The performance of crumb rubber modified asphalt pavements (CRMAPs)
depends on the mix design and aggregate gradation. Even though numerous success stories
about the CRMAPs have been reported in the literature, inadequate design can lead to less
than desired performance. Understanding the factors contributing to the improved
performance CRMAPs over unmodified asphalt pavements is important for developing
appropriate designs and also for possibly expanding the limits of aggregate gradation. The
objective of this study is to quantify the internal structure of the crumb rubber modified
asphalt binders (CRMABs) made with the wet process. 3D geometry of crumb rubber (CR)
particles was directly measured and quantified using a high-resolution synchrotron based XRay
Microtomography (XRM) imaging system. The CRMAB samples were instantly frozen
using liquid nitrogen and kept frozen until processing. The 3D internal structure images of
these specimens were acquired using the XRM available at Argonne National Laboratory
(ANL). These images were analyzed and the change in the size of particles was directly
quantified. It was observed that the mixing process causes the crumb rubber particles to
partially melt and separate to create small CR ‘chips’. These small CR ‘chips’
homogeneously mix with the asphalt binder creating a polymer-like structure, which
improves their engineering (e.g., fatigue) performance.