What is alkali -silica reactivity ( ASR )?

Alkali-silica reactivity is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis  , mostly from cement , but also from aggregates , pozzolans , admixtures and mixing water . External sources of alkali-silica gel that swells as it draws water from the surrounding cement paste , thereby inducing pressure , expansion and cracking of the aggregates and surrounding paste . This often results in map-pattern cracks ,sometimes referred to as alligator pattern cracking . ASR can be avoided through

1 ) proper aggregates selection
2 )zuse of blended cement
3) Use of proper pozzolanic material
4 ) Contaminant - free mixing water .