Alkali-Aggregate Reaction (AAR)

Summary

Alkali-Aggregate Reaction (AAR) - the chemical degradation of concrete where the alkaline cement paste reacts with reactive, non-crystalline silica or dolomite, which may be found in some common aggregates. The two reactions are called alkali silica reaction (ASR) and alkali carbonate reaction (ACR) respectively.

Technical Information

AAR is damaging for concrete because it can cause internal swelling leading to cracking, spalling and overall loss of strength in the concrete over time. The expansion in the concrete is caused by the formation of compounds which are bulkier than their reactants. This causes an expansive pressure which, when it exceeds the tensile strength of the cement paste causes cracking.

Some methods for limiting AAR are:

Limit the alkali metal content of the cement e.g. the Na2O content.
Use aggregate with a lower reactive silica content.
Add very fine siliceous particles (e.g. silica fume or fly ash) to the concrete mix to provoke the alkali silica reaction away from the aggregate. As the particles are very fine, they don’t require an expansive gel layer to react with the alkali, therefore limiting swelling damage.
Ensuring external alkalis don’t come into contact with the concrete.
Avoiding the use of dolomite-rich aggregates.

ACR is much less common than ASR because siliceous aggregates are more common than aggregates containing dolomite.