What is alkali-silica reaction (ASR) and how is it prevented?

ASR is a deleterious reaction between alkalis (Na, K) in cement and reactive silica in certain aggregates, forming expansive gel that absorbs water and causes cracking (map cracking pattern). Prevention methods include: using low-alkali cement (Na2Oeq < 0.6%), avoiding reactive aggregates (petrographic examination), using supplementary materials like fly ash or slag (30-40% replacement), limiting concrete permeability, and using lithium-based admixtures that form non-expansive products.