Crumb rubber-modified (CRM) asphalt has exhibited beneficial properties under laboratory and field conditions, including enhanced rut-resistance, greater crack-resistance, and delayed aging (oxidation) compared to virgin asphalt mixtures. However, a new requirement for phase angle has been recently been adopted by many Departments of Transportation, including the South Carolina Department of Transportation (SCDOT). Phase angle is a relative measure of recoverable and non-recoverable deformation – the closer the phase angle is to zero, the more recoverable deformation the binder has, while the closer the phase angle is to 90, the more non-recoverable deformation the binder has. Initial reviews of existing CRM binder data show that CRM binders are not reliably meeting the new criteria for phase angle. This has led to the questions of what factors may be causing elevated phase angle values in CRM asphalt binders and whether or not phase angle is a reliable benchmark criteria for evaluating CRM binders (i.e., creep stiffness from bending beam rheometer testing may be a better measurement of CRM binder performance than phase angle).

This project will evaluate the effect of several variables on binder performance of CRM asphalt binders. The variables will include:

• CRM particle size
   o Nominal #40 mesh
   o Nominal #16 mesh
• Amount of CRM
   o 10%
   o 15%
   o 20%
• Reaction temperature
   o 350 °F
   o 335 °F
   o 325 °F
• Blending process
   o Adding rubber at a lower temperature and then raising       temperature to the desired temperature for blending
   o Adding rubber at the desired blending temperature and then       blending immediately