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 depth of a 100 g needle 25° C (77° F)
- Flash point temperature
- Ductility at 25°C (77°F)
- Solubility in trichloroethylene
- Thin film oven test(accounts for the effects of short-term aging that occurs during mixing with hot aggregate)
- Retained penetration
- Ductility at 25° C (77° F)
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.
|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.
|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)|
A few states still have provisions for the penetration grading system. These will most likely disappear as the Superpave PG system becomes more prevalent.