Lightweight concrete is concrete weighing substantially less than that made using gravel or crushed stone aggregates. This loose definition is generally agreed to cover a broad spectrum of concretes ranging in weight from 12 to 120 pounds per cubic foot. Many types of concrete fall within this range some are cellular concretes, some are made with lightweight aggregates. Other lightweight concretes may contain some normal weight sand. The compressive strength of these concretes covers an even broader spectrum, with structural lightweights at 6000 psi and higher at one extreme, and cellular fill concretes at 5 psi at the other extreme.
Lightweight aggregate concrete is usually chosen for structural purposes where its use will lead to a lower overall cost of structure than would be expected with normal weight concrete. The generally higher unit cost of lightweight structural concrete is offset by reduced dead loads and lower foundation costs. There may be a special advantage when existing structures are being altered or expanded. For example, four stories were added to an existing department store without modifying the foundation.
In general, procedures for mixing lightweight structural concrete are similar to those for regular weight concretes, but some of the more absorptive aggregates may require prewetting prior to addition of other mix ingredients. Water added at the batching plant should be sufficient to provide the specified slump at the building site; slump at the batching plant will probably be appreciably higher. The designer must consider the benefits of lighter weight and better insulation in relation to the extra cost of the lightweight mix; the builder must recognize the few different requirements relative to transporting, placing, and finishing.