Analysis of bitumen–crumb rubber affinity for the formulation of rubberized dry mixtures

Crumb rubber derived from end-of-life tires is nowadays considered as a valuable component to be employed for the production of high-performance bituminous binders and mixtures. Currently available technologies require such a material to be either pre-mixed with bitumen (‘‘wet’’ method), with the consequent production of the so-called asphalt rubber binders, or to be added during the mixing of aggregates and bitumen (‘‘dry’’ method), thereby leading to the preparation of rubberized dry mixtures. Depending upon the case, different interactions take place between bitumen and crumb rubber, resulting in mechanical properties of the final mixture which can increase its performance in the field under the effects of loading and environmental factors. The laboratory investigation presented in this paper focused on the issue of bitumen–crumb rubber affinity with the main purpose of developing a simple laboratory procedure which can be employed to support the formulation of rubberized dry mixtures. Affinity between bitumen and coarse crumb rubber, with particle sizes comprised between 1 and 4 mm, was evaluated by subjecting mortars of known composition to the volumetric expansion test (VET). Measured volumetric behavior was compared to that of reference blends prepared with the same base bitumen and inert silica sand. Obtained results were interpreted by taking into account phenomena which may occur during testing at bitumen-particle interfaces, allowing a quantitative comparison between crumb rubbers of different types. Moreover, an affinity parameter was defined, with the consequent possibility of assessing bitumen–crumb rubber affinity in absolute terms.