Designers and contractors often come across problems which call for special solutions involving concrete. A special concrete a concrete made with special ingredients or by a special process may be ideally suited to the need. This article discusses some of the broad possibilities of special concretes. Brief descriptions are given so the reader can be better prepared to explore them in depth when the unusual situations come up. Those concretes included are: high strength, flowing, special aggregate, fly ash, structural lightweight, shrinkage-compensating, colored, heavyweight or high density, steel fiber, and cellular concretes.

Cellular concrete (foam concrete) is produced by discharging a stream of preformed foam into a transit mix load of sand-cement grout or a cement-water slurry. The preformed foam resembles shaving cream or the foam used for firefighting. Most of the foam concentrates are hydrolyzed proteins and are available through proprietary sources. The companies distributing the foam typically license subcontractors, furnishing them with the foam generators, pumps for placing the foam concentrates and technical assistance. With some systems a compressor is required.

The sand-cement grout or neat slurry is a cement-rich mix and the resulting concrete will contain about six 94-pound bags of cement to the cubic yard with a density ranging from 15 to 120 pounds per cubic foot. The heavier densities (100 pounds per cubic foot) are used for toppings on floor systems. The major use in recent years has been over plywood on wood floor systems or over hollow-core precast slabs. The material is also used in light density for roof fills (30 pounds per cubic foot) and thus has good insulating properties. The heavier material is made by shooting foam into the sand-cement grout mix and the lighter concrete is made from the cement slurry.