Alkali-Aggregate Reaction

Alkali-aggregate reaction includes both alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). ASR occurs most commonly with siliceous sedimentary rocks such as chert, quartzite, and graywacke, siliceous intrusive and metamorphic rocks such as granites and gneisses, and siliceous volcanic rocks such as rhyolite. ACR occurs most commonly with dolomitic limestone aggregates. Generally speaking ASR is more common and understood better than ACR. Alkali aggregate reaction may cause significant cracking in a concrete element or it may be benign with no adverse affect on the concrete. ASR can also be associated with surface popouts on concrete slabs.

Cross-polarized transmitted light photomicrograph of microcracks (green and yellow arrows) in concrete undergoing alkali-carbonate reaction. The red arrows indicate deposits of brucite in voids.

EDS map of concrete from a sea wall undergoing a combination of alkali-silica reaction and marine water attack. The areas shown in purple, light pink and yellow represent ASR gel, the he blue areas represent brucite and the reddish brown areas represent hydrotalcite.

Cross-polarized transmitted light photomicrograph of thin section with quarter wavelength plate inserted. The red arrows highlight deposits of gel in microcracks.

Reflected light photomicrograph of microcracks (yellow arrows) in reaction rim (yellow bar) of a dolomitic limestone undergoing alkali-carbonate reaction.

Reflected light photomicrograph of polished surface showing void (red arrow) that is filled with gel next to a fracture graywacke (GW) particle.

Reflected light photomicrograph showing microcracks (green arrows) and voids (red arrows) that contain ASR gel.

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