Evaluating the substitution potential of SBS with crumb rubber-polypropylene blends as asphalt binder and mixture modifiers

Abstract

The study aimed to assess the suitability of substituting styrene–butadienestyrene with waste crumb rubber-polypropylene blends as modifiers to enhance the performance characteristics of asphalt binders and mixtures. Six crumb rubber-polypropylene blends including three styrene–butadienestyrene percentages were used to modify a base binder and then the modified binders were used to fabricate asphalt concrete mixtures. To obtain the physical and compatibility behavior of the unmodified and modified binders, various conventional binder tests such as softening point, penetration, and storage stability were employed. Dynamic shear rheometer, multiple stress creep recovery, and linear amplitude straintests were performed on unaged and long-term aged samples to achieve a broad understating of rheological characteristics at high and intermediate temperatures. In order to assess the influence of the modifiers on asphalt mixtures, various test methods were implemented including indirect tensile strength, resilient modulus, and four-point beam fatigue and dynamic creep tests. The findings of the physical tests represented a decline in penetration and a rise in softening point by the crumb rubber-polypropylene and styrene–butadienestyrene modifications compared with the base one. Results of the storage stability test proved that only 3% styrene–butadienestyrene modified was a storage-stable blend at high construction temperatures and the instability increased by incorporating more polymer content. According to the rotational viscosity test, all of the modifications increased the viscosity, reducing the workability of the modified mixture. Based on the rheological tests, the addition of the modifiers resulted in a reduction in phase angles, and an increase in complex modulus at high temperatures, thereby decreasing binder sensitivity to permanent deformation. With increasing the percentage of the polymer, more improvement would be obtained regarding rutting resistance. At intermediate temperatures, however, the complex modulus of modified binders decreased while no notable changes were observed for phase angle compared to the base binder. Considering the mechanical test results, the crumb rubber-polypropylene and styrene–butadienestyrene modified mixtures resulted in an enhancement in indirect tensile strength, resilient modulus, four-point beam fatigue, and dynamic creep tests results which would cause better resistance against rutting, fatigue damage, and moisture susceptibility compared with the base one. Besides, a radial bias function neural network model was designed to predict tensile strength ratio, and fracture energy ratio. The results showed the high performance of this method in the estimation of these values.