Viscosity Grade Classifications of Bitumen and Their Significance
Bitumen is a thermoplastic material whose stiffness varies significantly with temperature. This temperature-stiffness relationship depends on the crude oil source and the refining process used. In India, the Bureau of Indian Standards (BIS) first introduced paving grade bitumen specifications (IS: 73-1950) in 1950, classifying bitumen based on penetration values. These specifications were revised in 1962 and again in 1992 to reflect evolving quality standards.
To enhance bitumen quality, BIS updated the specifications in July 2006, shifting the classification from penetration grades to viscosity-based grades measured at 60°C. The revised IS: 73-1992 standard defines four viscosity grades: VG-10, VG-20, VG-30, and VG-40. Some tests from the earlier standard—such as specific gravity, water content, ductility, loss on heating, and Fraass breaking point—were removed because they were found unrelated to product quality or performance.
Since August 2009, IndianOil has marketed bitumen according to these viscosity grade specifications across all its refineries, fully replacing the penetration-based classification. Viscosity grading reflects the fluidity of bitumen: higher grades indicate stiffer material. Tests are performed at 60°C and 135°C, representing typical summer road surface and mixing temperatures, respectively. Additionally, penetration at 25°C, approximating average annual pavement temperature, continues to be measured to assess bitumen performance.
Bitumen Grade Variations
VG-10 Bitumen: This grade is commonly used in surface dressing and paving applications in very cold climates, replacing the old 80/100 Penetration grade. It is also a key ingredient in the production of bitumen emulsions and modified bitumen products.
VG-20 Bitumen: Suitable for paving in cold and high-altitude regions, VG-20 offers better performance under lower temperature conditions.
VG-30 Bitumen: Primarily used for constructing extra heavy-duty bitumen pavements, VG-30 is designed to withstand heavy traffic loads. It serves as a substitute for the older 60/70 Penetration grade.
VG-40 Bitumen: This high-viscosity bitumen is ideal for highly stressed areas such as intersections, toll plazas, and truck parking zones. It replaces the old 30/40 Penetration grade and provides stiffer mixes, enhancing resistance to rutting, shoving, and damage caused by high temperatures and heavy traffic.
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VISCOSITY GRADE (VG) BITUMEN SPECIFICATION AS PER IS 73:2006
