Bitumen is a viscous liquid or a solid consisting essentially of hydrocarbons and their derivatives which is soluble in trichloro-ethyelene and is substantially nonvolatile and softens gradually when heated. It is black or brown in color & possesses waterproofing and adhesive properties.
Bitumen is an effective binder for most road surfacing applications and mainly used in spray seals and asphalt. It is of petroleum origin and behaves as a thermoplastic material. It is produced from the refining of crude petroleum oil and its properties of chemical inertness, water resistance and natural adhesion make it an ideal material for a variety of applications. Bitumen is obtained by vacuum distillation or vacuum flashing of an atmospheric residue.
Road construction requires a variety of bitumen grades with strict quality specifications, such as narrow ranges for penetration and softening point. These special grades are manufactured by blowing air through the hot liquid bitumen.
The desirable properties of bitumen depend on the mix type and construction. In general, Bitumen should possess following desirable properties.
The bitumen should not be highly temperature susceptible: during the hottest weather the mix should not become too soft or unstable, and during cold weather the mix should not become too brittle causing cracks.
The viscosity of the bitumen at the time of mixing and compaction should be adequate. This can be achieved by use of cutbacks or emulsions of suitable grades or by heating the bitumen and aggregates prior to mixing.
There should be adequate affinity and adhesion between the bitumen and aggregates used in the mix.
Bitumen are classified by a broad range of properties including by the midpoint of their specified viscosity range at 60°C measured in Pascal seconds.
Viscosity Range at 60°C
170 140–200 Pa.s
Different forms of bitumen used in roadmaking
Bitumen in its original form: Bitumen in its original form which has no flux or cutter added is used in most hot asphalt production and in some spray seal applications. Class 320 is used in asphalt and class 170 bitumen is used in sprayed sealing applications.
Cutback bitumen: Cutbacks comprise a bitumen which has its viscosity reduced by the addition of a relatively volatile material such as kerosene. The purpose of adding a 'cutter' is to temporarily lower the bitumen viscosity to either improve its wetting ability and also allow more time for sealing crew to apply cover aggregate before it hardens.
There are different types of cutback bitumen like rapid curing (RC), medium curing (MC), and slow curing (SC). RC is recommended for surface dressing and patchwork. MC is recommended for premix with less quantity of fine aggregates. SC is used for premix with appreciable quantity of fine aggregates.
Bitumen Emulsion: Bitumen emulsion is a liquid product in which bitumen is suspended in a finely divided condition in an aqueous medium and stabilised by suitable material. Normally cationic type emulsions are used. The bitumen content in the emulsion is around 60% and the remaining is water. When the emulsion is applied on the road it breaks down resulting in release of water and the mix starts to set. The time of setting depends upon the grade of bitumen. Three types of bituminous emulsions are available, which are Rapid setting (RS), Medium setting (MS), and Slow setting (SC). Rapid setting emulsions are used for surface dressing work. Medium setting emulsions are preferred for premix jobs and patch repairs work. Slow setting emulsions are preferred in rainy season.
Modified Bitumen: Certain additives or blend of additives called as bitumen modifiers can improve properties of Bitumen and bituminous mixes. Bitumen treated with these modifiers is known as modified bitumen. Polymer modified bitumen (PMB)/ crumb rubber modified bitumen (CRMB) should be used only in wearing course depending upon the requirements of extreme climatic variations. It must be noted that the performance of PMB and CRMB is dependent on strict control on temperature during construction.
The advantages of using modified bitumen are as follows:
Lower susceptibility to daily and seasonal temperature variations
Higher resistance to deformation at high pavement temperature
Better age resistance properties
Higher fatigue life for mixes
Better adhesion between aggregates and binder
Prevention of cracking and reflective cracking
Tests on bitumen
There are a number of tests to assess the properties of bituminous materials. The following tests are usually conducted to evaluate different properties of bituminous materials.
Softening point test
Specific gravity test
Flash and Fire point test
Water content test
Loss on heating test