What are the Advantages
of Viscosity Grade Bitumen?
VG bitumen is globally preferred to penetration grades (30/40, 60/70, 80/100, and so on), mainly because it is tested and classified more accurately.
This type of classification is based on the understanding of bitumen behavior and its functional properties, with different characteristics of cracking and deformation due to cold, heat, and fatigue. To do so, experts in the lab measure the consistency of bitumen in two phases.
In the first phase of viscosity grading test, the penetration of bitumen at 25° C is measured. Since the penetration test only measures the stiffness of bitumen at an average temperature, its results are not reliable in hot summer days. That’s why we complete the testing procedure by calculating the viscosity of bitumen in the second phase.
In the second phase, we calculate the viscosity of bitumen at 60° C (Absolute Viscosity), and then the viscosity ratio to density is calculated at 135° C (Kinematic Viscosity).
Based on this new method, engineers introduced four grades of bitumen as VG 40, VG 30, VG 20, and VG 10, to the market. The higher the number of the grade, the stiffer the Bitumen. For more information on viscosity grades click here.
VG bitumen is also manufactured from different raw materials. Refineries produce viscosity grade bitumen from a high-quality vacuum bottom. This is why the performance of VG bitumen is more than other grades of bitumen.
VG bitumen’s advantages to penetration grade bitumen are:
Viscosity grading system is more accurate. For example, a specific grade of viscosity bitumen gives a similar rutting result in hot temperatures, unlike a specific grade of penetration bitumen that behaves differently in the same temperature
The test temperature predicts bitumen’s behavior in an average temperature pavement (25° C), in a hot temperature pavement (60° C), and in the mixing plant for using as hot mix asphalt (135° C)
For softer types of bitumen like polymer modified bitumen, penetration grading cannot be employed. We use viscosity testing for measuring the specifications of these types of bitumen
We can have a better knowledge of the physical properties of bitumen. With considering bitumen’s temperature susceptibility (Change in bitumen rheology* with temperature), the durability of the final asphalt is increased
Although viscosity grade bitumen has a number of advantages, it has some problems that newer forms of testing and grading are trying hard to solve. Some of the disadvantages are:
- Viscosity testing is more expensive and takes longer than the penetration test,
- The aged VG bitumen has the given specifications in a short term period, long term aging is not considered here and may result in unpredicted changes in bitumen properties,
- The standard grades of VG10, VG20, VG30, and VG40 has been introduced without consideration of the climate in a specific location,
- In comparison to the modernized grades of bitumen, known as Superpave Performance, Viscosity grade bitumen is less accurate.