Effects of rubber absorption on the aging resistance of hot and warm asphalt rubber binders prepared with waste tire rubber

Recycled from waste tires, crumb rubber modifier (CRM) is added into bitumen to produce the asphalt rubber (AR) with superior performances but a more complex aging mechanism. With much attentions paid to the complex composition of CRM, the effect of rubber absorption on the aging resistance of AR remains unclear, especially when warm mix asphalt (WMA) technology is combined. Therefore, this study aims to investigate the role of CRM absorption in improving the aging resistance of AR and warm asphalt rubber (WAR) binders. One AR and two WAR binders were prepared, as well as a direct mixing-AR (DAR) binder, which did not have significant CRM absorption, for comparison. The prepared binders under different aging conditions were separated into binder liquid phases and absorbed gel layers for various chemical and rheological analyses. Chemical characterization indicated that CRM could hinder the attack of oxygen on bitumen, and the absorbed fractions in the gel layers were less oxidized after aging. During aging, the rubber tended to preserve the light fractions inside the gel layer and expel the large molecules into the liquid phase, which maintained the relatively softer environment around the swelling rubber. Furthermore, the contribution of rubber absorption to the aging resistance was reflected by the less changed rheological properties of AR compared with the raw bitumen and DAR. The incorporation of WMA additives affected the rheological properties of AR but the aging resistance is not compromised. Overall, it was concluded that sufficient rubber absorption should be emphasized during the production of AR for better aging resistance, and the use of WMA additives can be considered for a more environmental-friendly production.