Viscosity Grade bitumen is tested differently from other types and grades of bitumen. Globally known as viscosity testing, this testing method has been introduced to the industry in order to make the behavior of bitumen predictable at various temperatures.
In comparison to the well-known penetration testing, viscosity testing is more accurate. This test results in grading of bitumen into four grades named as VG10, VG20, VG30, and VG40.
Due to its durability and reliability, this type of bitumen is highly used in Australia and India. Applying these grades of bitumen provides you with less temperature susceptibility and better performance of the asphalt, as a result.
Below you can find a full description of viscosity grade bitumen and its advantages.
What is Viscosity Grade Bitumen?
Known also as VG, this type of bitumen is the result of testing the viscosity level of bitumen. By this method of testing, the consistency of bitumen, meaning bitumen’s ability to flow, is measured in both 60° C and 135° C. This test temperature stands for the maximum bitumen temperature in a hot summer and the temperature of bitumen when mixed.
Viscosity grade bitumen was introduced in 1970s. In order to increase the reliability of bitumen in higher temperatures and prevent the pavement from rutting, engineers identified more rational parameters (less empirical) for measuring bitumen behavior.
From 1970 onwards, viscosity grading become popular among both traders and engineers. There are several advantages that convinced consumers across the world to prefer this grade of bitumen to the old-fashioned penetration grading system.
In what comes below, we provide you with more details on the difference between penetration and viscosity grade bitumen.
What are the Advantages
of Viscosity Grade Bitumen?
Bitumen viscosity grade is globally preferred to penetration system of grading, mainly because it is more accurate.
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 in 25° C is measured. Since penetration test only measures the stiffness of bitumen in 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 in 60° C (Absolute Viscosity), and then the viscosity ratio to density is calculated in 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.
Viscosity Grade has more advantages over penetration grade including:
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)
More Control over Temperature Related Behaviour
In viscosity grading system the temperature susceptibility (Change in bitumen rheology* with temperature) is measured, in comparison to penetration grading in which controlling the temperature susceptibility is not possible
Better Performance in Various Temperatures
In viscosity method of grading bitumen, both low and hot temperature that bitumen can endure is considered
With a better knowledge about the physical properties of bitumen, the durability of the final asphalt is increased
Better for Polymer-modified Bitumen
For softer types of bitumen like polymer modified bitumen, penetration grading cannot be employed
What are Different Standards for
Viscosity Grade Bitumen?
Standard Absolute and Kinematic Viscosity
For measuring viscosity of bitumen, there are two methods known as absolute (or dynamic) viscosity and kinematic viscosity.
In absolute viscosity, the level of bitumen’s resistance to flow is measured in 60° C, but kinematic viscosity is the viscosity to density ratio in 135° C.
The difference between these two types of viscosity grading is that the absolute viscosity predicts the characteristics of bitumen in a hot summers, but the kinematic viscosity predicts the bitumen performance in a hot mix asphalt.
This grading test results are VG10, VG20, VG30, and VG40 that are more popular in Asian and African countries.
Viscosity Grading Based on AASHTO Standard
Another type of viscosity testing bitumen is based on the specifications defined by American Association of State Highway Officials (AASHTO).
This organization introduced two series of viscosity grade bitumen.1) Standard viscosity grade or (AC grades) which is the result of measuring the viscosity of the standard bitumen (asphalt) at 60° C.
Six asphalt cement viscosity grades were established as AC-2.5 (softest), AC-5, AC-10, AC-20, AC-30, AC-40 (hardest). Low viscosity grades are used in cold climate while the high ones are generally suitable for the hot climate.
2) Aged residue or (AR grades) in which the viscosity test is done on an aged residue sample of bitumen at 60° C.