BITUMINOUS MATERIALS
Bituminous materials may be broadly classified as
bitumen and tar.
BITUMEN
Bitumen is a hydrocarbon
material of either natural or pyrogenous origin, found in gaseous, liquid,
semi-solid or solid form. It is a complex organic material that is completely
soluble in carbon disulphide. It occurs either naturally or may be obtained
during the distillation of petroleum. Bitumen consists of colloidal hydrocarbon
materials of asphalte nets, resins and oils. Bitumen may be further divided as
native asphalt or petroleum asphalt (generally called bitumen).
1. Native Asphalt
Native asphalts are those which occur in a pure or
nearly pure state in nature. These are further designated as (i) Lake asphalt,
(ii) Asphaltites and (iii) Rock asphalt based on their origin of occurrence.
(i) Lake Asphalt
These are found in depressions on the earth’s surface which
have accumulated in lakes. Most of the lake asphalts possess moisture in some
form. The refined asphalt has very low penetration. This material can be
softened by fluxing with a petroleum flux. This may be blended with asphalts
produced from petroleum to produce blended asphalts.
(ii) Asphalites
This is an asphalt-like substance presumably derived from the metamorphosis of petroleum. It is hard, brittle and practically a pure bitumen.
These types of asphalts are further divided into three classes, viz.,
gilsonite, glance pitch and grahamite. It is obtained from crevices and seams
in the rock formation.
(iii) Rock Asphalts
These are deposits of limestone or sandstone naturally
impregnated with asphalt. This is extensively available in many parts of the
world. The asphalt content in the rocks is about a maximum of 20% and varies
from soft to hard.
2. Petroleum Asphalts
Petroleum asphalts are derived from petroleum crude oil
which is taken from oil wells. Crude oil is transported to refineries where it
is separated into various components. The character of asphalt depends on the
nature of crude oil. Crude petroleum is divided into three classes, viz.,
asphaltic base, semi-asphaltic base and paraffin base.
(i)
Cutback Bitumen
When the viscosity of bitumen is reduced by a volatile
dilutant, the bitumen is called cutback bitumen. The viscosity of the
cutback and the rate at which it hardens depends on the characteristics and
quantity of both the bitumen and volatile oil.
(ii)
Bituminous Emulsion
When the bitumen is suspended in a finely divided
condition in an aqueous medium and an emulsifier is used to stabilise, the end
product is known as a bituminous emulsion.
Bituminous materials are widely used in highway
construction because of their dual qualities, viz., binding property and water
proofing property.
The use of bitumen for road construction depends on the type
of mix and the type of construction.
The
following are the desirable properties bitumen should have in paving mixes:
(i) Easy to get mixed,
(ii) Attainment of desired stability,
(iii) Should maintain stability even under adverse
weather condition,
(iv) Adequate flexibility to avoid cracking,
(v) Sufficient adhesion with aggregates.
TAR
Tar is the viscous liquid produced when natural organic
materials such as coal, petroleum, wood, etc., are carbonised. Depending on the
material from which tar is obtained, it is called wood tar or coal tar. Coal
tar is used mostly for road work because of its superior quality. Road tar is
produced by undergoing three stages, viz., carbonisation of coal to produce
coal tar, refining or distillation of crude tar and blending of distillation
residue with distillate oil fraction.
There are five grades of road tar, viz., RT-1 to RT-5
based on their viscosity and other properties. Uses of different grades of road
tar are given below.
RT-1:
Used for surface painting under adverse cold conditions.
RT-2:
Used for standard surface painting under normal weather conditions.
RT-3:
Used for surface painting, renewal coats, pre-mixing chips for the top course and
light carpets.
RT-4:
Used for pre-mixing tar macadam in base course. RT-5: Used for grouting.
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