Bitumen foam is a composite material composed of bitumen to about 97/98% (by mass) water vapour (1 to 3%), and a small quantity of additives in variable proportions which only have a transitory existence.

An expansion chamber receives injected hot bitumen (at 170 to 180°C) and cold and/or lukewarm water.

The two liquids become intimately mingled and are force through a small orifice (nozzle) out of the chamber.

At atmospheric pressure, the water vaporizes instantly, the molecules of bitumen stretch and form a film of bitumen having a thickness of 0.01 mm according to JENKINS (1999). The coefficient of expansion of bitumen is variable, depending on the type of bitumen used.