What is Tar || Asphalt || Bitumen

Bitumen and bituminous materials have been known and used in construction works since ancient times, approximately 6000 B.C. Asphalts were used as cements to hold stonework together in boat building and as waterproofing in pools and baths. Some asphalt was mixed with stand and used to pave streets and palace floors. The Egyptians made use of asphalt in the mummification process and as a building material. The Greeks and Romans not only used asphalt as a building material but also used burning asphalt as a military weapon. The asphalt used by these ancient civilizations was natural asphalt formed when crude petroleum oils rose to the earth’s surface and formed pools. The action of the sun and wind drove off the lighter oils and gases, leaving a heavy residue. The residue was asphalt with impurities such as water and soil present. Using crude distillation process, cementing and waterproofing materials were obtained.

The word bitumen comes from the original sanskrit word Gwitumen applied to native asphalts as fuel. Bitumens are mainly composed of a mixture of high-molecular hydrocarbons,methane, napthane and other aromatic series and their oxygen or sulphur derivatives. Tar and asphalt are the two varieties of bituminous materials. Tars are bituminous condensates obtained in the process of destructive distillation of coal, petroleum, wood and other organic materials at high temperature without access of air. They are composed of hydrocarbons and their sulphurous, nitrous and oxygen derivatives. Asphalt on the other hand is a naturally occurring bitumen which is a combination of an inorganic mineral matter either calcareous or siliceous and an organic matter—a chemical compound of carbon and hydrogen.

Bitumens and bituminous materials are being extensively used in damp proofing the basements, floors, roofs, damp proof courses; painting timber and steel structural elements as adhesives and caulking compounds, and tars are used as binders in road works. When combined with aggregate these are also used to provide floor surfaces. Bitumens are now more commonly used for building purposes than is tar.

Tar

Tar

It is a dark (deep black) viscous liquid produced by destructive distillation of organic material such as coal, oil, lignite and wool. Depending upon the source of origin it is classified as coal tar, wood tar and mineral tar. Tar is restraint to petroleum-based solvents. It has very low bitumen content.

coltar is obtained, as a by product in the destructive distillation of coal, or in the manufacture of coal gas. It is heavy, strong smelling and black. These generally have high specific gravities and viscosities, and good adhesive properties. On the further distillation of coal tar (from coal gas) coal naphtha, creosote oil, dyes, etc. are obtained, coal tars for road works are obtained by coking coal or melting together coal pitch with oils or dehydrated raw tar.

A typical composition of coal tar from coke oven plant is true pitch 72 per cent, heavy oils 15 per cent, medium oil 6 per cent, light oil 6 per cent, moisture and ash 1 per cent. For coating of wooden poles, sleepers, iron-poles, latrine walls, etc.

Wood Tar is obtained by the destructive distillation of resinous wood (pine, etc.). It contains creosote and as such is a very strong preservative. On further distillation wood tar produces wood creosote. Compared to coal tar creosote, it is an inferior preservative for wood. The residue left after the distillation is known as pitch.

There are five grades of road tars:

RT-1 : For surface painting under exceptionally cold weather conditions, hill roads at very high elevationsy 

RT-2 : For standard surface painting under normal climatic conditions.

RT-3 : For surface painting and renewal coats and is also used for premixing chips in top courses.

RT-4 : For premixing tar macadam (base course).

RT-5 : For grouting.

Asphalt

asphalt

Asphalt is a natural or artificial mixture in which bitumen is associated with inert mineral matter. It is black or brownish black in colour. At temperature between 50–100°C it is in liquid state whereas at temperature less than this it remains in solid state. Because it is a thermoplastic material it softens as it is heated and hardens as it is cooled. It is the basic paving material in use today.

Bitumen

bitumen

Bitumen is a noncrystalline solid or viscous material derived from petroleum, by natural or refinery process and substantially soluble in carbon disulphide. It is asphalt in solid state and mineral tar in semi fluid state. Bitumen is brown or black in colour.The main constituent is petrolene—a yellowish oily substance, an excess of which makes bitumen to melt at low temperature and, asphaltene—hard black substance, an excess of which makes bitumen brittle and non-plastic. Its compositions is carbon 87 per cent, hydrogen 11 per cent and oxygen 2 per cent.

Bitumen is not affected by light, air or water individually, but in combination they can make it brittle, porous and susceptible to oxidation forming blisters and cracks. It becomes soft at temperatures between 30°–100° C (no sharp melting point), and therefore must be protected from exposure to heat. It is insoluble in water and fairly resistant to most acids. Although bitumen is combustible, composite products, such as mastic asphalt, are not readily ignited.