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Asphalt, Bitumen and Tar are hydrocarbons, termed as bituminous materials.
Asphalt and bitumen are petroleum products, whereas tar is a dark coloured product collected from destructive distillation of organic substances like coal, wood or bituminous rocks.
These materials have been using since the Indus Valley Civilization period. They are preferred very much in building and road constructions.
Asphalt is defined as a mixture of bitumen (as binding material) with a substantial proportion of inert minerals like sand, gravel and crushed stone.
It is blackish-brown in colour, available in a solid-state at low temperature and in liquid form from 50°C and above temperatures. Asphalt appears in nature as natural deposits in many regions and also manufactured artificially too.
The following are the types of asphalt-
Natural Asphalt: There are two varieties of natural asphalt - Lake asphalt and Rock asphalt. Lake asphalt occurs as fossil deposits in places like lakes in Trinidad at depths of 3 to 60 m. It constitutes 40 to 70 % pure bitumen with about 30 % water content in it.
In some countries, certain rocks contain bonding bitumen in the form of rocks that are preferred to use in road works after slight heating.
Residual Asphalt: It is obtained from distillation of crude petroleum oil with an aspheric base.
Mastic Asphalt: It is also known as artificial asphalt, prepared by mixing the required minerals like limestone, dust, fine and coarse aggregates with black bitumen heated to a liquid form.
On cooling, it combines to a hard elastic block. It is reheated on sites and used for waterproofing works, pavements in building construction. Mastic asphalt is durable, tough, damp proof, non-absorbent, non-inflammable and noiseless.
Asphalt Cement: It is a blend of both bitumen and asphalt with flux oils having adhesive qualities suitable for making mastic asphalt. It is preferred to use in the manufacture of bituminous pavements.
Cutback Asphalt: It is available in a liquid state with a combination of asphalt cement and petroleum solvent. They are used in bituminous paints, repairing roofs, etc., because they reduce asphalt viscosity for lower temperatures.
Asphalt Emulsion: It is a suspension of small asphalt cement globules in 50 to 60% of the water in 1% emulsifying agent (such as soap). They are used for low temperatures like tack coats, fog seals, slurry seals, bituminous surface treatments and for stabilizing material.
Tar has low bitumen content and is obtained by destructive distillation of coal, wood or other bituminous materials. It is different from bitumen and is classified into the following three types.
Coal tar: It is obtained as a by-product during the production of coal gas, by heating coal in closed iron vessels to form coke. Escaping gases are condensed to get coal tar. Coal tar is mostly used for road works.
Wood tar: It is obtained by distillation of pine wood and other resinous wood. It has a strong preservative property for wood as it consists of creosote oil. It acts as an excellent anti-termite coat when it is applied to wooden poles buried underground.
Mineral tar: It has less volatile material than the wood tar and is obtained by distilling bituminous shales.
Because of its very good adhesive power, it is used as a waterproofing paint in many situations.
