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What is AAR? How does it affect concrete quality? What can we do about it?
Alkali-Aggregate Reaction or AAR in short refers to the reaction between active silica constituents of the aggregate and alkalies of of the cement. These alkalis are Na2O and K2O.
Aggregates in concrete are mostly inert. However some aggregates do react with the alkali hydroxides in concrete forming expansive alkali-silicate gels. This will result in progressive manifestation by swelling or expansion causing disruption of concrete with the spread of pattern cracks leading to eventual failure of concrete structures.
One way to control AAR is by adding pozzolanic additives such as crushed stone dust and fly ash. This is an effective and practical way to inhibit AAR.
Follow the steps in the flowchart below to determine if potential for ASR exists and to select materials to control it.
Source: http://www.cement.org/learn/concrete-technology/durability/alkali-aggregate-reaction