Investigation of cracking mechanisms in rubber-modified asphalt through fracture testing of mastic specimens

Rubber modification of asphalt leads to improvement in certain asphalt mixture properties in lab studies, and extension of service life has been reported in a number of field studies. However, modern laboratory cracking tests often produce conflicting results in terms of the expected cracking performance of rubber-modified asphalt mixtures. To better understand cracking/toughening mechanisms in rubber modified asphalt materials, a fracture mechanics based test was devised for various unmodified and modified binder systems (unmodified control, polymer- and rubber-modified at various treatment levels). The results of this testing showed that unmodified and polymer-modified specimens failed in a brittle fashion, while the rubber-modified specimens displayed increasing ductile-type failures as the rubber modification level increased. Analysis of the fractured faces of the specimens under a Scanning Electron Microscope also revealed the presence of characteristic tails and cavities in the rubber-modified specimens associated with a crack-pinning-type toughening mechanism.