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Grading and Surface Area
This discussion will be limited to the effect of grading and maximum size of aggregate on the properties of concrete used in ordinary construction. No-slump, lightweight, and heavy concrete will not be considered, although much of the discussion is applicable to all types of concrete because aggregate, cement, water, and entrained air, of which concrete is composed, fit together as absolute volumes and in no other way. Aggregate comprises about 55 per cent of the volume of mortar containing aggregate graded up to size and about 85 per cent of the volume of mass concrete containing aggregate graded up to 6-in.-max size. Thus, it is not surprising that the way the particles of aggregate fit together in the mix, as influenced by their gradation, shape, and surface texture, has an important effect on the workability and finishing characteristics of the fresh concrete and the properties of the hardened concrete. The influence of aggregate grading on the properties of concrete has been studied since the invention of portland cement, and many methods have been proposed for arriving at an “ideal” grading that would be applicable for all aggregates. None of these has been universally successful because of economic considerations, effect of particle shape and texture of the aggregate, and differences in cements from different mills . Grading specifications have been developed, however, which on the average will produce a concrete of satisfactory properties from materials available in a particular area.
Price, W. H.
Technical director, Am. Cement Corp., Los Angeles, Calif.