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Alkali-aggregate reaction

Alkali-aggregate reaction can develop in concrete containing siliceous aggregate and alkali-carbonate reaction can occur in some rare limestone aggregates.

For alkali aggregate reaction to occur, it is necessary to have a source of moisture, a reactive aggregate and a source of alkalis. Many of the aggregates in use in the UK are potentially reactive with alkalis in cement paste. However, in practice, alkali-aggregate reaction is rare in the UK and where encountered there are often additional risk factors such as:

  • The penetration of externally derived alkalis 
    for example, from de-icing salts
  • The presence of a highly reactive aggregate 
    such as an aggregate containing opal or glass
  • The use of a cement with a high alkali content.

Concrete suspected of undergoing damage as a result of alkali-aggregate reaction may contain large cracks. However there are many causes for cracking in concrete and using thin sections it is possible to detect the characteristic forms of microcracking that develop as a consequence of alkali aggregate reaction on a microscopic scale. Petrographic examination is the definitive method for detecting alkali aggregate reaction in concrete and for assessing the potential for the development of alkali-aggregate reaction. By measuring the chemical composition of gel with the electron microprobe it is often possible to assess whether or not alkali-aggregate reaction is ongoing.

A photomicrograph showing a gel-filled crack in a flint aggregate particle in concrete with a high alkali content.