04a concrete defects
DESCRIPTION
concreteTRANSCRIPT
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RECO2006 Construction IV
Concrete Defects and Repair Strategy
Edward CY YIUDepartment of Real Estate and Construction
January 2007
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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
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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.
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Corrosion Promotors
Oxygen (cracks, honeycombs) Water (cracks, honeycombs) Acidic environment (carbonation) Chlorides (salts, atmosphere, water) Insufficient concrete covers (penetration
path)
Emmons, 1993, p.9
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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
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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
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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
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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
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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
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Fire Damage on Concrete
Normal Pink Whitish Grey
Proportion of Strength
0.7
0.3
300 600
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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
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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
,,,,)(
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+=
==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
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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=
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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
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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+=
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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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DegradationProcessDegradation factor
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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DegradationProcessDegradation factor
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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DegradationProcessDegradation factor
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
For achievement of a further service life of the order of 10 years (T2)
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
For achievement of a further service life of the order of 50 years (T4)
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