Soundness of Aggregate:
Soundness of aggregate is the resistance of aggregate to disintegration and to change in volume with the change in the environmental/physical condition such as variation in temperature, thawing and freezing, alternate wetting and drying etc. Soundness of aggregate also indicates the durability of aggregate. Aggregate, which undergoes greater volume change under the above conditions, is called unsound aggregate. Aggregates with large amount of water reactive material (e.g. lime) and porous and weak aggregates undergo large volume change. Alkali-aggregate reaction is very important in this regard.
Alkali-aggregate Reaction
Earlier the aggregates were considered as inert material but later on it was found that these are not fully inert material. Some of the aggregates may contain some amount of reactive silica, which reacts with alkalies present in the cement. It produces the alkali silicate gel, which is of unlimited swelling type. Due to the swelling property of such gel, cracks are developed in concrete hence durability of concrete reduces.
It is interesting to note that alkali-aggregate reaction can take place only if the reactive silica of particular fineness and that too in particular proportion is present. The factors responsible for alkali-aggregate reaction are reactivity of aggregate, high alkali content of cement (i.e. > 0.6%, expressed as soda equal), availability of excess moisture and the optimum temperature conditions (i.e.10-38oC). So, it is quite possible to reduce the probability of such reaction by changing the proportion or fineness of reactive silica or both and by controlling the availability of excess moisture or temperature. A Mortar bar expansion test [IS 2386 (part vii): 1963] is popularly used to determine the potential reactivity of the aggregate.
Comments
Use of unsound aggregates in concrete will degrade the quality of concrete. The change in volume of concrete may appear in the form of scaling, surface cracks and/or disintegration of concrete over a considerable depth that not only hamper the appearance but also may cause structural damages. Therefore, it is always preferred to avoid the use of unsound aggregates.