Bitumen in different forms is used as a binder in road construction. At ambient temperature, bitumen is an extremely high viscous liquid that is not workable. It can be transferred into a workable state in three ways:
• by heating
• by blending with petroleum solvents (cutback)
• by emulsifying in water to form bitumen emulsion
The first alternative is normally used for hot mixes on medium and large size jobs, where equipment is available for heating, storing, transporting and applying the bitumen. This is, however, less suitable on small jobs or when equipment is not available. The second alternative, cut-back, is generally more expensive than hot mix since solvents, which do not play any part in the function of the binder, are often quite costly. Furthermore, the solvents pollute the environment and they are fire hazardous. The third alternative, bitumen emulsion, does not require heating when applied and it has the advantage over hot bitumen that it can be used with cold and even damp aggregate. Most emulsions have satisfactory adhesion properties as they are especially cationic emulsions.
An emulsion can be defined as a dispersion of small droplets of one liquid in another liquid. Bitumen emulsions are an oil-in-water type of emulsion where the bitumen is dispersed in water. The size of the droplets is generally in the range 0.001 to 0.02 mm. The bitumen content depends on the intended application of the emulsion, but is rarely lower than 40% or higher than 70%.

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