Definition of a laboratory optimization protocol for road bitumen improved with recycled tire rubber

Abstract

Bearing in mind the need to answer one of the most frequent needs for civil construction, in general, and major project such as road infrastructures in particular, this paper presents the results of a laboratory experimental study in order to evaluate the performance improvement that is possible to achieve when modifying road bitumen with CRM (Crumb Rubber Modifier) from discarded tire rubber (TR), using a Wet Process. The environmental advantage is double, since the aim of the research is to obtain high performance road bitumen thanks to the re-use of a discarded material, such as the rubber tires, which otherwise would be dumped in scrap-yard. In particular, this experimental study aimed to define a laboratory optimization protocol that allowed to discriminate amongst the different asphalt–rubber blends, in relation to the different content of rubber, as well as to the blending times, with the possibility of carefully controlling the production temperature throughout the production process. In order to do this, simple and widespread laboratory equipments and procedures have been adopted, such as the Brookfield rotational viscometer, capable of ensuring the representativeness of the final product to be tested. After the optimization study, a comparison was made between the conventional and rheological properties of the optimized rubber–asphalt blend with those of two commercially available PMBs, modified with synthetic rubber type Styrene–Butadiene–Styrene, with two different level of modification: a medium and a high level of polymer, respectively termed as ‘‘Medium’’ and ‘‘High’’ in this study. This comparison allows one to appreciate to what extent control of the modification process is able to affect the properties of the final product and shows that the preliminary optimization of the mixing parameters carried out with the proposed protocol allows one to obtain a rubber–bitumen blend with improved performances, above all reduced temperature susceptibility, as proved by an increase in the Superpave™ high temperature performance grading, up to a two grade jump, which it is possible to obtain only for SBS modified bitumen with high content of polymer. Therefore, this study proves that, thanks to the appropriate re-use of crumb rubber form scrap tires, with the proposed optimization protocol it is possible obtain improved bitumen with high content of recycled materials that, with respect to specific needs of regions with warm climates such as the Mediterranean area, can be considered as ‘‘high-performance blends’’.