Bitumen for road- Grade and properties of bitumen

Bitumen for road is an important topic to understand the construction of roads. I will walk you through how important bitumen is for road construction. Also, we will see bitumen road layers, different bituminous materials, cutback bitumen, grade of bitumen and properties of bitumen.

There are two types of bituminous binder for road construction.

  1. Bitumen (distillation of crude oil)
  2. Tar (Produced from coal)

So, what are the difference between them?

Comparison between tar and bitumen

The table below shows a comparison between tar and bitumen.

Petrolium productDistillation of coal or wood
Soluble in carbon disulphide and carbon tetrachlorideSoluble in toluene only
Temperature succeptibility is lowTemperature succeptibility is higher than bitumen
Free carbon content is lessFree carbon content is more
Comparison between tar and bitumen

Now, let’s sneak into the manufacturing of tar, being one of the important bituminous materials

Tar manufacturing

Figure shows the road construction using bitumen
Road construction using tar

Generally, tar is made by heating coal inside a chemical apparatus. Most tar is produced from coal as a byproduct of coke production, but it can also be produced from petroleum, peat or wood.

The major steps in tar manufacturing are,

  • Coal undergoes carbonation and produces crude tar
  • Crude tar undergoes distillation/ refining and produces a residue
  • The residue blends with distilled oil fraction and produces tar

I am going to tell more about the properties of bitumen now.

Also read: Classification of roads-5 types of roads full details

Desirable properties of bitumen- important topic in bitumen for road

figure shows bitumen oozing out from pipe
Bitumen oozing out from pipe

The desirable properties of bitumen are,

  1. Viscosity of bitumen during mixing and compaction is adequate
  2. Bituminous material should not highly temperature and susceptible
  3. In presence of water the bitumen should not strip off from aggregate
  4. The adhesive property of bitumen binds together all the components without bringing about any positive or negative changes in their properties
  5. Bitumen is insoluble in water and can serve as an effective sealant
  6. Due to versatility property of Bitumen it is relatively easy to use it in many applications because of its thermoplastic property
  7. Bitumen play a vital role in distributing the traffic loads on the pavement to the layers beneath

Okay. So, what are the types of bituminous materials that are used in flexible pavement construction? Below is the list for you.

  1. Paving grade material
  2. Modified bituminous binder
  3. Cutback bitumen
  4. Bitumen emulsion

Among the list, cutback bitumen is the major. Let me tell you more details about cutback bitumen.

Cutback bitumen

Cutback bitumen is the bitumen the viscosity of which is reduced by a volatile diluent. It is used in low-temperature mixing.

Three types of cutback bitumen are available

  1. Rapid curing
  2. Medium curing
  3. Slow curing

The diluent while mixing varies with the type of cutback bitumen.

Type of cutback bitumenDiluent
Rapid curingNafthal, gasoline
Medium curingCarosine or diesel oil
Slow curingHigh boiling point gas oil
Type of cutback bitumen and suitable diluent

Bituminous emulsion

figure shows bitumen emulsion
Bitumen emulsion

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

Three types of bitumen emulsions are available

  1. Rapid setting
  2. Medium setting
  3. Slow setting

Also read: Alignment of road: Factors affecting- obligatory points with figures

Grade of bitumen- must learn in bitumen for road topic

To determine the grade of bitumen, penetration test is conducted. The results are expressed in 1/10 mm. When penetration value is represented as 80/1000, it is called grading of bitumen.

The old method of grading is viscosity test. Two viscosities kinematic and absolute and penetration value by penetration test results are collected. Based on this, bitumen is graded. The tables shows the grade of bitumen and values of viscosity in accordance with penetration.

Grade of bitumenAbsolute viscosityKinematic viscosityPenetration
VG 1080025080- 100
VG 20100030060- 80
VG 30240035050- 70
VG 40320040040- 60
Grade of bitumen and viscosity

Let me tell you the application of each of the grade of bitumen now.

VG- 10- Used in spray application since viscosity is very less

VG- 20- Used in cold area

VG- 30- Commonly used in India

VG- 40- High grade bitumen used in high traffic areas

Okay. So, lets’ learn about the bituminous layers.

Bitumen road layers

Let’s first look into the road layers to understand bitumen road layers.

figure shows bitumen road layers
Road layers

The bitumen road layers come in the surface layer shown in the figure above. The figure below shows that. Bituminous mix consists of aggregate and binder. Aggregate consists of coarse aggregate, fine aggregate and filler less than 0.075mm.

figure shows bitumen mix
Bitumen road layers
  • Bituminous concrete consists of aggregate and bitumen.
  • Thickness of base course depends on grading of aggregate
  • Dense graded aggregates are provided in base course. That is the permeability will be very less
  • Number of voids should be very less
  • Dense bituminous macadam should be given as a binder course

So, the trip is over. Hope the time you spend for reading about the bitumen for road was worth it.

MUST READ: Road margins- 6 types of road margin in highway

Happy learning!

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