Evaluation of use of dry-process rubber modification in asphalt mixtures

Abstract

This study explores four different facets of using dry-process rubber modification in asphalt mixtures. The first investigation was motivated by the need to understand the cracking mechanism prevalent in the dry-process rubber-modified asphalt mixtures. It has been observed that dry-process rubber-modified specimens are characterized differently by different cracking tests, and thus, understanding the cracking mechanism is of immense importance, especially when mixture design is evolving to be based on mixture performance tests through implementation of Balanced Mixture Design (BMD) method. The study devised a new test method to ascertain the cracking mechanism of dry-process rubber modification. Results from the novel test methods, coupled with image analysis, revealed the presence of crack pining mechanism in the binder-rubber mastic. The next part of the study investigated the long-term aging of dry-process rubber-modified asphalt mixtures and compared it to unmodified, and polymer-modified asphalt mixtures in terms of their cracking potential determined by three different, widely-used cracking tests, namely, Disk-shaped Compact Tension (DC(T) test, Illinois Flexibility Index Test (IFIT), and indirect Tensile Asphalt Cracking test (IDEAL-CT). Results from this study showed the adequacy of DC(T) test to correctly characterize rubber-modified specimens, both short-term and long-term aged, while IFIT test was seen lacking in producing reliable results for long-term aged specimens. Analysis of test data revealed that rubber modification might decrease the aging susceptibility of asphalt mixture. This could be attributed to the enhanced cracking resistance due to presence of crack pinning mechanism. The third part of the study focused on the use of dry-process rubber-modified dense-graded asphalt mixtures as a popular pavement preservation technique, specifically, thin HMA overlay. Standard and modified mixture tests, both cracking and rutting, were used to compare the unmodified and rubber-modified mixtures. Additionally, a specialized software, TxACOL, specifically used for asphalt overlay design, was employed to investigate the benefits of rubber-modification. The results were mixed, in the sense that the rubber-modification showed great advantage when used over existing asphalt overlay, but a contrasting trend was observed when the existing pavement was jointed concrete. It was acknowledged that a more compliant mixture would perhaps be a better option for an asphalt overlay on existing concrete pavement. This study highlighted the high criticality of using proper calibrated factors for these tools to reflect the field reality of mixture performance. The final part of this study looked at a novel method to grade asphalt mixtures' performance, by using mixture performance tests instead of binder tests. The study included four plant-produced Missouri mixtures and a number of other mixtures, including rubber -, polymer-, and fiber-modified asphalt mixtures. The results highlighted the importance of considering effect of the aggregate structure is estimating mixture performance, as done with mixture performance tests. Rubber-modification showed huge improvements in the rutting resistance of mixtures. Comparison of recovered continuous binder grade to mix performance grade for rubber-modified mixtures revealed large differences in the useable temperature intervals (UTI = PG high temperature – PG low temperature, e.g. PG64-22 has UTI = 86[degrees]C) obtained from recovered binder testing and mixture testing, further indicating the caveats of determining the performance of a mixture using recovered binder, especially when recycled content is used.