Alkali-Silica Reaction (ASR), the reaction between alkalies in portland cement and certain siliceous rocks or minerals
Diagnosis: An expansive cracking condition with discoloration.
Cause: A chemical reaction between the naturally occurring sodium and potassium in Portland cement and certain silicaceous minerals. This reaction causes the expansion of these aggregates initially and as the reaction continues, the reaction breaks-down the minerals forming a gel like substance. This substance, ASR gel is even more expansive, compounding the problem.
Prevention: Avoid use of known potentially reactive aggregates. If this is unavoidable then adjustments to the mix design can reduce the risk. Consult your local concrete producer.
Possible concrete repairs: This one is a tough fix. Depending on the project, steps can be taken to relieve some of the stress imposed by the expansion (such as strategic cutting, retrofitting expansion joints and, conversely, anchoring for restraint). With this condition, it is more of a situation of managing the symptoms, as the only option for a total cure is complete demolition.