Alkali Silica Reactivity in Concrete
Rick Williams, P.G.
What Is ASR In Concrete?
Alkali Silica Reactivity
First discovered in California in the
1930’s
Expansive gels form as alkaline fluids
derived from the cement attack certain
reactive siliceous aggregates.
One of the leading causes of premature
concrete failure.
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Recipe For ASR
- 3 Ingredients -

1. Alkali Ions


2. Silica


Na2O and K2O found in all cement in varying
concentrations.
Reactive SiO2 primarily from amorphous or
poorly crystallized siliceous aggregate.
3. Moisture

H2O
All Three Ingredients Must Be Present
For Reaction To Initiate
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Reaction
Equation:
SiO2 + 2NaOH → Na2SiO3 + H2O
(Reactive Silica)
(Alkali Hydroxide)
(Alkali Silica Gel)
(Water)
The ASR Process



In the presence of moisture, an alkali hydroxide
solution can form.
As the solution migrates, the hydroxides begin
attacking the Si-O-Si bonds of reactive siliceous
aggregates.
The result is the formation of an expansive
alkali-silica gel within the concrete structure.
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Potentially Reactive Aggregates


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Chert
Opal
Flint
Quartzite
Volcanic glass
Strained quartz
Quartz-Arenite
Ordered “Non Reactive” Silica Vs.
Disordered “Reactive” Silica
Si
O
OH–
Na+
In the case of non-reactive quartz, the alkali ions cannot
penetrate the well-ordered crystal lattice.
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The Term Alkali-silica Reaction Is
Somewhat Misleading


The reaction itself is actually between the OHions in the pore solution and certain siliceous
components of the aggregates.
The alkalies, specifically sodium and potassium,
do not actually attack the reactive silica.
Importance of Alkalis to ASR



The importance of the alkalies is that their
presence in high concentrations in the pore
solution results in an equally high concentration
of OH- ions (to maintain charge equilibrium).
It is this high OH- concentration, and thus high
pH, that leads to the initial breakdown of
reactive silica components in the aggregates.
The alkalies also ultimately contribute to the
expansive ASR gel formation.
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Why Is ASR So Destructive?
Reaction products (Expansive Gel)
resulting from the interaction of Alkali
Hydroxide Ions and Siliceous
Aggregate occupy more space than the
original Silica Aggregate causing tensile
forces to build within the concrete until
cracking occurs.
Worst Case Scenario

High Alkaline Cement

Reactive Aggregate


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>= 0.6% NaOe (NaO Equvialvent)
Chert
Chalcedony
Opal
Humid Environment



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Bridge pier submerged in water
Dam Site
Area that floods; i.e. poor drainage
Internal Humidity >= 85%
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What Does ASR Look
Like In The Field?
Map Cracking
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Map Cracking
Map Cracking In 5
Year Old Bent Cap
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Backscattered
Electron Image
FRACTURE FILLED WITH
ASR GEL
REACTIVE AGGREGATE
ASR In Thin Section
ASR GEL
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ASR In Polished Section
ASR In Thin Section
Cement
ASR Gel
Reactive SiO2
Aggregate
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Test Methods For Predicting ASR
Aimed At Aggregates

ASTM Test Methods

ASTM C1260

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Mortar bar is immersed in a NaOH solution for 14
days. Expansion of more than .2% after 14 days
indicates the aggregate is reactive.
ASTM C1293

Actual concrete specimen tested for 1 year.
Expansion of more than .04% after 12 months
indicates a reactive aggregate.
Mortar Bar Test
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ASTM C1260

Pros:



Can be completed in
14 Days
If aggregate passes
this test it is virtually
assured to be nonreactive.
Cons:

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Very aggressive – High
NaOH concentration and
high temperature.
Performed on Mortar bars –
not concrete specimens
It is not uncommon for
good aggregate to fail and
be classified as reactive.
Many specifying agencies
modify this ASTM Standard
but use same expansion
limits.
ASTM C1293

Pros:



Performed on actual
concrete specimens
More representative of
field conditions than
C1260
Much less aggressive
than C1260

Cons:

Takes 1 year to
complete
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Outdoor Exposure Site
The University Of Texas At Austin
What Can Be Done To Prevent
ASR?



Use low alkaline cement
Use non-reactive aggregate
Use of SCMs and Admixtures

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Fly Ash (Class F) – Most widely used; Derived from
the combustion of coal, Very Effective
Lithium – Effective before and after ASR has formed
Silica Fume
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Fly Ash



Generated from the combustion of coal.
Fly ash replacement by mass of cement is
an effective way to reduce expansion due
to ASR.
The reaction can be slowed down to the
point that it will prevent ASR for the
design life of the pavement.
Lithium


Lithium forms a gel similar to ASR, however it is not
expansive – Lithium Silicate.
Research has proven this to be an effective way to
combat ASR before and after the reaction has initiated.
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Benefits Of Admixtures And SCM’s
 Permit the use of local, cost-effective
aggregates.
 Increase the life span of concrete structures.
If you see cracking like this…
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……You Can Say:

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1. “That’s A Result Of Alkali Silica Reactivity.”
2. “Alkali Silica Reactivity is driven by the attack
of alkali hydroxides in the cement on certain
reactive siliceous aggregates in the
concrete.”
3. “Had the cement, aggregate and exposure
condition been properly considered during the
design phase, this might have been avoided.”
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