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The alkali-aggregate reaction is receiving renewed attention for the following reasons:
1. Aggregates previously considered innocuous have shown expansions after several years.
2. The alkali content of portland cements has increased because of the process changes prompted by energy and environmental constraints.
3. The traditional use of fly ash to reduce the expansion from the alkali-aggregate reaction may not be beneficial if the fly ash has a low amorphous silica content and high alkali (particularly sodium) content.
The general mechanism for the alkali-silica reaction, the most common type of alkali-aggregate reaction, involves the attack of the reactive form of silica by the highly alkaline pore solutions to form a gel which imbibes water, creating internal stress and disruption of the concrete.
Factors affecting the alkali-aggregate reaction include the nature and amount of reactive aggregates, the alkali content of the cement and fly ash, the mix proportion, and the environment of the concrete.
With the increasing use of marginally reactive aggregates in combination with higher alkali cements and possibly fly ashes that will not deter the expansive reaction, it is very possible that concrete structures are being built today that in the years to come will begin to exhibit map cracking and eventually become unsafe.
Present test methods do not always predict when combinations of aggregate and cementitious materials will react to form expansive products. More research is needed on the long-term effects of high-alkali (particularly high-sodium) fly ash with a low amorphous silica content.
alkali-aggregate reaction, cement, fly ash, expansion, amorphous silica, pozzolan
ENRECO Laboratories, Amarillo, TX
Raba-Kistner Consultants, Inc., San Antonio, TX