Superpave Performance Grading

Penetration grading and viscosity grading are somewhat limited in their ability to fully characterize asphalt binder for use in HMA pavement. Therefore, as part of the Superpave research effort new binder tests and specifications were developed to more accurately and fully characterize asphalt binders for use in HMA pavements. These tests and specifications are specifically designed to address HMA pavement performance parameters such as rutting, fatigue cracking and thermal cracking.

Superpave performance grading (PG) is based on the idea that an HMA asphalt binder’s properties should be related to the conditions under which it is used. For asphalt binders, this involves expected climatic conditions as well as aging considerations. Therefore, the PG system uses a common battery of tests (as the older penetration and viscosity grading systems do) but specifies that a particular asphalt binder must pass these tests at specific temperatures that are dependant upon the specific climatic conditions in the area of use. Therefore, a binder used in the Sonoran Desert of California/Arizona/Mexico would have different properties than one used in the Alaskan tundra. This concept is not new – selection of penetration or viscosity graded asphalt binders follows the same logic – but the relationships between asphalt binder properties and conditions of use are more complete and more precise with the Superpave PG system. Information on how to select a PG asphalt binder for a specific condition is contained in Superpave mix design method. Table 1 shows how the Superpave PG system addresses specific penetration, AC and AR grading system general limitations.

Related Tests

Superpave performance grading uses the following asphalt binder tests:

• Rolling thin film oven (RTFO)
• Pressure aging vessel (PAV)
• Rotational viscometer (RV)
• Dynamic shear rheometer (DSR)
• Bending beam rheometer (BBR)
• Direct tension tester (DTT)

Performance Grade Nomenclature

Superpave performance grading is reported using two numbers – the first being the average seven-day maximum pavement temperature (°C) and the second being the minimum pavement design temperature likely to be experienced (°C). Thus, a PG 58-22 is intended for use where the average seven-day maximum pavement temperature is 58°C and the expected minimum pavement temperature is -22°C. Notice that these numbers are pavement temperatures and not air temperatures (these pavement temperatures are estimated from air temperatures using an algorithm contained in the LTPP Bind program). As a general rule-of-thumb, PG binders that differ in the high and low temperature specification by 90°C or more generally require some sort of modification (Figure 1).

Figure 1. Prediction of PG grades for different crude oil blends.

Performance Grade Specification

The table in Figure 2 is the standard summary table presented in the AASHTO MP 1 specification for performance graded asphalt binder. The following items may help to decipher this table:

• The top several rows (all the rows above the “original binder” row) are used to designate the desired PG grade. For instance, if the average 7-day maximum pavement design temperature is greater than 52oC but less than 58oC then you should use the “< 58” column. The temperatures directly under the “< 58” cell are selected based on the minimum pavement design temperature in oC.
• No matter what the desired PG binder specification, the same tests are run. The PG specification (e.g., PG 58-22) just determines the temperature at which the tests are run.
• Tests are run on the original binder (no simulated aging), RTFO residue (simulated short-term aging) and PAV residue (simulated long-term aging) in order to fully characterize the asphalt binder throughout its life. Notice that often the same test is run on different simulated binder ages. For instance, the dynamic shear test is run on all three simulated binder ages.
• The tests run on the binder are listed in the left-hand column. They are not necessarily listed by their common names but the applicable AASHTO test procedure is listed. For instance, “Flash Point Temp. T 48, Minimum (oC)” means that the flash point is measured according to AASHTO T 48 and that the value in the adjacent column represents the minimum allowable in degrees Centigrade.

Figure 2. PG binder specification taken from NAPA

Reliability

Reliability is defined as the percent probability in a single year that the actual temperature (seven-day high or one-day low) will not exceed the corresponding design temperatures. The animation in Figure 3 describes the basic process for selecting the pavement temperature extremes for a PG asphalt binder. Note that pavement temperatures are more extreme than air temperatures.

Limitations of Penetration, AC and AR Grading Systems

Penetration and ductility tests are empirical and not directly related to HMA pavement performance.

Tests are conducted at one standard temperature without regard to the climate in which the asphalt binder will be used.

The range of pavement temperatures at any one site is not adequately covered. For example, there is no test method for asphalt binder stiffness at low temperatures to control thermal cracking.

Test methods only consider short-term asphalt binder aging (thin film oven test) although long-term aging is a significant factor in fatigue cracking and low temperature cracking.

Asphalt binders can have significantly different characteristics within the same grading category.

Modified asphalt binders are not suited for these grading systems

Superpave Binder Testing and Specification Features that Address Prior Limitations

The physical properties measured are directly related to field performance by engineering principles.

Test criteria remain constant, however, the temperature at which the criteria must be met changes in consideration of the binder grade selected for the prevalent climatic conditions.

The entire range of pavement temperatures experienced at a particular site is covered.

Three critical binder ages are simulated and tested:

1. Original asphalt binder prior to mixingwith aggregate.

2. Aged asphalt binder after HMAproduction and construction.

3. Long-term aged binder.

Grading is more precise and there is less overlap between grades.

Tests and specifications are intended for asphalt “binders” to include both modified and unmodified asphalt cements.