Causes of Damage to Concrete-Alkali aggregate reaction
Some types of sand and aggregates that have high silica content such as opal, chert, and flint will react with the calcium, sodium, and potassium hydroxide alkalis. The reaction of this kind of aggregates and cement will adversely affect the concrete and cause destructive expansion. Petrographic examination shows that the reactive concrete will form a gel around alkali reactive aggregate. The gel will expand in the presence of water or water vapor (relative humidity of 80 to 85 percent is all the water required). The expansion of this gel will generate tensile stress that can cause concrete cracks and spall. Figure no:1 showing the gel formed as a result of alkali aggregate reaction. Cracked concrete due to alkali-aggregate reaction usually will have a whitish exudation in and around cracks, the presence of this whitish exudation will be evident that the destructive expansion occurred due to the presence of reactive alkali aggregates in concrete. The width of crack can reach up (2 in) 5 cm in extreme instances, as showing in figure no:2. In large structures, the destructive expansion may occur in portions of the structure, which might be confusing. Using multiple aggregate sources may be the reason for the concrete deterioration in some parts of the structures. Only the portions of the structure constructed with alkali reactive aggregates or sand will exhibit the destructive expansion due to alkali aggregate reaction. In dams, the expansion of concrete due to the alkali-aggregate reaction can control or seizure the control gates.
In a new structure, the threat of alkali-aggregate reaction can be significantly reduced or eliminated by the using of pozzolans such as fly ash and the using of low alkali cement. In existing concrete structures, there are no reliable solutions for the deteriorating of concrete due to alkali aggregates reaction. There are no proven methods of eliminating the deterioration of alkali-aggregate reaction. However, the reaction can be reduced by maintaining the concrete dry as much as possible. The repairing of concrete actively undergoing aggregate alkali reaction considered useless because the continues expansion of concrete will disrupt and damage the repaired portion. Structures undergoing active deterioration should be monitored for the rate of expansion and movement, and only the repairs necessary to maintain safe operation of the facility should be made. A wire saw cut has been used to cut the expanded concrete on both sides of the dam gate to relieve and allow gates to operate safely. The cut locations were sealed to water leakage using polyurethane resin injection techniques. The relief cutting of concrete might have to be repeated several times as the expansion of concrete continues.
In many structures, the expansion associated with the reactive aggregate’s reaction slows down and ceases. The slowing down of reaction will occur when the alkali components are consumed by the reaction. Once the expansion of concrete ceases, repairs can be performed for the deteriorated concrete to restore the structure to full operation and serviceability. In extreme conditions, the structure might be replaced due to alkali aggregates deterioration.