Alkali release from aggregates: contribution to ASR

The alkali–silica reaction in concrete is normally associated with alkalis from cement. However, some aggregates have alkalis in their crystalline or amorphous phases that can be released during long-term exposure to the water in concrete structures. Previous research has demonstrated that alkaline feldspars in aggregates can supply a significant amount of alkalis when exposed to alkaline solutions. Other research indicates that micas can release alkalis and promote alkali–silicate reaction. The main problem is the evaluation of the potential contribution of the alkalis released by aggregates during the service life of a concrete structure. This paper describes accelerated tests that can determine the level of alkalis released from aggregates, but finds that the results are not representative of the long-term behaviour of concrete structures. Alternative tests are being developed, but these could take much longer, and it is concluded that numerical methods may be the most practical solution.