Bitumen is viscous, nonvolatile liquid or solid. Bitumen is a complex colloid system the chemical properties of which are dependent on the properties of crude oil from which it is produced. Pure bitumen is a colloid dispersion of microscopic asphalt particles (dispersion phase). The chemical composition of bitumen is a mixture of various hydrocarbons with molecules of oxygen, sulphur, and nitrogen. Hydrocarbons present in bitumen are mostly condensed naphthene and aromatic rings with small number of side paraffin chains. Mass fraction of hydrocarbons is 75-85%, hydrogen 9-10%, oxygen 2-8%, sulphur 5-7%, and nitrogen 0.1-0.5%. Bitumen is partially or completely soluble in various organic solvents. Dissolved fractions of bitumen in solvent are called maltenes or petrolenes, and undissolved fractions are called aspaltenes. The maltenes are a mixture of resins and oil, and they are a disperse agent. For the most part, physical properties of bitumen depend on dispersion degree of asphaltenes in maltenes.

Properties of Bitumen

  • Adhesion: Bitumen has the ability to adhere to a solid surface in a fluid state depending on the nature of the surface. The presence of water on the surface will prevent adhesion.
  • Resistance to Water: Bitumen is water resistant. Under some conditions water may be absorbed by minute quantities of inorganic salts in the bitumen or filler in it.
  • Hardness: To measure the hardness of bitumen, the penetration test is conducted, which measures the depth of penetration in tenths of mm. of a weighted needle in bitumen after a given time, at a known temperature. Commonly a weight of 100 gm is applied for 5 sec at a temperature of 77 °F. The penetration is a measure of hardness. Typical results are 10 for hard coating asphalt, 15 to 40 for roofing asphalt and up to 100 or more for water proofing bitumen.
  • Viscosity and Flow: The viscous or flow properties of bitumen are of importance both at high temperature during processing and application and at low temperature to which bitumen is subjected during service. The flow properties of bitumen vary considerably with temperature and stress conditions. Deterioration, or loss of the desirable properties of bitumen, takes the form of hardening. Resultantly, decrease in adhesive and flow properties and an increase in the softening point temperature and coefficient of thermal expansion.
  • Softening point: Softening point is the temperature at which a steel ball falls a known distance through the bitumen when the test assembly is heated at a known rate. Usually the test consist of a (3/8)in dia steel ball, weight 3.5 gm, which is allowed to sink through a (5/8) in dia, (1/4) in thick disk of bitumen in a brass ring. The whole assembly is heated at a rate of 9 °F per min. Typical values would be 240 °F for coating grade asphalts, 140 °F to 220 °F for roofing asphalt and down to 115 °F for bituminous water proofing material.
  • Ductility: Ductility test is conducted to determine the amount bitumen will stretch at temperature below its softening point. A briquette having a cross sectional area of 1 in2 is placed in a tester at 77 °F. Ductility values ranges from 0 to over 150 depending on the type of bitumen.
  • Tar and Natural bitumen

    Petroleum bitumen is often confused with tar. Although bitumen and coal tar are similarly black and sticky, they are distinctly different substances in origin, chemical composition and in their properties. Coal tar is produced by heating coal to extremely high temperatures and is a by-product of gas and coke production. It was widely used as the binding agent in road asphalt in the early part of the last century, but has since been replaced by refined bitumen. Tar sands, also called oil sands, are deposits of a mixture of fine clay, sand, water, and variable amounts of bitumen which is a black, high-sulfur, tar-like, heavy oil. Tar sands are sedimentary rocks and the bitumen is an asphaltic substance. A typical tar sand is composed approximately of 83% sand, 13% bitumen, and 4% water (by weight percent). Tar sands containing more than 6% by weight of bitumen are considered to have commercial potential. The difference between oil shale and tar sand is that the bituminous matter in shale is a solid, whereas, the bituminous matter in tar sand is highly viscous liquid. Oil which is particularly thick and viscous is called heavy oil, or more colloquially, tar or asphalt.