Asphalt cement grade of bitumen is one of the main categories of viscosity grade bitumen. The American Association of State Highway Official (AASHTO) has classified viscosity grade bitumen into two categories as follow. First one is AC, which stands for Asphalt Cement, and the second one is AR, which is the abbreviated form of Aged Residue. The difference between AR and AC grade is in the meaning of the numbers following them in written form. AC grade is followed by a number which indicates the viscosity at 60 c while AR grade is followed by a number which indicates the viscosity at 60 c after the bitumen has been aged.
The viscosity grading system is more comprehensive compared to the penetration grading system. Viscosity is the conversion of fluidity. It’s a general concept for the texture of substances. Chemically speaking, viscosity is applied as a consistency measurement. Viscosity test is conducted at 60 c (the maximum pavement temperature). Good to know that the measurement unit of viscosity grading system is Poise. The viscosity grading can be done on both original (as-supplied) asphalt binder which is called AC grading and aged residue binder which is called AR grading. The AC grading is related to absolute viscosity at 60 c in 100 poises.
Here are six asphalt cement (AC) viscosity grades: AC-2.5 (which is the softest grade), AC-5, AC-20, AC-30, and AC-40(which is the hardest grade). As a brief explanation, AC-2.5 is asphalt cement with a target viscosity of 250 poises at 60c. (250 are divided by 100).
As mentioned above, for other grades, the divided number for each grade stands for the target viscosity poises.
Low viscosity grades are AC-2.5 and AC-5. They are recommended for cold climate whereas other grades AC10 to AC40 are suitable for hot climate.