be the minimumwidth plus the minimum depth, i.e. only half of each side ofthe groove is counted since the groove sides cannot normallybe prepared to as high a standard as an exposed face. Ifspecial methods and particular care are used on the sides ofthe groove, it may be appropriate to increase the usefulperimeter to the width plus twice the depth, i.e. the grossperimeter of the groove. If there is a layer of weak laitancenear the surface of the concrete the value for the depthshould be reduced appropriately.

6.4.6 Anchorage design

Two main approaches have been developed in order topredict the anchorage length required and the maximumstress that can be carried by NSM reinforcement. In the firstapproach data from bond slip tests are used to model thebond behaviour of NSM anchorage(118). These equations canpredict the anchorage length required and the resulting stressin the bar. In the second method an average bond stress isassumed and the radial pressure exerted on the surroundingconcrete is related to the ultimate cracking stress in theadhesive or concrete interface(117).

However, in view of the variability and relative novelty ofthe technique, a cautious approach is suggested, which maybe relaxed if a more rigorous design method based onspecialist advice is carried out. The guidelines are presentedhere for a subset of possible NSM schemes based upon

published test data for the performance of NSM baranchorage with a wide range of properties. The guidance islimited to the following:

• circular bars of up to 16mm diameter• epoxy adhesive used to bond bars into grooves• surface preparation of the bar (e.g. peel ply or a deformed

surface) such that manufacturer’s testing demonstratesthat the bar will not pull out from the adhesive used

• grooves such that the installed bar is straight• grooves having square cross-section, with both width and

depth of groove at least twice the bar diameter• existing structural metallic reinforcement not intersecting

the groove• where grooves are spaced on the structure, the clear

spacing between grooves should be at least the width ofthe groove and the last groove should be at least fourtimes the bar diameter from an edge of the structure

• grooves having a surface preparation that provides arough gripping surface for bonding (i.e. not simplydiamond-sawn).

For conditions outside these criteria, specialist advice shouldbe sought. Detailed analysis as described in the referencesmay be required to confirm anchorage, or it may beappropriate to undertake testing to confirm the assumedperformance.

To avoid adhesive splitting failure, bar force should belimited to no more than:

for plain FRP bars (includingspirally wound and sand coatedbars) (Equation 19)

for deformed FRP bars(Equation 20)

whereTnsm,ad = characteristic adhesive bond failure force

bbarperim = perimeter of FRP bar

lnsm = anchorage length provided for NSM bar

fat = design adhesive tensile strength.

To avoid concrete splitting failure, the maximum ultimateanchorage force, Tnsm,max, and corresponding maximum

anchorage length, lnsm,max, can be calculated from the follow-

ing expressions:

(Equation 21)

(Equation 22)

atnsmbarperimadnsm, 1.0 flbT =

atnsmbarperimadnsm, 3.0 flbT =

ctmnotchperimffdmaxnsm, 9.1 fbAET =

ctmnotchperim

ffdmaxnsm, 5.4

fb

AEl =

Strengthening members in flexure

39

whereTnsm,max = maximum NSM anchorage force (N)

lnsm,max = anchorage length required to generate Tnsm,max

Efd = design FRP modulus of elasticity (N/mm2)

Af = area of FRP (mm2)

bnotchperim = effective perimeter of notch (mm) (making

allowance for surface preparation and/or weaklaitance layer)

fctm = concrete tensile strength (N/mm2)

In situations where the maximum anchorage length is notpossible or necessary, the anchorage force generated by ashorter length can be assessed from:

(Equation 23)

whereTnsm = characteristic anchorage force for NSM

lnsm = anchorage length provided for NSM.

While the use of NSM strips, rather than rods, appears to bean efficient strengthening technique, there is at present insuf-ficient information available for specific design guidelinesfor such situations to be included and specialist adviceshould be sought.

6.5 FLEXURAL STRENGTHENINGPLATE LOCATION

Flexural strengthening is usually achieved by bonding plates(or fabric) to the tensile face of the member beingstrengthened (i.e. the soffit of a sagging simply supportedbeam). This achieves maximum efficiency in use of material,since the FRP is subjected to the maximum possible tensilestrain. However, in certain situations it may be appropriateto bond plates to other parts of the section.

If this is undertaken, there is reduced utilisation of the FRPas it is less highly strained since it is located nearer to theneutral axis of the section. Other issues must also beconsidered. Most significantly, if it is proposed to bond theFRP to the sides of the beam, the strip will now be subjectedto bending about its strong axis, in which it may havesignificant flexural stiffness. Although this may make asmall additional contribution to the flexural stiffness of thesection as a whole, the most significant effect is thatdebonding of the plate may be precipitated, since in additionto tensile peeling and longitudinal shear stress, the adhesiveinterface is now also subject to transverse shear stresses.There has been no significant research with plates in thisorientation. If this arrangement is considered, the adequacyof the design should be confirmed by testing, unless thebeam is very deep and therefore the differential strainbetween the edges of the plate relatively small

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

maxnsm,

nsm

maxnsm,

nsmmaxnsm,nsm 2

l

l

l

lTT

6.6 THICK AND MULTI-LAYERLAMINATES

The FRP strengthening systems available in the UK are gene-rally pultruded plates of less than 2mm thick (1.2mm and1.4mm are the most common) and fabrics with an effectivethickness of between 0.1mm and 0.3mm (see Appendix B).

Some manufacturers can supply thicker plates, either manu-factured in a single process or by bonding together pre-viously manufactured pultruded plates to produce a thickerlaminate. In addition, it is possible to bond plates in stackson site. Fabrics are laminated on to the structure by applyingsuccessive layers of resin and fabric until the requiredthickness (and hence strength) is obtained.

However, there are limitations to the thickness of FRP thatcan be usefully employed. Frequently, the prevention of adebonding failure of the FRP from the concrete will limit thethickness, since stacking two laminates will almost double thelongitudinal shear stresses in the FRP–concrete adhesive bond.

Additional layers increase the number of potential failuremodes, since failure can occur in the adhesive betweenlayers and exacerbates potential failure within the FRP. If thestacked layers are not of the same length (which is normal,in order to reduce stress concentrations in the curtailmentzone), there are also additional anchorage zones that must bechecked for debonding.

It is preferable not to stack pultruded plates. In some situa-tions (e.g. T-beams with narrow webs that have insufficientwidth of soffit to accommodate the required areas of FRP) itmay be the only way that an otherwise suitable scheme canbe detailed. In these situations stacking of plates in situ, oruse of thicker laminated plates may be appropriate if thefollowing conditions are met:

• The plates are intended by the manufacturer for such use,and have sufficient inter-laminar strength that they willnot suffer internal shear failures.

• The plates that are bonded to both sides have suitablepreparation to both faces; some plates incorporating peelply are manufactured with this on only one face, andwould not be suitable for stacking without further prepa-ration work.

• Peeling failure is checked at the curtailment of everyplate, and allowance is made in the design calculationsfor the non-prismatic section.

• The construction sequence is carefully specified to ensurethat bonding subsequent plates does not disturb or damagethe bond of the lower plates.

For stacks of plates formed in situ, the stack should notnormally be more than two high. There is some limited UKexperience with stacks three high, but in this case specialist

Design guidance for strengthening concrete structures using fibre composite materials

40

advice should be sought. For stacks formed by the manu-facturer under factory conditions, higher stacks may bepossible. Testing may be necessary to demonstrate the perfor-mance of the stack, but it is unlikely that greater than 5mmthickness will be useful, due to the limits of the bondstrength to the concrete.

When fabrics are used, multiple plies can be overlaid toachieve the necessary strength of the FRP component. Thesame overall limitations apply, but these limits will normallybe reached when many plies are installed. If large numbersof plies are overlaid, it is likely to be the achievable qualityof workmanship that limits the design, and a trial installationshould be considered in order to demonstrate that a void-freelaminate can be produced under the site conditions relevantto the particular project.

6.7 REDISTRIBUTION

Moment redistribution in any continuous concrete structurerelies on adequate rotation capacity of critical sections(122). Ifa structure displays ductility, it will also display rotationcapacity. However, if a structure displays rotation capacity, itwill not automatically display ductility(123–125). This is impor-tant in the context of FRP-strengthened concrete structuresbecause research has shown that ductility is usually limitedin such structures(126), even though adequate levels of rota-tion capacity often exist(127,128).

Therefore, moment redistribution into FRP-strengthenedzones is permitted up to a maximum of 30% if it can bedemonstrated that there exists sufficient rotation capacitywithin the strengthened zone and within the surroundingstructure to allow such redistribution to be take place (seeDenton(129)).

However, moment redistribution out of FRP-strengthenedzones relies on a substantially higher level of rotation capa-city being available in the strengthened zone. Therefore,moment redistribution out of FRP-strengthened zones is notrecommended.

6.8 SERVICEABILITY

6.8.1 Crack widths

The causes of any existing cracking in the concrete substrateshould be ascertained, and resolved if possible, prior toinstallation of the FRP strengthening system. Any suchcracks should be repaired prior to installation of the FRPstrengthening system, for example by resin injection; allmaterials used should be compatible with the FRPstrengthening system.

Normally, crack widths will not be excessive providing theFRP strengthening system has been properly installed.Where necessary crack widths at service loads should bechecked against the limits recommended in BS 8110 or BS5400: Part 4, as appropriate. Guidance on calculating crack

widths in steel-reinforced concrete structures is given inSection Three of BS 8110: Part 2, and Section Five of BS5400: Part 4. These methods can be adapted for FRP-strengthened structures simply by calculating the strain inthe tension steel, using a suitable concrete compressionstress–strain curve, and crack width under permanent loadand transient load separately. The FRP strengthening canbetaken into account by using the transformed area of theFRP laminate in calculating the strain in the tension steelunder transient loading (unless the construction sequence issuch that the FRP laminate will be subject to permanentloads, for example where surface finishes are applied overthe FRP laminate). The second moment of area of the sectionshould be determined assuming that the long-term modularratios of steel to concrete, αe, and FRP to concrete, αf, aregiven by:

αe = Es /(½ Ec) or αe = Es /((1/1+φ) Ec) (Equation 24)

αf = Efd /(½ Ec) or αf = Efd /((1/1+φ) Ec) (Equation 25)

whereφ = creep coefficient.

It is worth noting that calculating crack widths is not asstraightforward as suggested here. This is because, when theFRP strengthening system is placed on the surface, the crackspacing is defined by the load on the unstrengthenedstructure. The crack width due to live load would be signi-ficantly reduced due to the presence of the FRP strength-ening. However, only a limited amount of experimental andtheoretical work defining the extent of this reduction incrack width has been carried out. In the interim, therefore,the procedure outlined above is recommended, which willprovide a conservative estimate.

Where a wet lay-up FRP strengthening system is used tofully cover the concrete surface the durability of the concretestructure may be significantly improved in the region of theFRP strengthening. However, additional appropriate construc-tion details, for example, to prevent the build-up of moisturein the substrate adjacent to the wet lay-up FRP strength-ening, should also be considered.

6.8.2 Deflections and material stresses

For buildings, deflections due to the projected increased loadshould not exceed the limits recommended in BS 8110. Toavoid excessive deformations in bridges, the stresses in thesteel reinforcement and concrete at working loads should notnormally exceed 0.8fy and 0.6fcu (or 0.6 times the worst

credible strength), respectively. See also Sections 5.6.5 and5.6.6. Such deflections and material stresses can beestimated using elastic principles. The presence of the FRPstrengthening will only reduce the live-load deflections(assuming that the FRP is not prestressed and jacking is notused). The material stress limits in Sections 6.8.3 and 6.8.4are applicable. The equivalent transformed section for long-term loading should be obtained by assuming that the

Strengthening members in flexure

41

modular ratios of steel to concrete, αe, and FRP to concrete,αf, are given by Equations 24 and 25, respectively, and forshort-term loading are based on the short-term elastic moduliof the concrete and steel.

6.8.3 Fatigue

For bridges, the designer should consider the effect ofrepeated live loading on the fatigue strength of the steelreinforcement and the FRP. Checks for fatigue failure shouldbe carried out in accordance with the recommendations inClause 4.7 of BS 5400: Part 4. The stress range in the FRPshould be limited to the appropriate value given in Table 8.The failure mode for fatigue is typically yield of the steelfollowed by spalling at the concrete soffit and debonding ofthe FRP.

Highways Agency Advice Note BA 30(9) controls the fatiguebehaviour of steel plate bonding applications by limiting thecyclic stresses that may be applied to the steel plate.

Table 8: Maximum stress ranges as a proportion of the design

ultimate strength (%).

Material Stress range (%)

Carbon FRP 80

Aramid FRP 70

Glass FRP 30

Lap splices of FRP plates

Wherever possible, lap splices of FRP plates should beavoided in regions where the fatigue stress range is high. Inaddition, lap splices in adjacent plates should be staggeredand alternated to avoid the concentration of lap splices in asingle region.

Preliminary research(130) has shown that the fatigue life of alap splice is dependent on the length of the lap, among otherfactors. Therefore, the minimum length of any lap spliceshould be 300mm or the manufacturer’s minimum recom-mended lap splice length, whichever is the greater.

Delaminations/voids

Concrete Society Technical Report 57(5) provides guidanceon the acceptable limits of delamination and voids in anadhesive bond.

6.8.4 Stress rupture

Rupture of the FRP may occur at service loads due to sus-tained stresses in the material. Applications where sustainedstresses may be present in the FRP include:

• temporary dead load removal by jacking, followed byFRP strengthening

• physical removal of dead load, followed by FRPstrengthening and then reinstatement of dead load.

It is recommended that the maximum stress in the FRP atservice loads, as a proportion of the design strength, shouldnot exceed the values given in Table 9.

Table 9: Maximum stress under service loads to avoid stress rup-

ture as a proportion of design strength (%).

Material Maximum stress (%)

Carbon FRP 65

Aramid FRP 40

Glass FRP 45

6.8.5 Strengthening under non-static live load

One of the great benefits of using FRP for retrofit strength-ening is the ease, speed and short period of time required forthe works. In order to maximise these benefits during thestrengthening of concrete bridges, it is clearly desirable thattraffic be allowed to flow over the bridge during the works.In the USA, strengthening under such live load is permitted,as long as vehicle speeds are controlled(131).

The obvious caveat to strengthening under live load is theeffect that intermittent loading has on the curing process inthe adhesive. Hejll et al.(132) have carried out laboratory testson concrete bridge girders strengthened under static condi-tions and under simulated live-load conditions, in which aload causing 60% of the yield strain in the steel reinforce-ment was applied cyclically every 108 seconds. Their testsshowed that there was effectively no difference in ultimatecapacity and behaviour between bonded systems (eitherlaminate or NSM) that had been installed under static or undersimulated live-load conditions. What is more, the level ofstrengthening was 80–150% of the original capacity of theunstrengthened bridge girders, adding confidence to theirclaim that traffic need not be stopped during FRP strength-ening works on bridges.

This tends to confirm work by Barnes and Mays(133) whoconducted tests on strengthened beams with a 1Hz cyclicload during curing of between 20 and 170 microstrain. Theirresearch showed that for strengthening concrete beams underlive load there was no effect on the ultimate strength. This wasbecause failure in their test specimens was due to the con-crete cover pulling off, rather than failure of the adhesive.However, further tests designed to investigate failure in theadhesive itself showed a significant reduction in strength ifthe adhesive interface governed.

It is therefore recommended that the strength of the adhesivebe reduced in accordance with Table 10. If this reducedadhesive strength is less than the design strength of theconcrete, then the live load during adhesive curing should berestricted. This should be taken into account when calcu-lating achievable anchorage force in NSM-strengthenedstructures.

Design guidance for strengthening concrete structures using fibre composite materials

42

Table 10: Reduction in strength of adhesive for given live-load

strains at FRP–concrete interface (%).

Live-load strains at

FRP–concrete interface

during curing (10–6)

Reduction in strength of

adhesive (%)

20 10

50 12

100 16

150 22

200 32

6.9 STRENGTHENING PRESTRESSEDSTRUCTURES

Many of the same principles outlined in the previousSections of this Chapter can be applied to the strengtheningof prestressed concrete structures. However, note that service-

ability issues govern prestressed design whereas these guide-lines focus predominantly upon ultimate limit state. There are,therefore, a number of factors to consider when strength-ening a prestressed structure such as:

• the need to accurately assess current and future stressstates

• the sensitivity of the strengthened section to the initialstress state compared with that of an uncracked rein-forced concrete section

• the lower ductility of prestressing tendons compared withreinforcing steel

• different anchorage behaviour due to reduced crackingpreventing the full generation of anchorage force.

A fuller discussion of strengthening prestressed concretestructures is given in the FIB Technical Report(90).

Strengthening members in flexure

43

Design guidance for strengthening concrete structures using fibre composite materials

44

Strengthening members in flexure

45

Design guidance for strengthening concrete structures using fibre composite materials

46

7 SHEAR STRENGTHENING

7.1 INTRODUCTION

Externally bonded FRP laminates and fabrics can be used toincrease the shear strength of reinforced concrete beams andcolumns. FRP may be bonded to the concrete in variousconfigurations. Ideally FRP should be wrapped around thewhole perimeter of the member (fully wrapped). Alter-natively, it can be applied only to the sides of the member(side-only) or to the sides and the tension face of the member(U-wrapped). Figure 24 shows examples of possible FRPshear strengthening configurations. This Chapter focuses onrectangular beams and columns. Guidance on strengtheningof circular columns in shear is included in Section 8.3.4.

Side Only U-Wrapped Fully Wrapped(n=2) (n=1) (n=0)

Side Only U-Wrapped Fully Wrapped(n=2) (n=1) (n=0)

Figure 24: Shear reinforcement configurations.

The orientation of the FRP fibres can affect the performanceof the strengthening system. Theoretically fibres that areinclined to resist the formation of shear cracks can be moreeffective than fibres aligned perpendicular to the longi-tudinal axis of the member. However, if the shear forcedirection can reverse, or if the FRP is partially or fullywrapped around the beam, systems with fibres alignedperpendicular to the longitudinal axis of the member aremore convenient and are typically used in practice.

In understanding the behaviour of FRP strengthening inshear, it is important to recognise that the bond behaviour ofFRP differs markedly from conventional embedded steelreinforcement. As discussed in Section 6.3, it has been foundin tests on the anchorage of externally bonded FRP thatbeyond a limiting bonded length, no further increase in theultimate anchorage load-capacity occurs with increasingbonded length. This maximum anchorage capacity can bevery much less than the ultimate tensile capacity of the FRP.The contribution that the FRP makes to the shear capacity

can therefore be governed by separation of the FRP from theconcrete, and it is not sufficient to assume that fracture of theFRP will occur. Such separation is typically associated withthe propagation of a failure plane in the concrete close to thesurface.

Side-only or U-wrapped members will be more prone toseparation failures than fully wrapped members. Full wrap-ping is therefore preferable and should always be used whenit is feasible. However, it is generally not practicable forbeams, because the top of the beam is inaccessible. In mostcases it will be possible to fully wrap columns.

The behaviour of reinforced concrete in shear is complex.Furthermore, considerably less research has been undertakeninto FRP shear strengthening than flexural strengthening. Itis therefore appropriate at present to adopt a cautious designprocedure for shear strengthening. The design procedure givenis based upon that proposed by Denton et al.(112). In deve-loping their proposals, they reviewed numerous alternativedesign approaches and provide a detailed justification fortheir proposed method.

The majority of experimental testing of reinforced concretemembers strengthened in shear has used carbon rather thanaramid or glass fibre. Although the underlying principlesshould be common to all materials, the design procedurepresented below is best suited to designs using carbon FRP.The approach should be conservative if applied to aramid orglass FRP, and in some cases may be significantly so.

As with flexural strengthening, the assumptions made in thedesign should be reflected in the installation work on site. Itis therefore important to consider the issues outlined inSection 6.1 in developing the design for shear.

7.2 DESIGN PROCEDURE

7.2.1 Maximum shear capacity

Irrespective of the amount of conventional or FRP shearreinforcement provided, the shear strength of a section canbe limited by a diagonal compression shear failure in theconcrete. Such a failure can be avoided by limiting the maxi-mum shear stress in the concrete. According to Clause 3.4.5.2of BS 8110, the maximum permissible shear stress, vmax,

should be taken as the lesser of 0.8√fcu or 5N/mm2, whatevershear reinforcement is provided. Similar limits are includedin BS 5400 Part 4, where vmax is the lesser of 0.75√fcu or

47

Design guidance for strengthening concrete structures using fibre composite materials

48

4.75N/mm2. It is recommended therefore, that such a criterionbe used in the design of FRP shear-strengthened members.The maximum allowable design shear force due to ultimateloads, VR,max, at any cross-section, is thus obtained from:

VR,max = vmax b d (Equation 26)

whereb = width of sectiond = effective depth of sectionvmax = maximum concrete shear stress

(determined from the appropriate Standard)

Tests on FRP-strengthened beam and slab structures haveindicated that the shear capacity of such structures can belimited by the longitudinal shear capacity at the interfacebetween the beam and slab. This interface should thereforebe checked to ensure its adequacy, using conventional designor assessment Standards.

7.2.2 FRP shear strengthening design

The ultimate shear capacity of an FRP strengthened beamcan be expressed as:

Vu = Vc + Vs + Vf (Equation 27)

whereVc = contribution from the concrete to the shear capacity

(N)Vf = contribution from the FRP to the shear capacity (N)

Vs = contribution from the steel to the shear capacity (N)

Vu = ultimate shear capacity of FRP strengthened section

(N).

Vc and Vs can be determined from conventional design stan-

dards, such as Clauses 3.4.5.3 and 3.4.5.4 of BS 8110: Part 1:1997. For structures designed to the 1985 edition of BS8110, it would be more appropriate to assume a steel stressof 0.87fy, rather than 0.95fy, as in the 1997 edition. Alterna-

tively, equivalent criteria are included in BS 5400 Part 4.

Assuming a 45° shear crack, it follows from equilibrium thatthe contribution of the FRP to the shear capacity is given by:

(Equation 28)

wheren = 0 for a fully wrapped beam, 1.0 when FRP is bonded

continuously to the sides and bottom of a beam and2.0 when it is bonded to only the sides of a beam

β = angle between the principal fibres of the FRP and aline perpendicular to the longitudinal axis of themember. β is positive when the principle fibres ofthe FRP are rotated away from the direction inwhich a shear crack will form

( )ββε sincos3

f

max,tf

fsfsefdf +⎟⎠⎞⎜

⎝⎛ −

=s

ln

d

AEV

εfse = effective strain in the FRP for shear strengthening

Afs = area of FRP (mm2) for shear strengthening mea-

sured perpendicular to the direction of the fibres.When FRP laminates are applied symmetrically onboth sides of a beam, Afs is the sum of the areas of

both laminates, i.e. Afs = 2 bf tfbf = width of the FRP laminate (mm) measured perpen-

dicular to the direction of the fibres. For continuousFRP sheet, sf is taken as 1.0 and bf is taken as cos β

df = effective depth of the FRP strengthening, measured

from the top of the FRP to the tension reinforcement(mm)

Efd = design tensile modulus of the FRP laminate

(N/mm2) (see Section 5.6.3)lt,max = anchorage length required to develop full anchorage

capacity (see Section 6.3) (mm)sf = longitudinal spacing of the FRP laminates used for

shear strengthening (mm). For continuous FRPsheet, sf is taken as 1.0 and bf is taken as cos β

tf = thickness of the FRP laminate (mm).

The notation is illustrated in Figure 25.

α

sf

bfh

FRP Laminates on both sides

d

Afs = 2bf tf

α

sf

bfh

FRP Laminates on both sides

d

Afs = 2bf tf

α

sf

bfh

FRP Laminates on both sides

d

Afs = 2bf tf

Figure 25: General notation for shear strengthening.

The effective strain in the FRP, εfse, accounts for the varia-tion in strain in the FRP along the shear crack when the ulti-mate limit state is reached. It should be taken as the minimumof:

(i) εfd / 2

(ii)

(iii) 0.004

wherefctm = tensile strength of the concrete (N/mm2)

εfd = design ultimate strain capacity of FRP (see Section5.6.4)

The first strain limit of half the ultimate strain capacityrepresents the average FRP strain when fracture of the FRPoccurs. Alternative limits have been suggested for this condi-tion. Chen and Teng(134) propose half the ultimate strain capa-city, while Täljsten(92) proposes 0.6 times the ultimate strain

ffd

ctm64.0tE

f

Shear strengthening

49

capacity. It appears that Täljsten’s limit applies when thebehaviour of the member is predominantly elastic and thatChen and Teng’s limit is more suitable when the behaviourof a member is characterised by rigid body movements of theregions of the member either side of a shear crack. The lowerof the two values has been adopted.

The second strain limit corresponds to debonding of the FRP,and is based on Neubauer and Rostasy’s anchorage model(111),as described in Section 6.3. In other design approaches, FRPseparation has been considered primarily for FRP bondedeither to the sides of beams or to the sides and the tensionface of beams. Here it is recognised that this condition shouldalso be applied to fully wrapped beams to ensure that theintegrity of the concrete is maintained and Vc does not dimi-

nish before Vf reaches its design value, as explained by

Denton et al.(112). For small beams such an approach may beconservative, but importantly it should be safe for the casesmost frequently encountered in practice.

The final 0.004 strain limit was proposed in early designmethods to ensure that the concrete integrity is maintained.This convenient rule of thumb appears to have limitedrational justification, and as is shown by Denton et al.(112),does not necessarily prevent the development of wide cracks.It is retained because it seems sensibly cautious to do so.

In Equation 28, the effective depth is reduced by a lengthequal to (n/3)lt,max. This adjustment accounts for the reduc-

tion in force that can be sustained by the FRP in the anchor-age regions. The Neubauer and Rostasy anchorage model, asdescribed in Section 6.3, assumes a parabolic variation ofstress with distance (see Figure 26). The force correspondingto the area under the stress curve in the anchorage region istherefore only 2/3 of the maximum stress multiplied by theanchorage length. This reduction in the FRP contribution canbe modelled by subtracting (n/3)lt,max from the effective

depth, as in Equation 28. The adjustment is made at the topfor U-wrapped beams (n=1) and at the top and bottom forbeams with FRP bonded only to the sides (n=2). No adjust-ment is necessary for fully wrapped beams (n=0).

0

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600

Bonded Length (mm)

t f = 0.5mm

t f = 1.5mm

t f = 1mmParabolic Curve

E fd = 230GPa

ε fd = 0.015

f ctm = 3MPa

Figure 26: Typical variation in ultimate strain capacity withbonded length (after Neubauer and Rostasy(111)).

Figure 27 shows a comparison of experimental measuredvalues of Vf and those determined using the design method

described above (after Denton et al.(112)), with all partialsafety factors taken as unity. There appears to be reasonableagreement. Furthermore, the substantial majority of cases lieon the safe side, with the experimentally measured valuesexceeding those determined using the proposed designmethod.

020406080

100120140160180200

0 100 200 300 400 500 600

V f (Experimental)

Vf (

Des

ign

Met

hod)

Side and U-Wrapped

Fully Wrapped

Design = Exp

Figure 27: Experimental verification of design method (afterDenton et al.(112)).

7.3 SPACING OF FRP STRIPS

As in the case of steel shear reinforcement, the centre-to-centre spacing of strips of FRP should not be so wide as toallow the full formation of a diagonal crack without inter-cepting a strip. In addition, Equation 28 is based on theapproximation that the FRP contribution to the shearresistance is distributed across the whole crack, rather thanin discrete locations, which becomes invalid at large stripspacings. For these reasons, if strips are used, their centre-to-centre should not exceed the least of:

(i) 0.8df

(ii) df – (n/3)lt,max

(iii) bf + df /4

where the variables are as defined after Equation 28.

Alternatively, the contribution of FRP strips to shear capa-city may be evaluated with a rigorous analysis, accountingfor the critical location for a shear crack and the effect ofanchorage of the FRP strips. If this approach is used, thelimits on strip spacing in (i) to (iii) need not apply.

7.4 ADDITIONAL AXIAL FRP

Using the truss analogy it can be shown that beam andcolumn elements subjected to a shear force will experienceaxial tensile forces, additional to those due to bending.Additional axial reinforcement may therefore be requiredwhen strengthening for shear.

Design guidance for strengthening concrete structures using fibre composite materials

50

The standard method is to simply extend the axial FRPreinforcement a distance of half the effective depth beyondthe point at which it is no longer required for bending.

If there is insufficient FRP to sustain the additional axialtensile forces then extra axial FRP, Afa, should be provided.

The amount of additional FRP required can be determinedfrom:

Afa = Vs / 2ff (Equation 29)

whereVs = shear force due to ultimate loads

ff = stress in the FRP at the same location determined

from a flexural analysis.

Clearly these approaches are not relevant when no axial FRPis present for bending. In this case, the ultimate bendingcapacity of the member should be re-evaluated assuming thearea of each axial reinforcing bar between the tension faceand the mid-depth of the section is reduced by an amountequal to:

(Equation 30)

wherene = total number of effective axial reinforcing bars

between the tension face and the mid-depth of thesection.

Any shortfall in bending capacity should be compensated forby providing axial FRP reinforcement.

msye

s

2 γfn

V

Shear strengthening

51

Design guidance for strengthening concrete structures using fibre composite materials

52

53

8 STRENGTHENING AXIALLYLOADED MEMBERS

8.1 INTRODUCTION

Concrete columns in existing structures such as bridges andbuildings may require upgrading to enhance the followingproperties:

• axial load capacity• flexural capacity• shear capacity• ductility.

Increased axial load capacity, for example, may be neededfor compression required to carry higher loads than origi-nally envisaged or where the loading requirements havechanged. Enhanced flexural strength may be required forbridge supports that:

• are not capable of fully sustaining design loads fromheavy vehicle impact

• have insufficient lap lengths• have incorrect termination of longitudinal reinforcement.

Some columns designed to older codes may be incapable ofwithstanding the large horizontal displacement that occursbetween member ends during an earthquake. They may there-fore require ductility enhancement in order to hold the coverconcrete in place and prevent buckling of longitudinal rein-forcement under axial load. Shear strength must also beconsidered in any proposed column upgrading.

Where a deficiency exists, upgrading can be achieved bybonding layers of axial and/or hoop FRP to the columnperimeter.

Generally, bonding axial FRP to the column surface enhancesthe flexural strength of the member. Hoop wrapping will alsobe necessary and should be placed over the axial FRP. Thehoop wrapping increases the concrete compressive straincapacity, which can significantly improve the efficiency ofthe strengthening design. It also prevents buckling of theaxial fibres, potentially enabling them to contribute incompression. However, this contribution will be small and istherefore generally neglected.

Bonding hoop FRP to the column surface enhances axialload capacity and ductility of columns. The hoop FRP resistslateral deformations due to the axial loading, resulting in aconfining stress to the concrete core, delaying rupture of theconcrete and thereby enhancing both the ultimate compres-sive strength and the ultimate compressive strain of the con-

crete. As noted in Section 4.2.2, this process is significantlymore efficient with circular than with square or rectangularcolumns. This is because, with the latter, the confining actionis mostly concentrated at the corners. Measures for, and limi-tations of, strengthening columns of non-circular cross-sectionare discussed in Section 8.6. It should be noted that for thecase of concentrically loaded columns, the bond is notcritical since the composite does not need to transfer forcesalong the FRP–concrete interface. However, bond becomesimportant when moment is also applied to the column andanchorage of the localised confining hoop FRP is required.

Column strengthening is normally carried out using fabric,which may be applied dry or be preimpregnated with anepoxy resin. The use of preformed shells made from a rangeof fibre types, including glass and carbon, is another option.However, at present, most of the studies that have beencarried out using FRP shells have concentrated on theirpotential for new construction, where the FRP acts both aspermanent formwork to the wet concrete and as externalreinforcement, rather than in repair or strengthening work.

The following sections deal with the design of circularcolumns for enhanced compressive strength, flexural strength,shear strength and ductility. A feature common to the designprocedures discussed is the stress–strain model for FRP-confined concrete, which is examined next.

It should be noted that the basic principles of strengtheningcolumns given in this Chapter are applicable also tostrengthening with shells. However, this is a more complexdesign process that includes aspects such as the performanceof the grout annulus and is beyond the scope of this Report.

8.2 STRESS–STRAIN MODEL FOR FRP-CONFINED CONCRETE

Concrete in circular columns confined by hoop FRP displaysan approximately bilinear stress–strain response, as shown inFigure 28. Initially, the behaviour is similar to that of plainconcrete since the FRP exerts a limited confining pressure onthe concrete. As the axial stress increases, however, the rateof lateral deformation of the concrete also increases, whichresults in a concomitant reduction in stiffness of the con-crete. Once the concrete reaches the ultimate strain limit ofunconfined concrete, typically 0.0035, the material becomeshighly fissured and the confinement provided by the FRP isfully activated. At this stage, the stress–strain responsebecomes approximately linear with a slope dependent uponthe stiffness of the hoop FRP.

Design guidance for strengthening concrete structures using fibre composite materials

54

Figure 28: Idealised stress–strain curve for FRP-confinedconcrete.

Several authors have proposed models that attempt to predictthe compressive stress–strain behaviour of confined concrete.A large body of this work actually relates to steel rather thanFRP-confined concrete, with most of the formulations beingfounded on the pioneering work of Richart et al.(135) ontriaxially confined concrete. Based on tests on cylindricalspecimens subjected to constant hydrostatic pressure, theseauthors showed that both axial strength and ductility of theconcrete increases with increasing confinement pressure.Further analysis of their data revealed that the compressivestrength increase could be predicted using the expression:

fccd = fc0 + 4.1 fr (Equation 31)

wherefccd = confined concrete compressive strength

fc0 = unconfined concrete compressive strength

= 0.67 fcu / γmc

fr = confinement pressure

Later work on the behaviour of steel-confined concretetended to confirm the validity of Equation 31. This is some-what surprising considering the differences between the twomodes of confinement actually used. Richart’s test speci-mens were subjected to active confinement due to the hydro-static pressure, which remained constant throughout the test.However, lateral steel reinforcement provides passive con-finement since the confining stresses only develop as a resultof the lateral expansion of the concrete. In this case, theconfining stress is neither uniform nor constant.

Over recent years, certain disadvantages of steel confine-ment have emerged, such as corrosion, incompatibilitiesbetween the modulus of elasticity and Poisson’s ratio of steeland concrete, and aesthetics, which have led researchers toconsider FRP composites as an alternative confinementmedium. Strength models for FRP-confined columns, similarto Equation 31, have been proposed by a number ofresearchers. While these models may be adequate for pre-dicting ultimate strength, they grossly underestimate ultimatestrain. Consequently, there are large discrepancies betweenthe actual and predicted stress–strain responses.

A large number of researchers have developed theoreticalstress–strain models validated by experimental testing(136–139).While these models are reported to show quite good agree-ment with test results(140), they are in some instances rathercomplex and can be unconservative. There is also a need toaccurately predict ultimate stress and strain conditions. Arecent model developed by Lam and Teng(141) captures themain aspects of the behaviour of FRP-confined circularconcrete columns in a simple form. The model has beencalibrated against all the currently available test data.

The model is made up of two parts, an initially parabolicsection, similar to that of unconfined concrete, followed bya straight line section, the slope of which is dependent uponthe level of confinement. The initial slope of the parabolicsection is the same as that for unconfined concrete. Theparabola and straight line meet at the same slope and theprojection of the linear portion intercepts the stress axis atthe unconfined strength, fc0, as shown in Figure 29. The

model converges to a stress–strain model similar to that inBS 8110 for the unconfined case. The model is defined asfollows:

fcc = Ecεcc – (Ec – E2)2εcc

2/4fc0 for 0 < εcc < εt

(Equation 32)

and

fcc = fc0 + E2 εcc for εt < εcc < εccu

(Equation 33)

whereεt = position of transition region between parabola and

straight line= 2fc0 /(Ec – E2) (Equation 34)

E2 = slope of linear portion of confined stress–strain

curve= (fccd – fc0)/ εccu (Equation 35)

Ec = initial modulus of elasticity of concrete

= 5.5√(fcu/γmc) (kN/mm2)εcc = confined concrete axial strainfcc = confined concrete axial compressive stress

εccu = confined concrete ultimate axial strain, given byEquation 40

fccd = confined concrete ultimate strength, given by

Equation 39.

It is clearly evident that in order use this model, the ultimatedesign failure stress, fccd, and ultimate compressive failure

strain, εccu, of the concrete must be defined. Suitable modelsare given in Section 8.3.2.

It should be noted that the proposed model is only suitable ifthe strength of the confined concrete increases withincreasing strain. For low levels of confinement, followingthe initial parabolic increase, the stress may decrease with a

– –

– –

55

Strengthening axially loaded members

further increase in strain and, therefore, the ultimate strengthmay be lower than the peak strength. It is therefore recom-mended that this stress–strain model only be used when thefollowing condition, based upon the work of Xiao andWu(142), is met:

(mm2/N) (Equation 36)

wheretf = thickness of FRP (mm)

Efd = design modulus of elasticity of FRP (N/mm2)

D = diameter of column (mm)

If the condition is not met then no strength enhancementshould be assumed and the stress–strain model in BS 8110 orBS 5400 for unconfined concrete should be assumed.

183.02

2c0

fdf >Df

Et

Unconfined concrete

Confined concrete

1E2

fc0

fccd

fcc

εccεccuεt 0.0035

Ec

Figure 29: Stress–strain model.

It should also be noted that Equations 32 and 33 are bothcalibrated against test results obtained by subjectingcylindrical specimens to substantially concentric compres-sion. In most practical situations, columns are subject to bothaxial load and flexure, either due to load eccentricity orapplied moments. In particular, columns in bridge structurescan be loaded horizontally as a result of vehicle impact. Inthis case, the column must sustain a combination of axialload and bending moment. Methods for analysing sectionsunder combined axial and flexural loads are detailed inSection 8.4.

8.3 COMPRESSION

8.3.1 Introduction

As previously mentioned, providing hoop FRP around theperimeter of the column can increase the compressive strengthof circular columns. All members strengthened in compres-sion should meet the following conditions:

• Tensile rupture of the FRP should be considered.• Failure of the FRP jacket at lap joints should be

examined.• The shear capacity of the column should be checked.

• Compliance with relevant serviceability limit states, suchas axial shortening, lateral deformation, loss of strength-ening effectiveness, fatigue and creep rupture, should beinvestigated.

8.3.2 Tensile rupture of FRP

Generally it has been assumed that compression membersstrengthened by hoop wrapping will fail if the circum-ferential stress in the composite exceeds its design tensilestress capacity. On this basis, various equations have beenproposed for predicting the strength of FRP-confinedcolumns. A number of these equations are actually based onthe failure stress criterion for hydrostatic pressure proposedby Richart et al.(135), given in Equation 31, and take thefollowing form:

fccd = fc0 + k fr (Equation 37)

where the confining pressure, fr, is given by:

fr = 2ffd tf / D (Equation 38)

andk = confinement effectiveness factor.

Various values and expressions have been suggested byresearchers for the value of the effectiveness factor k

(typically less than the value of 4.1 given in Equation 31),based on experimental results(137,139).

While such equations are reported to give good correlationwith experimental results, they do not satisfy lateral straincompatibility requirements and are therefore deemed unsuit-able. According to Lillistone and Jolly(143) this problemwould be overcome if the failure criterion were based on theconfining stiffness rather than the confinement pressure.Such an approach would offer the following advantages:

The failure criterion does not require prior knowledge of thelateral expansion of the concrete core. Unlike tensile strength,the elastic modulus of fibres is not reduced by mechanicalabrasion during manufacturing processes.

On this basis, they recommended that the design strength ofconcrete-filled filament-wound glass FRP circular tubes, fccd,

should be estimated using:

fccd = fc0 + 0.05(2tf/D) Efd (Equation 39)

Comparative studies by Lillistone(140) show good agreementwith the results presented by Samaan et al.(137) for concrete-filled E-glass-fibre filament-wound tubes, Howie andKarbhari(136) and Picher et al.(138) for carbon-fibre-wrappedconcrete cylinders, and Saafi et al.(139) for concrete-filled E-glass and carbon-fibre filament-wound tubes.

Design guidance for strengthening concrete structures using fibre composite materials

56

In view of the above, it is recommended that Equation 39 beused to calculate the axial failure stress of FRP-confinedconcrete. It is worth noting that this equation is also based onthe characteristic unconfined cube strength of concrete andthat a partial safety factor for concrete of 1.5 is assumed.

The corresponding strain at rupture is required for deve-loping the stress–strain curve in Section 8.2. Studies haveshown that the behaviour of confined concrete depends notonly on the confining pressure but also the stiffness of theconfining FRP(137). As a result, ultimate strains are differentfor different confining FRP materials, even when the con-fining pressure is the same. Lam and Teng(141) have shownthat correlation with experimental results is poor if stiffnessof the confining FRP is neglected in the model of ultimatestrain. Based on experimental results, Lam and Teng havedeveloped an empirical equation for ultimate concrete strainthat takes into account the stiffness of the FRP as follows:

(Equation 40)

whereE0 = secant modulus of concrete = 0.67fcu/(γmc εc0)εc0 = axial strain in unconfined concrete at peak stress =

2.4×10–4√(fcu/γmc)εfd = design ultimate strain of FRP.

It should be noted, however, that at concrete compressivestrains of over approximately 0.01, the concrete will havebeen crushed and have lost all cohesion. It is thereforerecommended that if the ultimate strain, εccu, is greater than0.01, then the failure stress should be taken as the value forfccd corresponding to the value of εcc=0.01 from thestress–strain curve, rather than the failure stress at rupture ofthe FRP.

8.3.3 Lap joint failure

Failure of the FRP jacket can occur at lap joints due todebonding, if the lap length is inadequate. This type offailure is brittle and can be avoided simply by providing anadequate lap length. The actual length of overlap required islikely to vary between strengthening systems and so it isrecommended that individual manufacturers are consulted.Where necessary, independent testing should be carried out.

When two or more plies of FRP are applied to a column, thelap joints should be arranged so they are staggered evenlyaround the column perimeter, as shown in Figure 30. Theminimum overlap for fabric materials, in the direction of thefibres, should be in accordance with the manufacturer’srecommendations, but not less than 200mm.

45.1

c0

fd

0

ffd

c0

ccu 6.021275.1 ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+=

εε

εε

DE

tE

column Lap to be determined

column

Figure 30: Laps in columns.

8.3.4 Shear

The presence of hoop FRP can increase the shear strength ofconcrete columns. Guidance on shear-strengthening ofsquare or rectangular beams and columns is included inChapter 7. The guidelines included in this Section relate tocircular columns.

The maximum shear strength of the section should bedetermined in accordance with Section 7.2.1. This capacitygives the upper limit on the degree of strengthening that canbe achieved.

The ultimate shear capacity of an FRP strengthened columncan be expressed as:

Vu = Vc + Vs + Vf (Equation 41)

whereVc = contribution from the concrete to the shear capacity

Vf = contribution from the FRP to the shear capacity

Vs = contribution from the steel to the shear capacity

Vu = ultimate shear capacity of FRP strengthened section.

As described in Section 7.2.2, Vc and Vs can be determined

from conventional design standards with account taken ofthe material partial safety factors appropriate to the originaldate of construction. Additional guidance on evaluating thecapacity of circular columns is given by Clarke andBirjandi(144).

As discussed in Section 7, for rectangular or square sectionsit is important to take account of debonding of the FRP in thedesign of shear strengthening, even if the member is fullywrapped. However, for circular members, the significance ofdebonding is reduced because the development of tensilestresses in the hoop FRP tends to improve the bond behaviourby providing a lateral confining pressure.

For continuous hoop FRP wrapped around a circular columnwith fibres aligned perpendicular to the longitudinal axis ofthe member, Vf is given by:

Vf = (π/2) tf d Efd εfse (Equation 42)

Strengthening axially loaded members

57

where the effective strain in the FRP, εfse, should be taken asthe lesser of:

(i) εfd / 2or(ii) 0.004

andεfd = design ultimate strain capacity of FRPd = effective depth (distance from the extreme compres-

sion fibre to the centroid of the tension reinforce-ment)

Efd = design tensile modulus of the FRP

tf = thickness of the FRP.

As discussed in Section 7.4, additional axial reinforcementmay be required when strengthening for shear. Section 7.4also outlines how this area of reinforcement can be deter-mined.

8.3.5 Serviceability

Axial shortening/lateral deformation and loss of strength-

ening effectiveness

Axial shortening due to projected load increases will giverise to lateral deformation of compression members. Thisdeformation, if excessive, may cause problems of appear-ance, damage to brittle finishes and/or loss of structuralefficiency. Also, at service loads the maximum compressivestrain in the concrete should not be excessive otherwise lossof confining pressure due to accidental damage, fire,vandalism, etc., may result in brittle collapse, because theconcrete is fissured. To prevent the possibility of eitherproblem arising, it is recommended that the axial compres-sive strain of the concrete should not exceed 0.0035 underworking loads.

Fatigue

For bridges, the designer should consider the effect ofrepeated live loading on the fatigue strength of the FRP.Checks for fatigue should be carried out in accordance withthe recommendations in BS 5400: Part 4. The stress range inthe FRP should be limited to the appropriate values given inTable 8 of this Report.

Stress rupture

Rupture of the FRP may occur at service loads due to thesustained stresses that exist in the material. This type offailure can be avoided simply by limiting the stress level inthe FRP. It is therefore recommended that the stress in theFRP should not exceed the values given in Table 9 of thisReport.

8.4 FLEXURE

8.4.1 Introduction

Bonding axial FRP over-wrapped with hoop FRP to columnsurfaces can enhance the flexural strength of columns. Themain benefit of the axial FRP is to increase the flexuralstrength of the member, and the problem in design is to deter-mine the thickness of axial FRP fibre required to resist thecombined design axial load and moment. The hoop wrap-ping confines the concrete, increasing its compressivestrength and strain to failure. This can significantly improvethe efficiency of the strengthening design by increasing thestrain that can develop in the FRP. Hoop wrapping alsoenhances the shear capacity of columns and prevents buck-ling of the axial fibres, enabling them to contribute incompression. This contribution will be small since FRPmaterials are weaker in compression than tension.

To calculate the required thickness of axial FRP, the effect ofhoop wrapping on compressive strength and strain to failureof the concrete must be known. As discussed in Section 8.2,the stress–strain behaviour of confined concrete is ratherdifferent to that of unconfined concrete. It is perhaps worthcomparing the stress–strain response of the two types ofbehaviour.

For unconfined concrete BS 8110 approximates the behaviouras a parabola reaching a plateau at a value of 0.67fcu/γmc withstrain terminating at 0.0035. However, while the initialparabolic behaviour is reasonable, tests show that this isnormally followed by a descending stress–strain response,shown by the solid line in Figure 31. The model in BS 8110is a convenient approximation used for design purposes. Forconfined concrete, the stress–stain behaviour is again initiallyparabolic, until sufficient concrete dilation occurs for thehoop reinforcement to start confining the concrete. Providedsufficiently stiff hoop FRP is used (i.e. the condition inEquation 36 from Section 8.2 is met), this results in anapproximately linear ascending stress–strain branch, shownby the dashed line in Figure 31. The additional confinementalso greatly increases the maximum achievable concretestrain to failure (this can be taken conservatively as 0.01 andis usually limited by rupture of the FRP). Equations 32 and33 approximate this stress–strain behaviour, while Equations39 and 40 correspond to the ultimate confined compressivestress and strain, respectively, of the concrete. For a lightlyconfined section (i.e. low stiffness hoop wrapping) the initialparabolic stress–strain response may be followed by a branchthat initially descends, as shown by the dotted line in Figure31. It is evident from Figure 31 that the three types ofbehaviour are quite different. While it is not unreasonable toassume a rectangular stress block approximation for uncon-fined concrete since the centre of pressure remains in approxi-mately the same place, for the confined concrete this wouldbe conservative since, depending on the exact shape of thestress–strain curve to failure, the centre of pressure of thestress block may move considerably to the right, comparedwith the relative shape of the unconfined stress block.

Design guidance for strengthening concrete structures using fibre composite materials

58

Fully confined

Unconfined

Partially confined

εcc

fcc

Figure 31: Stress–strain behaviour of variably confinedconcrete.

In axially loaded columns subject to moment, the confine-ment of the concrete varies. Rather than full mobilisation ofthe confining hoop FRP due to dilation of the full cross-section, as would occur under concentric loading, whenmoment is applied the dilation varies around the section inproportion to the axial strain at any given point. Therefore,the achievable level of confinement will vary. Fam et al.(145)

have proposed a variable confinement method where thefully confined stress–strain relationship is reduced inaccordance with the eccentricity of the load (where the axialforce, N, and the moment, M, are related by the eccentricity,e, i.e. e=M/N). While the general method seems rational,although conservative, the procedure is rather complicated.However, based upon their test results, there is little diffe-rence between using the fully confined stress–strain modeland a reduced confinement model provided that the position,x, of the neutral axis from the outermost compression fibre isgreater than the radius of the column (i.e. the area ofconcrete in compression is no less than half the total cross-sectional area of the column):

x > D/2 (Equation 43)

If this condition is not met then a reduced stress–strain curveshould be used, referring to Fam et al.(145), or, conserva-tively, the unconfined concrete stress–strain model of BS8110 or similar may be assumed. However, it is permissibleto extend the curve to a maximum ultimate strain of 0.01, asshown in Figure 32. This is effectively the stress–strainrelationship of Equations 32 and 33 but with the slope of thelinear part equal to zero, i.e. E2=0. It should be noted that

this relationship is only applicable to columns of circularcross-section.

Generally, the design of compression members strengthenedin flexure should consider the following:

• at critical points the combination of maximum momentand coexistent axial load

• the risk of debonding• the risk of anchorage failure• the shear capacity of the column (see Section 8.3.4).

Extended unconfined forx < D/2 (i.e. E2 = 0)

Fully confined for x > D/2

1E2

fc0

fccd

fcc

εccεccuεt

Ec

0.01

Figure 32: Recommended stress–strain curves for combinedflexure and axial load.

It should be noted that flexural enhancement can only beachieved within the span of the column. It cannot be achievedat connections to beams or footings.

8.4.2 Moment capacity with axial load

To calculate the maximum moment and coexistent axial loadcapacity of an FRP-strengthened column of circular cross-section, the following assumptions can be made:

• Sections that are plane before bending remain plane afterbending.

• Slip does not take place between the FRP and the concrete.• Axial fibres are placed in a layer of even thickness all

round the column.• The stress–strain response for unconfined concrete

follows the idealised curve for concrete presented incurrent codes and standards, with γmc=1.5

• The maximum allowable compressive strain in the con-crete is 0.01 or εccu, whichever is less.

• The stress–strain response for steel reinforcementfollows the idealised curves presented in current codesand standards, with γms=1.15. (As indicated in Section6.2.3, many of the structures that require strengtheningwill have been built before the 1997 edition of BS 8110.Hence it would seem appropriate to use the earlier partialsafety factor of 1.15 rather than the value of 1.05 nowrecommended.)

• FRP has a linear elastic response to failure in tension.• The tensile strength of the concrete is ignored.• Confinement provided by any existing hoop steel is

ignored.• FRP in compression is conservatively neglected.

The procedure outlined below for calculating the requiredthickness of axial FRP is based on the Highways Agency BD84(10), Strengthening concrete bridge supports using fibre

reinforced plastics. The overall approach is, in principle, thesame as that used in conventional reinforced concrete columndesign. However, because at the outset it is not clear whetherit is the (confined) compressive strain in the concrete or thetensile strain in the FRP that is critical, the design procedure isslightly more complex. The actual steps involved are givenbelow.

Strengthening axially loaded members

59

1. Select a thickness of axial FRP.2. Using Equation 40, find the ultimate failure strain, εccu.

Set the strain in the outermost compression fibre of theconcrete to this value or 0.01 (to prevent excessive con-crete crushing), whichever is less.

3. Assuming a linear strain distribution, perform sectionalanalyses to determine by trial and error the depth, x, ofthe neutral axis from the outermost compression fibrewhen the axial load equals the applied ultimate load. If x> D/2 then the force in the concrete in compression ismodelled using the fully confined stress–strain model ofEquations 32 and 33. If x < D/2 then the extendedunconfined stress–strain model should be used (Equations32 and 33 with E2=0). The forces due to both the concrete

and FRP are most easily found using a layer-by-layerapproach, i.e. by subdividing the section into a number ofhorizontal layers, parallel to the axis of bending, andcalculating the compressive force contribution of theconcrete (dependent on the average strain, and henceaverage stress in the layer, from the stress–strain model)and tensile force contribution of the FRP appropriatelyfor each layer. Tensile and compressive forces due toexisting steel reinforcement can be added separately.

4. Calculate the corresponding ultimate moment capacity.5. If the ultimate moment capacity is less than the design

moment, M, increase the thickness of axial FRP andrepeat from Step 3 until the bending moment capacityequals or exceeds M.

6. Evaluate the tensile strain in the FRP. If it is less than theultimate design strain then the axial FRP thickness isadequate. However, if the strain in the FRP exceeds theultimate strain capacity, the FRP will fail before M isreached. This implies that it is the strain in the FRP andnot the concrete, as assumed in Step 2, that is critical.Therefore, repeat Steps 3 to 5 but this time limit the strainin the FRP to its ultimate design value. Obviously, themaximum strain in the concrete will be less than εccu butthe same stress–strain model may be used.

7. In order to ensure ductility, check the maximum com-pressive strain in the concrete. Provided it is not less than0.0035, the axial FRP thickness is adequate. Otherwise,increase the strain in the outermost compression fibre ofthe concrete to 0.0035 and proceed to step 8.

8. Increase the axial fibre thickness and determine by trialand error the depth of the neutral axis when the axial loadequals the applied ultimate load. Determine the corres-ponding ultimate moment capacity.

9. Evaluate the tensile strain in the FRP. If it exceeds the ulti-mate capacity, repeat step 8 until the tensile strain in theaxial FRP is equal to or less than its ultimate strain limit.

This design procedure has been set up so that the strain in theconcrete does not fall below 0.0035 in order to ensureductility under dynamic loading. However, lower concretestrains may be acceptable where flexural strengthening isrequired for reasons other than dynamic loading.

Use of this procedure to determine the thickness of axialFRP required to strengthen columns in flexure is verytedious. For convenience, therefore, design charts have beendeveloped by Denton et al.(146). The design charts form partof the Highway Agency Design Document BD 84(10), inwhich their application is explained and examples arepresented. They are based on the BS 5400: Part 4 models forconcrete and steel reinforcement. Also, any contribution tothe flexural strength of the column by the FRP in com-pression has been neglected. Note that the confined concretestress–strain model assumptions are not the same as thosedescribed in the above procedure, but are conservative.

8.4.3 Debonding

The work by Cuninghame et al.(147) appears to show that,providing the number of axial layers is not excessive, theprovision of hoop wrapping over the axial fibres can preventdebonding failure. However, debonding failure may occur inwrapped columns (below the load capacity expected on thebasis of the area of FRP) and it is important therefore that adegree of caution is exercised until there is more reliableinformation on this aspect of design.

8.4.4 Anchorage

The axial FRP can fail at the base and/or top of columns andat cut-off points. Anchoring the FRP by extending beyondthe point at which it is theoretically no longer required canprevent this. The anchorage length can be determined usingthe principles described in Section 6.3. In most situations,for example where a column is built into a rigid base, it is notfeasible to provide sufficient anchorage length. In suchsituations, the axial wrapping may be enclosed within acollar constructed of steel or concrete.

8.5 DUCTILITY

Lack of ductility is largely an issue for compressionmembers that are located in seismic regions. Upgradingnormally involves confining the concrete at column ends(where bending moments are greatest) with hoop FRP. Toensure that column bar buckling does not control the flexuralfailure mode, additional checks on the transverse reinforce-ment ratio need to be performed, particularly for slendercolumns where M/VD > 4, in which M and V are themaximum column moment and shear, respectively (seePriestley et al.(101)). Ductility enhancement may increase therisk of shear failure both at column ends and column centres,and the risk of flexural failure due to lap splice debonding atthe junction between the footing and column base(102).

As noted in Section 5.7.2, seismic loading is not a majorloading case for most UK structures and will therefore not bediscussed further.

Design guidance for strengthening concrete structures using fibre composite materials

60

8.6 STRENGTHENING COLUMNS WITHNON-CIRCULAR CROSS-SECTION

Confining square columns is generally acknowledged to beless efficient than confining circular columns. This is largelyattributable to the fact that, with square columns, confine-ment is concentrated at the corners rather than over the entireperimeter. Current research on small columns has shown thatthe maximum achievable increase in compressive stress forFRP-confined square columns with reasonable levels ofrounding of corners is about 50%, compared with up to200% for circular columns. For larger columns, found moreoften in practice, the level of increase may be less than this.The efficiency decreases further with columns of rectangularcross-section and/or large side dimensions. (The differencein confinement performance between circular and rectangularcolumns is similar to the difference between the use of hoopsand stirrups (without cross-ties) in conventional steel-reinforced column design).

Several models of the strength enhancement of rectangularcolumns have been proposed and compared with the limitednumber of experimental results available. However, it shouldbe noted that most models are semi-empirical in nature andhave been calibrated with small-scale test specimens.Typically these specimens are 150mm x 150mm for squarecolumns and up to 150 mm x 225mm (i.e. an aspect ratio of1.5) for rectangular columns. As rectangular columns getlarger in size the length of the unconfined regions along thesides of the column increases creating a size effect, which isnot evident for circular columns. It should also be noted thatthe confinement model is based upon concentrically loadedcolumn behaviour. It is likely that if there is significant loadeccentricity then the stress distribution will be such that theeffectively unconfined areas will be more highly stressedthan assumed in the model. Caution should be used in sucha situation.

The generally accepted theoretical approach is to develop anarea of effective confinement defined by four parabolaswithin which the concrete is fully confined and outside ofwhich negligible confinement occurs. The shape of the para-bolas and the resulting effective confinement area is a func-tion of the dimensions of the column and the radius of thecorners. From this effective area a shape factor can be defined.An effective confining pressure must be calculated basedupon the same thickness FRP wrap confining an equivalentcircular column of diameter D. The effective strength of theconfined concrete is then given by the unconfined strengthplus the product of the shape factor, the effective confiningpressure and an effectiveness coefficient, which is derivedempirically and conservatively taken as 2.0. The variousmodels differ mainly in the calculation of the effective areaand the diameter of the equivalent circular column. Use ofany current state-of-the-art explicit method for analysingstrengthened square or rectangular columns is notrecommended unless the following conditions are met:

• Loading is essentially concentric.• The smaller edge dimension is no greater than about

200mm.• The aspect ratio is no greater than 1:1.5.• The corners can be provided with a radius of at least

15mm to avoid stress concentrations.

Of the various models suggested, that by Lam and Teng(148)

predicts the observed behaviour most accurately. However,while their model suggests that the initial slopes of theparabolas should be the same as the slope of the diagonallines between the column corners, a more common assump-tion(149) is that the initial slopes of parabolas start at 45° tothe face of the column, as shown in Figure 33. Although thisassumption may be conservative, it is recommended thatsuch an assumption be made in calculating the confined area.Thus, the ratio of the effective area of confinement(contained within the parabolas) and the total cross-sectionalarea of the column is given by:

(Equation 44)

whereρsc = is the ratio of longitudinal steel to the cross-sectionb = length of short sideh = length of long sideRc = radius of corner

Ae = effectively confined area

Ag = total cross-sectional area

= bh –(4-π)Rc2

Aol = area of overlap of the parabolas

= 0 if 2b > (h – 2Rc)

= if 2b < (h – 2Rc)

wherelol = length of overlapping region

=

( ) ( )[ ]( )sc

scgol2

c2

c

g

e

1

3/3221

ρρ

−−−−+−−

=AARbRh

A

A

–

( ))2(2)2(3

4col

c

3ol Rhbl

Rh

l −−+−

2

)2(

4

)2( c2

c RhbRh −−−

45o

effectively confined

region, Ae

Rc

b

h

Figure 33: Assumed confined region for FRP-wrappedrectangular column.

Strengthening axially loaded members

61

Subtraction of the area Aol is required since, as the aspect ratio

of the cross-section increases such that the limit 2b < (h – 2Rc)

is reached, the larger parabolas corresponding to the longeredge will overlap in the middle of the section, as shown inFigure 34. This resulting area is not confined and thereforemust be subtracted from the total area enclosed by theparabolas.

45o

effectively confined

region, Aeb

h

lol

Aol

Figure 34: Overlapping parabolas in confined region.

Thus, a shape factor is defined as:

(Equation 45)

The aspect ratio in this equation is necessary in order to reflectproperly the reduction in confinement as the aspect ratioincreases. The shape factor relates the effective confiningpressure to that provided by an FRP wrap of the samethickness to an equivalent circular column of diameter D.The diameter of the equivalent circular column is defined byLam and Teng as the diagonal distance across the section, i.e.D=√(b2+h2). Thus, using Equation 38, the equivalentconfining pressure is given by:

(Equation 46)

Given the equivalent confining pressure and the shapefactor, the strength of the confined rectangular column isgiven by:

(Equation 47)

wherefc0 = 0.67 fcu / γmc

Again, it should be emphasised that this model has only beencalibrated against small-scale concentrically loaded columnsand caution should be taken if applied to columns not typicalof those tested in the literature due to the size effect of theunconfined sides resulting in premature failure.

Shear strengthening of rectangular columns should becarried out according the methods outlined in Chapter 7, inthe same way as for beams.

g

e

A

A

h

bgs =

22

ffd2

hb

tffr

+=

rsc0cc 0.2 fgff +=

Confinement efficiency can be improved by rounding thecorners of the column or by casting circular or oval concreterings around the column perimeter. However, it should benoted that the confined strength of oval columns reduces asthe aspect ratio increases. Teng and Lam(150) suggest usingEquation 47 but with an empirically derived shape factor:

(Equation 48)

where a and c are the major and minor dimensions,respectively.

The equivalent circular column is defined as one with thesame volumetric FRP ratio as the elliptical column and, there-fore, the equivalent lateral confining pressure is given by:

(Equation 49)

Again, this model should be used with caution, since it iscalibrated against a small number of concentrically loadedspecimens. The greatest aspect ratio tested was 5/2.

Rectangular carbon fibre jackets are also reported to providesufficient confinement and bar buckling restraint to achievehigh flexural displacement ductility levels(102). Perhaps analternative approach may be to use preformed circular FRPshells and fill the intervening space with grout. Other methodsmay also be employed in order to improve the efficiency ofconfined rectangular columns. Where it is difficult to shapecorners to a radius of at least 15mm significant stressconcentrations may occur resulting in premature failure ofthe FRP. In such situations it has been suggested that addi-tional localised wrap reinforcement be provided at the cornersprior to the application of the continuous layers, thus redu-cing corner stresses(151). It has been suggested that internalFRP ties can be used through the width of the section toincrease confinement where side lengths are large(152). Itmay be beneficial to incorporate additional longitudinal FRPin a wrapping scheme to reduce the likelihood of burstingfailures. The design of strengthening schemes that use any ofthese systems should be verified by independent testing.

To summarise the information currently available:

• A significant increase in the axial load capacity of squareor rectangular columns by wrapping with FRP may bedifficult to achieve in practice.

• An increase in the flexural strength of square columns bywrapping with axial and hoop FRP should be feasible,but any proposed design should be verified by indepen-dent testing.

• Ductility enhancement of square columns may provedifficult.

• FRP can be used to increase the shear capacity of squareand rectangular columns.

2

s ⎟⎠⎞⎜

⎝⎛=

a

cg

[ ]ac

tfaccafr 2

)(5.1 ffd−+=

Design guidance for strengthening concrete structures using fibre composite materials

62

Strengthening axially loaded members

63

Design guidance for strengthening concrete structures using fibre composite materials

64

65

9 EMERGING TECHNOLOGIES

9.1 EMERGING STRENGTHENINGTECHNOLOGIES ALREADY USED INPRACTICE

9.1.1 Prestressing using FRP composites

The use of prestressed FRP composites for flexural strength-ening of concrete structures has been developing over recentyears, and a number of proprietary FRP systems are nowavailable commercially. However, design guidance in thisarea is still developing, and as such it is advisable to seekspecialist advice for the design of prestressed FRP compositestrengthening systems.

There are many reasons for strengthening of concrete struc-tures using prestressed FRP composites:

• increasing live load capacity• reducing dead load deflections (i.e. mobilising locked in

stresses)• reducing crack widths and delaying the onset of cracking• reducing serviceability problems such as excessive deflec-

tion, cracking of the concrete and tensile steel stresses atserviceability

• improving fatigue strength by reducing tensile steelstresses

• regaining prestressed conditions in the concrete that maybe lost by damage to the original prestressing tendons orother effects.

However, if the concrete stress at the serviceability limit stateis high, prestressing with FRP composites will not provide asignificant increase in the serviceability load.

For some prestressing systems, no reliance on an adhesivebond is required, which can be advantageous where very lowor very high service temperatures are present (or there is asignificant fire risk) and could significantly reduce the per-formance of the adhesive, or where the surface quality of theconcrete is inadequate for adhesive bonding.

The use of prestressed FRP composite strengthening systemsmay not be appropriate, or at least will require more detailedplanning and risk assessment, where there are limited avail-able installation periods (for example within railway posses-sions or in structures where there is a continuous industrialprocess), significant risk of vandalism, or exposure to a highlyaggressive environment that would affect the long-termproperties of the FRP composite.

It has also been suggested that the use of prestressed FRPcomposites can increase shear capacity by a confining effecton the concrete(153), although this has not been investigatedin great detail.

FRP composites generally exhibit superior durability andfatigue properties to those of steel. A number of types of FRPcomposite can be used to post-tension existing concretestructures: bonded or unbonded FRP composite plates orsheets, bonded FRP NSM reinforcement and external FRPcomposite tendons.

Prestressing allows a greater proportion of the FRP compo-site tensile strength to be utilised, and can therefore be moreefficient than unstressed solutions. Carbon FRP composite isgenerally the most suitable type for prestressing applicationsdue to its superior creep and stress rupture propertiescompared with those of glass FRP and aramid FRPcomposite. It is advisable not to use glass FRP compositesfor prestressing, unless the prestressing force being appliedis quite low, where stress rupture will not be an issue.Aramid FRP composites can be used instead of carbon FRPcomposites, their lower tensile elastic modulus being anadvantage in achieving greater control of elongations in theprestressing process. However, as with glass FRP, aramidFRP can be susceptible to stress rupture, and therefore theprestress levels should be limited.

The basic principle of strengthening concrete structures bypost-tensioning with FRP composites is similar to that forconventional post-tensioned concrete structures. A portion orall of the existing dead load in the concrete member istransferred to the FRP composite by creating a tensile forcein the FRP prior to application, bonding and/or anchoring theFRP to the concrete substrate, and then releasing the pre-stress load. The bonded joint or mechanical anchors at theends of the plate then transfer the prestress into the concretemember. The prestressed FRP composite carries both aportion of dead load, and also live load, in comparison tounstressed FRP strengthening where the FRP compositecarries only a portion of live load.

The method of prestressing the FRP composite is crucial to thefeasibility of a practical FRP prestressed strengthening appli-cation. The method can be described in a number of stages:

1. application of tensile force to FRP composite2. anchorage/bonding of FRP composite to concrete substrate3. release of prestress into concrete member and redistribu-

tion of forces throughout section.

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66

The method of jacking out the dead-load deflection of aconcrete member, or physically removing existing dead loadtemporarily, prior to the application of unstressed FRPcomposite is also essentially a prestressing solution as theFRP composite carries a portion of the dead load in additionto live load.

The ROBUST project demonstrated the benefits of pre-stressing the FRP prior to bonding to the concrete, on 1.0mand 4.5m beams in the laboratory, and with 18.0m beams inthe field. Specially developed glass FRP end tabs weredeveloped to enable the carbon FRP plate to be pulled, priorto anchoring the tabs into the concrete using resinanchors(29).

Anchorage of the prestressed FRP composite is a criticalaspect of the application. Large shear stresses are presentwithin the adhesive bond when the prestress in the FRPcomposite is released and transferred into the concretemember. Therefore, in most cases a mechanical anchorage atthe ends of the FRP composite is required, althoughtechniques are under development to avoid the need forbulky end anchorages.

A number of anchorage types have been developed,generally based on jacking the FRP composite plate througha steel anchorage and bolting through the plate (which islocally strengthened with steel or glass FRP end tabs toavoid failures due to stress concentrations around the boltholes) into the anchorage and the concrete substrate. Theplate is then cut and the transfer of prestress occurs via thebolted anchorage.

In Switzerland, a system has been developed whereby acarbon FRP plate is stretched over a set distance betweentwo large wheels(154). The entire mechanical system is thenlifted up to the soffit of the concrete and the laminate isbonded to the structure. This method overcomes many of theproblems associated with successfully gripping the plate.Furthermore, this method also allows the problem of highlongitudinal shear stress at the concrete–plate interface to beovercome. The longitudinal shear stresses originally led toanchorage failures in laboratory tests. However, a gradually-anchored system was devised to reduce the longitudinalshear stresses and delay the onset of anchorage failure.Using this method, the plate is bonded to the concrete fromthe centre and then moved outwards in stages. As eachportion of the plate is bonded to the concrete at successivestages, the prestressing force is slightly reduced to a nominalvalue at the end of the plate. In order to speed up the curingprocess, so that the step-wise technique is economic andpractical, heating devices within each portion of the plate areused to reduce the adhesive bond curing time.

On transfer of the prestress to the concrete member, somelosses occur in a similar manner to those for conventionalprestressed post-tensioned structures. The losses in prestressin the short term are due to:

• elastic shortening of the concrete member• creep effects in adhesive for prestress of systems with no

mechanical anchorage• ‘drawing in’ within the anchorage system (if a mecha-

nical anchorage is used).

The relaxation loss for prestressed FRP composite in com-parison to high strength steel is generally small.

The losses in prestress in the long term are essentially thesame as those for post-tensioning with low-relaxation steel,due to the reduction in elastic modulus of the concrete andshrinkage in the long term.

The ultimate load capacity of concrete members post-tensioned with FRP composites can be analysed based onconventional theory for reinforced-concrete structures(29,155),but only if flexural failure is the dominant failure mode. Thefailure mode may be either concrete crushing or FRPcomposite rupture, depending on the degree of prestressapplied to the FRP.

Design checks are also required at the serviceability limitstate. The level of prestress should be such that the followingconditions are acceptable:

• tensile stress and cracking at the concrete edge awayfrom the prestressed FRP composite under dead andsuperimposed dead load, after transfer of the prestress

• compressive stress in the concrete and dead, super-imposed dead and live loading

• tensile stress in the FRP composite under dead and super-imposed dead loads (for durability and stress rupture),and live load (for fatigue).

The achievable prestress levels vary depending on the typeof FRP composite used, based on their susceptibility to stressrupture. Carbon FRP composite is not susceptible to stressrupture and therefore the greatest degree of prestress can beachieved, followed by aramid FRP composite and glass FRPcomposite, which both exhibit stress rupture. As an indi-cation, carbon FRP composite can typically be stressed up to50% of the design tensile stress, aramid FRP composite upto 30% of the design tensile stress and glass FRP compositeto only 15–20% of the design tensile stress. For prestressingsystems where the adhesive bond is relied upon to transferprestress forces, the permanent stress in the adhesive shouldbe limited to 25% of the design strength to avoid creep anddurability problems, as stated in Section 5.6.8.

A number of commercial prestressing systems are currentlyavailable, using the following methods of prestressing:

• Stressing and anchoring of carbon FRP plates in a steelanchorage, placed in a recess in the concrete, containinga base plate for force transfer (bonded and bolted to theconcrete), tensioning plate for the hydraulic jack andlevelling aids: the stressing process is undertaken in twostages with temporary and permanent anchorages.

67

Emerging technologies

• Stressing and anchoring in a steel and carbon FRPcomposite anchor block, placed in a recess in the con-crete substrate: the steel anchorage is bonded and boltedto the concrete substrate; the prestressing operation iscarried out in a single stage.

Some prestressing systems are designed such that platefailure would always occur prior to anchorage failure.

A number of reinforced concrete and conventionally pre-stressed and post-tensioned concrete structures throughoutEurope, particularly in Germany and Switzerland, have beenstrengthened using prestressed FRP composites. The firstfull-scale application of a FRP composite prestressing systemin the field was on Lauterbridge, Gomadingen, Germany inOctober 1998. In this particular case, the prestressing systemwas installed to reduce crack widths, and increase theflexural strength and rigidity.

A number of existing concrete bridges have been strength-ened using carbon FRP prestressing tendons or aramid FRPropes(156,157). FRP tendons were chosen due to easy handlingon site, small-diameter bundles allowing simple deviatordetailing, and corrosion resistance. Anchorage systems forFRP tendons have been developed that allow full designstrength of the tendon to be assumed, under both static andcyclic loading conditions(158). Furthermore, research hasshown that external FRP tendon prestressing systems allowsubstantial rotation to occur over supports in continuousspans near the ultimate limit state. Based on this research, itis possible for full moment redistribution to be assumed(159).

Examples of commercial prestressing systems are shown inFigures 35 and 36.

9.1.2 Unstressed FRP anchorage techniques

The use of transverse FRP U-wraps around the soffit of abeam is a common laboratory technique to anchor longitu-dinal FRP laminates(160). This technique has also been carriedout in practice on many occasions, with satisfactory resultsbeing achieved(161).

Figure 35: Dead-end prestressing system (left) and stressing anchorage (right).

Figure 36: Prestressing system used on Lauterbridge,Gomadingen.

Another well-founded laboratory technique to anchor wetlay-up FRP laminates is to cut a transverse groove in theconcrete, insert the end portion of the fabric into the grooveand anchor into place (through resin) an FRP NSM bar(127).Again, this technique has also been used in practice, parti-cularly where sufficient anchorage length for the FRP cannotbe provided by bond alone. Examples where this techniqueis particularly useful include anchorage of U-wrap laminatesfor shear strengthening of T-beams(114) and anchorage oflongitudinal laminates for flexural strengthening of members,which vary in cross-section near supports(54).

9.1.3 Bolted plate anchors

Nurchi et al.(162) have assessed the enhancement of anchor-age provided by bolting the ends of an FRP plate to concrete,in addition to adhesive. The primary use of the bolts is toprevent or delay the onset of debonding and concrete coverfailure. However, the use of bolts requires multi-directionalFRP in order to prevent longitudinal splitting failure of theFRP, and in order to provide sufficient bearing stiffness atthe position of the bolts. The bolts, about 200mm in length,are fixed into predrilled holes using epoxy adhesive. Anadditional FRP reinforcement layer is added at the ends ofthe FRP plates to increase strength at these positions.

Design guidance for strengthening concrete structures using fibre composite materials

68

The results of their tests indicate that the use of bolts withinthe shear span of the beam significantly postpones debonding.Following debonding, they are still capable of anchoring theends of the FRP so that it acts as an unbonded tension member,resulting in a less brittle mode of failure at the ultimatecondition. The load capacity of the beam following debondingis reported to be at least equal to the capacity of the beamwith the bond intact, although greater deflections occur.

9.1.4 Concrete masonry walls

FRP composite systems can provide solutions for the strength-ening of masonry. The dynamic properties of the existingstructure remain unchanged because there is minimaladdition of weight, and stiffness changes may be engineeredcase by case.

FRP composites used for flexural and/or shear strengtheningof masonry structures are similar to those used for strength-ening of concrete elements. A number of research projectshave demonstrated the effectiveness of FRP composites toimprove the structural performance of masonry walls, parti-cularly in situations of high slenderness(163). Availableliterature shows that walls strengthened with FRP in thelaboratory usually fail due to debonding of the laminates.

An alternative to FRP laminates is the use of NSM FRP bars.In this way, FRP bars have been used successfully to increasethe flexural strength of concrete masonry walls. FRP systemscan also be used to improve the performance (strength orpseudo-ductility) of masonry walls subject to in-planeloads(164). Strengthening by FRP structural repointing (inser-tion of small diameter FRP bars in the bed joints) can alsoincrease shear capacity and provide pseudo-ductility to walls.

9.2 EMERGING STRENGTHENINGTECHNOLOGIES AT THE RESEARCHSTAGE

9.2.1 Prestressed NSM bars

One way in which the problem of high shear stress at theends of prestressed FRP plates might be overcome is to useNSM bars instead. This is because they are bonded overmost of their perimeter, leading to a spreading of longitu-dinal shear stress in comparison with the one-side-bondedlaminate case. Research conducted in Sweden and in theUSA has confirmed their suitability(165). Naturally, the mainconcern is how to prestress the NSM bars prior to insertioninto the grooves. At present, this is being considered in somedetail because of the potential for this form of strengthening.

9.2.2 NSM bars for shear strengthening

Because NSM bars offer potentially superior bond perfor-mance over plates or sheets, their use for shear strengtheninghas been attempted, with success. It has been found that theuse of either glass or carbon FRP NSM bars increases shear

resistance of concrete beams considerably. In particular, deLorenzis and Nanni(166) have shown that, in the case ofstrengthening T-beams in shear, if it is possible to anchor theNSM bars into the compression flange, shear resistance isenhanced greatly. They have also shown that angling theNSM bars at approximately 45° (rather than placing themvertically) is advantageous. While such angled reinforce-ment is problematic for plates or sheets, it is relativelystraightforward for the NSM case.

9.2.3 FRP anchor systems

In situations where FRP laminates need to be mechanicallyanchored, due to insufficient bond length being available,there are various options already available. For instance,bolted steel plate anchor systems, as described in Section9.1.3, may be used for FRP plates. Alternatively, FRP NSMbars may be used to anchor wet lay-up FRP laminates withinslots(127), as described in Section 9.1.2.

Another anchorage system in the process of developmenthas proved to be very effective in preliminary tests at theUniversity of Missouri-Rolla(114). The anchors themselvesconsist of many glass fibres of overall length 150mm. Theyare dipped into resin to a depth of 75mm, and the resultingglass FRP portion (of approximate diameter 12mm) is allowedto cure. Holes 75mm deep and 200mm apart are drilled intothe soffit of the concrete structure to be strengthened, andresin is inserted into each hole. The first layer of wet lay-upsheet is adhered to the surface of the concrete, followingstandard procedures. At each hole location, the glass FRPend of each anchor is inserted into the hole through the sheet(by locally realigning the sheet fibres to skirt around theanchor). The ‘dry’ end of each anchor is then fanned out overthe surface of the first layer of sheet and fixed into place withresin. Subsequent layers of sheet are added above the fannedanchors so that, after curing, the fan anchor is located withinthe thickness of the FRP laminate. This enhances anchorageand bond behaviour.

9.2.4 Steel-reinforced polymers

The use of steel-reinforced polymer (SRP) materials hasrecently been considered in the USA(167). Part of the moti-vation for use of this novel material is that discarded tyrescontain significant quantities of ‘Hardwire’ steel strand,which can be used to make the SRP. This clearly hasenvironmental benefits, which is advantageous. However,the thin steel strands are susceptible to corrosion. Variousmatrix materials have been looked at to protect and bind thesteel strands. A cementitious grout presently appears to offera good compromise between structural strength and adurable composite.

A parking garage in Indiana, USA, was recently condemned.Prior to its demolition, parts of it were strengthened usingSRP, with encouraging results. The strengthened sectionsshowed distinct improvement over the original sections in

Emerging technologies

69

terms of capacity and ductility. Failure occurred by SRPpeeling, in much the same way that might be expected tooccur in the FRP situation.

9.2.5 Deep embedded bar for shear strengthening

Particularly when working on concrete bridges, it is desir-able that any strengthening be carried out rapidly, with as littledisruption as possible. In order to expedite shear strength-ening, it is possible that vertical holes could be drilled intothe bridge from soffit level, and FRP bars fixed into the holeswith resin to provide additional shear resistance(168).

Although similar in concept to the use of stainless steelthreaded bar for such applications, the FRP alternativerequires access only from the soffit (rather than from top andbottom), and the possibility of corrosion of reinforcement inclose proximity to the stainless steel bars is eliminated. Thistechnique has the further benefits that no surface preparationof beam webs is required in beam-and-slab situations and thewebs need not be accessible. This is very useful in situationswhere precast beams are closely spaced.

This shear strengthening technique has been shown to befeasible, preventing shear failure in laboratory tests andinstead forcing a ductile flexural failure to occur.

9.2.6 Prestressed carbon FRP straps for shear

strengthening

As it is difficult to anchor U-wraps around T-beam webs,slots could be drilled in the flange at regular intervals, andfull strap-wrapping of the equivalent rectangular sectioncarried out. Since local overstrain in bonded FRP at cracklocations can lead to local failure, it would be best not tobond these straps to the concrete. Furthermore, in order toresist shear crack openings (and hence enhance aggregateinterlock effects), the straps should be prestressed.

Such a system has been developed for commercial use(169).The straps consist of five or ten individual carbon FRP tapes,heat-welded together in order to create continuity. The strapsare prestressed using a patented system, and accurately-machined wedges are inserted between strap and concrete inorder to maintain the prestress. Results from tests show thatshear strength is enhanced considerably through making useof a debonded, prestressed system.

9.2.7 Mechanical fastening techniques

Some of the limitations associated with FRP strengtheningof concrete structures are the modest longitudinal shearstrength of adhesives, the time taken to prepare the concretesurface and the brittle form of failure, which occurs when theFRP peels off under severe overload. The use of a mecha-nical bonding system might be an approach to addressingthese problems.

Research carried out in the USA has shown the feasibility ofusing such a technique. Originally intended for extremelyrapid strengthening of civilian concrete bridges to allow heavymilitary vehicles to pass, the technique is to mechanicallyfasten a precured FRP laminate to the concrete soffit usingpower-driven bolts. No surface preparation of the soffit isrequired. The FRP is bidirectional in nature, to preventlongitudinal splitting. Preliminary tests carried out inMadison, Wisconsin, have shown the potential for this rapidform of construction(170), although integrity of the coverconcrete following the power-driving activities has beenshown to be crucial to successful implementation.

9.2.8 Strengthening for torsion

Limited tests have been conducted on torsional strength-ening of concrete structures using FRP materials. Publishedresults show that such strengthening is possible and that FRPcan contribute substantial torsional resistance(171). Prelimi-nary indications are that the wrapping of the torsion elementshould be as full as possible, as the confinement that iscreated in this way is particularly beneficial in resistingtorsion in a controlled, ductile manner.

9.2.9 Life expectancy modelling

Research carried out in Italy is attempting to rationalise thelife expectancy of FRP-strengthened concrete schemes(172).In order to do this, a series of degradation states has beendefined, each component relating to a particular intervention.These interventions are, in order of least to most severe:

• inspection• cleaning• repair• partial substitution• total substitution.

A database is being developed on a range of FRP-strength-ening schemes throughout the world. By categorising perfor-mance levels and long-term mechanical properties of theconstituent materials, it is hoped that this research will leadto a rational approach to life-expectancy predictions, one ofthe most important pieces of information in any design.

9.2.10 Inorganic adhesives

Because of concerns over the performance of organicadhesives at elevated temperatures, there are moves todevelop inorganic adhesives, more akin to cement-basedmaterials. Toutanji and Deng(173) have reported tests on beamsstrengthened using ‘Geopolymer’ adhesives consisting ofalumino-silicate with a water-based activator. These performsignificantly better at high temperatures and, being water-based, are somewhat easier to use on site.

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10 WORKMANSHIP ANDINSTALLATION

10.1 INTRODUCTION

The design guidance in Chapters 5 to 8 is only valid if thecomponent materials used are in accordance with the speci-fication and the installation is carried out correctly. ThisChapter is not intended to be a specification for strength-ening with composites but gives background information onthe standards of workmanship and the installation proce-dures required. Further information on the requirements forinspection during the installation process, along with adviceon the necessary records that need to be maintained, is givenin Chapter 4 of TR57(5), which should be read in parallelwith this Chapter.

The Client should be satisfied as to the competency of thecontractor. All installations should comply with the require-ments of the Health and Safety at Work etc Act(174), theControl of Substances Hazardous to Health Regulations(27)

and the Construction (Design and Management) Regula-

tions(26). In addition, all materials must be used in accor-dance with the manufacturer’s requirements.

Only limited post-application inspection is possible, so thesuccess of the application relies heavily on the quality of theworkmanship. It is therefore crucial to the success of theinstallation that an experienced contractor, with suitablytrained and supervised staff experienced in the technique, isappointed. The contractor should have quality assuranceprocedures in place, accredited and audited in accordancewith ISO 9002(175). In the UK, the contractor should have aproven track record in the installation of composites andpreferably be a member of the Concrete Repair Association.The contractor must be able to demonstrate competency andbe approved for the application of the system. This approvalmay be obtained by providing evidence of the training of theoperatives who will undertake the work and by documentaryevidence of experience on similar projects.

It is strongly recommended that the following issues betaken into account when selecting a contractor:

• The contractor should provide a full method statementand risk assessment for the works.

• Operatives should be trained and qualified in applicationtechniques by the manufacturer of the system.

• Personnel should be supplied with the correct personalprotection equipment for use when handling the materials.

• The contractor should provide a safe means of access tothe work location and maintain an environment suitablefor the successful use of structural adhesives.

• Procedures should be in place to minimise the risks to theworkforce and to any other persons (especially children)who may be affected by the work.

The following sections give general guidance on theinstallation of plate and fabric materials, which are bondedto the surface of the concrete, and of NSM material. Theinstallation of shells around columns, which are generallybonded to the concrete by means of a secondary process suchas grout injection, should be in accordance with themanufacturer’s requirements and are not covered here. Forall materials and processes, quality assurance proceduresshould ensure that each stage is approved before starting thenext stage. It is vitally important that the manufacturer’srecommendations are followed throughout.

The sequence of the subsections in this Chapter follows thestep-by-step procedures that would be followed on site by acompetent contractor.

10.2 EVALUATION OF CONCRETECONDITION

An investigation of the condition of the structure(7) should becarried out prior to the decision to undertake strengthening.This will identify any deterioration processes (e.g. rein-forcement corrosion due to the presence of chlorides) likelyto affect the performance of the structure within its residualdesign life. The investigation should also include a thoroughinspection of the concrete surfaces on which the bonding isto be carried out, a visual inspection, an assessment of theconcrete strength (see Figure 37) to assess whether it is suffi-cient for strengthening to be carried out (see Section 2.2),chemical analysis and a sounding survey to identify defect.

If defects are identified, repairs should be carried out usingan appropriate concrete repair system in accordance with themanufacturer’s recommendations. Cementitious repairsshould be cured for at least 28 days before undertakingbonding work. As indicated in Section 2.2, the cause of anydeterioration should, as far as possible, be eliminated beforethe structure is strengthened.

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72

Figure 37: Use of pull-out test to determine concrete strength.

10.3 CONCRETE PREPARATION

10.3.1 Concrete surface for plates and fabric

Good preparation of the concrete surface is of paramountimportance to the long-term success of the bonding andstrengthening operation, though this is less crucial for fullywrapped systems. Before adhesive is applied the concretesurface must be cleaned so that it is free of laitance, loosematerial, fungal or mould growth, oil or grease, corrosionproducts, previous coatings and, in the case of new concrete,mould release agents and curing membranes.

It is important that the preparation process selected is suchthat it removes the surface layer to expose small particles ofaggregate without causing micro-cracks or other damage inthe substrate. The surface should not be polished or rough-ened excessively. Sharp edges, shutter marks or other irregu-larities should be removed to achieve a flat surface.

Figure 38: Surface grinding.

Effective preparation techniques include:

• wet-, dry- and vacuum-abrasive blasting• high-pressure washing, with or without emulsifying

detergents, and using biocides (where necessary)

• steam cleaning alone or in conjunction with detergents• for smaller areas, mechanical wire brushing or surface

grinding (see Figure 38).

Mechanical impact methods such as needle gunning andbush hammering are very effective but are often too aggres-sive and produce a deeper texture in originally smoothconcrete. In addition they may shatter aggregate particlescausing micro-cracks.

Mechanical methods may be effective in removing deeplypenetrated greases, oils and paints, but may remove an un-acceptable depth of concrete. Washing techniques may beineffective and can simply spread the contaminant further. Insuch instances the use of solvent-based and sodiumhydroxide-based products in the form of a gel or poultice canbe effective in drawing out the contaminants. Such productsmust be used with great care; if they are not thoroughlyremoved, debonding of the strengthening system may occur.

Wet grit-blasting or vacuum dry-blasting are commonly usedbecause they reduce the dust created by ‘open’ dry-blastingwhich is unacceptable for health, safety and environmentalreasons. However, wet techniques may create a water disposalproblem and the concrete surface needs to be allowed to dryout to a degree that is suited to the intended adhesive.

Whichever method is adopted it is always advisable to carryout trials to select and optimise the technique in conjunctionwith the material supplier.

The preparation of the surface should be to a standard suchthat the adhesive layer is of uniform thickness when thestrengthening material is in place. Any steps in the surfaceshould be removed and hollows filled with a suitable quick-setting repair mortar (see Figure 39). Generally the flatnessof the surface should be such that the gap under a 1mstraight-edge does not exceed 5mm. The thickness of theadhesive layer is commonly between 1 and 5mm dependingon the material, though thickening to 10mm may be accept-able to accommodate local defects such as dislodgedaggregate.

Figure 39: Filling imperfections with quick-setting repairmortar.

Workmanship and installation

73

When fabric is to be wrapped round corners, e.g. round asquare column or round the bottom of a beam, the cornersshould be rounded to a minimum radius of 15mm, or asrecommended by the supplier, to avoid local damage to thefibres.

Minor imperfections in the concrete surface can be treated atthis stage with epoxy materials which can be applied in thinlayers and whose rapid strength gain permits over-bondingto be carried out after a short time. Some bonding systemsrequire the use of a primer on completion of the surfacepreparation. This primer, which seals the surface, should beapplied in strict accordance with the manufacturer’sinstructions.

The final assessment for surface quality can take the form ofa series of pull-off tests. (If a surface primer is used, the testsshould be carried out on the primed surface.) Figure 40shows a ‘dolly’ after being pulled off, with the concrete stilladhering to it. A minimum of three tests per representativearea should be carried out, as described in BS 1881: Part207(176), to give an indication of the tensile strength of thesubstrate and the quality of the surface preparation. The con-crete surface should be dry for normal applications. Wherethis is not possible, because of the nature of the structure,special consideration should be given to the adhesive to beemployed.

Figure 40: Pull-off specimen after removal from concretesurface.

10.3.2 Slots in concrete surface for NSM material

Before starting to form slots for NSM reinforcement, acovermeter survey should be carried out to check the posi-tion and depth of the existing steel reinforcement in the areato be strengthened, to avoid damaging it. Slots are formed bymaking parallel cuts in the surface to the required depth, atan appropriate distance apart, and removing the interveningconcrete using a chisel or similar. The slots should be cleaned,using a vacuum cleaner or high-pressure air, to remove anyloose material but otherwise should not require any furtherpreparation to ensure that the adhesive has adequate bond.

10.4 MATERIAL CONFORMITY

The design process will involve certain assumptions aboutthe properties of the FRP. It is therefore important that allmaterials are in accordance with the specification. FRPplates, rolls of fabric, etc., should carry identification labelsto indicate their type and grade. To ensure that materials arecompatible, they are generally specified as part of a system,e.g. pultruded plate and adhesive. Other materials should notbe substituted without the approval of the specifier.Guidance on the approaches for ensuring conformity and onacceptance tests are given in TR57(5), which includes aproforma for recording details of the materials used. TheReport also includes guidance on permissible tolerances inpultruded plate geometry.

10.5 STORAGE OF MATERIALS

Thick fibre composite plates and NSM reinforcement areusually delivered to site in the lengths required for installa-tion. Thinner plates and fabrics are delivered in the form ofa long roll, which can be cut easily to the required lengths atsite. Materials should be stored at site in such a way thatdamage or contamination is avoided.

Adhesives should be stored in dry conditions in accordancewith the manufacturer’s instructions, paying particularattention to the specified maximum and minimum storagetemperatures. Adhesive and material delivery dates shouldbe recorded and these items should be used in rotation.

10.6 SITE CONDITIONS

Temperature, relative humidity and surface moisture at thetime of installation can affect the performance of the FRPsystem. It is therefore necessary to maintain the appropriateenvironment in the work area during surface preparation,application of the adhesive and the subsequent curing period.Environmental control during surface preparation generallyconsists of a system to extract dust from the work area andthe exclusion of any material that might contaminate theprepared surface. A clear access path should be maintainedfrom the area where the adhesive is applied on the plates tothe location of the concrete surface to which the plates are tobe applied. This is to minimise the risk of contamination ofthe adhesive surface while the plate is being handled.

During the curing period it is necessary to maintain the tem-perature in the adhesive at an appropriate value and withinspecified limits. Exceeding the maximum specified tempera-ture may result in a joint with poor long-term properties.Curing temperatures below the specified minimum may resultin an adhesive with a low strength. Of equal importance iskeeping the work dry.

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10.7 MIXING AND APPLICATION OFADHESIVE

10.7.1 General

All equipment used for the mixing and application of theadhesive and materials should be kept clean and maintainedin good operating condition. All operatives should be suitablytrained in the use of such equipment.

The mixing and application of the adhesive should be strictlyin accordance with the manufacturer’s instructions. (Figure41 shows adhesive being mixed using a power tool with asuitable attachment.) In particular, the amounts of materialsmixed at any one time should not exceed the specifiedamounts, as larger volumes will lead to higher temperaturesbeing generated, which will reduce the pot life. Resin andhardener have to be mixed together in defined proportions orthe properties of the cured adhesive will be impaired. Henceprebatched quantities of resins and hardeners should be used.

Figure 41: Mixing adhesive.

The components should be thoroughly mixed together. Someadhesives are supplied with resin and hardener of differentcolours. This makes it easier to check that thorough mixinghas been achieved.

The volume of adhesive mixed at one time must be such thatit may be applied and the surfaces brought together withinthe pot life of the adhesive. Any adhesive remaining at theend of the specified pot life must be discarded.

10.7.2 Application to substrate prior to plate installation

Where the concrete surface is to be strengthened using FRPplates, the mixed adhesive is applied to the bonding area byhand, using plastering techniques (see Figure 42). The thick-ness of the adhesive should be maintained at 1–2mm.

Figure 42: Application of adhesive to concrete surface.

10.7.3 Application to FRP plates

Before installation, FRP plates should be checked visuallyfor signs of damage, such as cracks or delamination. Thesurface of the plate should be prepared immediately beforeapplication of the adhesive, in accordance with the manu-facturer’s recommendations. This may involve light abrasionand cleaning with a solvent. Some materials are manu-factured with an additional peel ply, which, on removal,exposes a clean surface with the appropriate roughness. Thisis the preferred approach since no additional treatment at siteis required.

The adhesive layer should be applied to the plates to form aslightly convex profile across the plate. The extra thicknessalong the centre-line helps to reduce the risk of voidformation. A method of application is shown in Figure 43.

Figure 43: Application of adhesive layer on to fibrecomposite plate.

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10.7.4 Application to substrate prior to fabric installation

Where the concrete surface is to be strengthened using FRPfabric, the bonding adhesive is applied using a hand-heldfoam roller or brush. This should be evenly applied tosaturate the concrete surface and promote adhesion of thefabric material.

10.7.5 Application to FRP fabrics

Fabric can be readily cut to size using simple tools (seeFigure 44). Dry fabric can be directly applied to the resin-saturated concrete surface without adhesive being applied tothe fabric. For wet fabric, the resin is applied to the fabricbefore it is installed. This resin can be applied to the fabricusing hand-held foam rollers (see Figure 45), brushes or im-pregnation machines (see Figure 46). Alternatively, vacuum-assisted resin infusion can be used to form the composite insitu. The fibres are applied to the structure dry. The area issealed with a rubber sheet and a vacuum used to draw in theresin.

Figure 44: Cutting fabric.

Figure 45: Applying resin using roller.

Figure 46: Impregnation of fabric.

10.7.6 Inserting adhesive into slots for NSM

reinforcement

When NSM reinforcement is installed on the top surface ona member, adhesive is simply poured into the slot to a depthequal to approximately the eventual mid-section of the NSMrod or strip. The adhesive needs to be sufficiently fluid toflow into the slot without entrapping air. For installation over-head or on vertical surfaces, a stiffer adhesive is requiredwhich will not ‘slump’ significantly. Installation will be bymeans of an adhesive ‘gun’.

10.8 ASSEMBLY AND VISUALINSPECTION

10.8.1 Installation of FRP plates

Immediately after application of the adhesive the fibrecomposite plate should be brought into contact with theconcrete substrate. There is sufficient ‘grab’ in the adhesiveto hold the fibre composite material in position, and no othertemporary support is usually needed.

Even pressure is applied by roller (as shown in Figure 47)starting at one end along the longitudinal centre-line andworking outwards to expel excess adhesive at the edges andto produce an even glue line. A final adhesive thickness of1.5–2mm is ideal in most cases. Excess adhesive is removedusing scrapers, cloths and solvents.

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Figure 47: Installing FRP plates, using a roller to applypressure.

Where plates are lapped, the minimum overlap, in thedirection of the fibres, should be in accordance with themanufacturer’s recommendations, but not less than 200mm.Chapter 6 gives more detailed information on lap lengths.The spacing of FRP plates on the soffit or top surface of aslab should not exceed 0.2 x span or 5 x slab thickness.

Immediately after assembly, the joint should be inspected.The aim is to check that a continuous and uniform layer ofadhesive is visible. In some situations the soundness of theinstalled adhesive layer can be checked by tapping thecomposite with a small object such as the edge of a coin.Voids or gaps give a characteristic sound. Further informa-tion is given in Section 4.3 of TR57(5). If defects are foundtechniques such as vacuum filling with a suitable resin orplate overlapping could be used as a repair. Further informa-tion is given in Section 6.2 of TR57.

10.8.2 Installation of FRP fabrics

The dry fabric is wrapped tightly over the concrete substrate,avoiding any wrinkles. Figure 48 shows fabric being wrappedround an arched member and Figure 49 shows the wrappingof a column. After application, the fabric is rolled to forcethe adhesive through the fibres and to expel any air (seeFigure 50).

If required, a second and further layers of fabric can be appliedin a similar fashion, making sure that each successive layeris fully saturated with resin. Finally, a layer of epoxy adhe-sive may be applied to encapsulate and protect the compositematerial. Alternatively, the fabric can be impregnated withresin, i.e. wet fabric, and then wrapped around the member.As before, the surface should be rolled to remove wrinklesand to expel air.

Figure 48: Wrapping fabric round an arched member.

Figure 49: Wrapping fabric round column.

Figure 50: Rolling fabric to consolidate layers.

The minimum overlap for fabric materials, in the direction ofthe fibres, should be in accordance with the manufacturer’srecommendations, but should not be less than 200mm. TheFIB guide(90) suggests a maximum of five layers in a givendirection. However, some suppliers suggest that more layersmay be used. Caltrans (the California Department of Trans-portation) permits up to 14 layers. Advice should be soughtfrom the supplier.

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10.8.3 Installation of NSM reinforcement

The NSM reinforcement is placed in the slot and pressed intothe adhesive, which partly fills the slot, to the correct depth.The remaining void in the slot is then filled with moreadhesive (taking care to avoid trapping air) and finishedflush with the concrete surface.

10.9 CONTROL SAMPLES

Tests should be carried out on samples obtained from eachbatch of adhesive and on the composite materials for testingby an independent laboratory to confirm the properties of thematerials used. These tests should be in accordance withagreed national or international standards, as detailed inAppendix B of TR57(5).

Figure 51 shows a double-lap shear test, which is closelyrelated to the actual site use of fibre composite plates. As theembedded studs may cause premature failure of the concreteblocks, alternative methods are being developed for loadingthe specimens, for example by the University of Glamor-gan(177). The test can be carried out using a standard universaltesting machine and can be carried out, relatively easily, atdifferent temperatures. Compatibility of fibre, or composite,and adhesive can be tested, as well as adhesion to concrete.Additionally, pull-off tests can be performed to check theadhesion of the adhesive to the substrate.

Figure 51: Double-lap shear test.

BS EN 1504(17) will be a product standard for materials forthe repair and protection of concrete. (Currently some partshave yet to be published as British Standards.) It will detailthe required properties of the materials and the tests that arerequired to demonstrate conformity.

10.10 NON-DESTRUCTIVE TESTS

Various non-destructive tests may be used to inspect acompleted and cured bond for surface-bonded materials.However, there is no regime of non-destructive testing that

can guarantee the soundness of an application. Full details ofavailable tests are given in TR57(5). The most common isacoustic sounding (hammer tapping). Thermography may beused to survey large areas. Other methods, such as ultrasonictesting, are being developed. However, there are currently nonon-destructive methods that are capable of detecting pooradhesion, which might lead to failure of the joint in the longterm. To provide assurance about long-term performance,additional pull-off dollies could be installed at the time ofstrengthening. Pull-off tests could be performed on these atvarious times in the future to monitor the adhesion betweenthe adhesive and the substrate. Similarly, additional double-lap shear test specimens could be prepared at the time ofstrengthening, placed securely on site, and tested at varioustimes in the future to monitor the adhesion between the FRPmaterial and the adhesive.

Some documents, such as TR57(5) and ACI 440.2(2), suggestthe extent of delamination that may be acceptable. However,the acceptable extent will be very dependent on the type ofstrengthening and the location in the structure. For example,an area of delamination in the wrapping of a column willprobably have a limited effect on the performance whiledelamination of a plate on the soffit of a beam, particularlyat points of high shear, will have a significant effect.

Currently there are no techniques for inspecting NSM strength-ening systems after installation, apart from a visual check formajor voids.

For major structures, it may be appropriate to install instru-mentation prior to the strengthening. Measurements of thedifference in the response of the structure under a load testbefore and after strengthening can be compared with predic-tions and the instrumentation used to monitor changes withtime.

10.11 APPLICATION OF OVER-COATINGS

When over-coatings are to be applied, these should be com-patible with the underlying composite material and approvedfor use by the manufacturer. These over-coatings may beapplied for the following reasons:

• Fire protection. Regulations may require the applicationof an over-coat layer, which has been tested on the fully-cured composite system.

• Protection against vandalism. Where the FRP materialmay be vulnerable to damage, it may be encapsulated ina cementitious or epoxy mortar either spray- or hand-applied.

• Appearance. A cosmetic over-coating can be applied tothe composite material to match the existing structure.

• Protection against ultraviolet radiation. The manufac-turer should be consulted for advice on the UV resistanceof the FRP. If the manufacturer recommends UV protec-tion, a cementitious, or other, over-coating can be applied.

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• Reducing solar gain. In exposed situations the FRPcould be painted white or shrouded to reduce solar gainand the consequent temperature rise.

• Other reasons. In certain circumstances the FRP mate-rial may be encapsulated by other structural finishes or,on bridge decks, covered by the waterproofing.

Figure 52 shows a sprayed mortar over-coat being applied toFRP plates on the soffit of Dudley Port Bridge.

Figure 52: Spray application of mortar over-coating.

10.12 IDENTIFICATION/WARNING SIGNS

There may be a risk that the fibre composite will be damagedby other work carried out on the structure. For example,holes drilled though to take the fixings for a false ceilingwould seriously affect the capacity of the fibre composite;because of stress-concentrations around the hole, drillingwill lead to a loss in strength equivalent to a reduction insection of two- or three-hole diameters. When FRP is bondedto the upper surface of a member and covered by a surfacingwith a limited life, removal of the surfacing may lead todamage of the FRP. In such cases suitable identifi-cation/warning plates or other markings should be fixed onor adjacent to the composite (see Figures 53 and 54). Wherean over-coating layer is applied, the plates should, wherepossible, be placed on the exposed surface.

Figure 53: Example of warning printed on carbon fibre plate.

Figure 54: Examples of proposed warning plates fixed tostructure adjacent to strengthened area.

10.13 RECORDS

Detailed records should be kept of the work carried out, asrequired under the CDM Regulations(26), and added to theHealth and Safety File, which should include details of anyfuture inspection and testing regime that is considered appro-priate. Details of the records that should be kept, along withsuggested proformas and check lists, are given in TR57(5).

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11 LONG-TERM INSPECTION,MONITORING AND MAINTENANCE

11.1 INSPECTION AND MONITORINGREGIME

Full details on the requirements for inspecting and moni-toring structures strengthened with FRP are given in TR57(5),which includes checklists for the aspects to be considered atvarious stages and a standard proforma for recording inspec-tion data. The following Sections summarise the informationin TR57, which should be read in parallel with this document.

As with all structural elements there will be a need to checkthe fibre composite strengthening system as part of the regularinspection and monitoring of the structure. Such inspectionsare already carried out for bridges, with general (visual)inspections annually and detailed inspections every six yearsor so. However, buildings are rarely inspected on a regularbasis, inspections often being carried out only when there isa change of use or of ownership. It is strongly recommendedthat all building owners should instigate a regular inspectionregime for strengthened elements.

Information on the materials used in the strengthening shouldbe included in the Health and Safety File for the structure.This File should also include details of any initial faults inthe fibre composite strengthening, such as minor areas ofdelamination, and should indicate those regions of thestrength-ening that are critical, such as anchorage zones. Thestructural engineer responsible for designing the strength-ening should indicate the action to be taken in the event ofany likely forms of damage to the composite material. Anexample of this might be damage to fibre composite materialon the soffit of a bridge following impact by an over-heightvehicle. The action to be taken will be specific to the parti-cular structure as it will depend on the amount of damageand the extent to which the structure has been strengthened.Hence no general guidance can be given in this Report.

It is strongly recommended that additional samples of thefibre composite material should be bonded to the structureaway from the region to be strengthened. (This approach hasbeen adopted on a number of structures including the BarnesBridge in Manchester and the John Hart Bridge in BritishColumbia (see Section 4.3).) Additionally, or alternatively,FRP can be bonded to concrete samples, such as shortbeams, which can be stored on or adjacent to the structure.Samples can be inspected and tested as part of the inspectionregime. To aid inspection, some or all of the samples shouldnot be covered with any protective layer. They should thus

indicate a lower bound to the performance of the compositesbonded to the main structure. Details should be included inthe Health and Safety File along with recommendations forthe frequency of testing.

Finally, the Health and Safety File should include details ofany instrumentation that was installed as part of the strength-ening exercise, along with any data obtained before and afterstrengthening.

11.2 FREQUENCY OF INSPECTIONS

The intervals between inspections recommended below,taken from Chapter 1 of TR57(5), should be taken only as aguide. Structures in aggressive environments will requiremore frequent inspection. Special structures may require aspecial inspection regime, the frequency and extent of whichbeing determined by a risk assessment.

Routine, visual inspection

The recommended intervals for routine visual inspection areas follows:

Bridges Every yearBuildings Every yearOther structures Depends on the use of the structure

but ideally every year.

Detailed inspection with testing

In the absence of other guidance, detailed inspections shouldbe carried out at intervals as follows:

Bridges At least every six yearsBuildings At change of occupancy or change

of use, when structural work orrefurbishment is carried out on thebuilding, but at intervals not excee-ding ten years

Other structures Depending on the use of the struc-ture but at least every ten years.

Detailed inspections should be carried out more frequentlyin the first few years after installation, to give the owner ofthe structure confidence that the strengthening has beencarried out satisfactorily.

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11.3 ROUTINE VISUAL INSPECTION

Information on routine visual inspection is given in Section5.2 of TR57. The surface of the fibre composite should beinspected for signs of crazing, cracking or delamination,which would indicate some level of overall deterioration.The composite should be inspected for local damage, forexample caused by impact or abrasion. In addition, of course,the inspection should look for signs of the deterioration ofthe concrete structure itself, such as additional cracking orcorrosion.

Where the composite has been covered with over-coating, itwill not be possible to directly inspect the composite.Damage to the protective layer will suggest the possibility ofdamage of the composite. In general, it will not be appro-priate to remove the protective layer as this may cause damageto the fibre composite. Thus any inspection of the compositewill have to be limited to the control samples.

Identification/warning labels (see Section 10.12 and Figures54 and 55) should be checked and missing ones should bereplaced. This is particularly important where there is thelikelihood of future work that could damage the fibre compo-site material, such as the installation of fixings for services.

The fibre composite may have been covered by paint orother form of protective layer, e.g. for protection from ultra-violet light, which will have a limited life. It will be necessaryto check the condition of this layer and to replace it whenrequired, in accordance with the supplier’s recommendations,with a material that is compatible with the fibre composite.

11.4 DETAILED INSPECTION

Information on detailed inspection and testing is given inSection 5.3 of TR57. Debonding of the fibre compositematerial from the concrete may be determined by tapping orthermography, as indicated in Section 10.10. However, thereare currently no simple, non-destructive tests that can beused to assess the condition of the adhesive. This is bestdetermined by carrying out pull-off tests on the controlspecimens at regular intervals. These tests should be carriedout as part of the detailed inspection, though there may be arequirement to test samples more frequently, at least duringthe early period after the strengthening.

Instrumentation may have been installed as part of theassessment process, for example to measure strains due tolive loading on the structure. In addition, instrumentationmay have been installed on the structure at the time ofstrengthening, to enable the response to be compared withthat predicted. Such instrumentation can be used to indicatechanges in the response. If significant changes are observed,it will be necessary to identify whether they are due tochanges in the strengthening system (such as delamination)or due to overall changes in the concrete structure (such asadditional cracking or corrosion) so that appropriate actioncan be taken. It will be necessary for the structure to be re-analysed by a structural engineer to determine what remedialaction may be required.

When local areas of damaged composite are identified, theymay be repaired by techniques such as vacuum filling with asuitable resin (taking care not to further damage the mate-rial) or plate overlapping. When major damage is identified,such as peeling and debonding of large areas, it may benecessary to remove the defective material and adhesive.The defective material should be removed over a sufficientlylarge area such that material on the periphery is fully bonded.The concrete surface should then be prepared again andfurther FRP installed. It will be necessary to provide anadequate overlap between the new and old material at theperiphery of the repaired area. Where these repair techniquesare used, it is crucial to check the compatibility of the repairmaterial with the materials already in place. In addition tocompatibility, the repair material must have similar charac-teristics to the material in place. Such characteristics includefibre orientation, volume fraction, strength, stiffness and over-all thickness. Some additional information on repair is givenin Section 6.2 of TR57.

11.5 MAINTENANCE

The nature of FRP materials means that they should needlittle or no maintenance while in service. However, as indi-cated in Section 2.3 of TR57, moisture is one of the mostdamaging elements and so all gutters, drains, etc., must bekept clear of debris, so that rainwater is carried off thestructure and away from the FRP. If any cleaning is carriedout near the FRP, it must be checked that any solvents usedwill not cause damage. Cleaning techniques such as water-jetting or grit-blasting are not appropriate as they are likelyto cause damage to the FRP.

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production, installation and servicing, 1994, BSI,London, 20pp.

176. BRITISH STANDARDS INSTITUTION. BS 1881:Testing concrete, Part 207. Recommendations for theassessment of concrete strength by near-to-surfacetest, 1992, 20pp.

177. UNIVERSITY OF GLAMORGAN. Design of

bespoke testing equipment to evaluate lap shear test

on concrete specimens/FRP specimens, School ofTechnology Report, November 1999.

178. BRITISH STANDARDS INSTITUTION. BS EN 923,Adhesives – terms and definitions, BSI, London, 1998.

179. AMERICAN SOCIETY FOR TESTING ANDMATERIALS. ASTM D 907. Standard terminology of

adhesives, American Society for Testing and Mate-rials, Philadelphia, USA.

89

APPENDIX A

GLOSSARY OF TERMS

As many readers of this Report may be unfamiliar with fibrecomposites and with adhesive technology, many of the termsused are defined below. A more extensive glossary of adhe-sive terms is given in BS EN 923(178) and ASTM D 907(179).

Adhesive – A polymeric material which is capable of holdingtwo materials together by surface attachment.

Aramid – A manufactured fibre in which the fibre- formingsubstance consists of a long-chain synthetic aromatic poly-amide.

Bond – The adhesion of one surface to another, with the useof an adhesive or bonding agent.

Carbon fibre – Fibres produced by the pyrolysis of organicprecursor fibres such as rayon, polyacrylonitrile (PAN) orpitch in an inert atmosphere. The term is often used inter-changeably with graphite. However, carbon fibres and gra-phite fibres differ in the temperature at which the fibres aremade and heat-treated, and the carbon content.

Composite or composite material – A combination of high-modulus, high-strength and high-aspect-ratio fibre reinfor-cing material encapsulated by and acting in concert with apolymeric matrix.

Cure – To change the properties of an adhesive irreversiblyby chemical reaction into a more stable condition and todevelop the desired properties.

Epoxy resins – Resins which may be of widely differentstructures but which are characterised by the reaction of theepoxy group to form a cross-linked hard resin.

Fabric – Non-woven – A textile structure produced bybonding or interlocking of fibres, or both, accomplished bymechanical, chemical, thermal or solvent means and combi-nations thereof.

Fabric – Woven – A generic material construction con-sisting of interlaced yarns or fibres, usually a planar structure.

Filament winding – A reinforced plastics process that em-ploys a series of continuous resin-impregnated fibres appliedto a mandrel in a predetermined geometrical relationshipunder controlled tension.

Filler – A relatively inert substance added to an adhesive toalter its physical, mechanical, thermal, electrical or otherproperties or to lower the cost.

FRP – Fibre-Reinforced Plastics (or Polymers)

Glass fibre – A fibre spun from an inorganic product of fusionwhich has cooled to a rigid condition without crystallising.

Glass transition temperature (Tg) – The approximate mid-

point of the temperature range over which a polymeric adhe-sive changes from a relatively stiff and brittle material to aviscous material.

Hand lay-up – A process in which resin and reinforcementare applied either to a mould or to a working surface andsuccessive layers built up by hand.

Hardener – The curing agent or catalyst, which promoteschemical cross-linking with the resin in two-componentadhesive systems.

Laminate – A layer of fibre composite, either preformed orformed in situ.

NSM – Near-surface-mounted reinforcement.

Peel ply – The outside layer of a reinforced plastic material,which is removed to aid bonding.

Plate – Preformed prismatic FRP plate, formed by pultru-sion or manufacturing process, generally with all the fibresarranged in the longitudinal direction.

Polymeric – Adjective describing a material (most com-monly organic) composed of molecules characterised by therepetition of one or more types of monomeric units.

Pot life – The period of time during which a multi-part adhe-sive can be used after mixing the components. (Note: Thepot life varies with the volume and temperature of the mixedadhesive and the ambient temperature. The term ‘pot life’ isalso used for the application of hot-melt adhesives for theperiod for which an adhesive, ready for use, remains usablewhen kept at normal operating temperature.)

Prepreg – Reinforcing fibres in sheet or roll form impreg-nated with resin and stored for use.

Primer – Material used to protect a surface prior to theapplication of the adhesive, improve adhesion and/orimprove the durability or to stabilise/protect the substrate.

Pultrusion – A continuous process for the manufacture ofcomposite profiles by pulling layers of fibres, impregnatedwith a thermoset resin, through a heated die, thus formingthe ultimate shape of the profile.

Resin – The reactive polymer base in adhesive and prepregmatrix systems.

Substrate – The material of the adherend adjacent to theadhesive layer.

Thermoset – A resin that is substantially infusible andinsoluble after being cured.

UHM – Ultra high modulus.

Wet lay-up – A method of making a reinforced product byapplying a liquid resin system while the reinforcement is putin place, layer by layer.

91

APPENDIX B

SYSTEMS AVAILABLE IN THE UK

The following Tables give information on some of thestrengthening systems currently available in the UK fromsupporters of the Project. The properties given are takenfrom manufacturers’ and suppliers’ data sheets and arethought to be correct at the time of publication. For designpurposes, actual properties must be obtained from the manu-

facturer. As test methods vary, the information should detailthe basis for the information (e.g. frequency of testing,standard deviation).

In addition, strengthening materials are available from othermanufacturers and suppliers, who should be contacted forthe appropriate information.

Table B1: Suppliers of strengthening materials.

Supplier Trade name Type of material

Exchem Selfix Carbofibe Carbon FRP platesCarbon fibre rodsCarbon fibre sheetAramid fibre sheetGlass fibre sheet

Degussa MBrace Carbon fibre sheet(formerly MBT Feb) MBrace Carbon FRP plates

MBrace Ultra high modulus carbon platesMBrace Glass fibre sheetKevlar* Aramid fibre tape and sheetMBar Carbon fibre rods

Fyfe Tyfo Fibrwrap Systems Carbon fibre sheetCarbon FRP platesCarbon/aramid hybrid fibre sheetCarbon bi-directional fibre sheetGlass fibre sheetGlass FRP platesGlass/aramid hybrid fibre sheetGlass bi-directional fibre sheetAramid fibre sheetFRP anchors

Sika Sika CarboDur Carbon FRP platesSika CarboDur DML UHM bespoke carbon fibre platesSikaWrap Hex 230C Carbon fibre sheetSikaWrap Hex 100G Glass fibre sheet

weber building solutions Enforce Carbon FRP platesCarbon fibre sheetGlass fibre sheetAramid fibre sheetUltra high modulus carbon FRP plates

Toray Europe Ltd. Torayca UT70 Carbon fibre sheet

* In association with Du Pont.

Table B2: Properties of fibre composite sheet materials.

Trade name Fibre Strength Modulus Areal weight Effective Width

(N/mm2) (kN/mm2) (g/m2) thickness† (mm) (mm)

Enforce Carbon 3800 240 200 0.117 300

Carbon 2650 640 400 0.190 300

Glass 1700 65 350 0.135 680

Aramid 2400 120 290, 420, 650, 0.2, 0.29, 0.45, 300850 0.59

MBrace C130 Carbon 3550 235 300 0.11, 0.165 300, 500

MBrace C530 Carbon 3000 390 300 0.165 300, 500

MBrace* Carbon 3800 240 200 0.117 300

MBrace* Carbon 2650 640 400 0.190 300

MBrace AR55 Glass 1550 74 915 0.118 500, 1000

MBrace Kevlar Aramid 2100 120 320, 450, 650 0.193, 0.286, 100, 300, 500AK40, 60, 90 0.430

Selfix Carbofibe E Glass 3450 73 432 0.167 150, 300 & bespoke

Selfix Carbofibe C Carbon 4900 230 300 0.167 150, 300& bespoke

Selfix Carbofibe AR Aramid 2900 100 240 0.167 150, 300& bespoke

SikaWrap 230C Carbon 4100 230 220 0.12 300, 600

SikaWrap 103C Carbon 3900 230 610 0.34 600

SikaWrap 100G Glass 2300 76 935 0.36 600

SikaWrap 450A Aramid 2880 100 450 0.31 300

SikaWrap 300A Aramid 2880 100 300 0.21 300

Torayca UT70-20 Carbon 4090 230 200 0.111 100, 250, 500, 1000

Torayca UT70-30 Carbon 4220 235 300 0.167 100, 250, 500, 1000

Tyfo SEH -51 Glass/Aramid 3238 72 915 0.36 1372hybrid

Tyfo SEH-51A Glass 3238 72 915 0.36 1372

Tyfo SEH-25A Glass 3238 72 505 0.19 1372

Tyfo WEB Bidirectional 3238 72 295 0.116 1270glass

Tyfo BC Bidirectional 3238 72 813 0.32 1372±45° glass

Tyfo SCH-41 Carbon 3789 230 644 0.28 610

Tyfo SCH-41s Carbon/Aramid 3789 230 644 0.28 610glass

Tyfo SCH-11UP Carbon 3789 230 298 0.127 610

Tyfo SCH-7UP Carbon 3789 230 200 0.08 610

Tyfo CWEB Bidirectional 3445 228 162 0.116 1270carbon

Tyfo WAB Aramid 3098 114 176 0.116 1270

Tyfo SAH-41 Aramid 3098 114 650 0.36 300

Notes: The properties are for dry fibres. The values should be treated as indicative only. See also note to Table B3.*Available in Europe and the Middle East; not available in the UK.†The effective thickness is the total cross-sectional area of the fibres divided by the width of the sheet or tape.

Design guidance for strengthening concrete structures using fibre composite materials

92

Appendix B

93

Trade name Fibre Strength Modulus Thickness Width

(N/mm2) (kN/mm2) (mm) (mm)

2800–3000 165 1.2, 1.4 10, 50, 80, 90, 100, 120Enforce Carbon 2400–2600 210 1.2, 1.4 50, 80, 90, 100, 120

Bespoke plates >300 Bespoke up to 50mm Bespoke up to 1m

MBrace LM Carbon 1800 140 1.2, 1.4 50, 80, 100, 120

MBrace MM Carbon 3100 170 1.4 50, 80, 100, 120, 150

MBrace HM Carbon 2400 250 1.2, 1.4 50, 80, 100, 120, 150

MBrace UHM Carbon 4500 150 Bespoke up to 48mm 50, 100, 150, 200

MBrace 150* Carbon 2800–3000 165 1.2, 1.4 10, 50, 80, 100, 120

MBrace 200* Carbon 2400–2600 210 1.2, 1.4 10, 50, 80, 100, 120

Selfix Carbofibe S Carbon 2800 150 1.2, 1.4 & bespoke 50, 80, 120

Selfix Carbofibe M Carbon 3200 200 1.2, 1.4 & bespoke 50, 80, 120

Selfix Carbofibe H Carbon 1600 280 1.2, 1.4 & bespoke 50, 80, 120

Sika CarboDur S Carbon 3100 165 1.2, 1.4 50, 60, 80, 90, 100, 120, 150

Sika CarboDur M Carbon 3100 210 1.4 50, 60, 90, 100

Sika CarboDur DML Carbon 1110 360 Bespoke Bespoke

Tyfo UC Carbon 2790 230 1.2, 1.4, bespoke 50, 100, bespoke

Tyfo UG E-glass 890 41 1.2, 1.4, bespoke 50, 100, bespoke

Table B3: Properties of composite plate materials.

Note: These properties are taken from manufacturers’ data sheets and are thought to be correct at the time of publication(2004). For design purposes, actual properties must be obtained from the manufacturer. As test methods vary, the informationshould detail the basis for the information (e.g. frequency of testing, standard deviation).*Available in Europe and the Middle East; not available in the UK.

Table B4: Properties of NSM rods and strips.

Supplier Trade name Dimensions Strength Stiffness

(mm) N/mm2 kN/mm2

Degussa Mbar Galileo 7.5 dia. 2300 130

Mbar Leonardo 7.5 dia. 2000 200

Hughes Brothers Aslan 2 x 16 2070 130

weber building solutions Enforce 1.4 x 10 2800 165

Design guidance for strengthening concrete structures using fibre composite materials

94

Table B5: Properties of epoxy adhesives.

Property Supplier and trade name

Degussa Exchem Fyfe weber building Sika

solutions

MBrace Resifix 31 Selfix Tyfo WS Tyfo TC Epoxy Plus SikaDur

laminate Carbofibe

adhesive Adhesive

Tensile strength (N/mm2) 32 24 23 51 47 19 30

Flexural strength (N/mm2) >35 55 50 86 80 35

Shear strength (N/mm2) 22 Currently 18 15under test

Flexural modulus (kN/mm2) 10 6.5 Currently 2.2 2.1 9.8 12.8under test

Shear modulus (kN/mm2) 3.8 Currentlyunder test

Glass transition temperature, Tg, (°C) 65 60 Currently 93 82 60, 120 82

under test

Notes: See note to Table B3.

Table B6: Properties of laminating resins.

Property Supplier

MBT weber Fyfe Tyfo S Exchem SikaDur SikaDur

building 330 300

solutions

Tensile strength (N/mm2) 50 17 72 60 30 45

Flexural strength (N/mm2) 120 28 123 100

Flexural modulus (kN/mm2) 3 5 3 3 3.8 3

Glass transition temperature, Tg, (°C) 55 60, 120 93 64 53 60

Notes: See note to Table B3.

95

APPENDIX C

QUALITY CONTROL OF MATERIALS

This section is concerned with the quality control of thematerials used for strengthening. It is not intended to be aspecification, but covers some of the significant points thatshould be included in a specification. The manufacturershould supply characteristic values of the mechanicalproperties to be used for design purposes (e.g. strength,elastic modulus) which should be taken as the mean valueminus 2 standard deviations. Sufficient tests should becarried out at regular intervals to ensure that this isstatistically valid.

Strengthening materials

General

• All materials should be produced under an approvedquality scheme, such as ISO 9000.

• All fibres, resins, composites and other materials shouldbe in accordance with the relevant ISO specifications,Euronorms or other equivalent international standard.

• Traceability of all materials should be ensured; all mate-rials should be supplied with a certificate of conformity.

• All external or independent testing should be carried outin Approved Laboratories in accordance with internationalstandards or by the manufacturer under an approvedquality scheme.

• All testing should be carried out in accordance with therelevant ISO Standard, Euronorm or other internationallyaccepted standard. Where no such standards exist, anindustry or company standard or method with a recog-nised history should be used.

• Testing should consist of visual checks on the basicmaterials (though this will provide limited information)and, where appropriate, physical tests on the finishedelements as detailed below.

Fabric materials

• The properties of a specified width of finished materialshould be checked by testing samples.

• The frequency of testing should be stated in the QualityPlan. A minimum of one sample should be taken at thestart and finish of each production run.

• The sample should be weighed to determine the weightper square metre.

• The elastic modulus and tensile strength should bedetermined directly by testing the sample.

• Samples of unidirectional fabric sheet may be made intolaminates, using the appropriate specified resin and thelaminate tested to determine the properties.

• Where appropriate, properties should be determined in thetransverse direction as well as in the longitudinal direction.

• All sheets and tapes should be supplied with a certificateof conformity.

• All individual rolls of material should be appropriatelylabelled.

Pultruded plates

• For pultrusion, the supply of fibres to the pultrusion lineshould be monitored on a regular basis; this should be atleast once per hour.

• All resins, hardeners, etc., should be used strictly inaccordance with the manufacturer’s instructions.

• The speed of processing, processing temperature, etc.,should be maintained within agreed limits and should bechecked and recorded regularly.

• The properties of the plate should be checked by testingsamples.

• The frequency of testing should be stated in the QualityPlan. A minimum of one sample should be taken at thestart and finish of each production run.

• The samples should also be checked for dimensionalaccuracy.

• Plates should be marked with a unique batch number atregular intervals.

• All pultruded plates should be supplied with a certificateof conformity.

Prepreg plates

• Cure schedules should be monitored and recorded.Records should be maintained to demonstrate compliancewith the required procedure.

• The finished product should be checked visually fordefects.

• The finished product should be checked dimensionallyand by mass to ensure compliance.

• Plates should be marked with a unique batch number.• All plates should be supplied with a certificate of

conformity.

Shells

• Shells may be fabricated by filament winding or by handlay-up (or similar process) against a positive or negativemould.

Design guidance for strengthening concrete structures using fibre composite materials

96

• The finished product should be checked visually fordefects such as pin-holes and blisters.

• The dimensions of the finished product should be checked.• The thickness should be measured at agreed locations.• Trial pieces should be made and tested. These may be

either sacrificial parts of the unit that can be removed fortesting or specially prepared samples made at the sametime and by the same process. In the latter case, thesamples may be formed on a flat surface to aid testing. Inboth cases care must be taken to ensure that the sample isrepresentative of the material in the finished unit.

• The frequency of testing should be stated in the QualityPlan.

• The samples should be weighed to check the density ofthe product.

• The samples should be tested in direct tension, in theappropriate direction, to determine the elastic modulusand the tensile strength.

Specials (shear straps, etc.)

• Specials may be fabricated by any appropriate method.• Testing should be in line with the recommendations for

shells.• Testing should be carried out on complete elements by an

appropriate method to determine the tensile strength andelastic modulus.

• The frequency of testing should be stated in the QualityPlan.

Site requirements

As detailed in Chapter 10 and TR57, some site testing willbe required. The following points should be included in anyspecification.

• All work should be carried out in accordance with anagreed Quality Plan, in accordance with ISO 9000 orsimilar.

• All materials should be accompanied by a certificate ofconformity.

• All materials, adhesives, laminating resins, etc., should bestored and used strictly in accordance with the manu-facturer’s instructions.

• Accurate records should be maintained of all materialsused (e.g. delivery notes, batch numbers) and, whererequired, the ambient conditions (e.g. temperature,relative humidity).

• Any independent testing that is required by the Clientshould be carried out in Approved Laboratories in accor-dance with international standards or by the manufacturerunder his approved quality scheme.

Plates

• Visual checks should be carried out on the plate to ensurethat the plate material is as specified.

• The plate should be checked for local damage.• When bonded, the plate should be checked by tapping or

other means to ensure continuous adhesion.

Wet lay-up laminates

• Visual checks should be carried out on mats, unidirec-tional tapes/fabrics, woven rovings and multi-axial fabricsto ensure uniformity and conformity.

• The completed laminate should be checked visually fordefects.

• When required by the contract, trial pieces should bemade at the same time and by the same process. Careshould be taken to ensure that the trial pieces are repre-sentative of the material in the finished unit.

• The frequency of testing should be as agreed with theClient.

• The samples should be tested to determine the elasticmodulus and the tensile strength.

97

APPENDIX D

SPECIALIST SUPPLIERS, CONTRACTORS, CONSULTANTS, UNIVERSITIES ANDOWNERS

This Appendix gives contact details of the organisations involved with the preparation of this Report. It is believed to be correctat the time of going to press, but readers should check the information at the time of use.

Specialist suppliers

Degussa (formerly MBT Feb)

Albany HouseSwinton Hall RoadSwintonManchester M27 4DTTel: +44(0)161 727 2731Fax: +44(0)161 794 0944www.mbtfeb.co.uk

Exchem Mining and Construction

Ventura WorksP O Box 7AlfretonDerbyshire DE55 7RETel: +44(0)1773 540440Fax: +44(0)1773 607638e-mail: emc@exchem.comwww.exchem.com

Fyfe Company LLC

“The Fibrwrap® Company”Nancy Ridge Technology Center6310 Nancy Ridge Drive Suite 103San DiegoCA 92121-3209USATel: +1(1) 858 642 0694Fax: +1(1) 858 642 0947e-mail: info@fyfeco.comwww.fyfeco.com

Fyfe Europe Ltd

28 Aristippou StreetGlyfada 16674Athens, GreeceTel & Fax: +(30) 210 064 3402e-mail: fyfe.europe@mail.gr

Fyfe Asia Pte. Ltd

10 Toh Guan Road #03-10 T.T.International Tradepark608838 SingaporeTel: +65 95 898 5248Fax: +65 95 898 5181e-mail: fyfeasia@singnet.com.sg

Sika

WatchmeadWelwyn Garden CityHerts AL7 1BQTel: +44(0)1707 394444www.sika.co.uk

Toray Europe Ltd

3rd Floor7 Old Park LaneLondon W1Y 4ADTel: +44(0)20 7663 7779Fax: +44(0)20 7663 7777

weber building solutions

Saint-Gobain WeberDickens HouseEnterprise WayFlitwickBedford MK45 5BYTel: +44(0)8703 330070Fax: +44(0)1525 718988e-mail:mail@weberbuildingsolutions.co.ukwww.weberbuildingsolutions.co.uk

Specialist contractors

Concrete Repair Association

(Secretary Mr J Fairley)Association House99 West StreetFarnhamSurrey GU9 7ENTel: +44(0)1252 739145Fax: +44(0)1252 739140e-mail: info@associationhouse.org.ukwww.concreterepair.org.uk

Concrete Repairs Limited

Cathite House23a Willow LaneMitchamSurrey CR4 4TUTel: +44(0)20 8288 4848Fax: +44(0)20 8288 4847Email: mail@concrete-repairs.co.ukwww.concrete-repairs.co.uk

Specialist consultants

Arup Research & Development

13 Fitzroy StreetLondon W1T 4BQTel: +44(0)20 7636 1531Fax: +44(0)20 7755 3669Email: RandD@arup.comwww.arup.com

Mouchel Parkman

West HallParvis RoadWest Byfleet KT14 6EXTel: +44(0)1932 337000Fax: +44(0)1932 356122www.mouchelparkman.com

Design guidance for strengthening concrete structures using fibre composite materials

98

Parsons Brinckerhoff Ltd

Queen Victoria HouseRedland HillBristol BS6 6USTel: +44(0)117 933 9300e-mail: FRPcomposites@pbworld.comwww.pbworld.com

Tony Gee and Partners

TGP House45-47 High StreetCobhamSurrey KT11 3DPTel: +44(0)1932 868277e-mail: ac@tgp.co.ukwww.tgp.co.uk

Universities

University of Bath

Department of Architecture & CivilEngineeringBath BA2 7AYTel: +44(0)1225 386908/6Fax: +44(0)1255 386691email: A.P.Darby@bath.ac.uk orT.J.Ibell@bath.ac.ukwww.bath.ac.uk/ace

University of Bristol

Dept of Civil EngineeringQueen’s BuildingUniversity WalkBristol BS8 1TRTel: +44(0)117 928 7707Fax: +44(0)177 928 7783www.cen.bris.ac.uk

University of Glamorgan

School of TechnologyDivision of Built EnvironmentPontypridd CF37 1DLTel: +44(0)1443 482121Fax: +44(0)1443 482169e.mail: dbtann@glam.ac.ukwww.glam.ac.uk/sot

Owners

British Energy plc

Civil Design Group – EngineeringDivision3 Redwood CrescentPeel ParkEast Kilbride G74 5PRTel: +44(0)1355 262000

Health & Safety Executive

Nuclear Safety DirectorateRoom 304, Balliol RoadBootleMerseyside L20 3JZ

Highways Agency

Safety Standards & Research DivisionRoom 201, Heron House49/53 Goldington RoadBedford MK40 3LLTel: +44(0)1234 796107Fax: +44(0)1234 796060

London Underground Ltd

84 Eccleston SquareLondon SW1V 1PXTel: +44(0)20 7027 9510

Network Rail

Civil Engineering40 Melton StreetLondon NW1 2EETel: +44(0)20 7557 8000Fax: +44(0)20 7557 9000www.networkrail.co.uk

Oxfordshire County Council

Speedwell HouseSpeedwell StreetOxford OX1 1NETel: +44(0)1865 815 641

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INDEX

Aadhesives 12-13, 69

curing 28, 41-2, 66, 73definition 89delamination 41design considerations 28design life 7electrical currents and 10flexural strengthening 31, 41-2health and safety 12, 13NSM reinforcement 37, 38, 73, 75, 77prestressed FRP composites and 65, 66properties 26, 28, 94storage and site conditions 73surface preparation 72, 73temperature 10, 12, 28, 65, 73testing 77, 80thickness 72, 74, 75workmanship and installation 12, 74-5,

76alkali-resistant glass fibre 9, 27anchorages

axial FRP 59flexural strengthening 34, 36, 37, 67material selection 13maximum capacity 34, 47, 49NSM reinforcement 37, 38-9, 41prestressed FRP composites 65, 66-7specials 12tendons and U-wraps 67wet lay-up laminates 67see also bolted plate anchors; glass fibre

anchorages; mechanical fasteningaramid fibre, definition 89aramid fibres and composites 9-10

application examples 17, 18, 20, 21, 22blast protection 18health and safety 13impact loading 29NSM reinforcement 21partial safety factors 27prestressing 65ropes 67shear strengthening 47stress rupture 65, 66suitability 14

assessment, structures 3-4, 24, 31-2, 71axial FRP 49-50, 53, 57-9, 61axial shortening 57axially loaded members 53-64

Bbeam-column connections 7, 12, 18, 58, 59beams and slabs

application examples 16-17, 18-20bridges 8, 18-20, 69buildings 16-17deep embedded bars 69flexural strengthening 31-46FRP advantages/disadvantages 5NSM bars 68shear strengthening 12, 17, 47-52, 67-8wrapping 67, 73see also moment capacity; shear capacity

bending moment capacity see momentcapacity

blast protection 14, 18, 29bolted plate anchors 66, 67-8

see also mechanical fasteningbond, definition 89bonding, electrical 10bonds

columns 53delamination 41maximum anchorage capacity 47non-destructive tests 77NSM reinforcement 37, 73prestressed FRP composites 65, 66surface preparation 72-3thick plates 39see also adhesives; anchorages;

debonding; separation failurebridges

application examples 7-8, 15-16, 18-21, 22, 28

appraisal 24beams and slabs 8, 18-20, 69deflection 40deformation 40design life 7fatigue 41, 57flexural strengthening 8, 41, 53FRP advantages/disadvantages 5, 6joints 21partial safety factors 24plates 8, 21prestressed FRP composites 67shear strengthening 7-8, 69stress-strain model 55wrapping 21see also impact damage

brittle failure 25, 31, 56, 57, 68, 69buildings 7, 15, 16-18, 40

see also beam-column connections; beams and slabs; columns; structural assessment; walls

Ccar parks 1, 7, 17, 18carbon fibre, definition 89carbon fibres and composites 9-10, 14

application examples 7-8, 16-22, 28costs 14, 18, 19durability 26, 27fabrics 18, 19, 20health and safety 13jackets 61mats 19, 28NSM reinforcement 11, 17, 19, 37, 68partial safety factors 27plates 11, 16-22, 66, 67preformed shells 53prestressed 65, 66, 69rectangular columns 61shear strengthening 17, 19, 20, 47, 68sheets 16-22stability 25stress rupture 65, 66strips 17, 18, 20tapes 21, 69

chemical resistance, fibres 9chimneys 21

see also towerscircular columns 17-18, 21, 29, 53-9, 62-4classification, material properties 6coastal structures see marine structurescoatings see over-coatingscollars 5, 59columns 5, 53-64

application examples 17-18, 20-1, 22preformed shells 11-12, 53see also beam-column connections;

circular columns; moment capacity; rectangular columns; shear capacity

compressive strengths 10, 53, 55-7see also stress-strain model

concave surfaces, separation failures 34-5concrete confinement 5, 17, 53-64

earthquake damage protection 14preformed shells 11-12torsional strengthening 69see also wrapping

concrete preparation 72-3concrete properties 25, 26concrete splitting failure 37-8concrete strength see concrete properties;

structural assessmentconformity see quality controlconnections

preformed shells 12see also beam-column connections; joints

Design guidance for strengthening concrete structures using fibre composite materials

100

consultants 97-8contractors 71, 97control samples see samplescorrosion 1, 4, 17, 21, 80costs 4, 7-8

carbon systems 14, 18, 19NSM reinforcement 1, 14, 37preformed shells 12

cracks and crackingapplication examples 18, 19, 21, 67crack widths 25, 28, 40, 67design considerations 28flexural strengthening 40inspection 80NSM reinforcement 37, 38prestressed carbon FRP straps 69separation failure 34, 38shear strengthening 49

curingadhesives 28, 41-2, 66, 73see also temperatureconcrete repairs 71definition 89

Ddebonding

application example 17bolted plate anchors and 67, 68flexural strengthening 31, 39inspection for 80lap joints 56, 59NSM reinforcement 37shear strengthening 49, 56surface preparation 72wall strengthening 68wrapped columns 59

deep embedded bars 69deflection 17, 25, 28, 40-1deformation 25, 27, 29, 40, 53, 57delamination 41, 77, 80design life 7design modulus of elasticity 26-7

see also stiffnessdesign resistance moment, beams 32design strain 27, 35design strength 26, 27, 55docks see marine structuresdouble-lap shear test 77ductility 25, 40, 54ductility enhancement 53, 54, 59, 61, 68durability 6-7, 26, 27, 65

see also corrosion

Eearthquake damage

application examples 17, 18, 21, 22protection against 14, 20, 28-9, 53repair 18

economics see costselectrical hazards 10, 14electricity transmission poles 22environmental aspects 6, 13, 73-4

see also health and safetyepoxy adhesives see adhesives

epoxy resinsdefinition 89see also resins

extreme loadings 28-9

Ffabrics 10

adhesive application 75, 76advantages 1application examples 18, 19, 20-1column strengthening 53concrete preparation 72-3installation 76lap joints 56properties 25quality control 95surface regularity and 35see also multi-layer plates and fabrics;

woven fabricsfailure

design consideration 8ductility enhancement and 59FRP response 26NSM reinforcement 37-8post-tensioning systems 66thick and multi-layer plates 39see also brittle failure; deflection;

deformation; fatigue; fire protection; lap joints; separation failure; shear stress; splitting failure; stress rupture;tensile rupture; vandalism

fan anchors 68fatigue 25, 41, 57, 65fibre composites

advantages/disadvantages 1, 5-7, 41definition 89health and safety 13partial safety factors 27post-tensioning 65prestressed 65-7, 68, 69properties 5, 9, 25, 92shear strengthening 47sheets 7, 8, 13, 27, 65, 92strips 17, 18, 20, 49tapes 21, 27, 69tendons 65, 67wet lay-up systems 13see also aramid fibres and composites;

carbon fibres and composites; glass fibres and composites; NSM (near-surface-mounted) reinforcement

fibres 9-10, 13filament winding 11, 27, 55, 89fire damage 17fire protection

adhesive selection 12design considerations 10, 25, 26, 28FRP advantages/disadvantages 6over-coatings 77

flexural strengthening 31-46anchorages 67application examples 16, 17, 18, 20, 21axial FRP 57-9, 61beam-column connections 58

bridges 8, 41, 53columns 53, 57-9, 61materials 13walls 68see also NSM (near-surface-mounted)

reinforcementfull wrapping 47, 49, 69, 72

Gglass fibre, definition 89glass fibre anchorages 68glass fibres and composites 9-10

application examples 18, 20, 21durability 26, 27fabrics 20, 21health and safety 13NSM reinforcement 37, 68partial safety factors 27plates 21preformed shells 18, 21, 53prestressing 65shear strengthening 47, 68stress rupture 65, 66suitability 14see also filament winding

glass transition temperature (Tg) 12, 28, 89glossary 89grooves see slotsgrouts 12, 21, 37, 68

Hhand lay-up 11, 27, 89health and safety 10, 12, 13, 28, 71, 72

records 78, 79see also environmental aspects; fire

protectionHM (high modulus) carbon systems 9, 14hoop wrapping

application examples 20columns 53, 55, 56-7, 58, 59, 61earthquake damage protection 14rectangular columns 29

Iidentification 73, 78, 80impact damage 6, 19, 26, 29impact resistance 10, 20, 55inorganic adhesives 69inorganic resins 9inspection 2, 7, 29, 71, 77, 79-80

see also maintenance; visual inspectioninstallation 5-6, 71-8

costs 7-8NSM reinforcement 1, 37, 77prestressed FRP composites and 65surface regularity 34-5see also quality control; workmanship

instrumentation 77, 79, 80

Jjetties see marine structuresjoints

bridges 21see also connections; lap joints

Index

101

Llaminating resins see resinslap joints 5-6, 41, 56, 59, 76, 80lateral deformation 53, 57life-cycle costing see whole-life costinglife expectancy 69lighthouses 21limit state design 24-5

see also ultimate limit stateslive loads, strengthening under 41-2

Mmaintenance 7, 8, 80

see also inspectionmarine structures 1, 21-2masonry see wallsmaterials 9-14

compatibility 80design values 26-8properties 6, 9, 25-6, 92-4quality control 73, 95-6storage 73suppliers 91testing 77see also adhesives; concrete; fibre

composites; fibres; resins; SRP (steel-reinforced polymer) materials

mechanical fastening 34, 66, 68, 69moment capacity 3, 31-3, 58-9

see also beams and slabs; columnsmoment redistribution 31, 40, 67monitoring 2, 7, 29, 79-80mortars see groutsmulti-layer plates and fabrics 39-40, 76

Nnon-destructive tests 77non-woven fabrics 10, 89NSM (near-surface-mounted) reinforcement

1, 36-9adhesives 37, 38, 73, 75, 77anchorages 37, 38-9, 41application examples 17, 19, 21aramid fibres and composites 21beams and slabs 68carbon fibre and composites 11, 17, 19,

37, 68costs 1, 14, 37glass fibres and composites 37, 68inspection 77installation 1, 37, 77post-tensioning 65prestressed 68shear strengthening 68sizes and properties 11, 93slots for 13, 38, 73, 77suitability 13-14vandalism and 29wall strengthening 68wet lay-up laminates 67see also deep embedded bars

nuclear structures 7, 18, 28

Ooval columns 61over-coatings 10, 12, 25, 77-8, 80overlaps see lap joints

Ppaints see over-coatingspartial safety factors 26-7, 28

existing sections 24, 25, 32stress-strain 58tensile rupture 56

peel ply 19, 39, 74, 89piers see marine structuresplates 5, 9, 11

application examples 8, 16-22definition 89fatigue 41flexural strengthening 39-40lap splices 41partial safety factors 27post-tensioning 65prestressed FRP composites 66-7, 68properties 11, 25, 38, 93quality control 13, 95, 96resins 11stacking 39-40suitability 1, 13surface irregularities 35tolerances 73workmanship and installation 6, 13, 72-3,

74, 75-6see also separation failure; specials;

warning platespolyester/polyurethane adhesives 12post-tensioning 65-6preformed shells 11-12, 18, 21

columns 11-12, 53, 61installation 71partial safety factors 27quality control 95-6

prepreg, definition 89prepreg fabric 10, 21, 53prepreg plates 11, 27, 95prestressed FRP composites 65-7, 68, 69prestressed structures 42protective coatings see over-coatingspull-off tests see testingpultrusion 11, 89

see also plates

Qquality control 13, 25, 71, 73, 95-6

Rrailway structures 7, 10, 14, 19, 21, 22

see also tunnelsrecords 71, 73, 78, 79, 96rectangular columns 53, 60-1

application examples 17, 18impact loading 29shear strengthening 47-52, 61wrapping 29, 73

redistribution, moment 31, 40, 67reinforcement 19, 25, 27-8, 73

see also corrosion; NSM (near-surface-mounted reinforcement); SRP (steel-reinforced polymer) materials; strengthening

repairbefore strengthening 4, 40, 71by FRP composites 17, 18, 19, 21of FRP composites 80whole-life costing 8

resin, definition 89resins 9, 12-13

properties 26, 94pultrusion process 11sunlight and 10workmanship and installation 74, 75see also adhesives

Ssafety see health and safety; partial safety

factorssamples 7, 25, 28, 77, 79, 80

see also testingseismic loading see earthquake damageseparation failure 31, 33-6, 38, 45-6, 47serviceability 40-2, 57serviceability limit states 24-5, 26, 42, 65,

66shape factor 60-1shear capacity 31, 34, 47-8, 56, 65

see also beams and slabs; columns; shear strengthening; shear stress

shear-crack-induced FRP separation 34, 38shear strengthening 3, 5, 47-52

application examples 17, 18, 19, 20, 21axial FRP 49-50, 57beams and slabs 12, 17, 47-52, 67-8bridges 7-8, 69carbon fibre and composites 17, 19, 20,

47, 68circular columns 53-9debonding 49, 56deep embedded bars 69glass fibres and composites 47, 68NSM reinforcement 68prestressed carbon FRP straps 69rectangular columns 47-52, 61specials 12ultimate limit states 48-9, 50, 56-7walls 68wet lay-up systems 13wrapping 13, 47, 49, 56, 57, 67

shear stress 35-6, 47-8bond failure 34NSM reinforcement 38prestressed FRP composites 66, 68thick and multi-layer laminates 39wet lay-up systems 13see also shear capacity

shells see preformed shellsside-only wrapping 47, 49site requirements 73-4, 96

Design guidance for strengthening concrete structures using fibre composite materials

102

slabs see beams and slabsslots

useful perimeter 38wet lay-up systems 67workmanship 13, 37, 73, 75, 77see also NSM (near-surface-mounted)

reinforcementsolar gain 78spacing

plates 76strips 49

specials 12, 96splitting failure 37-8, 67square columns see rectangular columnsSRP (steel-reinforced polymer) materials

68-9stacking, plates 39-40steel reinforcement see reinforcementsteel stress 27-8stiffness

carbon systems 14, 26design considerations 25, 26, 39, 55, 56fibres 10, 26plates 11, 38preformed shells 12woven fabrics 10

storage, materials 73strain 26-7

flexural strengthening 31, 32, 33, 35NSM separation failure 38shear strengthening 48-9tensile rupture 56see also stress-strain model

straps 12, 69see also strips

strengthening 1-8, 15-22see also flexural strengthening; shear

strengthening; torsional strengtheningstrengthening systems 91-4stress rupture 12, 25, 41, 57, 65, 66stress-strain model 26, 32, 53-5, 56, 57-8,

59strips 17, 18, 20, 49

see also straps; tapes

structural assessment 3-4, 24, 31-2, 71structural design 23-9, 71substrate, definition 89substrates 13, 31, 72-3, 74, 75

see also cracks and cracking; surface irregularity-induced separation; surface preparation; surface regularity

suppliers 97surface irregularity-induced separation

34-5, 38surface preparation 1, 12, 37, 72-3surface regularity 34-5symbols 23-4

Ttapes 21, 27, 69

see also straps; stripstemperature, adhesives 10, 12, 28, 65, 73tendons 65, 67tensile rupture 55-6testing 7, 77, 79, 80

quality control 95surface quality 73wet lay-up systems 25see also samples

thick plates 39-40torsional strengthening 69towers 8, 21, 22tunnels 6, 12, 21, 28

UU-wrapping 13, 47, 49, 67UHM (ultra high modulus) carbon systems

9ultimate limit states 24-5, 26, 27

flexural strengthening 31-2NSM reinforcement 37post-tensioning 66separation failure 34, 35, 38shear strengthening 48-9, 50, 56-7stress-strain model 54, 56tensile rupture 56

ultraviolet radiation 10, 12, 25, 77underpasses 18

Vvandalism 6, 26, 29, 65, 77vinyl ester adhesives 12visual inspection 71, 74, 75-7, 79, 80, 96

Wwalls 18, 29, 68warning plates 29, 78, 80water-cured glass fabric 21wet lay-up, definition 89wet lay-up systems

anchorages 67, 68concrete durability 40partial safety factors 27quality control 96suitability 13testing 25wrapping 13, 76

whole-life costing 4, 8see also costs

workmanship 6, 12, 35, 40, 71-8see also installation; quality control

worst credible strength 24, 25, 26woven fabrics 10, 19, 26, 89wrapping 3, 9

application examples 17, 18, 20-1, 22beams and slabs 67, 73bridges 21impact loading 29rectangular columns 29, 73shear strengthening 13, 47, 49, 56, 57,

67torsional strengthening 69wet lay-up systems 13, 76see also columns; concrete confinement;

fabrics; filament winding; full wrapping; hoop wrapping; U-wrapping