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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

Na2O and K2O found in all cement in varying concentrations.

2. Silica

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

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 OH-ions 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 >= 0.6% NaOe (NaO Equvialvent)

Reactive Aggregate Chert Chalcedony Opal

Humid Environment 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 WITHASR GEL

REACTIVE AGGREGATE

ASR In Thin Section

ASR GEL

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ASR In Polished Section

ASR In Thin Section

Reactive SiO2

Aggregate

ASR Gel

Cement

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Test Methods For Predicting ASR Aimed At Aggregates

ASTM Test Methods

ASTM C1260

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 non-reactive.

Cons: 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

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:

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.”