Preliminary Investigation into the Use of Reclaimed Asphalt Pavement in Gap-Graded Asphalt Rubber Mixes, and Use of Reclaimed Asphalt Rubber Pavement in Conventional Asphalt Concrete Mixes

This report describes a preliminary study that investigated the potential implications of using reclaimed rubberized asphalt pavement (R-RAP) materials as partial binder and aggregate replacement in new conventional dense-graded asphalt concrete mixes, and using reclaimed conventional asphalt pavement (RAP) materials as partial binder and aggregate replacement in new gap-graded asphalt rubber mixes. The use of rubberized hot mix asphalt (RHMA) in pavements in California has been increasing since the early 1990s. As these RHMA layers reach the end of their design lives they are being milled off and replaced with new hot mix asphalt (HMA) or new RHMA. The millings are being added to RAP stockpiles, which in turn are reused in new conventional HMA. There is no published information or experience documenting whether the use of R-RAP influences mix performance. Although Caltrans currently does not permit the use of any RAP in asphalt rubber mixes, there is increasing interest in allowing it as binder replacement in gap-graded mixes in order to reduce the amount of virgin binder required. Laboratory test results indicate that adding R-RAP to dense-graded HMA could potentially yield some improvement in overall rutting performance, but it could also have a potentially overall negative effect on fatigue and low-temperature cracking performance. These findings are consistent with those from tests where conventional RAP was used. The degree of change in rutting and cracking resistance in the HMA mixes was dependent on the R-RAP source, with mixes containing millings only from RHMA layers performing slightly better than mixes containing both R-RAP and RAP. Based on these findings, there appears to be no reason or justification for separating R-RAP and RAP millings or maintaining separate stockpiles at asphalt plants. Test results from the gap-graded RHMA mixes containing RAP indicated that rutting performance is likely to improve, but that adding RAP could have a potentially overall negative effect on fatigue and low-temperature cracking performance, which would negate the benefits of selecting RHMA-G as an overlay to retard the rate of reflection cracking. Since only limited testing on asphalt rubber mixes containing RAP was undertaken in this study, further laboratory testing, followed by full-scale field testing in pilot projects or accelerated wheel load testing should be considered on a wider range of virgin binder, virgin aggregate, and RAP material sources to confirm the findings before any changes to current practice are considered