Before 1976, no concrete aggregates in the United Kingdom were thought to react harmfully with alkalies. But that year researchers found that some structures had indeed cracked because of an alkali-silica reaction (ASR). Most of the affected members are exposed to groundwater, rain, or heavy condensation and the concrete generally contained more than 760 pounds of cement per cubic yard. Chert particles present in natural aggregates, mostly from three sources, contained the reactive silica. The alkalies for the reaction came primarily from the portland cement. Because water is needed for the expansive reaction, coatings or cladding may affect the reaction by altering the concrete moisture content.
REDUCING THE RISK OF CRACKING
One preventative approach is limiting the alkali content of concrete made with potentially reactive aggregates. One of the following two procedures is used: limiting the concrete active alkali content from all sources to 5 pounds per cubic yard or less; and limiting the active alkali content of the cement or binder to 0.6% or less when the alkalies from other sources don't exceed 0.3 pound per cubic yard.