Crumb rubber asphalt binder characterization and specifications for road pavement applications

Over the past five years, Australia generated on average an estimated 450,000 tonnes of end-of-life tyres (EOLTs), annually. From 2018–2019, almost 70% of the 466,000 tons of EOLTs generated in Australia were recovered and recycled into tyre-derived products or thermal processing. Unfortunately, approx. 140,000 tons of tyres were landfilled, buried on-site, or illegally dumped.  Sustainability Victoria (SV) and the Department of Environment and Heritage Protection (EHP) Queensland are co-leading the development of a National market development strategy for used tyres. The Strategy seeks to provide a framework for a national approach to market development for tyre-derived products (TDPs) in Australia. Several initiatives are underway to introduce new crumb rubber modifier (CRM) asphalt technology to Australia and increase the use of CRM binders in both asphalt and seals. However, the asphalt and seal technology are relatively well understood; there is currently no uniform framework to characterize and specify the CRM bitumen performance, hindering broad use of the technology. Therefore, this research is dedicated to developing CRM Binder Specifications for Road Pavement Applications. This study is divided into two major phases 1) fundamental investigation and 2) performance-based specifications. Phase 1 was focused on developing a blending protocol, selecting an appropriate CR size, and durability assessment of crumb rubber modified bitumen (CRMB) against UV ageing. CR was added into C-320 bitumen for blending protocol in three different percentages (7.5, 15, and 22.5% (wt.)) at 177+10 oC using a high shear mixer, with a mixing rate of 700 and 3500 rpm for 30, 60, and 90 minutes. To establish a blending protocol, blends were evaluated for physical, chemical, and rheological properties. Once the protocol was established, the blending was carried out using the established blending parameters for the consequent part of the experiments. Results showed that a blending rate of 700 rpm is appropriate for blending CR because higher mixing rates were found to age the CRMB samples and thus lower the material's fatigue resistance. Furthermore, a mixing duration of 60 minutes was enough to swell the CR particles in the bitumen if mixed at 700 rpm and 30 minutes if 3500 rpm was used. This provided a clear understanding of the influence of the blending factors on the performance of CRMB and helped in the establishment of the blending protocol. Moreover, blends prepared with #30 mesh CR improved the elastic response and storage stability compared to the #50 mesh CR modified binder. The presence of a greater number of coarser CR particles - which mostly remained in the swollen form at the end of the blending process - as compared to finer particles - which instead were depolymerized and released the absorbed oils back into the binder phase – proved to increase the flexibility of the binder and improve its rheological performance. Overall, a coarser CR gradation (within certain limits of maximum size of the rubber particles) generally performs better than a finer gradation for what concerns physical and rheological properties of the rubber-modified binder. The extra costs associated with the refinement of the CR gradation to a smaller size can only be justified by the lower viscosity associated with using the finer mesh in CRMB; in fact, #50 CR modified bitumen halved viscosity at 165 oC compared to the coarser mesh. This could improve workability in the field even at high CR content (i.e., 22.5%). To evaluate the durability of CRMB, the ageing mechanisms of UV radiations, thermal ageing, and pressure vessel ageing (PAV) were also investigated. To this scope, samples were aged in the Q-Lab weatherometer for 869 hours at 0.89 Wm-2, equivalent to one year of UV radiation energy in Melbourne, Australia, in 2019. Long-term ageing via pressure ageing vessel was also used to age CRMB. The ageing index developed from the crossover modulus of the modified binder showed that applying a 1-year equivalent period of UV radiations in Melbourne (Australia) provides 1.51 times more ageing to the neat bitumen compared to its unaged state, while ageing is up to 2.64 times after 20-hour PAV. When the same neat bitumen was modified with CR, the ageing resistance against both PAV and UV radiations was enhanced by limiting the level of ageing to 1.16 and 1.12 times than its unaged state, respectively. Linear regression analysis showed that a good correlation exists between the chemical properties (FTIR) and Glover-Rowe (G-R) parameter and thus could be used for the ranking of binders in terms of resistance to ageing. For the development of performance-based specifications, a process was established based on the multiple stress creep & recovery (MSCR) and linear amplitude sweep (LAS) tests performed at 60 oC and 25 oC, respectively. This process can be used to allocate different blends to various traffic conditions. In the current study, modification of C-320 bitumen with 7.5% CR makes it suitable for withstanding the ‘Heavy’ traffic category. The 15 % and 22.5 % CRMB are suitable for ‘Extreme’ traffic. Since the type of base bitumen also plays an essential role in defining the final performance, any change in the base binder type (soft or hard bitumen) can easily be accommodated in the proposed performance chart.