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VISCOSITY BITUMEN
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 the key advantages and disadvantages of the viscosity grading system.
Advantages and Disadvantages of Viscosity Grading as stated below
ADVANTAGES
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Unlike penetration depth, viscosity is a fundamental engineering parameter.
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Test temperatures correlate well with:
25°C (77°F) – average pavement temp.
60°C (140°F) – high pavement temp.
135°C (275°F) – HMA mixing temp.
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Temperature susceptibility (the change in asphalt binder rheology with temperature) can be somewhat determined because viscosity is measured at three different temperatures (penetration only is measured at 25°C (77°F)).
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Testing equipment and standards are widely available.
DISADVANTAGES
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The principal grading (done at 25°C (77°F)) may not accurately reflect low-temperature asphalt binder rheology.
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When using the AC grading system, thin film oven test residue viscosities can vary greatly with the same AC grade. Therefore, although asphalt binders are of the same AC grade they may behave differently after construction.
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The testing is more expensive and takes longer than the penetration test
The new method of grading the product has now rested on the viscosity of the Bitumen (at 60°C and 135°C). The new grades have thus evolved with nomenclature:(IS 73:2013) and (ASTM 3381-09)
