Published in 2022
Ground tire rubber (GTR) from scrap tires is used in asphalt mixtures (rubber modified asphalt [RMA]) for improving the performance of pavements. There are different ways to add GTR in asphalt mixtures, but the two primary methods are referred to as the “wet” and “dry” processes. The dry process incorporates GTR directly into the asphalt mixture during production (directly to the aggregates through the reclaimed asphalt pavement collar). The Virginia Department of Transportation (VDOT) has limited experience with RMA mixtures in Superpave dense-graded mixtures using the dry process, but the relative ease of mixture production makes the dry process an attractive option for RMA. In the fall of 2019, VDOT placed a dense-graded RMA mixture, SM 12.5 (GTR), on US 60 in VDOT’s Richmond District (New Kent County). This was the first use of a SM 12.5 (GTR) mixture in Virginia using the dry process method. The purpose of this study was to establish a performance baseline for a GTR modified dense-graded asphalt mixture that was designed and produced using the dry process. The US 60 project also included the use of a thin hot mix asphalt concrete overlay (THMACO) as an interlayer. An assessment of the THMACO as an interlayer was a secondary objective of the study.
The study found that dry process SM 12.5 (GTR) mixture can be produced and placed with no significant field-related concerns and that the special provision developed for its use was effective. Density requirements were achieved, and the as-placed mat had excellent (very low) permeability characteristics. Laboratory performance testing showed the SM 12.5 (GTR) mixture to be more crack resistant than conventionally modified polymer (SM 12.5E) mixtures. Conventionally modified SME mixtures had slightly better rutting performance. However, this conclusion was based on performance testing and thresholds that were developed for non-modified asphalt mixtures. Additional laboratory and field performance comparison is needed to develop mixture acceptance criteria for GTR mixtures. Further, THMACO mixtures had excellent laboratory reflective cracking resistance properties. They performed particularly well in the Texas overlay test. Grading of extracted (from the asphalt mixture) binder may not provide an accurate representation of the binder performance for the dry process GTR modified asphalt. Continued monitoring of performance will be needed to quantify any benefit of SM 12.5 (GFR) mixtures in comparison with regular SM E mixtures. The study recommends additional field trials with SM 12.5 (GTR) mixtures for performance evaluation. Further, the study recommends continued use of a THMACO as an interlayer to mitigate reflective cracking for composite pavements.
Last updated: November 7, 2023