With a typical unit weight of 90 to 120 pounds per cubic foot (pcf) and a compressive strength from 2500 psi to more than 8000 psi, structural lightweight concrete is a versatile building material. Since it is generally 20% to 40% lighter than normalweight concrete, a structure's dead load can be reduced, its foundation costs lowered, and its concrete and rebar needs lessened. Structural lightweight concrete also resists fire better than normalweight concrete because of its lower thermal conductivity and its lower coefficient of thermal expansion. In many structures, these benefits justify the use of lightweight aggregate concrete, which generally costs more than normalweight concrete.
By definition, structural lightweight concrete contains aggre- gates that are either all-lightweight or a combination of lightweight and normalweight aggregate. Lightweight aggregates suitable for structural concrete may be natural materials such as pumice or scoria, or they may be processed aggregates such as expanded shales, clays, slates, and slags. More porous than normalweight particles, lightweight aggregates that are not presaturated will absorb the water in the concrete mix. To control the slump, the lightweight aggregates should be prewetted before being used in a mix. There are three ways to prewet them: thorough sprinkling, thermal quenching, and vacuum saturation. If the aggregate particles in lightweight concrete are properly prewetted, then the concrete should behave much like normalweight concrete.