Comparing the critical aging point of rubber-modified bitumen and plastic-modified bitumen

Abstract

The critical aging point (CAP) of an asphalt binder is defined as the longest aging period at which rejuvenation has been shown to be effective for all indicators. The increasing use of novel modified asphalt binders necessitates research on their CAP to optimize asphalt-mix service life through timely rejuvenation. This paper investigated the aging evolution and restoration capacity of bitumen modified with either crumb rubber (CR), a recycled thermoset, or waste polyethylene terephthalate (PET), a recycled thermoplastic. By using the CAP concept, this research hypothesized that the ability of modifiers to delay bitumen aging was influenced by their capacity to block the diffusion of free radicals and their capacity for adsorption of volatile compounds. This study used a rejuvenator to establish the CAP values for two modified bitumens: CR-modified bitumen, where CR was modified with waste vegetable oil (WVO); and PET-modified bitumen, where the PET was modified with WVO and then coated with wood-pellet biochar. Throughout the aging and rejuvenation processes, the intermolecular interactions and thermo-mechanical properties of each modified bitumen were monitored. Results indicated that, irrespective of the modifier type, the Glover-Rowe parameter (indicating bitumen's susceptibility to cracking) increased with aging, more notably for CR-modified bitumen than for PET-modified bitumen. Additionally, the healing index, crossover modulus, and stress relaxation capacity decreased with aging, with higher rates of change initially that slowed as aging progressed. The rejuvenator effectively restored mildly aged bitumen (up to 40 h) but not severely aged bitumen (60 h or more). PET-modified bitumen showed greater responsiveness to the rejuvenator in terms of restoring the healing index, while CR-modified bitumen responded more in restoring the crossover modulus. This suggests that a combined modifier using PET and CR could be beneficial for restoring both characteristics. The study's findings highlighted the efficacy of using recycled rubber and waste plastics in delaying bitumen aging, emphasizing timely rejuvenation before exceeding the CAP in order to get optimal restoration. In addition, this study addresses a significant gap in existing research by focusing on the limited exploration of the critical aging point of asphalt binders and the absence of a standardized measurement method. Our efforts not only contribute valuable insights to the understanding of this crucial parameter but also highlight the need for collective endeavors to develop a standardized approach. This, in turn, is intended to propel advancements in the field of asphalt engineering.