CHARACTERIZATION OF SIMPLE AND COMPLEX CRUMB RUBBER MODIFIED BINDERS

Abstract

This research focused on the evaluation of using crumb rubber modifier (CRM) in asphalt binders. The increasing usage of CRM in asphalt pavements requires a better understanding of the physical, chemical, and rheological properties of CRM so that its performance in the field can be more accurately predicted. The existing literature regarding CRM in asphalt binders has been focused on using either conventional binder testing procedures or the Superpave binder testing protocol. It is believed that Superpave binder specifications cannot be extended to all modified binders, since certain simplifications are used in the current specification that cannot estimate their contribution to pavement performance under different traffic and pavement structure conditions. Therefore the benefits of using crumb rubber for modification of asphalt needs more detailed evaluation using an extended set of testing methods. This report shows the results of the binder testing and analysis of these results to quantify the effects of size and concentration of CRM on critical binder properties related to construction and performance of hot mix asphalt in pavements. The evaluation was conducted by measuring the change in performance-related properties of the selected asphalt binders as a result of mixing various levels of CRM content and particle with asphalts. Testing was conducted at different temperatures, testing frequencies, and strain conditions using methods recently developed as part of the NCHRP 9-10 project (Superpave Protocols for Asphalt Binders). Statistical analysis was used to develop models that predict the nature of effects on the performance-related properties of asphalts and that can quantify the role of different variables. The results of the research indicate that the inclusion of crumb rubber does affect some of the properties of the asphalt binder, and that the effects differ based on the size and concentration of crumb rubber added. It was found that the viscosities of asphalt binders increase when there is an increased concentration of crumb rubber, and when the size of the crumb rubber used is decreased. It was also found that crumb rubber modified asphalt binders have increased dependency on mechanical working, higher strain dependency, increased strain at failure, and increased stress at failure. On the mixture end, it was found that the belief that crumb rubber modification of asphalt binders would result in a rebound effect, or increase in mix volume, was unfounded. The presence of crumb rubber modifier did affect the amount of air voids and the frictional resistance of a mixture. The effects were, however, dependent on the stage of densification, and were also influenced by other material characteristics in some cases.