Penetration Grading

The penetration grading system was developed in the early 1900s to characterize the consistency of semi-solid asphalts. Penetration grading quantifies the following asphalt concrete characteristics:

Penetration grading’s basic assumption is that the less viscous the asphalt, the deeper the needle will penetrate. This penetration depth is empirically (albeit only roughly) correlated with asphalt binder performance. Therefore, asphalt binders with high penetration numbers (called “soft”) are used for cold climates while asphalt binders with low penetration numbers (called “hard”) are used for warm climates. Penetration grading key advantages and disadvantages are listed in Table 1.

Table 1. Advantages and Disadvantages of the Penetration Grading from Roberts et al., 1996[1])

Advantages Disadvantages
The test is done at 25° C (77° F), which is reasonably close to a typical pavement average temperature. The test is empirical and does not measure any fundamental engineering parameter such as viscosity.
May also provide a better correlation with low-temperature asphalt binder properties than the viscosity test, which is performed at 60° C (140° F). Shear rate is variable and high during the test. Since asphalt binders typically behave as a non-Newtonian fluid at 25° C (77° F), this will affect test results.
Temperature susceptibility (the change in asphalt binder rheology with temperature) can be determined by conducting the test at temperatures other than 25° C (77° F). Temperature susceptibility (the change in asphalt binder rheology with temperature) cannot be determined by a single test at 25° C (77° F).
The test is quick and inexpensive. Therefore, it can easily be used in the field. The test does not provide information with which to establish mixing and compaction temperatures.

Penetration grades are listed as a range of penetration units (one penetration unit = 0.1 mm) such as 120 – 150. Penetration grades specified in AASHTO M 20 and ASTM D 946 are listed in Table 2.

Table 2. AASHTO M 20 and ASTM D 946 Penetration Grades

Penetration Grade Comments
40 – 50 Hardest grade.
60 - 70 Typical grades used in the U.S.
85 - 100
120 – 150
200 – 300 Softest grade. Used for cold climates such as northern Canada (Roberts et al., 1996[1])

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.