A study has been conducted on the changes to in-service asphalts. This study represents ongoing extensive research efforts at Purdue University to investigate the changes to in-service asphalts and to evaluate the recently developed SUPERPAVE asphalt binder specifications. During the 1985 construction season, samples from the original asphalt binders and the truck mix samples (original mixtures) were retained from a number of paving projects in Indiana. In 1992/1993, after 7 to 8 years in-service, cores from these pavements were taken. During the same period of time, distress surveys were conducted. In a later stage, six projects were selected for detailed analysis using the SUPERPAVE binder test equipment and procedures. A major interest of this research was to characterize the changes in the asphalt binders in-service and to correlate these changes with the pavement distresses, mainly: permanent deformation (rutting), fatigue cracking, and low temperature cracking. Consequently, the performance grade (PG) binder specifications could be evaluated. The SUPERPAVE binder tests and associated criteria were used to determine the high, intermediate, and low temperature characteristics (stiffness, (Pa)) of the asphalt binders tested. The results compare the relation between the conditioned (aged) asphalt binders from the Rolling Thin Film Oven (RTFO) test and the asphalt binders extracted from the original mixtures and the relation between the asphalt binders conditioned in the Pressure Aging Vessel (PAV) to the asphalt binders extracted from the in-service (field) cores. The results indicate that the current SUPERPAVE RTFO conditioning does not simulate short term aging. Similarly, the results show that the PAV aging procedure does not correspond to the aging encountered after eight years in-service.

A major result of the research conducted under the Strategic Highway Research Program from 1987 to 1993 was the development of the Superpave (Superior Performing Asphalt Pavement) system for the comprehensive design of asphalt pavements. These 14 contributions describe the experience to date in the

Design related project level pavement management - Economic evaluation of alternative pavement design strategies - Reliability / - Pavement design procedures for new construction or reconstruction : Design requirements - Highway pavement structural design - Low-volume road design / - Pavement design procedures for rehabilitation of existing pavements : Rehabilitation concepts - Guides for field data collection - Rehabilitation methods other than overlay - Rehabilitation methods with overlays / - Mechanistic-empirical design procedures.