Types of bitumen and bituminous materials

Bituminous materials are used as the binder in road construction. The main types are as follows.

1. Natural bitumen

Found in nature, in lakes and as rock asphalt (Trinidad lake asphalt, Dead Sea, Gilsonite). It occurs mixed with mineral matter and is used as a modifier or for waterproofing. It is not common in Nepal.

2. Petroleum bitumen (straight-run bitumen)

The residue left after distilling crude petroleum. It is the most common binder in Nepal (imported from India). Grades: penetration grades 30/40, 60/70, 80/100 and viscosity grades VG-10, 20, 30, 40. Used for bituminous macadam, DBM, bituminous concrete, seal coats and tack coats. Variants: blown (oxidised) bitumen for roofing and waterproofing, and modified bitumen (PMB/CRMB) for heavy traffic.

3. Cutback bitumen

Bitumen dissolved in a volatile solvent (petroleum distillate) to reduce its viscosity so that it can be used cold:

• Rapid curing (RC) - gasoline/naphtha solvent,
• Medium curing (MC) - kerosene,
• Slow curing (SC) - diesel/heavy oil. Used for prime coat, tack coat, surface dressing and patching, where heating is difficult, or in cold weather. The solvent evaporates after laying.

4. Bitumen emulsion

Fine droplets of bitumen dispersed in water with an emulsifier. Cationic (positively charged) types are used with most aggregates. Types by setting speed: rapid (RS), medium (MS) and slow setting (SS). No heating is needed; used for tack coat, surface dressing, slurry seal, cold mix and in wet conditions; it is environmentally friendly and safe.

5. Tar

Obtained by destructive distillation of coal, wood or similar organic matter (coal tar). Road tar grades are RT-1 to RT-5 (RT-1 and RT-2 for surface painting, RT-3 for surface painting/renewal coats and light premixing, RT-4 for premix tar macadam, RT-5 for grouting). It has good adhesion with aggregate, but it is more temperature susceptible, ages quickly and is carcinogenic, so it is now rarely used.