Bitumen is used in road construction as a binder for the purpose of retaining the cover aggregate, and providing a waterproof seal to the pavement. It is a natural constituent of petroleum is refined to separate the various fractions (petrol, kerosene, distillate etc.) and recover the it.
There are also natural deposits of bitumen . Often these deposits are mixed with variable quantities of mineral matter, water and other substances. Bitumen is a semi-solid material at normal temperatures and can be produced in a variety of classes.
One means of determining viscosity in the laboratory is to measure the time for a fixed of the bitumen to be drawn up through a capillary tube by means of a controlled vacuum
or accurately reproducible head.
Bitumen performs the functions of a binder because it possesses the following properties :-
(1) it has a relatively strong cementing action,
(2) it will readily adhere (under favorable conditions) to stone aggregate,
(3) it is highly waterproof,
(4) it is durable,
(5) it is resistant to the action of most acids, alkali and salts,
(6) it can be softened or liquified by –
(a) the application of heat
(b) the addition of kerosene or oil (i.e. “cutting” or “fluxing“)
(c) emulsification with water.
Bitumen alone could be used as a binder in which case the only preparation necessary would be to raise its temperature just sufficiently to make it liquid enough to pump and spray. However, the usual practice is to use a cutback bitumen binder.