Question: Can you give me some information on insulating concrete for roof decks?

Answer: Insulating concrete is made with special lightweight aggregates meeting ASTM specifications C332 or made by using preformed or formed-in-place foam with cement, cement-sand mortar and also with lightweight aggregates. The amount of insulation required with the specific structural deck will determine the type of concrete used. The thermal conductivity (K) of the dry concrete depends on its oven-dry weight per cubic foot. A general relation of weight to thermal conductivity is given in the accompanying chart.Lowest thermal conductivity is obtained by foam concrete or concretes made with perlite or vermiculite. These concretes are readily placed by means of suitable pumps. Perlite and vermiculite concretes contain 4 to 8 cubic feet of aggregate per sack of cement, the proportion depending on desired thermal conductivity and strength.Concrete strength is also related to the oven-dry weight of the concrete. An oven-dry weight of 20 to 30 pounds per cubic foot gives a 28-day compressive strength of 100-350 psi; a weight from 50 to 60 pounds per cubic foot gives a 28-day strength of 500 to 600 psi.Expanded shale, clay, slag aggregates or pumice will give a range of weights and can be used with or without foam. The aggregate manufacturers will furnish data on mixes suitable for specific weight requirements. All lightweight insulating concretes require a large amount of mixing water, so it is essential to use the minimum amount of water compatible with the necessary workability. Excess water may cause undesirable shrinkage and cause damage to the waterproofing membrane. When insulating concrete is placed on galvanized steel decks, do not use accelerators containing calcium chloride.