Foamed bitumen is made via a procedure in which cold water (2 - 3%) is injected into hot bitumen (~180°C), which causes spontaneous foam formation. Upon contact with the hot bitumen the water is turned to vapour and trapped in thousands of small bitumen bubbles, thus temporarily altering the physical properties of the bitumen. When the foam dissipates the bitumen resumes its original properties and so it is essential to apply the bitumen to the surface, usually via spraying, whilst still in the foamed state.
Foaming surfactants are often used to improve foam stability and ensure that the bitumen stays in its foamed state long enough for transportation and application onto the aggregate surface. The highest quality foamed bitumen has a very low viscosity and a large surface area.