alkali-aggregate reaction (aar) of concrete structures · internal concrete within buildings,...
TRANSCRIPT
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Alkali-Aggregate Reaction (AAR) of Concrete Structures
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Outline
• What is AAR?
• How does AAR work?
• How to assess AAR?
• How to minimise AAR risk in new concrete structures?
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AAR
• Alkali-silica Reaction (ASR): - only type of AAR occurred in HK
• Alkali-silicate Reaction
• Alkali-carbonate Reaction
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Concrete failure due to ASR
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Concrete failure due to ASR
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Effect of ASR
• Concrete Quality • Loss of strength, stiffness, impermeability • Affect concrete durability and appearance • Premature failure of concrete structures
• Economic Costs • Maintenance cost increased • The life of concrete structure is reduced
• Overall Result • No concrete structures had collapsed due to ASR damage • Some concrete structures/members were demolished because of
ASR
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Example: Daqing Railway Bridge, China
Alkali-Aggregate Reactions in Railway Bridge
• Railway Bridge built in 1987 • Precast prestressed concrete beams cast at
Gaogezhuang, North of Beijing
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Example: Daqing Railway Bridge, China
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Example: Daqing Railway Bridge, China
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Local Example in HK
Shek Wu Hui Treatment plant
Built in early 1980s ASR was reported in 1991
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Example in HK
Shek Wu Hui Treatment plant
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Local Examples of ASR
Structures Affected by ASR:
Site Year Year ASR Approx. time for constructed reported ASR to develop
Shek Wu Hui Treatment
1980 - 1983 1991 about 9 - 12 years
Works
Fan Ling Footbridges
1982 1998 16 years
North Point Govt. School
1987 about 1999
about 12 years
Hill Road Flyover
1982 1997 15 years
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Local History of ASR
Background
• ASR was first diagnosed in HK (1991) • AAR Sub-Committee under SCCT was set up
• Hong Kong Specification
• WBTC 14/94 issued 3 Kg/m³ limit on reactive alkali content in concrete
• Quality Scheme for the Production and Supply of Concrete (QSPSC) Cl. 7.1.1(e) Use of chemical method ASTM C289
• Increasing use of AMBT (included in PS) in major concrete structures
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How to assess ASR
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Test for Potential ASR
• Reaction of alkali with silica is slow
• ASR can be accelerated by: - increasing temperatures, - increasing moisture availability, - increasing alkali concentrations
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Common Test Methods to assess ASR
• Petrographic Examination
• Chemical Test
• Accelerated Mortar Bar Test (AMBT)
• Concrete Prism Test (CPT)
• Accelerated Concrete Prism Test (ACPT)
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Common Test Methods to assess ASR
Test Method RILEM ASTM Standard
Canadian Standard
British Standard
Petrographic Examination
AAR-1 ASTM C295 -- BS 812:Part 104
Accelerated Mortar Bar Test (AMBT)
AAR-2 ASTM C1260 CSA A23.225A
DD 249: 1999
Concrete Prism Test (CPT)
AAR-3 ASTM C1293 CSA A23.214A
BS 812:Part 123
Accelerated Concrete Prism Test (ACPT)
AAR-4 -- -- ---
Chemical Method ASTM C289
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Petrographic Examination
Petrographic examination of concrete cores from Shek Wu Hui Treatment Plant
Crack running through aggregate particle and into surrounding cement paste
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Use of Chemical Method in HK
Use of Chemical Method (ASTM C289)
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Use of AMBT in HK
• PWL CON 5.5 Method
• RILEM AAR-2
• Others: ASTM C1260, CSA A23.2-25A
• GEO Technical Note TN 6/2002 issued in December 2002
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AMBT : RILEM AAR-2 Public Works Central Laboratory
Flow Chart for Rilem TC-106-2
Cure at 20 ± 1°C and Immerse in distilled water Immerse in 1M NaOH at 80 ± 2 °C RH >90% at room temp and put in
oven at 80 ± 2 °C
Ln 24 ± 2 hrs 24hrs 1day 4days 2days 5 days 2 days
L1 L5 L7 L12 L14 Demould 1day 5day 7day 12day 14day
(Li) (L0)
CASTING 1
DAY 2
DAY 3 DAY
7 DAY
9 DAY 14
DAY 16 DAY
gauge length = L0 - 2studs length
Ln − L0Expansion (%) = ×100 %Gauge length
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AMBT : RILEM AAR-2
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AMBT: RILEM AAR-2
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AMBT: RILEM AAR-2
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AMBT Results of Local & Reference Aggregate
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Legend: PWCL CON5.5 ASTM C1260 (94) CSA A23.2-25A (94) RILEM AAR-2
AMBT Results of Local & Reference Aggregate
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Cure at Cure inside the container in environment of 38 + 2 oC, 20 + 2 oC Cure inside the container 24 hours before each measurement,
and in environment of 20 + 2 oC allow container with prism to cool at 20 + 2 oC. RH > 90%
under moist covers Measurement of weight (Wn) and length comparator reading (Rn).
24 + 0.5 hours 24 hours 5 days 1 week 2 weeks 9 weeks 13 weeks 26 weeks
Demould, wrapped with cotton cloth, Measure
polyethylene initial length (Li), Measurement Measurement Measurement Measurement Measurement Measurement tubing, and initial weight (W2) at day 7 at 2 weeks at 4 weeks at 13 weeks at 26 weeks at 52 weeks
& length comparator polyethylene bag, ( R7 , W7) ( R14 , W14) ( R28 , W28) ( R91 , W91) ( R182 , W182) ( R364 , W364) reading (R2)
stored in container.
Casting 1 day
2 days Percentage increase in weight ( % ) = Wn – W2 x 100 % W2
Where Wn is the weight at n days age. 1 week
Percentage increase in length ( % ) = Ln –L2 x 100 % 2 weeks Li
Where Ln is the length of prism with studs at n days age. ( Ln = Rn + calibrated length of reference bar). 4 weeks
13 weeks
26 weeks
52 weeks
Rilem AAR-3 ( TC 106-3 ) Detection of Potential Alkali-reactivity of Aggregates
Flow Chart of Measurement
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CPT: RILEM AAR-3
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CPT: RILEM AAR-3
• Steel Mould with Gauge Studs
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CPT: RILEM AAR-3
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CPT: RILEM AAR-3
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CPT: RILEM AAR-3
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CPT: RILEM AAR-3
• Interior of Container
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CPT: RILEM AAR-3
• Interior of Oven
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CPT: RILEM AAR-3
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ACPT
• A Faster test when compared with CPT • Determination of alkali threshold limit • Performance test for concrete mix
• Development of ACPT will be the next task of PWCL
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ASR Testing Methods
• Hong Kong Experience
• Chemical test is not reliable • AMBT show reliable result • CPT is under development by PWCL • Development of ACPT will be the next task
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Preventive Measures against ASR in New Concrete Structures
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Factors affecting ASR Reactivity
• Reactivity of the siliceous material
• total alkali content of the concrete
• supply of moisture
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Preventive Measures against ASR
• Cements
• Limiting the alkali content of the mix, means that a low-alkali cement have to be used
• most countries adopted the limit of 0.60% Na2O equivalent
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Preventive Measures against ASR
• Restriction of alkali level in concrete
Canada, Denmark 1.8 to 3.0 Kg/m³ Na2O equivalent, depending on information about AAR reactivity of aggregate and the level of risk
South Africa 2.0 to 4.5 Kg/m³ Na2O equivalent, depending on information about AAR reactivity of aggregate
New Zealand 2.5 Kg/m³ Na2O equivalent
UK 2.5 to 5.0 Kg/m³ Na2O equivalent, depending on information about AAR reactivity of aggregate
Belgium, China, Japan, USSR
3.0 Kg/m³ Na2O equivalent
France 3.0 to 3.5 Kg/m³ Na2O equivalent, depending on information about variability of cement alkalis
Ireland 4.0 Kg/m³ Na2O equivalent but 4.5 Kg/m³ Na2O equivalent if reactive aggregate is only Carboniferous chert
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Preventive Measures against ASR
• Supplementary Cementitious Materials (SCM)
• Pulverized-fuel ash (PFA) • Ground granulated blastfurnace slag (GGBS) • Microsilica, also called silica-fume
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Preventive Measures against ASR
• Use of Cementitious Replacement Materials
• PFA, GGBS used in most countries
• microsilica used in Australia, Belgium, Canada, Denmark, Iceland, New Zealand, South Africa & USA
• Usual Range of Replacement Levels – 15 to 40% PFA – 25% to 70% GGBS – 7% to 15% microsilica
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Preventive Measures against ASR
• Aggregates
• If the aggregate is non-reactive, no other precautions are necessary
• If not, a variety of precautions are invoked
• Preclusion of reactive aggregates from particular uses.
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Preventive Measures against ASR
• Moisture
• Reduce the access of moisture and maintain the concrete in a sufficiently dry state
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Preventive Measures against ASR
• Others
• Modify the properties of any gel such that it is non-expansive, e.g. using lithium salts
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Review of International Practice with ASR
• 3 Basic Approaches
• (1) aggregates are classified as reactive or innocuous; mitigation measures are prescribed for use with reactive aggregates.
• (2) the reactivity of an aggregate is first classified; mitigation measures are prescribed for use of the aggregate according to the nature of the structure andthe environment it is in.
• (3) consideration starts with the nature of structure to be constructed and the service environment; aggregate reactivity is considered for the choice ofsupply sources and the mitigation measures needed toprevent AAR
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Typical Framework
• Determination the level of precaution
• Characterisation of the structural needs
• Characterisation of the environment
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Classes of Structures
• Structures classified by risk category
• S1 – some deterioration from AAR is acceptable, e.g. temporary or short service life structures,easily replaceable elements
• S2 – minor AAR and resulting cosmetic cracking is acceptable, e.g. most building and civilengineering structures
• S3 – no AAR damage is acceptable, even if only cosmetic - long service life or highly critical structures, e.g. nuclear installations, dams,tunnels, exceptionally important bridges orviaducts, structures retaining hazardous materials
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Classes of Environment
• Characterisation of the environment
• E1 – protected from external moisture, e.g. internal concrete within buildings, externalconcrete protected by cladding
• E2 – exposed to external moisture, .e.g. internal concrete within buildings where humidity is high(laundries, swimming pools), external concreteexposed to atmosphere
• E3 – exposed to external moisture + aggravating factors such as de-icing salts, freezing andthawing or a marine environment
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Level of Precaution
• An example of Level of Precaution • P1 – no special precautions against ASR • P2 – normal level of precaution • P3 – special level of precaution
Environment Category E1 E2 E3
Category of Structure
S1 P1 P1 P1
S2 P1 P2 P2
S3 P2 P3 P3
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Example of Control Framework: RILEM: Draft International Specification to Minimise Damage from ASR in Concrete • Level of Precaution • P1 – no special precautions against ASR • P2 – normal level of precaution
M1: Restrict the alkalinity of pore solution, e.g. limit the alkali content of concrete, use of low alkali cement, include PFA, GGBS, etc.
M2: Avoid the presence of a critical amount of reactive silica, e.g. identify non-reactive aggregate
M3: reduce the access of moisture and maintain the concrete in a sufficiently dry state, e.g. use external cladding or tanking
M4: modify the properties of any gel such that it is non-expansive, .e.g. use lithium salts
• P3 – special level of precaution Combined application of at least two of the precautionary measures from level 2
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What shall we do?
What we know:
• Which reactants involved and their sources • How alkali-silica gel is created • ASR prevention can be achieved by using low
alkali cement and non-reactive aggregate • Cement replacement such as PFA, GGBS and
microsilica help mitigate ASR damage
What we don’t know:
• Cement Replacement: its mechanism of inhibition, which compounds work best, how much of each compound is needed to prevent expansion, its long term performance
• Which test method most suit local aggregate
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What shall we do?
• SCCT: Review of Concrete Related Standards
• (1) Cement Standard • (2) Aggregate Standard • (3) Unifying Concrete Specification • (4) CS1 - Concrete Testing • (5) CS2 – Reinforcement Bar
• Proposed framework for controlling risk of ASR in Hong Kong is being prepared and will becirculated to relevant parties
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THANK YOU