At the lower end of the lightweight concrete spectrum, concretes weigh as little as 12 to 15 pounds per cubic foot, and have compressive strengths ranging down to 5 psi or less. When normal weight sand is included in these mixes, increased weight and strength usually can be achieved, but the information in this article focuses on concretes weighing 50 pounds per cubic foot and less, a range where thermal insulation is most effective. Cellular concretes in this density ranges are also used for highway and foundation fills where their weight-strength combination offers advantages over natural soils.
Within the spectrum of lightweight concretes now available, the lightest ones provide the best insulation k-values from 0.4 to 0.7 Btu inch per hour square foot degree F but little strength. The designer must consider not only the insulating value of the concrete material but also its other properties and how it will be combined with adjoining materials, whether in a roof assembly or a foundation fill. Perlite concrete, expanded polystyrene bead concrete, cellular concrete, and vermiculite concrete are discussed in this article.
Vermiculite is a soft, laminated, mica-like mineral that when heated and exfoliated becomes a lightweight aggregate of great value for fill and insulating concrete. The components of vermiculite insulating concrete are expanded vermiculite aggregate, air-entraining admixture, Portland cement, and water, all mixed and applied according to precise procedures. The resulting concrete mixture is usually pumped to the roof site and screeded into place over the structural base. Vermiculite concrete is installed in thicknesses of 2 inches or more, depending on design needs and strength requirements. It weighs from 20 to 40 pounds per cubic foot, with compressive strengths from 90 to 500 psi.