Effect of temperature on permanent deformation of polymer-modified asphalt mixture

This study aims to evaluate the permanent deformation of unmodified and styrene-butadiene-rubber (SBR) modified asphalt mixtures using dynamic creep test. The purpose was to assess the effect of SBR as one of the most plentiful and low-cost polymers in Iran on rutting resistance of asphalt mixtures in the regions with hot climate, such as the areas around the “Persian Gulf”, and the central deserts of the “Iran Plateau”. First, the critical gradation of aggregates with higher permanent deformation was determined. Then, the aggregates with critical gradation were mixed with the optimum amount of bitumen modified by different amounts of SBR, and tested to obtain an optimum SBR content with lower permanent deformation. Finally, the unmodified and SBR-modified asphalt mixtures with optimum SBR content (6%wt) were tested at 40 and 50 ˚C as the simulated ambient temperatures in order to evaluate the effect of SBR and rising temperature on rutting resistance of the asphalt mixtures. In addition, the flow number (FN) of asphalt mixtures is calculated according to Goh and You method. Results showed that with addition of 6% SBR, the permanent strains of asphalt mixtures decreased by 39 and 60%, and the creep modulus increased by 64 and 133% at 40 and 50 ˚C, respectively. Furthermore, with the temperature rising from 40 to 50 ˚C, the permanent strains of asphalt mixtures containing 0 and 6% SBR increased by 61 and 5%, and their creep modulus decreased by 34 and 6%, respectively. The FNs of unmodified samples were obtained 8416 and 9728 loading cycles at 40 and 50 ˚C, respectively. In contrast, up to the last loading cycle, the SBR-modified samples did not experience the tertiary flow at both ambient temperatures. These results let us conclude that the SBR-modified bitumen is able to significantly reduce the permanent deformations, and enhance the resilience and creep modulus of asphalt mixtures; moreover, it can minimize the negative effects of rising temperature on their engineering properties.