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Aggregates constitute about 75 per cent of the volume of concrete, and, consequently they have an important influence on the strength and durability of concrete. Numerous laboratory investigations and field condition surveys have shown that certain aggregates contribute to the lack of durability of concrete exposed to the elements. However, the exact cause of the lack of durability is not always clear. Other studies have shown that modifications must be made in design or other steps taken to compensate for the low strengths obtained with some combinations. Durability of concrete, as considered in this discussion, is confined to the consideration of the resistance of the concrete to (1) normal weathering action, (2) chemical reaction between the cement and the aggregates, (3) fire, and (4) the action of salts used for ice control. The compressive strength and modulus of rupture may give sufficient information for protected structural concrete, providing there is no chemical reaction between the cement and aggregates, but for concrete exposed to the elements the materials should be subjected to exposure tests and, if possible, the service record of the materials in question used under similar conditions should be determined. Numerous investigations have shown that various concrete aggregates differ widely in their effect upon the durability of concrete even though producing practically identical strengths.
Allen, Charles W.
Research Engineer, Columbus, Ohio