Technical Questions

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Protecting against excessive expansion due to alkali-aggregate reaction (AAR)

Q. What precautions are necessary to protect against excessive expansion due to alkali-aggregate reaction (AAR)?


A. Alkali-aggregate reactions (AAR) can be either alkali-carbonate reactions (ACR) or alkali-silica reactions (ASR). In ACR the reaction is between the alkalies (sodium and potassium) and certain carbonate rocks, particularly calcitic dolomite and dolomitic limestones, present in some aggregates. In ASR, the reaction is between alkalies and certain siliceous rocks or minerals, such as opaline chert, strained quartz, and acidic volcanic glass, present in some aggregates. These reactions may cause abnormal expansion and cracking of concrete in service. If the use of reactive aggregate can be avoided, no precaution is needed.

For ASR, it may be sufficient to merely use the amount of pozzolan or slag that is appropriate for economic reasons. In other cases low-alkali cement may be sufficient; however, if an effective pozzolan or slag is available; its use should be considered (ACI 221, 201.2R).

For ACR, avoidance or reduction in proportion of the reactive phases is the only recommended practice. The other methods listed, though proven effective with alkali-silica-reactive aggregates, may not be a sufficient remedy for the alkali-carbonate rock reaction. Testing the job materials is recommended to ensure that the reactivity can be controlled adequately.


References: SP-1(02); ACI 221.1R-98; ACI 201.2R-16

Topics in Concrete: Aggregate; Alkali Aggregate Reaction; Durability; Testing of Concrete

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