Impact of High Asphalt Binder Replacement on Level Binder Properties for Controlling Reflective Cracking

A thin hot-mix asphalt (HMA) level binder (LB) course or interlayer has been used for decades in Illinois. Originally, the LB was simply a layer to provide a working platform that corrected minor variations in the profile of the pavement before HMA overlay. The performance of LB has become of increasing concern with respect to whether high asphalt binder replacement (ABR) could be incorporated into the LB mixes to reduce costs by adding reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS). This study investigated three LB courses with varying ABRs and sources that had been used in five field HMA overlay rehabilitation projects in Illinois. The performances were characterized by a comprehensive testing suite that consisted of tests for dynamic modulus, rutting (flow number and Hamburg wheel tracking tests), moisture susceptibility (tensile strength ratio), fracture resistance [Illinois Flexibility Index Test (I-FIT) overlay tester, and four-point beam fatigue]. The role that these LB mixes play in HMA overlay performance also was examined in field performance. It was found that the rutting and the moisture resistance performances were good for all mixes. Three cracking performance tests showed a consistent trend that higher use of ABR in LB mixes resulted in worse cracking resistance. On the basis of the field performance, I-FIT is recommended as a simple performance test to evaluate the cracking resistance of LB mixes with high ABRs, given its testing simplicity, reliability, and ease of specimen preparation. The LB mixes with the best cracking resistance exhibited the least amount of transverse cracking. The addition of RAP and RAS to an LB course must be reexamined without jeopardy to the main purpose for the use of LB and without a negative impact on its performance.