04a concrete defects

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RECO2006 Construction IV

Concrete Defects and Repair Strategy

Edward CY YIUDepartment of Real Estate and Construction

January 2007

2

Deterioration Theories of Reinforced Concrete

Design, Materials and Workmanship Embedded Metal Corrosion-induced cracking

and spalling Reduction in Structural Capacity Chloride Penetration Carbonation

Thermal and Moisture Fire Loading

23

Corrosion Process

Concrete is a highly alkalinity material (pH=12).

Embedded steel is protected from corrosion by a passivating film bonded to the bar surface.

Corrosion-an electrochemical process is accelerated in an acidic environment

Emmons, 1993, p.9

The rate of corrosion increases sharply from 0.25mm/year to 0.8mm/year when the alkalinity of the concrete drops from pH=4 to 1.

4

Corrosion Promotors

Oxygen (cracks, honeycombs) Water (cracks, honeycombs) Acidic environment (carbonation) Chlorides (salts, atmosphere, water) Insufficient concrete covers (penetration

path)

Emmons, 1993, p.9

35

Reduction in Structural Strength

More than 1.5% corrosion of re-bar, the ultimate load capacity began to fall,

At 4.5% corrosion, the ultimate load was reduced by 12%

Al-Sulaimani, Kaleemullah, Basunbal and Rasheed, (1990) Influence of Corrosion and Cracking on Bond Behavior and Strength of Reinforced Concrete Members, ACI Structural Journal, Mar-Apr, p.220

6

Chloride Penetration

Chlorides penetrate into concrete due: Surface moisture Crack Construction joint Cast-in chloride

Corrosion begins when chlorides contact steel

Delamination and spalling are resulted

Emmons, 1993, p.12

47

Corrosion Threshold

Standard Threshold: The concentration level of chloride ions at which the

protective passivity layer on the surface of the embedded steel breaks down and corrosion initiates.

An international recognized threshold of the chloride concentration is of 0.40% by weight of cement for a concrete with 400 kg/m3 of portland cement (i.e. corresponding to a critical chloride content of about 0.05-0.07% by weight of concrete).

Source: Grace Construction Products

8

Carbonation

pH is lowered by:CO2+H2O+Ca(OH)2->CaCO3+2H2O

Carbon dioxide penetrates into the pores of concrete by diffusion

Concrete protection of the steel is LOST!

The process proceeds by 1mm annually, 15years -> 15mm threshold

Emmons, 1993, p.15

59

Fire Damage Temperature gradient

is built up (21C-800C)

Spalling of expanding concrete

Cement mortar converts to quicklime at 400C

Re-bar loses tensile capacity at 700C

Emmons, 1993, p.45

10

Fire Damage on Concrete

Normal Pink Whitish Grey

Proportion of Strength

0.7

0.3

300 600

611

Load Effects

Re-bars are placed in the concrete to provide tensile strength

Concrete is poor in tension, good at compression

Tension Cracks are formed

Emmons, 1993, p.48

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Improper placement of re-bars

Compare Slab and Cantilever canopy Albert House Case

713

Durability Assessment of Concrete

Sarja and Vesikari (1996) a general theory of stochastic durability design based on the probability of failure of serviceability limit state

model. The degradation process is modeled by the interaction of a

resistance R and a load S, which are assumed to be normally distributed, then the failure probability, Pf(t) can be determined using the

safety index : Eurocode 1 (EN, 2000) specifies the acceptable failure

probability of not exceeding 7%, and that in the NS-3490 (Norwegian Standards, 1999) of not exceeding 10%( ) ( ) ( ){ } ( )

[ ] [ ][ ] [ ]tStR

tStRt

wheretStRPtPf

,,,,)(

22

+=

==where and denote the mean and the standard deviation of the variables, and is the cumulative density function of the standard normal distribution N(0,1) .

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Embedded steel corrosion induced degradation of concrete structure

TimeOnset of corrosionCracking

Spalling / Delamination

Structural failure

Source: Ferreira et al., 2004

Corrosion-induced dam

age

t0

tL

Limit state

t1

815

Ficks Second Law of Diffusion

The rate of chloride penetration into concrete is modeled by:

where C(x,t) is the gradient of chloride content, i.e. the chloride ion concentration at a distance x from the concrete surface after being exposed for a period of time t, and Dc is the chloride diffusion coefficient

( ) ( )2

2 ,,dx

txCdDdt

txdCc=

16

Sarja and Vesikari (1996, p.130)

solved and simplified to the following formula by using a parabola function:

Normal values of Dc and Cs are 10-7 10-8cm2/s and 0.3 0.4 by weight of concrete, respectively

( )2

321,

= tDxCtxC

cs

917

Taking the threshold chloride concentration to be 0.4% of the weight of cement (0.07% of the weight of concrete) the three rates of chloride diffusion represent the three initiation time of corrosion at 9-year, 17-year and 41-year respectively Chloride Content over time

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51

Year

Chl

orid

e co

nten

t in

% o

f con

cret

e

x=2, Cs=0.1, Dc=10-8x=2, Cs=0.1, Dc=2.5x10-8x=2, Cs=0.1, Dc=5x10-8

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Sarja and Vesikari (1996, p.23) provides a stochastic durability assessment model of carbonation in concrete

structures

where (D) = the mean of the depth of carbonation in mm; Kc = the carbonation rate factor in mm/year1/2; and t = time in years

where cenv = the environmental coefficient; cair = the coefficient of air content; fck = the characteristic cubic compressive strength of concrete (MPa);

and a, b = carbonation constants

tKD c=)(

( )bckairenvc faccK 8+=

10

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Theoretical safety index of carbonated concrete

Durability Analysis on Carbonation - conc strength sensitivity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Age (years)

Prob

abili

tyof

Failu

re

conc strength =10MPaconc strength =20MPaconc strength =30MPaconc strength =40MPa

Durability Analysis on Carbonation - concrete cover sensitivity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Age (years)

Prob

abili

tyof

failu

re

conc cover =5mmconc cover =15mmconc cover =25mmconc cover =35mmconc cover =45mm

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Concrete conditions of ageing buildings in Hong Kong

Surface chloride contents averages at 0.35% and 0.40% by weight of cement at beams and columns respectively, which reached the threshold;

Carbonation rates ranged from 7.8 -14.5mm/year1/2 are far greater than the standard rate (6.2mm/year1/2)

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Sarja and Vesikari (1996, p.68) Durability Risk Factors

Target / Design Service Life

Environmental Effects

Degradation Mechanisms

Mechanical Design

Parameters

Durability Parameters

Depth of deterioration of

concrete

Corrosion of Reinforcement

Concrete Cover Diameters of Rebars

Other Factors

Strength of Concrete

Permeability of Concrete

Type of Cement & Reinforcement

Curing Method Structural Dimensions

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Degradation Risk Factors

Leaching Acid productionMicro-organisms

Biological

Vibration, deflection, cracking, failure

FatigueImpact loading

Deflection, cracking, failure

Fatigue, deformationCyclic loading

Deflection, cracking, failure

DeformationStatic loading

Mechanical

DegradationProcessDegradation factor

Source: developed from Sarja and Vesikari (1996, p.102)

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Expansion, disintegrationCarbonate reactionCarbonate aggregateExpansion, disintegrationSilicate reactionSilicate aggregate, alkalisDisintegration of concreteCrystal pressureSulphates

Failure in prestressingtendons

Stress corrosionStress / chlorides

Expansion of steel, loss of diameter in rebars, loss of bond

Corrosion Steel depassivation, oxygen, water

Steel depassivationPenetration, destruction of passive film

Chlorides

CarbonationCarbon dioxideSulphur dioxideNitrogen dioxide

Steel depassivationNeutralizationAcidifying gasesSteel depassivationNeutralizationAcidDisintegration of concreteLeaching AcidDisintegration of concreteLeachingSoft water

Chemical

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CavesCavitationTurbulent water

Surface damageErosionRunning water

Rutting, wearing, tearingAbrasionTraffic

Cracking, scalingAbrasionFloating ice

Scaling of concreteHeat transferDeicing salt, frost

Disintegration of concrete

Ice formationLow temperature, water

Shortening, lengthening, restricted deformation

Shrinkage, swellingRH change

Shortening, lengthening, restricted deformation

ExpansionTemperature change

Physical

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Cannot be ascertainedErrors in mathematical and statistical modelling

Cannot be ascertainedUncertainties of manufacture and

execution

Cannot be ascertainedErrors of communication

Cannot be ascertainedUncertainties of design

Human errors and uncertainties

Cannot be ascertainedAbuse / vandalism

DeteriorationManagement

Deterioration and obsolescence

Maintenance / upkeep

Wear and tearNormal use

Use

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Repair Strategy Repair strategy is dependent of the intended

building service life. Very different repair tactics are devised for

different intended life spans. Five different tactics are set out:

T1: hazards removal; T2: Repair of defective elements; T3: Repair of deteriorated elements; T4: Rehabilitation; and T5: Redevelopment of the whole building.

Source: Yiu (2007)

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Minimum Life Short Life Medium Life Long Life Very Long Life

Check Intended Building Life

Tactic 1(Hazard Removal)

Tactic 2(Defects Repair)

Tactic 3(Deteriorated Repair)

Tactic 4(Rehabilitation)

Tactic 5 (Redevelopment)

Identification of defects

DiagnosisIdentification of hazards

Stability of elements Financial viability Acceptability of disruption

Determine the extents of deterioration

Determine the cause(s) of deterioration

Protective and Prevention Measures (T4)

Replacement of deteriorated elements (T3, T4)

Removal of hazards(T1)

Patch repair of defective elements (T2, T3)

Replacement of defective elements (T2, T3, T4)

Repair Monitoring

Repair System

Repair Methods

Repair Tactics

Source: Yiu (2007)

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oDemolish the buildingoRebuild the building to the required standards

RedevelopmentVery long life (> 50 years)

T5

oT1 + T2 + T3oUpgrade to the latest / a better standardsoApply preventive / protective measures

Rehabilitation Long life (21-50 years)

T4

oT1 + T2oRepair / replacement of deteriorated elementsoRemove all carbonated and chloride contaminated concreteoMinimize the source(s) / cause(s) of deterioration

Repair of deteriorated elements

Medium life (11-20 years)

T3

oT1oRemoval of defective elementsoRepair / make good the defects

Repair of defectsShort life (2 -10 years)

T2

oRemove hazardsoApply cosmetic repairoFulfill statutory / minimum requirements

Hazard removal only

Minimal life (< 2 years)

T1

DescriptionsTacticsIntended Further

Building Life[1]

Repair Tactic Codes

[1] For a 50 years old building in reinforced concrete framed structure

Source: Yiu (2007)

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29Repair options to be included in the repair methods(7)

Mildly carbonated, i.e. carbonation front < 15mm depth and did not exceed reinforcement

M=(6)

Fully carbonated, i.e. carbonation front > 15mm or exceeded reinforcementF=(5)

No water ingressN=(4)

Water ingressY=(3)

Low chloride content, i.e. Cl- < 0.4% (by weight of cement)L=(2)

High chloride content, i.e. Cl- > 0.4% (by weight of cement)H=(1)

Legends:

---MNL8--FNL7--MYL6-FYL5--MNH4-FNH3-MYH2FYH1

Protective coating

Prevention of Cl- attack

Stop water source

Repair

For T4 onlyFor T3 and T4CarbonationWater Ingress

Chloride Concentration

Repair MethodsCausesCase

Source: Hong Kong Housing Authority (1999) MTE1-1.2 Issue 1

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Diagnose the cause(s) of the deteriorations;Make good the source(s) / cause(s) of the deteriorations.

Minimize the source(s) / cause(s) of deterioration

T3c

Remove all carbonated and chloride contaminated concrete;Replace with sound concrete;Replace all rusty steel bars.

Retain the original properties of rcconcrete

T3b

T2 and replacement of building services installations;Complete replacement of finishes and re-roofing;Complete replacement of plumbing / drainage systems;Repair / replacement of other defective components / systems

Repair / replacement of defective components

T3a

For achievement of a further service life of the order of 20 years (T3)

Repair of building services installations;Patch repair of finishes;Patch repair of plumbing / drainage systems;Patch repair of other defective components / systems

Repair of defective components / systems

T2c

Demolish a part or the whole of a structural element, replace all seriously corroded reinforcement and then cast back with the designed grade (or better quality) concrete substrate concrete

Partially recasting of concrete structural elements

T2b

Damaged concrete is removed and patched up with the application of repair mortar systems selected from an authorizedapproved list

Patch repair of spalled concrete

T2a


DescriptionsRepair MethodsItem

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Repair Strategy (contd)DescriptionsRepair MethodsItem

Fire services installations;HVAC, electricity, transportation, communication, security plumbing and drainage systems;

Upgrade of servicesT4b

T3 and strengthening of concrete structural elements;Re-alkalization of concrete;

Upgrade of structural elements

T4a


Source: Yiu (2007)

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A electrochemical method preventing future corrosion in carbonated concrete. Chloride ions are transported out of the concrete to increase the pH level, so as to stop corrosion.

Chloride extraction / Desalination

T4f

A electrochemical method preventing future corrosion in chloride contaminated concrete. Alkalis are transported into the concrete to increase the pH level, so as to stop corrosion.

Re-alkalization T4e

Stop the setting up of anodes on the reinforcement by applying a low voltage electric current or by a sacrificial anode.

Cathodicprotection

T4d

Produces a thin outer layer to protect the substrate concrete by forming an impermeable barrier or slowing the rate of penetration of aggressive components from the environment

Protective coating

T4cFor achievement of a further service life of the order of 50 years (T4)

DescriptionsPreventive Measures

Item

Source: Hong Kong Housing Authority (1999) MTE1-1.2 Issue 1

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References Al-Sulaimani, Kaleemullah, Basunbal and Rasheed, (1990) Influence of Corrosion

and Cracking on Bond Behavior and Strength of Reinforced Concrete Members, ACI Structural Journal, Mar-Apr, p.220

Buildings Department, (2002), Building Maintenance Manual, The Government of the Hong Kong SAR.

Buildings Department, (1998), Interim Technical Guidelines on The Inspection, Assessment and Repair of Buildings for The Building Safety Inspection Scheme, The Government of the Hong Kong SAR.

CEN (2000) EN 1991 Eurocode 1:Basis of design and actions on structures, CEN. Emmons P.H. (1994) Concrete Repair & Maintenance. R.S. Means Co. Inc., Kingston,

MA. Ferreira, M., Jalali, S. and Gjrv, O.E. (2004) Probabilistic assessment of the

durability performance of concrete structures, Engenharia Civil, 21, 39-48. HKHA (1999) Repair to Corrosion Damaged Concrete, MTE1-1.2 Issue 1. Norwegian Standard (1999) NS-3490 Design of structures requirements to reliability,

Oslo. Sarja, A. and Vesikari, E. (Eds) (1996) Durability Design of Concrete Structures,

RILEM Report 14, E&FN Spon, London, UK. Yiu, C.Y. (2007) Durability Assessment, A chapter in a Consultancy Report for

Structural Assessment of Ageing Buildings in Mongkok, REC, HKU, Hong Kong.

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The End

For enquiries, please send email to Edward CY YIU

Department of Real Estate and ConstructionThe University of Hong Kong

[email protected]

04a concrete defects

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