ABSTRACT
The parameter or coefficient gamma obtained from the E* master curve has been shown to relate to the width of the relaxation spectra and is related to the slope of the master curve. Incorporation of aged binders or binders with hardening modifiers to a mixture leads to an increase in gamma, which corresponds to a flattening of the master curve (lower susceptibility to frequency changes) and widening of the relaxation spectra. For a standard logistic function, the inflection point modulus (-beta/gamma) is controlled by the aggregate structure and volumetric effects. At high temperatures, the aggregate structure begins to dominate the behavior of the mixture, while volumetric properties and binder stiffness control the behavior at lower temperatures. Incorporation of aged binder or hardening modifiers to a mixture leads to a shift of the inflection point to lower frequencies. In this study, 15 asphalt mixtures containing Recycled Asphalt Shingles (RAS) where tested to evaluate their cracking susceptibility in the laboratory. Besides mixture characterization under dynamic modulus test protocols, the laboratory testing program evaluated recovered binder performance grade and fatigue cracking laboratory performance tests including Bending Beam Fatigue test, Energy Ratio, Overlay Tester and Fracture Energy (Flexibility Index test). The results of this study indicated good relationships between the master curve inflection point with the flexibility index and the dissipated creep strain energy (DCSE, from Energy Ratio testing). These results suggested that as the mixtures become more susceptible to cracking (lower Flexibility Index) and become more brittle (lower DCSE) the inflection point of the master curve decreases (due to stiffening of the binder in the mixture). In addition, when all the mixtures where ranked from top to bottom cracking susceptibility, the master curve inflection point agreed well with all the other cracking test results.
