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What is Alkali-Aggregate Reaction (AAR)

Alkali-aggregate reaction (AAR) is a chemical reaction between the alkali in Portland cement and reactive minerals in aggregate and additives that takes place when moisture is present. This reaction results in the formation of a hygroscopic gel that absorbs water and expands, causing significant expansion and characteristic cracking of the concrete and ultimately failure of the concrete in worst cases.

AAR often occurs in old concrete hydraulic structures, where the problem was not detected or not properly treated before and during construction. Many concrete dams and other hydraulic structures worldwide have suffered from AAR.

Today, the reactive mechanism and consequences of AAR are recognized by concrete and dam engineers, and high attention is paid to the problem. The potential for AAR in a new large hydraulic structure should be thoroughly explored before and during its construction. Necessary measures should be taken to prevent or suppress the potential expansion within a tolerable limit.

It is important to understand the current knowledge and practice in managing AAR in the concrete of hydraulic structures.

Types and prerequisites of AAR

There are three subsets of AAR:

  • Alkali-silica, a reaction between the alkali hydroxides in concrete and reactive forms of silica in aggregate (e.g., chert, quartzite, opal, strained quartz crystals);
  • Alkali-silicate, a reaction between the alkali hydroxides in concrete and reactive forms of silica present in the combined form of phyllosilicates (e.g., chlorite, vermiculite, micas); and
  • Alkali-carbonate, a reaction between the alkali hydroxides in concrete and the dolomite crystals present in aggregate.

Alkali-silica is the most common form of AAR. Alkali-silicate and alkali-carbonate reactions are relatively rare. AAR problems have been reported throughout the world, and intensive research has been performed.

Concrete dams are particularly monitored for AAR because of their size, large investment, important role, and severe consequences in case of failure.

Three approaches can be followed to identify possibility of AAR:

  • The first indicator  is  petrographic examination
  • The second indicator test is Mortar bar expansion test
  • The third indicator is Chemical  method test for Coarse and fine aggregates

Research and practice indicate that the following prerequisites must be simultaneously fulfilled for AAR expansion to occur;

  • High content of alkali in concrete (2.4 to 3 kilograms per cubic meter or more);
  • Presence of reactive minerals in aggregate; and
  • Sufficient moisture supply (at least 80 percent).

If any one of these three conditions is not met, expansion due to AAR cannot occur.
Therefore with aggregates falling in deleterious zone, Use of  low alkali cement is recommended to prevent any  chances of ASR occurrence.