| Since the mid-1960s, crumb rubber has been used to modify asphalt binders. The crumb rubber, also referred to as crumb rubber modifier (CRM), is produced by grinding scrap tires into fine powders with particulate sizes generally smaller than 2 mm. In most cases, CRM is incorporated into binder using the "wet" method, where the crumb rubber is blended with the asphalt binder. The binder modification that occurs during this blending process is physical in nature, where the lighter oils of the binder diffuse into the CRM particles. As the rubber particles absorb the oils, the particles swell; therefore, increasing the viscosity and stiffness of the CRM binder.; This research evaluated the viscosity and complex shear modulus (G*) of 36 different laboratory-produced CRM binder combinations (two CRM processing methods, two CRM contents, three CRM sizes, and three binder sources) and three base binders. From this investigation, a method was developed to quantify the effects of the crumb rubber modification. These effects were identified as the interaction effect (IE) resulting from the diffusion of the lighter binder factions into the rubber particles and the particle effect (PE), which is the result of the change in the rheological properties due to the filling effect of the particulate CRM.; Following statistical analyses of the results, it was determined that CRM binders had greater viscosity and complex shear modulus (G*) values than the base binders. These values increased with CRM content. The CRMs having higher surface area (i.e., ambient ground and finer CRM) generally had higher IE values, while the PE generally increased with particle size. HP-GPC testing indicated that the CRM binders also had higher large molecular size (LMS) fractions and lower small molecular size (SMS) fractions as determined from the HP-GPC profiles. As the LMS increased and the SMS decreased, the IE increased. Two of the three binder sources followed this trend, indicating that those sources were more compatible with the CRM, thus verifying the diffusion phenomenon. |
