Performance of Asphalt Rubber Gap-Graded Mixture Overlays Over Jointed Concrete Pavements

About the report:

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-GGM12.5) 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 than90.  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.

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