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Composition and Properties of Bitumen


Composition of Bitumen

Materials in bituminous family are:

1) Tar:

Coal tar is a brown or dark black liquid of high viscosity, which smells of naphthalene and aromatic hdydrocarbons. Being flammable, coal tar is sometime used for heating or to fire boilers. It can be used in coal tar soap, and is used in medicated shampoo to kill and repel head lice, and as a treatment for dandruff.

Depending upon its source of origin, TAR is classified as:

2) Coal Tar:

It is the liquid by-product of the distillation of coal to make coke. The gaseous by-product of this process is commonly known as town gas. It is used for coating of wooden poles and sleepers, iron poles.

3) Wood Tar:

See Also - Related See also: Construction Wood Properties

It is obtained by the distillation of resinous wood. Wood tar contains creosote and as such has strong preservative properties. Search for "resin" in the above search box.

4) Mineral Tar:

It is obtained by the distillation of bituminous shale.

5) Coal Tar Pitch:

It is the residue of the direct distillation of crude tar produced by the high temperature carbonization of coal. It is used as a water proofing compound in masonry, steel and timber structure. It is also used for water proofing concrete structures.

Chemical Composition of Bitumen:

Molecular weight wise, bitumen is a mixture of about 300 - 2000 chemical components, with an average of around 500 - 700. Elementally, it is around 95% carbon and hydrogen (± 87% carbon and ± 8% hydrogen), and up to 5% sulfur, 1% nitrogen, 1% oxygen and 2000ppm metals. Bitumens are composed mainly of highly condensed polycyclic aromatic hydrocarbons. They also contain several elements, a number of which are toxic.

Chemical Components in bitumen are:

  1. Asphaltenes
  2. Resinous components (polar aromatics)
  3. Non-polar aromatics (naphtene aromatics) and
  4. Saturates

Properties of Bitumen

See Also - Related See also: Applications of Bitumen

  1. Bitumen Laboratory Tests

    Adhesion: Bitumen has the ability to adhere to a solid surface in a fluid state depending on the nature of the surface. The presence of water on the surface will prevent adhesion.

  1. Resistance to Water: Bitumen is water resistant. Under some conditions water may be absorbed by minute quantities of inorganic salts in the bitumen or filler in it.

  1. Hardness: To measure the hardness of bitumen, the penetration test is conducted, which measures the depth of penetration in tenths of mm. of a weighted needle in bitumen after a given time, at a known temperature. Commonly a weight of 100 gm is applied for 5 sec at a temperature of 77 °F. The penetration is a measure of hardness. Typical results are 10 for hard coating asphalt, 15 to 40 for roofing asphalt and up to 100 or more for water proofing bitumen.

  1. Viscosity and Flow: The viscous or flow properties of bitumen are of importance both at high temperature during processing and application and at low temperature to which bitumen is subjected during service. The flow properties of bitumens vary considerably with temperature and stress conditions. Deterioration, or loss of the desirable properties of bitumen, takes the form of hardening. Resultantly, decrease in adhesive and flow properties and an increase in the softening point temperature and coefficient of thermal expansion.

  2. Softening point: Softening point is the temperature at which a steel ball falls a known distance through the bitumen when the test assembly is heated at a known rate. Usually the test consist of a (3/8)in dia steel ball, weight 3.5 gm, which is allowed to sink through a (5/8) in dia, (1/4) in thick disk of bitumen in a brass ring. The whole assembly is heated at a rate of 9 °F per min. Typical values would be 240 °F for coating grade asphalts, 140 °F to 220 °F for roofing asphalt and down to 115 °F for bituminous water proofing material.

  3. Ductility: Ductility test is conducted to determine the amount bitumen will stretch at temperature below its softening point.
    A briquette having a cross sectional area of 1 in2 is placed in a tester at 77 °F. Ductility values ranges from 0 to over 150 depending on the type of bitumen.

Download 2 Various Lab Tests on Bitumen

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