BITUMEN is the heaviest section of petroleum and one of the most complicated parts of it which can be seen in dark color and in the solid, semi solid or viscose states, whether natural or synthetic. The main components of BITUMEN are hydrocarbon compounds having high molecular masses which consist of oil, resin and asphaltine substances. This substance has a very complicated chemical composition, and among its physical characteristics are viscosity, and being damp proof; it is solvable in carbon disulphide and CO2.
In some countries the term asphalt is used as the equivalent of BITUMEN, but in Iran asphalt is commonly known as a mixture of BITUMEN and sand which is used in road construction process.
BITUMEN is a very dense and black material which is nearly in solid state in normal conditions. It has a complicated chemical structure, and the researches made so far have not been able to offer a definite theory regarding its chemical structure. The reason is that petroleum from which BITUMEN is generated is formed in various geophysical conditions thus it has many different forms. By definition “BITUMEN is a substance which can be solved in carbon sulfur. However, the BITUMEN used in road construction contains less than 0.1 % of materials which are insolvable in carbon sulfur. Though this material, known as carboide , has a slight impact in the BITUMEN impurity , if exposed to high heat its molecules will be broken and this has been realized as a damage to road asphalt.