Investigation of Binder Aging and Mixture Performance of In-Service RAP Mixtures

Abstract

In 2007, the Virginia Department of Transportation piloted a specification allowing up to 30% reclaimed asphalt pavement (RAP) in certain dense-graded asphalt surface mixtures while changing virgin binder grade requirements. The change affected only mixtures requiring an end binder grade of either PG 64-22 or PG 70-22. For mixtures specifying PG 64-22 binder, the virgin binder grade at RAP contents of 30% or less was no longer required to change. For mixtures specifying PG 70-22 binder, the virgin binder grade at RAP contents of 21-30% was no longer required to change from PG 64-22 to PG 64-28. Prior to this, both types of surface mixtures were allowed to contain only up to 20% RAP before binder grade adjustments were required. An initial laboratory study of mixtures produced under the pilot specification indicated that there were no significant differences for fatigue, rutting, and susceptibility to moisture between the higher content (21-30%) RAP mixtures and control mixtures (having 20% RAP or less). The current study evaluated the inservice performance of these mixtures after approximately 7 years and encompassed field visits and a laboratory investigation of a sample of 23 in-service pavement sites used in the initial laboratory evaluation. Cores were collected from each site and used to evaluate the binder and mixture properties. Binder data were compared to data from the original construction when available to assess the changes in properties over time. Overall study results revealed no systematic effect on field and laboratory performance with increasing RAP contents up to 30%. Test results from roadway cores showed no conclusive trends in performance with RAP content. Testing of extracted binder indicated that RAP content appears to have an influence on the rate of aging of virgin binder–RAP blends; initial grades were lower for blends having lower RAP contents, although after 7-8 years of service, all blends aged to similar grades. Binder analysis also revealed that depth within the surface layer (in this case, the top half versus the bottom half) significantly affects binder properties, with stiffness decreasing with depth. However, increasing RAP contents appeared to mitigate the difference in failure temperature before and after aging, possibly attributable to the preexisting aged composition of the RAP and its influence on the virgin binder properties.