Viscosity Grading

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. Viscosity grading quantifies the following asphalt binder characteristics:

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.

Table 1. Advantages and Disadvantages of Viscosity Grading (from Roberts et al., 1996[1])
Advantages Disadvantages
Unlike penetration depth, viscosity is a fundamental engineering parameter. The principal grading (done at 25° C (77° F)) may not accurately reflect low-temperature asphalt binder rheology.
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.
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.
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)). The testing is more expensive and takes longer than the penetration test.
Testing equipment and standards are widely available.

Viscosity is measured in poise (cm-g-s = dyne-second/cm2, named after Jean Louis Marie Poiseuille). The lower the number of poises, the lower the viscosity and thus the more easily a substance flows. Thus, AC-5 (viscosity is 500 ± 100 poise at 60° C (140° F)) is less viscous than AC-40 (viscosity is 4000 ± 800 poise at 60° C (140° F)). Table 2 shows standard viscosity grades for the AC and AR grading systems from AASHTO M 226 and ASTM D 3381. Typical grades used for HMA paving in the U.S. are AC-10, AC-20, AC-30, AR-4000 and AR 8000.

Table 2. AASHTO M 226 and ASTM D 3381 Viscosity Grades
Standard Grading based on Original Asphalt (AC) Grading based on Aged Residue (AR)
M 226
AC-2.5 AC-5 AC-10 AC-20 AC-30 AC-40 AR-10 AR-20 AR-40 AR-80 AR-160
D 3381
AC-2.5 AC-5 AC-10 AC-20 AC-30 AC-40 AR-1000 AR-2000 AR-4000 AR-8000 AR-16000

Footnotes    (↵ returns to text)
  1. Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W.  (1996).  Hot Mix Asphalt Materials, Mixture Design, and Construction.  National Asphalt Pavement Association Education Foundation.  Lanham, MD.