Alkali-Silica Reactivity in Concrete

One of the damaging defects caused in concrete is due to alkali-silica reaction (ASR) in the concrete. Typically, the reaction of finely-divided forms of silica in aggregates, mostly in gravel, and elevated concentration of alkali, mostly originating from the cement, in the presence of water/moisture causes a gelatinous material to be formed in the concrete. The reaction continues, and the volume of gelatinous material continue to grow, so long as moisture is available. Higher ambient temperatures accelerate the reaction and volume of deleterious reaction product.

In summary, ASR occurs when three conditions exist in concrete: (i) source of finely-divided silica, (ii) elevated alkali content in concrete (source: from cement) and (iii) moisture.

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A siliceous aggregate in a high alkali medium – due to cement – showing the reactive gel. (Petrographic photo)

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A flowing gel in concrete with the right conditions of formation, namely: i) reactive aggregate ii) high alkali medium & iii) moisture. (SEM Photo)

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Typical damage crack damage on a pavement.