QUESTION: With the price of asphalt rising despite lower crude oil prices, a contractor told us he may enter the roofing business by providing cellular concrete. What is cellular concrete?

Can it be made in a concrete plant?

ANSWER: ACI defines low-density cellular concrete as a material made with hydraulic cement, water, and preformed-foam. When it hardens, the concrete has an oven-dry density of 50 lb/cubic feet or less. Recipes may also include aggregates, fly ash, and admixtures.

ACI committee 523 has prepared a Guide for Cast-in-Place Low-Density Cellular Concrete that provides data and techniques pertaining to its batching and use. The material is placed to provide a roof's base, thermal insulation, and drainage slope for flat-roofed industrial and commercial buildings.

In geotechnical applications, the material is applied in thick sections of cellular concrete with low compressive strengths for replacing poor soils, fills for abandoned structures, and fills designed, mixed, and placed to meet specific job conditions and functional requirements.

For the most part, typical concrete mixing equipment can't produce low density cellular concrete mixtures. Drum mixers do not have the proper mixing action or mixing speed to combine the ingredients. But a high-speed paddle mixer can properly combine the ingredients while blending the preformed foam rapidly and efficiently to produce a uniformly consistent, low-density cellular concrete mixture.

Renewed interest in cellular concrete isn't limited to concrete contractors. Several precast producers have installed foam injection systems to cast lightweight elements.