Viscosity Grade (VG) bitumen is a classification system that grades bitumen based on its viscosity — a direct measure of how resistant it is to flow — rather than the older penetration-depth method.
Description
Developed in the early 1960s as a more scientific alternative to penetration grading, the VG system tests bitumen at two temperatures that closely mirror real-world conditions:
60°C ± 0.3°C — representing the average road surface temperature in direct sunshine
135°C — representing the average temperature of bitumen during asphalt mixing
The Core Principle
Viscosity and penetration move in opposite directions: the higher the viscosity, the lower the penetration, and vice versa. This relationship is the foundation for how VG grades are matched to climate:
- Hot climates call for bitumen with low penetration and high viscosity (e.g., VG40) — thick enough that it won't flow or deform under intense heat and heavy traffic loads.
- Cold climates call for bitumen with high penetration and low viscosity (e.g., VG10) — flexible enough that it won't crack or become brittle under freezing temperatures.
| Test | Unit | VG-10 | VG-20 | VG-30 | VG-40 |
|---|---|---|---|---|---|
| Dynamic viscosity @60°C | poise | 1000±200 | 2000±400 | 3000±600 | 4000±800 |
| Kinematic Viscosity @135°C, min | cSt | 250 | 300 | 350 | 400 |
| Penetration @25°C | 0.1 mm | 80 | 60 | 45 | 35 |
| Flash Point, min | °C | 220 | 220 | 220 | 220 |
| Softening Point, min | °C | 40 | 45 | 47 | 50 |
| Solubility in trichloroethylene, min | % | 99 | 99 | 99 | 99 |
| Ductility @25°C, min | cm | 75 | 50 | 40 | 25 |
| Test on residue from thin film oven test | - | - | - | - | - |
| Viscosity ratio @60°C | - | 4 | 4 | 4 | 4 |
Common Grades and Applications
The table below presents the different VG grades of bitumen and their respective applications.
| Bitumen Grade | Viscosity | Applications |
|---|---|---|
| VG-10 | Lowest | Cold / mountainous regions – low viscosity and high wetting capability |
| VG-20 | Low to medium | Temperate climates and lighter traffic conditions |
| VG-30 | Medium | Most widely used grade – general asphalt paving, national and provincial highways, and balanced performance across most climates |
| VG-40 | Highest | Hot climates and heavy-traffic areas – intersections, toll plazas, and truck parking areas; strong resistance to rutting and deformation |
Standards
VG bitumen specifications and testing methods are defined under:
- ASTM (absolute/kinematic viscosity test methods)
- AASHTO M226 (equivalent AC-graded system used in North America)
- IS 73:2013 (India)
