The optimal use of crumb rubber in hot-mix asphalt by dry process: a laboratory investigation using Marshall mix design

This paper aims to use the Marshall method to design and evaluate crumb-rubber modified (CRM) and conventional dense-graded (DG) hot mix asphalt (HMA), as there were limited previous experimental investigations that compared their properties by varying crumb rubber (CR) sizes, CR contents and aggregate types. Two types of crushed aggregate – diorite and granite – were mixed with 5.0% to 7.0% bitumen contents and 0%, 1%, 2% and 4% CR contents sized at 0.71 mm, 2 mm and 2.36 mm by dry process. The laboratory test results showed that the optimum bitumen content (OBC) and CR content that commonly satisfied international standards of CRM-DG-HMA by dry process and conventional DG-HMA were 5.5% and 1% respectively. CR content significantly contributed to the swelling of the CRM-DG-HMA, followed by the aggregate type and CR size. CRM-DG-HMA using granite aggregate was slightly more prone to swelling than that with diorite aggregate. Diorite-based CRM-DG-HMA with 2 mm CR size was more durable than conventional DG-HMA, granite-based CRM-DG-HMA and diorite-based CRM-DG-HMA with 0.71 mm CR size, with the last two being less durable than conventional DG-HMA. Granite-based CRM-DG-HMA was more stable and stiffer than conventional DG-HMA, while diorite-based CRM-DG-HMA was less stable but stiffer than conventional DG-HMA. Moreover, granite-based CRM-DG-HMA was more stable and stiffer than diorite-based CRM-DG-HMA with 0.71 mm CR size, with the latter being more stable and stiffer than diorite-based CRM-DG-HMA with 2 mm CR size. The findings from this paper have shown that partial replacement of mineral aggregate with CR in DG-HMA is a sustainable option, which could yield comparable or improved properties over conventional DG-HMA, provided that the CR size, CR content and aggregate type have been determined and selected in the mix design.