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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.
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