Alkali-Aggregate Reaction (AAR/ASR)

Alkali-Aggregate Reaction (AAR) — most commonly Alkali-Silica Reaction (ASR), sometimes called 'concrete cancer' — is a slow, expansive chemical reaction between the alkaline pore solution of cement (Na₂O, K₂O) and reactive forms of silica present in certain aggregates. It forms a hygroscopic alkali-silica gel that absorbs water and swells, generating internal tensile stresses that cause characteristic map (pattern) cracking, gel exudation, and long-term loss of strength + stiffness over years to decades.

Three conditions must coexist for damage: reactive aggregate, sufficient alkali content, and moisture — removing any one stops it. IS 383 requires aggregates to be tested for alkali reactivity (mortar-bar / chemical methods) and IS 456 Cl. 8.2.5.4 advises using low-alkali cement (≤ 0.6% Na₂O equivalent), supplementary cementitious materials (fly ash, GGBS, silica fume which bind alkalis), non-reactive aggregate, and limiting moisture ingress for at-risk structures (dams, bridges, marine works).