Comprehensive study on the sustainable technology of asphalt rubber for hot mix asphalt binders and mixes
Asphalt rubber (AR) technology is considered a sustainable technology. The high amounts of scrap tires that are in landfills, dumps and stockpiles present an environmental issue. By recycling and reusing ground tire rubber (GTR) in asphalt binders, this environmental issue is not only being addressed but also the overall performance of asphalt pavements is being improved. Although AR technology presents benefits to the fatigue performance of asphalt pavements as reported by the literature, it also presents certain challenges. One of these challenges is its workability, related to the higher viscosities of the binder at conventional mixing and compaction temperatures, leading to having higher mixing and compaction temperatures to obtain the desired workability. The second challenge is related to the rheology characterization of AR binders. While the addition of GTR to asphalt significantly improves the performance of AR mixes, the rheological characterization of AR binders using established performance grade methods for conventional or polymer-modified binders is not suitable to AR binders.
Chemical additives, as polyoctenamer, can be added to AR binders to improve their workability. The effects of polyoctenamer on the performance of AR mixes has not being widely evaluated. The approach of this dissertation is two-fold. The first approach focuses on the low temperature and fatigue performance of laboratory produced AR mixes prepared with AR binders containing polyoctenamer, and the effects and interactions with two types of GTR. The low temperature performance of these mixes was evaluated using the semi-circular bend test; while the fatigue performance was assessed using the four-point bending beam device. Results revealed that the addition of polyoctenamer does not have detrimental effects on the low temperature performance nor the fatigue performance of AR mixes. Interactions between polyoctenamer and rubber type were found in the fracture energy results. Whereas for the fatigue performance of these mixes, higher fatigue life was observed for mixes containing ambient GTR, but lower rate of damage accumulation was found for mixes containing cryogenic GTR.
The second approach presents a thorough binder study utilizing three different geometries to characterize the rheology of laboratory produced AR binders. The geometries used were 1 and 2 mm gap using parallel plates and concentric cylinders with 5.7 mm gap. Continuous performance grading, master curves, viscosities, mass loss, and storage stability were evaluated. Results demonstrated that the increase in testing gap does not isolate the interference of GTR particles in the rheological characterization of these binders. In general, results obtained with the AR binders that were centrifuged, meaning that rubber particles have been removed by the Binder Accelerated Separation Method, did not have the interference or influence of GTR particles when compared with the results obtained with the non-centrifuged AR binders tested using the aforementioned geometries.