Bitumen is a thermoplastic material, and its stiffness is dependent on temperature. The temperature-vs-stiffness relationship of bitumen varies depending on the source of crude oil and the method of refining.

The Bureau of Indian Standards (BIS) introduced paving grade bitumen specifications (IS: 73-1950) for the first time in the year 1950 and classified it based on penetration. These specifications were revised in the years 1962 and 1992. To improve the quality of bitumen, BIS revised IS-73-1992 specifications in July 2006 based on viscosity (measured at 60°C). As per these revised specifications, four viscosity grades were introduced: VG-10, VG-20, VG-30, and VG-40. Several qualification tests such as specific gravity, water content, ductility, loss on heating, and Farass breaking point were removed from IS: 73-1992 as they were found to have no correlation with the quality or performance of the product.

IndianOil commenced marketing bitumen as per viscosity grade specifications conforming to IS: 73-1992 from all its refineries starting August 2009. Consequently, penetration grades have been replaced by viscosity grade bitumen.

According to viscosity (degree of fluidity) grading, the higher the grade, the stiffer the bitumen. Tests are conducted at 60°C and 135°C, which represent the temperature of the road surface during summer (hot climates, such as in northern India) and the mixing temperature, respectively. Penetration at 25°C, which is the annual average pavement temperature, is also retained.

Different Grades of Bitumen:

VG-10 Bitumen:
VG-10 is widely used in spraying applications such as surface-dressing and paving in very cold climates, replacing the old 80/100 penetration grade. It is also used to manufacture bitumen emulsion and modified bitumen products.

VG-20 Bitumen:
VG-20 is used for paving in cold climates and high-altitude regions.

VG-30 Bitumen:
VG-30 is primarily used to construct extra heavy-duty bitumen pavements that need to endure substantial traffic loads. It can be used in place of the 60/70 penetration grade.

VG-40 Bitumen:
VG-40 is used in highly stressed areas such as intersections, near toll booths, and truck parking lots, replacing the old 30/40 penetration grade. Due to its higher viscosity, stiffer bitumen mixes can be produced to improve resistance to shoving and other issues associated with higher temperatures and heavy traffic loads.

VISCOSITY GRADE (VG) BITUMEN SPECIFICATION AS PER IS 73:2006