Aggregates constitute 60% to 75% of the volume of concrete. Therefore, both the cost and quality of a concrete are affected by the kind of aggregates used in it. Aggregates should be obtained from reliable dealers whose materials are of a quality known to make suitable concrete. They should comply with the requirements of ASTM C 33, which places limits on the allowable amounts of damaging substances and specifies requirements for grading, strength, and soundness of normal-weight aggregate.
The most commonly used aggregates—sand, crushed stone, and air-cooled blast-furnace slag—produce normal-weight concrete weighing between 135 and 160 1b./ft.³(2163 and 2563kg/m³). Aggregates of expanded shale, clay, slate, and slag are used to produce structural lightweight concrete weighing from 85 to 115 1b./ft.³ (1362 to 1842kg/m³). Other lightweight materials, such as pumice, scoria, perlite, vermiculite, and diatomite, are used to produce insulating concretes ranging in weight from 15 to 90 1b./ft.³(240 to 1442kg/m³).
Both fine and coarse aggregates should be uniformly graded from their finest particles up to their largest. They should also be clean and free of loam, clay, and vegetable matter, because these foreign particles prevent Portland cement paste from properly binding aggregate particles together. Concrete containing these objectionable materials will be porous, have low resistance to weathering, and have low strength. It can also develop surface defects such as popouts.