Viscosity Grade

In the early 1960s, an improved asphalt grading system was developed that incorporated a rational scientific viscosity test. The testing bath temperature must be 60° ± 0,3 °C, (average road surface temperature during sunshine) and 135 °C (average temperature of bitumen when mixing asphalt).

The viscosity of bitumen is of thermoplastic quality, which means that the material softens at high temperatures and hardens at lower ones. The viscosity of bitumen is important in determining how it will behave when used in road construction and other applications. The unique temperature/viscosity relationship means that the material can be soft and pliable when warm, but will harden when cool, which is crucial in en- suring that it adheres properly to surfaces and withstands traffic. Its rheology (the way it flows) can also be affected by temperature, and this needs to be considered when designing asphalt roads and other structures. Bitumen’s durability can also be affected by its viscosity – a higher-grade bitumen will last longer under repeated stress than a lower one.