Bitumen as Pavement Materials
Pavement materials are those which have great use in construction road pave for both Flexible and Rigid pavement. Some examples of pavement material are stone aggregate, earth, WMM, and also bitumen. Bitumen is used for the construction of flexible pavement, which gives more strength than other materials used for flexible pavement. Therefore, a piece of thorough knowledge is must in the fields of civil engineering. Here, I am trying to give an introductory discussion on Bitumen as a Pavement Material, and it’s different forms which are used in civil engineering fields.
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Bituminous materials or asphalts are extensively used for roadway construction, primarily because of their excellent binding characteristics and waterproofing properties and relatively low-cost Bituminous materials consists of bitumen which is a black or dark coloured solid or viscous cementitious substances consists chiefly high molecular weight hydrocarbons derived from distillation of petroleum or natural asphalt, has adhesive properties, and is soluble in carbon disulphide. Tars are residues from the destructive distillation of organic substances such as coal, wood, or petroleum and are temperature sensitive than bitumen. Bitumen will be dissolved in petroleum oils where unlike tar.
Production of Bitumen
Bitumen is the residue or by-product when the crude petroleum is refined. A wide variety of refinery processes such as the straight distillation process, solvent extraction process etc. may be used to produce bitumen of different consistency and other desirable properties. Depending on the sources and characteristics of the crude oils and on the properties of bitumen required, more than one processing method may be employed.
Vacuum steam distillation of petroleum oils
In the vacuum-steam distillation process, the crude oil is heated and is introduced into a large cylindrical still. Steam is introduced into the still to aid in the vaporization of the more volatile constituents of the petroleum and to minimize decomposition of the distillates and residues. The volatile constituents are collected, condensed, and the various fractions stored for further refining if needed. The residues from this distillation are then fed into a vacuum distillation unit, where residue pressure and steam will further separate out heavier gas oils. The bottom fraction from this unit is the vacuum-steam-refined asphalt cement. The consistency of asphalt cement from this process can be controlled by the amount of heavy gas oil removed. Normally, asphalt produced by this process is softer. As the asphalt cools down to room temperature, it becomes a semi-solid viscous material.
Different forms of bitumen
There are mainly four types of bituminous material available to use as a pavement material. These are-
- Cutback bitumen
- Bitumen Emulsion
- Bituminous primers
- Modified Bitumen
They are briefly discussed in below-
1. Cutback bitumen
Normal practice is to heat bitumen to reduce its viscosity. In some situations, preference is given to using liquid binders such as cutback bitumen. In cutback bitumen suitable solvent is used to lower the viscosity of the bitumen. From the environmental point of view also cutback bitumen is preferred. The solvent from the bituminous material will evaporate and the bitumen will bind the aggregate.
Cutback bitumen is used for cold weather bituminous road construction and maintenance. The distillates used for the preparation of cutback bitumen are naphtha, kerosene, diesel oil, and furnace oil. 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 an appreciable quantity of fine aggregates
2. Bitumen Emulsion
Bitumen emulsion is a liquid product in which bitumen is suspended in a finely divided condition in an aqueous medium and stabilized by a suitable material. Normally cationic type emulsions are used in India. 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 a release of water and the mix starts to set. The time of setting depends upon the grade of bitumen. The viscosity of bituminous emulsions can be measured as per IS: 8887-1905.
Three types of bituminous emulsions are available, which are Rapid setting (RS), Medium setting (MS), and Slow setting (SC). Bitumen emulsions are ideal binders for hill road construction. Where heating of bitumen or aggregates is difficult, rapid setting emulations are used for surface dressing work. Medium setting emulsions are preferred for premix jobs and patch repairs work. Slow setting emulsions are preferred in a rainy season.
3. Bituminous primers
In bituminous primer, the distillate is absorbed by the road surface on which it is spread. The absorption, therefore, depends on the porosity of the surface. Bitumen primers are useful on the stabilized surfaces and water bound macadam base courses. Bituminous primers are generally prepared on road sites by mixing penetration bitumen with petroleum distillate.
4. Modified Bitumen
Certain additives or the 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 (CRAMB) should be used only in rearing course depending upon the requirements of extreme climatic variations. The detailed specifications for modified bitumen have been issued by IRC: SP: 53-199. 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
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