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BITUMEN
 

DEFINITION

A non-crystalline solid or viscous material having adhesive properties derived from petroleum either by natural or refinery processes, and substantially soluble in carbon disulphide. Bitumen are black or brown in colour. This may occur naturally but are usually made as end products from distillation of, or extracts from, selected petroleum oils.

NOMENCLATURE

Bitumen and asphalt are both generic terms. In USA, the word asphalt is used as synonymous with bitumen- the refinery product which has now largely replaced the natural asphalts that occur in Trinidad, Venezuela, Cuba etc. Outside USA however, the word asphalt is generally taken to mean a mixture of refinery bitumen with a substantial proportion of solid mineral matter. Frequently used in road constructions are cutbacks, in which bitumen is mixed with a solvent such as Kerosene or gas oil which evaporates after the material is laid, and bitumen emulsions i.e. emulsions of bitumen with water.

Bitumen is always applied hot, cutbacks are applied either hot or cold and emulsion is always applied cold. When the emulsion breaks the water evaporates leaving bitumen.

END USE

The uses of bitumen are numerous. The chief one in most countries is for road construction. It is also used for surfacing airfield runways and taxi tracks, hydraulic applications such as canal lining, river bank protection, dam construction and sea defenses. There are also numerous industrial applications like roofing felt manufacture, printing inks, electrical cable / Junction boxes, mastic for roofing of terraces, duplex paper manufacture etc.

SIGNIFICANCE OF PROPERTIES

Asphaltic bitumen is valued for a variety of properties. It is water proof, ductile, adhesive, chemically inert and resistant to atmospheric exposure and the effects of dilute acids and alkalis. Obtained from the residues of naphthenic crude oils after distillation of the volatile products, it is marketed in a wide range of grades, ranging from soft to hard.

PENETRATION

The test determines the hardness of Bitumen by measuring the depth ( in tenths of a mm) to which a standard, and loaded needle will vertically penetrate in 5 seconds, a sample of Bitumen maintained at a temperature of 25 deg C ( 77deg F). Hence the softer the bitumen, the greater will be its number of penetration units.

SOFTENING POINT

This test is carried out by the Ring and Ball method, which consists of suspending a brass ring containing the test sample of Bitumen in water at a given temperature. A steel ball is placed upon the bituminous material, the water is then heated at the rate of 5 deg C increase per minute. The temperature at which the softened bituminous material first touches a metal plate at a specified distance below the ring is recorded as the Softening point of the sample.

FLASH POINT

In the interest of safety, legislation has been introduced in most countries fixing minimum flash point limits to prevent the inclusion of highly inflammable volatile fractions in kerosene distillates. According to Controller of Explosives classification it falls in the category of Class B Petroleum Products. Its flash point (Abel) is stipulated as Min.   35 deg C in the IS specification.

SPECIFIC GRAVITY

As with colour, specific gravity has no relation to burning quality, but it is a useful aid for quantity reckoning and identity.