Why do we Need to Change G*/Sinδ and How?

ABSTRACT In the United States, a number of State Highway Agencies claim that the Superpave specification has some critical gaps, most of them related to the performance characterization of modified bitumens (D'Angelo, 2002). Recognizing this fact AASHTO sponsored project NCHRP 9–10 and in 2001 NCHRP 459 (Bahia et al., 2001a) report was published. The report offered a revised system for testing and evaluating asphalt bitumens based on damage behavior. A scheme to conduct bitumen rutting tests that would allow a more direct qualification of bitumens for specific climate and traffic conditions was presented. The proposal, however, was only conceptual and lacked the details required for implementation, such as specific criteria and limits. This paper addresses the implementation of NCHRP 9–10 for bitumen rutting resistance. It reviews why G*/sinδ has to be changed for a new parameter, called viscous component of the creep stiffness Gv. It is shown how a mechanistic system based on damage behavior can be transformed in a new rutting specification. A proposal for specification limits in terms of the new Gv parameter is given. To develop the proposal, 19 asphalts of several grades currently used or marketed in Wisconsin were tested. The results of the bitumen testing were correlated with field performance using the past experience in the state of Wisconsin. The proposed limits are tentative and are mainly based on the ranking of the PG graded bitumens included in the study. The trial specification limits presented in this paper represent one step forward from the existing PG grading system towards a more reliable bitumen rutting characterization.