Q: We’re starting a job that calls for structural lightweight concrete. How does this material differ from normal weight concrete and what should we be prepared for in working with it?

A: Structural lightweight concrete (SLC) is made with lightweight aggregates (such as expanded clay, slate, or shale; pelletized or extruded fly ash; or expanded slag) either alone or combined with normal weight aggregates. It is used to reduce the dead load of concrete members, such as floor slabs in high-rise buildings. Its dry density ranges from 85 to 115 pounds per cubic foot, and its 28-day compressive strength exceeds 2500 psi (3000- to 5000-psi compressive strengths are typical). By comparison, the dry density of normal weight concrete ranges from 130 to 155 pounds per cubic foot.

Structural lightweight concrete can be mixed to achieve the same workability, finishability, and appearance of normal weight concrete, as long as it contains enough cement paste to coat each particle and enough fine aggregate to keep the mixture cohesive. Entrained air will improve workability, so it should be used whether or not it’s needed for freeze/thaw durability.

SLC slumps less than normal weight concrete of the same workability, and a 2- to 4-inch slump works best for finishing. You can begin finishing it earlier than comparable normal weight concrete, but keep floating and troweling to a minimum. Magnesium finishing tools are recommended for the best results.