Measuring and Defining Fatigue Behavior of Asphalt Binders

Fatigue damage is a distress mechanism observed in asphalt, particularly at moderate to low temperatures. Preliminary studies have shown that unmodified asphalts are sensitive to fatigue and that the use of modifiers in asphalt binders can dramatically improve the binder’s response to fatigue. One of the major challenges encountered has been the lack of a definition of fatigue failure consistent with the actual performance of the material regardless of testing conditions. Superpave® asphalt binder specification has improved the evaluation of modified and neat asphalts, but the definition of failure for fatigue damage is still unclear. A selected set of unmodified and modified binders was chosen and tested under a range of loading modes, stress or strain amplitudes, temperatures, and frequencies. The fatigue data were analyzed using the dissipated energy ratio concept. Np, the number of cycles to crack propagation, was used as the fatigue criterion for the analysis. Using the initial dissipated energy per cycle ( Wi) as the main independent variable for modeling fatigue of binders appears to be a promising technique to normalize some of the testing conditions. The parameter Np20, defined as the number of cycles at which the dissipated energy ratio shows 20% deviation from the no-damage ratio, appears to be a promising parameter to define failure. Using Np20 values, all modification methods used showed improvement in the fatigue behavior of unmodified asphalts. The level of improvement, however, was highly dependent on the modifier type and the testing conditions. Initial dissipated energy, testing frequency, and temperature were found to be important factors. If damage parameters are used in future specifications of binders, testing frequency and testing stress or strain levels should be carefully selected to represent pavement structural conditions and traffic speed.