Tyre rubber-modified bitumens development: the effect of varying processing conditions

Tyre rubber-modified binders (TR-MBs), produced through McDonald’s wet process and used worldwide (e.g. asphalt rubber), have been demonstrated to provide various benefits to pavements and, moreover, they represent a good opportunity for recycling tyre rubber (TR). However, this technology is still struggling to be fully adopted in Europe, mainly because of their poor stability during high temperatures storage, which leads to high initial costs in modifying existing asphalt plants. Storage stable TR-MBs are proving to be a great option and their development could be the key to spread the recycling of TR within paving applications in Europe. This paper aims to enrich this field of research by presenting the results of a study focused on the optimisation of laboratory procedures to better understand the effect of varying processing conditions on the rheology of TR-MBs. The experimental programme has been carried out by a preliminary selection of materials, followed by the production of the TR-MBs, via practical laboratory protocols. A continuous comparison between two commercially used styrene–butadiene–styrene (SBS)-MBs, with high and medium levels of modification, and the produced TR-MBs helped to understand the effect of varying the selected processing conditions on binder properties. Results have shown that TR-MBs are a very effective alternative to commercially used SBS-MB. Nevertheless, the rheology of TR-MBs is very sensitive to the variation of the processing conditions, so appropriate selection of materials and a superior binder design are mandatory to achieve the desired level of modification.