alkali silica reaction - cait · date 4 aar and preventive measures review of aggregate...
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Alkali Silica Reaction
Henry B. Prenger, P.E.
History
Date 2
Thomas Stanton, California Department of Highways
ASR Process
Date 3
Step One:Silica in aggregates reacts withalkali in cement to produce a gel.
Step Two:The gel absorbs water, causing expansion and hydraulic pressures sufficient to fracture and break apart the concrete.
Process Requirements:Sufficient moisture (80% RH). Reactive silica.Source of alkali.
Strategic Highway Research Program, Washington, D.C. 1991
Date 4
AAR and Preventive Measures
Review of Aggregate Alkali-Silica Reactivity
CEMENTsolution with ions OH- fromportlandite coming from cementhydration.pH basic (>12), ions OH-
alkali content (NaOH, KOH)
AGGREGATEsecondary or amorphous silica, potentially soluble
NaOH + SiO2 → NaSiO2 + H2O(alkali) (silica) (alkali silica gel)NaSiO2 + NaOH + H2O → Siliceous Gel(alkali (alkali) (water) alkaline hydratedsilica gel)
Swelling gelconcrete expansion, cracks formation
The Chemical Reaction:
Date 5
Alkalis from hydration of cementitious material containing KOH and NaOH provide OH- ions for reaction
Aggregates containing secondary or amorphous silica
Water
ASR
GEL
+ +
Cracking Mechanisms
Typical appearance of a concrete surface effected by ASR.
Expansion occurring in concrete tensile stresses are relieved by the formation of relatively wider cracks perpendicular to surface
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Bridge DeckU.S. Rt. 68 and MD Rt. 55
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Courtesy of Maryland State Highway Administration
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U.S. Rt. 1 over Little Gunpowder Falls
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Courtesy of Maryland State Highway Administration
U.S. Rt. 50 and Rt. 301
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Courtesy of Maryland State Highway Administration
MD Rt. 100 and Rt. 301
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Courtesy of Maryland State Highway Administration
Mid Atlantic Technical Committee
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Test Methods
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Test Methods
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1. Manufacturing mortar bars or concrete prisms
2. Storing either in a moist room or in high alkali solutions
3. Storing at standard or high temperatures
4. Measuring expansion
Most test methods involve:
Test Methods – ASTM C 227 Potential Reactivity of Cement Aggregate Combinations
High alkali cement used to make mortar bars
Length change tested at 3 and 6 months (expansions of .05% and 0.10%, respectively)
Not good for slowly reacting aggregates
Date 19
Test Methods – ASTM C 441Effectiveness of Mineral Admixtures or GBFS in Preventing Excessive Expansion in Concrete Due to Alkali Silica Reaction
Pyrex glass
Length change measured at 14 days
For fly ash, you can have the same expansion as the low alkali straight portland cement mortar bar
For slag, you can have up to 0.020% expansion compared to the straight portland cement mortar bar
There are questions about whether the limits in this test method are conservative enough
Date 20
Test Methods ASTM C 1260 - Potential Alkali Reactivity of Aggregates
(Mortar Bar Method)
Mortar bars stored in high alkali solution at high temperatures
Length measurements taken over a period of 14 days
Expansion less than 0.10% - Innocuous?
Expansion between 0.10 and 0.20% - May or may not be a problem
Expansion greater than 0.20% - Deleteriously reactive
May give false positives and false negatives
Some researchers feel that 0.08% is a better limit
Does not take into account the alkalis of the cementitious materials
Date 21
Test MethodsASTM C 1567 - Test Method for Determining the Potential Alkali Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated mortar Bar Method)
Mortar bars stored in high alkali solution at high temperatures
Length measurements taken over a period of 14 days
Expansion less than 0.10% - Innocuous?
Expansion between 0.10 and 0.20% - May or may not be a problem
Expansion greater than 0.20% - Deleteriously reactive
May give false positives and false negatives
Some researchers feel that 0.08% is a better limit
Does not take into account the alkalis of the cementitious materials
Date 22
Test MethodsASTM C 1293 - Test Method for Determination of Length
Change of Concrete Due to Alkali Silica Reaction
Concrete prisms cast and placed in high alkali solution
Measurements made out to 1 year
Expansion must be less than 0.04%
Does not take into account alkalis of cementitious materials
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Test Methods
ASTM C 295 - Petrographic Examination of Aggregate for Concrete
Field Performance
Date 25
Microscopic evidence of ASR
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ASR Gel Detection
Date 27
ASR Gel Detection
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Preventive Measures
There are four basic levers to control ASR in concrete:
•Use non-reactive aggregates
•Control the total alkalis in concrete
•Keep the concrete dry
•Use supplementary cementing materials
Lever #1 - Controlling Aggregates
Silica Sources in Aggregate
Highly Reactive
Poorly crystalizedOpalVolcanic glass
Meta-stablesCristobaliteTridymite
Potentially Reactive
CryptocrystallineChalcedonyChert
MicrocrystallineSiliceous network in some carbonatesVolcanic glass devitrifiedOther types of microcrystalline quartz: i.e. - metamorphic, sedimentary cement
Sedimentary siliceous cement re-crystallized
Granular with rolling extinction, with sutured joints or with micro-inclusions.
Date 30
Potentially Reactive Aggregates
Date 31
Greywacke SandstoneLi
mes
tone
Gravel
Date 32
Chalcedony in limestone
Date 33
Fine Aggregate
Date 34
Quartz Sand
Is it practical to control the source of aggregate to control ASR?
Date 35
Aggregate
Date 36
Lever #2 – Controlling Alkalis
Main source : Portland cement
Other sources :
Alkalies from other pozzolanic and cementitious materials :• Fly Ash• Slag• Silica Fume
Alkalis from chemical admixtures and mixing water
Soluble alkalis from aggregates
External sources : • sea water• deicing salts
What is Low Alkali Cement?
ASTM C 150 and AASHTO M85 has an optional requirement with a maximum of 0.60% Na2O equivalent for low alkali cement
Virginia DOT has a maximum limit for alkalis (expressed as Na2O equivalent) of 0.40%
NY DOT has a maximum limit for alkalis of 0.70% Na2O equivalent
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Effect of the alkali content of the concrete
0
0.1
0.2
0.3
0.4
0.5
1.0 2.0 3.0 4.0 5.0 6.0
Alkali Content of Concrete (kg/m3 Na2Oe)
Expa
nsio
n at
2 Y
ears
(%)
CSA Limit
expansion unlikely with most aggregates
if alkali content of concrete < 3.0 kg/m3
Laboratory testing of concrete indicates :
Under field conditions, expansion has sometimes been observed in concrete with less than 3 kg/m3 Na2Oe contributed by the Portland cement to the concrete
Alkali Loading in Concrete
Date 39
The concrete alkali content is calculated as follows:
Concrete alkali content
kg/m3 Na2Oe
= Cement alkalis% Na2Oe
Cement contentkg/m3
1100
x x
Example:Two different concrete mixtures with the same cement. The first mixture has a cement content of 350 kg/m3. The second has a cement content of 380 kg/m3. If the cement has a alkali content of 0.80% Na2Oe then the alkali content of the concrete is:
350 x 0.80100= = 2.80 kg/m3 Na2Oe
380 x 0.80100 = 3.04 kg/m3 Na2Oe=
Will limiting the alkali content control ASR?
Date 40
Lever # 3 Keep the Concrete Dry
Date 41
Is it practical to keep the concrete dry?
Date 42
Lever # 4 – Use Supplementary Cementitious Materials
Fly Ash
Slag Cement
Silica Fume
Ternary Blends
Date 43
Fly Ash
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Industrial by-product from coal fired electricity generating station
•F Ash – Low in Ca•C Ash – High in Ca
Date 45
0.00
0.05
0.10
0.15
0.20
0.25
0 6 12 18 24
Age (Months)
Exp
ansi
on (%
)No Fly Ash
30% CaO
22% CaO
14% CaO
6% CaO
25% Fly Ash
Effect of Fly Ash Chemistry on ASR
Calcium Content - ASTM C 1293
The efficiency of Fly Ash in controlling ASR tends to increase as the calcium content of FA decreases
Date 46
Slag Cement
Industrial by-productfrom the iron makingindustry
Grade 120
Grade 100
Grade 80
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Effect of Slag Cement Content on ASR
ASTM C 1293
The efficiency of Slag Cement in controlling ASR increases as the Slag Cement content increases
0.00
0.05
0.10
0.15
0.20
0.25
0 6 12 18 24
Age (Months)
Exp
ansi
on (%
)
Control
25% Slag
35% Slag
50% Slag
65% Slag
Limit (CSA): Exp ≤ 0.04%
Thomas & Innis, 1998
Date 48
Silica Fume
Industrial by-product from the silicon metal and silicon-iron makingindustry
Date 49
Effect of SF content on ASR
The efficiency of SF in controlling ASR increases as the SF content increases
0.00
0.10
0.20
0.30
0 6 12 18 24
Age (Months)
Exp
ansi
on (%
) Control
7.5% SF
10% SF
12.5% SF
Fournier et al. 1995
Date 50
Use of SCM ’s - Natural Pozzolans
Davis Dam
On the Colorado River in Arizona
Built in 1950’s
Used calcined opaline shale to control ASR
Ex : Types of Natural Pozzolan used in North America include:
• Calcined shale
• Calcined clay
• Diatomaceous earth
• Metakaolin
Spratt Aggregate Silica Fume or SlagASTM C1293
-0.02
0.04
0.10
0.16
0.22
0.28Ex
pans
ion
[%]
0 300 600 900Time (days)
8% SF
12% SF
25% Slag
50% Slag0.00
CSA LIMIT
Control
35% Slag
Spratt AggregateTernary BlendsASTM C1293
-0.02
0.04
0.10
0.16
0.22
0.28
Expa
nsio
n [%
]
0 300 600 900Time (days)
8/15 4/25 5/25 8/25 6/35
CSA LIMIT
ControlSilica Fume/Slag
0.00
Lithium
There are lithium nitrate and lithium hydroxide admixtures
Lithium nitrate works better than lithium hydroxide –available as a 30% solution
Date 53
What percentages of SCM’s should you use to mitigate ASR?
Fly Ash
Slag Cement
Silica Fume
Ternary Blends
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Aggregate Reactivity Class
Description of Aggregate Reactivity
14 Day Expansion in C1260
1 Year Expansion in C1293
R0 Non Reactive ≤0.10 ≤0.04
R1 Moderately Reactive >0.10 ≤0.30 >0.04≤0.12
R2 Highly Reactive >0.30 ≤0.45 >0.12≤0.24
R3 Very Highly Reactive >0.45 >0.24
ASTM Proposed Language
Determine Aggregate Reactivity
Date 56
Size and Exposure Condition Aggregate – Reactivity Class
R0 R1 R2 R3
Non-massive concrete in a dry environment
Level 1 Level 1 Level 2 Level 3
Massive elements in a dry environment
Level 1 Level 2 Level 3 Level 4
All concrete exposed to humid air, buried or immersed
Level 1 Level 3 Level 4 Level 5
All concrete exposed to alkalis in service
Level 1 Level 4 Level 5 Level 6
ASTM Proposed Language
Determine the risk level based on aggregate reactivity and size and exposure condition
Date 57
Class Acceptability of ASR Examples
S3 Minor Risk of ASR Acceptable
Foundations ElementsRetaining WallsLarge numbers of precast elements where economic costs of replacement are severeService life normally40-75 years
S4 ASR Cannot be Tolerated
Power PlantsNuclear FacilitiesCritical Elements that are very difficult to inspect and repairService life normally > 75 years
ASTM Proposed Language
Determine the class
Date 58
Level of ASR Risk Classification of Structure
S3 S4
Level 1 V V
Level 2 W X
Level 3 X Y
Level 4 Y Z
Level 5 Z ZZ
ASTM Proposed Language
Determine the level of risk
Date 59
Type of SCM
Alkali Level of SCM (% Na2Oe)
Minimum Replacement Level
W X* Y* Z* ZZ*‡
Fly Ash(CaO≤18%)
<3.0 15 20 25 35 35
3.0-4.5 20 25 30 40 40
Slag Cement
<1.0 25 35 50 65 65
Silica Fume <1.0 1.2xLBA†
1.5xLBA† 1.8xLBA† 2.4xLBA† 2.4xLBA†
ASTM Proposed Language
Date 60
Prevention Level
Maximum alkali content of concrete (Na2O eq.) lb/yd3
V No limit
W 5.0
X 4.0
Y 3.0
Z Use Option 1
ZZ
ASTM Proposed Language
Limit alkalis in concrete
The alkali content of concrete is calculated on the basis of the alkali contributed by the portland cement alone.
Simplified Version
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Aggregate Reactivity Class
Description of Aggregate Reactivity
14 Day Expansion in C1260
1 Year Expansion in C1293
R0 Non Reactive ≤0.10 ≤0.04
R1 Moderately Reactive >0.10 ≤0.30 >0.04≤0.12
R2 Highly Reactive >0.30 ≤0.45 >0.12≤0.24
R3 Very Highly Reactive >0.45 >0.24
ASTM Proposed Language
Determine Aggregate Reactivity
Date 63
Size and Exposure Condition Aggregate – Reactivity Class
R0 R1 R2 R3
Non-massive concrete in a dry environment
Level 1 Level 1 Level 2 Level 3
Massive elements in a dry environment
Level 1 Level 2 Level 3 Level 4
All concrete exposed to humid air, buried or immersed
Level 1 Level 3 Level 4 Level 5
All concrete exposed to alkalis in service
Level 1 Level 4 Level 5 Level 6
ASTM Proposed Language
Determine the risk level based on aggregate reactivity and size and exposure condition
Date 64
Class Acceptability of ASR Examples
S3 Minor Risk of ASR Acceptable
Foundations ElementsRetaining WallsLarge numbers of precast elements where economic costs of replacement are severeService life normally40-75 years
S4 ASR Cannot be Tolerated
Power PlantsNuclear FacilitiesCritical Elements that are very difficult to inspect and repairService life normally > 75 years
ASTM Proposed Language
Determine the class
Date 65
Level of ASR Risk Classification of Structure
S3 S4
Level 1 V V
Level 2 W X
Level 3 X Y
Level 4 Y Z
Level 5 Z ZZ
ASTM Proposed Language
Determine the level of risk
Date 66
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This concludes The American Institute of Architects Continuing Education Systems Program
Henry B. Prenger, P.E.
410-925-0411