Viscosity Grade Bitumen

Viscosity Grade Bitumen Description

Traditionally, paving bitumen have been specified in terms of their penetration, but the measurement of viscosity provides a more accurate method of specifying binder consistency and a more effective method of determining the temperature susceptibility of the bitumen. American Association of State Highway Officials (AASHTO) has two series of viscosity grade bitumen. One is denoted AC (asphalt cement), followed by a number indicating the viscosity in hundreds of poises at 60 C. The second series is denoted AR (aged residue) followed by a number indicating the viscosity in poises (not hundreds of poises) at 60 C after the bitumen has been aged.
In the early 1960s, an improved asphalt grading system was developed that incorporated a rational scientific viscosity test. This scientific test replaced the empirical penetration test as the key asphalt binder characterization. The viscosity grading system gave excellent performance results in the US for over twenty years. The viscosity grading system is more rational than the penetration grading system. Viscosity is defined as the inverse of fluidity. Viscosity thus defines the fluid property of the bituminous material. Viscosity is the general term for consistency and it is a measure of resistance to flow. Many researchers believe that grading of bitumen should be by an absolute viscosity in instead of the conventional penetration units. Viscosity grading is based on a fundamental, scientific viscosity test, which is conducted at 60˚C (near the maximum pavement temperature during summer) and its measurement unit is poise. The test equipment for measuring viscosity both at 60˚C and 135˚C.
Viscosity grading can be done on original (as-supplied) asphalt binder samples (called AC grading) or aged residue samples (called AR grading). The AC grading is based on absolute viscosity at 60 °C in units of 100 poises. Six asphalt cement viscosity grades were established as AC-2.5 (softest), AC-5, AC-10, AC-20, AC-30, AC-40 (hardest). AC-2.5 means asphalt cement with a target viscosity of 250 poises at 60˚C (250 has been abbreviated to 2.5). Similarly, AC-5, AC-10, AC-20, AC-30, and AC-40 mean asphalt cement with target viscosity of 500, 1000, 2000, 3000, and 4000 poises, respectively. Low viscosity grades AC-2.5 and AC-5 are used in cold climate while high viscosity grades AC-10 to AC-40 are generally suitable for the hot climate.
With AC grading, the asphalt binder is characterized by the properties it possesses before it undergoes the HMA manufacturing process. The AR grading system is an attempt to simulate asphalt binder properties after it undergoes a typical HMA manufacturing process and thus, it should be more representative of how asphalt binder behaves in HMA pavements. Table 1 lists key advantages and disadvantages of the viscosity grading system.
Viscosity grade bitumen have a thermoplastic property which causes the material to soften at high temperatures and to harden at lower temperatures. This unique temperature/ viscosity relationship is important when determining the performance parameters such as the adhesion, rheology, durability and application temperatures of bitumen. In the Viscosity Graded Bitumen specifications, further special emphasizes is placed on the Bitumen ductility.

Viscosity grading can be done on original (as-supplied) asphalt binder samples (called AC grading) or aged residue samples (called AR grading). The AR viscosity test is based on the viscosity of aged residue from the rolling thin film oven test. With AC grading, the asphalt binder is characterized by the properties it possesses before it undergoes the HMA manufacturing process. The AR grading system is an attempt to simulate asphalt binder properties after it undergoes a typical HMA manufacturing process and thus, it should be more representative of how asphalt binder behaves in HMA pavements. Table 1 lists key advantages and disadvantages of the viscosity grading system.

Advantages and Disadvantages of Viscosity Grading as stated below

Viscosity Grade Bitumen Specification (ASTM D3381-09)

Viscosity Grade Bitumen Specification (IS 73:2013)