VDOT Research Council Publishes Two Reports on Ground Rubber in Asphalt
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Performance of Asphalt Rubber Gap-Graded Mixture Overlays Over Jointed Concrete Pavements
Abstract:
The Virginia Department of Transportation (VDOT) maintains 3,343 lane-miles of composite pavements (asphalt over jointed concrete or continuously reinforced concrete pavements). Propagation of cracks from existing pavements into new asphalt concrete overlays (reflective cracking) is a major problem with composite pavements. Treatments that are used to reduce or mitigate reflective cracking include the use of asphalt mixtures with highly modified binders. One way of modifying asphalt mixtures is by using ground tire rubber (GTR), also referred to as rubber modified asphalt. There are three ways of adding GTR to asphalt mixtures: (1) traditional wet process, (2) terminal-blend wet process, and (3) dry process. The traditional wet process blends GTR with asphalt binder or bitumen on-site at the asphalt mixture plant prior to mixing the GTR modified asphalt binder with aggregate. The traditional wet process, along with a gap-graded stone structure, is typically used for incorporating higher GTR concentrations (>15%). VDOT has limited experience with rubber modified asphalt mixtures in general and even less experience with GTR content that exceeds 10%.
The purpose of this study was to establish a performance baseline for an asphalt rubber gap-graded mixture (AR-GGM 12.5) using the wet process on I-85 in the Richmond District (I-85 Southbound, Dinwiddie County). Another objective was to compare its performance with VDOT’s stone matrix asphalt (SMA) mixture, which is also a gap-graded mixture.
This study found that AR-GGM mixtures can be placed with no special field accommodations (compared with SMA mixtures), and the special provision developed for AR-GGM mixtures is effective. Further, based on laboratory performance testing, both the AR-GGM and SMA control mixtures tested in this demonstration project were crack and rutting resistant, with the AR-GGM mixture exhibiting more flexibility (i.e., lower stiffness). Both sections are performing as expected after 3 years of traffic and exhibiting minor to no distresses, with a Critical Condition Index greater than 90. However, at this early stage of field service, it is too soon to quantify a performance advantage of AR-GGM mixtures in comparison with conventional SMA mixtures. This study recommends continued use of AR-GGM mixtures for suitable projects as a reflective cracking mitigation tool. Further, the study recommends continued performance monitoring of the study sections to evaluate the cost-effectiveness of AR-GGM mixtures in comparison with SMA mixtures.
Abstract:
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 SM E 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 (GTR) 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.
