Performance-Graded High-Temperature Selection Criterion: Can We Do It Better?

The Superpave binder grading system provides a selection process based on the potential performance characteristics of the binder as related to the climatic considerations of the area, principally the temperature. The selection criterion for the high-temperature grade uses the average of the maximum consecutive 7-d high pavement temperatures at a 20-mm depth with requisite grade bumps. This selection criterion is examined as to how well it actually relates to the performance of the asphalt concrete under high temperatures, principally permanent deformations in the mixtures, which is a fundamental connection that must be established before adjustments are made to the binder grade on the basis of the traffic using the pavement. The current temperature criterion is compared with the actual temperature distributions of a number of cities across the United States to demonstrate the disconnected nature of this value in relation to the actual pavement temperatures. The temperature data are further analyzed to present the performance differences that could be incurred in different climates when the 7-d high pavement temperature average is used as the selection criterion with rather arbitrary grade bumps. Georgia loaded wheel rut data are used to illustrate the possible performance differences that may be incurred by ignoring the temperature frequency distribution and using only the maximum consecutive 7-d high pavement temperatures at 20 mm for the selection criterion. A procedure is outlined that would allow inclusion of traffic loads and temperature distribution to develop a simple selection procedure that is based on binder differences.