Development of dynamic shear-rheology-based method to improve evaluation of swelling degree of recycled crumb rubber in asphalt rubber

Inappropriate disposal of substantial scrap tires has become an unavoidable and imminent environmental concern. Asphalt rubber (AR) facilitates the value-added recycling of scrap tires into eco-friendly and durable pavement, showcasing immense potential for pollution alleviation and resource conservation. In preparing AR, the swelling effect of rubber plays a dominant role in the gel-like network formation and performance enhancement of the modified asphalt. Accordingly, it is crucial to accurately evaluate the rubber swelling degree for guiding AR preparation and mixture design. Existing research mainly relies on measuring the volume of expanded rubber particles to assess the rubber swelling degree, which requires sophisticated instruments or a cumbersome process of extracting the crumb rubber. In this study, a method was proposed to conveniently evaluate the swelling degree of crumb rubber based on the dynamic shear rheological indicators. Firstly, the base asphalt was mixed with crumb rubber at different levels of time to prepare AR binders with varying swelling degrees. The mechanical properties (i.e., elastic recovery) and physical properties (i.e., volume expansion) were measured and analyzed to investigate the impacts of swelling time on AR properties. Secondly, the rheological properties of AR with different swelling degrees were measured via temperature sweep (TS) test, multiple stress creep recovery (MSCR) test, and linear amplitude sweep (LAS) test. Thirdly, the particle effect (PE) and interaction effect (IE) values were calculated, and the swelling degrees were determined based on the variations of PE values. Relationships between swelling degrees and PE-related indicators were established to facilitate the development of the new evaluation method. Finally, the swelling degrees evaluated by the newly developed method were compared with volume expansion to validate its reliability. The results from different rheology tests were also used to verify the feasibility of the proposed method, and the prediction of swelling degree against the swelling time was established. The findings of this research provide references for evaluating the swelling degree of crumb rubber conveniently and accurately.