before they are used in concrete. The most commonly used aggregates—sand, gravel, crushed stone, and air-cooled blast-furnace slag—produce freshly mixed normal-weight concrete with a density (unit weight) of 2200 to 2400 kg/m. 3 (140 to 150 lb/ft. 3). Aggregates of expanded shale, clay, slate, and slag (Fig. 5-3) are used to produce struc-