Evaluation of the Effects of Engineered Crumb Rubber (ECR) on Asphalt Mixture Characteristics

Abstract

Abstract This study examined the effects of modification of dense-grade asphalt mixtures with chemically engineered crumb rubber (ECR) via the dry process. The ECR mixtures were compared to unmodified and polymer-modified mixtures in various criteria. The study mainly had three objectives: (1) comparison of mixture performance, (2) investigation of warm mix equivalence, i.e., to determine if the chemical coating on the ECR resulted in benefits similar to warm mix additives, and (3) to determine the effects of ECR on mixture draindown. For the first objective, the disk-shaped compact tension test was conducted to determine resistance to thermal cracking, and the Hamburg wheel tracking test was performed to determine resistance to permanent deformation (rutting) and moisture damage. To fulfill the second objective, the mixtures were prepared with warm mix asphalt (WMA) additives wherever necessary, and for the third objective, a stone mastic asphalt (SMA) was employed instead of the dense-grade mixture that was used for the rest of the study. Findings showed that modification with ECR greatly improved resistance to rutting and cracking, indicating that ECR pavements will exhibit longer service life. In terms of warm mix equivalence, a comparison of volumetrics data showed that ECR without WMA additive compacted in a similar manner as a polymer (PG 76-22) mixture containing WMA additive. Additional mix cost savings may be realized with ECR modification because of the ability to avoid the cost of WMA additive for the goal of achieving mix workability and compaction at lower temperatures. Finally, an ECR-SMA mix was used for draindown assessment and was found to pass the draindown test without the need for fibers, nearly matching the draindown resistance of the polymer-WMA mix containing cellulose fibers, thereby presenting another cost-saving avenue for contractors. The study also includes details of three field projects that used ECR and reported excellent field performance.