Evaluating fracture properties of crumb rubber modified asphalt mixes

Discarded vehicle tires pose a severe environmental problem in the United States. Incorporating shredded tire rubber as crumb rubber modifier (CRM) into asphalt paving materials has shown potential for great benefits. This study was undertaken to evaluate the effect of CRM, base virgin binder stiffness, total binder content, and aggregate gradation on fracture performance of CRM asphalt mixes using the semi-circular bend (SCB) fracture test. All mixes were long term oven aged (LTOA) before testing at the effective temperature for cracking resistance. The results were analyzed within the context of several response parameters: a modified version of Illinois flexibility index (FI), fracture energy, and the load at failure (peak load). Fracture performance of virgin and CRM binders were also evaluated using linear amplitude sweep (LAS) test and the Glover-Rowe (G-R) parameter obtained from dynamic shear rheometer (DSR). As expected, adding CRM into asphalt mixes increased the design binder content. In addition, design binder content increased as the CRM content increased in the mix. Fracture test results showed that among dense graded mixes, CRM mixes with the highest binder content showed superior flexibility and cracking resistance compared with those with lower binder content, in spite of the fact that CRM mixes delivered lower peak load (i.e. strength). It was also shown through this study that gap graded CRM mixes are preferred to dense graded CRM mixes as they deliver higher flexibility thus better cracking resistance. Finally, results from the binder tests demonstrated superior cracking resistance of CRM binders compared with virgin binders.