What Is Alkali-Silica Reaction in Concrete?
Alkali-silica reaction (ASR) is a slow chemical reaction between the alkalis in cement and reactive silica in certain aggregates, forming an expansive gel that cracks concrete from the inside over years or decades. Known as 'concrete cancer,' it produces characteristic map-pattern cracking and is irreversible once started. Prevention at mix design is the only real defense — which is why specifications matter.
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How it develops
The reaction needs three things: reactive aggregate, high-alkali cement, and moisture. The gel absorbs water and swells, generating internal pressures that crack the concrete in a distinctive random map pattern, often with gel exuding from the cracks. Structures in wet environments with susceptible aggregates — dams, bridges, pavements — show it first, typically 5-20 years after construction.
Why it matters for new construction
ASR cannot be fixed, only prevented: low-alkali cement, non-reactive aggregates (verified by ASTM C1260/C1293 testing), and supplementary cementitious materials (fly ash, slag, silica fume) that suppress the reaction. Specifications for public infrastructure routinely require ASR mitigation; private commercial specs should too. The testing and mitigation add modest cost at batching — the alternative is a structure with a shortened service life.
Diagnosis and management
Petrographic examination of cores confirms ASR and rules out look-alikes (freeze-thaw, sulfate attack, delayed ettringite formation). Management of affected structures focuses on keeping water out — sealers, membranes, drainage — to slow the reaction, plus structural monitoring. In severe cases, affected elements are replaced. There is no chemical treatment that reverses established ASR.
The bottom line
ASR is an irreversible expansive reaction between cement alkalis and reactive aggregates — map cracking over years. Prevent it with tested aggregates, low-alkali cement, and SCMs; manage existing cases by excluding water.
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