Alkali Aggregate Reaction is the reaction between the active mineral constituent of some aggregate and the sodium and potassium alkali hydroxide in the concrete.

Harmful only when it produce significant expansion.

Alkali aggregate reaction is a chemical reaction of alkali in concrete and certain alkaline reactive mineral in aggregate producing a hygroscopic gel which when moisture present absorb water and expand.

Gel expansion causes cracking in the concrete.

The number of structure affected by Alkali aggregate reaction is relatively small comparing to the total number of concrete structure built but the problem has been found in many countries around the world.

Source of alkali in concrete

1) Cement

2) Pozzolans

3) Aggregate

4) Admixture

5) Water

6) Alkali from outside the concrete

General type of aggregate alkali aggregate reaction

1) alkali silica reaction

2) Alkali silicate reaction

3) Alkali carbonate reaction.

Test of aggregate re activity

1) Petrographic examination

2) Chemical test

3) Expansion test

Form of alkali aggregate reaction

1) alkali silica reaction

2) Alkali carbonate reaction