6 Important Question on Tar (Highway Material)

6 Important Question on Tar (Highway Material)

Q1. What do you know about Bitumen Emulsion?

A1. A bitumen emulsion is a liquid product in which a substantial amount of bitumen is suspended in a finely divided condition in an aqueous medium and stabilized by means of one or more suitable materials.

  1. Emulsion is a two-phase system consisting two immiscible liquids.
  2. The bitumen/tar content in emulsion range from 40-60% and the remaining portion is water.
  3. The average diameter of globules of bitumen portion is about 2mm.
  4. Emulsion are used especially in maintenance and patch repair works. The main advantage of emulsion is that is can be used in wet weather even when it is raining.
  5. Emulsions can be used for soil stabilization in desert.

Q2. How the tars can be produced?

A2. Tar can be produced in 3 stages-

  1. Carbonization of coal to produce crude tar.
  2. Refining or distillation of crude tar.
  3. Blending of distillation residue with distillate oil fraction to give desired road tar.
6 Important Question on Tar (Highway Material)

Q3. What are the tests which can be done on tars?

A3. The various tests that are carried out on the road tars are listed below-

  1. Specified gravity test
  2. Viscosity test on standard tar viscometer
  3. Equiviscous temperature (EVT)
  4. Softening Point
  5. Softening point of residue
  6. Float Test
  7. Water content
  8. Distillation fraction on distillation up to 2000C, 2000C to 2700C, and 2700C to 3300C.
  9. Phenols, percent by volume
  10. Naphthalene, percent by weight
  11. Matter insoluble in toluene, percent by weight


  • RT-1 has the lowest viscosity and is used for surfacing painting under exceptionally cod weather as this has very low viscosity.
  • RT-2 is recommended for standard surface painting under normal Indian climatic conditions.
  • RT-3 may be used for surface painting, renewal coasts and premixing chips for to course and light carpets.
  • RT-4 is generally used for premixing tar macadam in base course.
  • For grouting purposes RT-% may be adopted, which has the highest viscosity among the road tars.

Q4. Do the comparison of Tar and Bitumen.

A4. Bitumen is a petroleum product where as tar is produced by the destructive distillation of coal or wood.

  1. Bitumen is soluble in ‘Carbon disulphide’ and ‘carbon tetra chloride’ but tar is soluble in only toluene.
  2. Bitumen is more resistant to water than tar.
  3. Tar is more temperature susceptible resulting in great variation in viscosity with temperature.
  4. The free carbon content is more in tar as seen from the solubility test.

Q5. Define about Bituminous Paving Mixes.

A5. The desirable properties of a good bituminous mix are stability, durability, flexibility and workability.

  • Stability is a function of friction and cohesion; it is the deformation under loading.
  • Durability is resistance against weathering and abrasive actions.
  • Three design methods for mix are available namely:
  • Marshall Method
  • Hveem Method
  • Hubbard-field method

Q6. Define Marshall Method of Bituminous design.

A6. This stability test is applicable to hot-mix design of bitumen and aggregates with maximum size 2.5cm.

  • In India, the Bituminous concrete mix is commonly designed by Marshal Method.
  • In this method, the resistance to plastic deformation of a cylindrical specimen of the bituminous mixture is measured when the same is loaded at the periphery at a rate of 5cm per minute.
  • There are two major features of the Marshall method of designing mixes namely,
    1. Density-voids analysis
    2. Stability-flow test
  • The stability of the mix is defined as a maximum load carried by a compacted specimen at a standard test temperature of 600C.
  • The flow is measured as the deformation in units of 0.25mm between no-load and maximum load carried by the specimen during stability test.
  • In this test, an attempt is made to obtain optimum binder content for the aggregate mix type and traffic intensity.
  • A compaction pedestal and hammer are used to compact a specimen by 4.54 kg weight with 45.7cm height of fall.

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