Influence of ground tire rubber on stone mastic asphalt mixtures and preliminary sustainability studies on rubber-modified asphalt pavements

Over the last few decades, the use of Stone Mastic Asphalt (SMA) has been adopted by several states in the U.S. as a specialty mix for high-traffic volume purposes. Extensive research on these mixes has revealed its unique characteristics, along with significant performance benefits, such as enhanced cracking and rut resistance, that are essential to mitigate critical pavement distresses. However, certain economic issues pertaining to SMA mixtures resulting from the need for high-quality aggregates and elevated binder content make it less favored by state transportation and highway agencies. To offset these costs, numerous studies have been conducted that encourage the incorporation of recycled material, such as recycled asphalt pavement (RAP), recycled asphalt shingles (RAS) and recycled rubber into SMAs. Of these, the rubber modified SMA mixes have exhibited superior performance, and economic and sustainability benefits. The incorporation of recycled scrap tires, as ground tire rubber (GTR), into the asphalt mixtures helps reduce the accumulation of end-of-life vehicle tires in landfills, which is a growing environmental concern. At present, the state of Missouri does not allow the use of recycled material in its SMA mixes. This thesis was proposed to assess GTR as viable means modification suitable for SMA pavements, with respect to the extreme climatic conditions of Missouri. To achieve this, two GTR-modified SMA mixes with 10 percent modification were compared against an unmodified SMA mix. A suite of performance tests was conducted to address prime pavement distresses, namely, Disk-Shaped Compact Tension test (DC(T)) to assess low-temperature cracking, Hamburg Wheel Tracking test (HWTT) for high-temperature deformations, and indirect tensile asphalt cracking test (IDEAL-CT) to determine intermediate-temperature fracture resistance. Further, a performance space diagram was also used to evaluate the overall performance or balance of these mixes. All experimental results concluded that the GTR-modified mixes performed better than the unmodified mix. Nevertheless, all three SMA mixes were within satisfactory performance threshold. The performance space plot clearly indicated that the GTR-modified SMA mixes were ideal for high-traffic volume pavements, in terms of thermal cracking and rutting distresses. To understand the potential of recycled rubber modification in a holistic manner, apart from performance analyses, sustainability studies on rubber-modified asphalt (RMA) pavements were conducted on a preliminary level, as a part of this thesis work. Life Cycle Assessment (LCA) is an environmental impact evaluation tool that has played a significant role in the recent years, for promoting advances in the use of recycled material in asphalt pavements to reduce the overall environmental burden and energy consumption. A widespread and comprehensive literature review was performed with an intention to obtain significant findings and learn the varied approaches used in these pavement LCAs. The defining LCA aspects such as the goal, functional unit, system boundaries and impact categories were analyzed and compared. This study established the following key knowledge gaps and recommendations: the inclusion of the maintenance phase of pavements and end-of-life phase of scrap tires in the system boundaries are critical for RMA pavements, there is a need to assign standardized eco-credit for RMA, using up-todate performance data including functional characteristics, and quantifying additional impact categories can significantly improve sustainability analysis outcomes for rubbermodified pavements. Addressing such issues could contribute to apprehend the full sustainability potential of rubber as a recycled material for pavement application.