A unified method for the analysis of nonlinear viscoelasticity and fatigue cracking of asphalt mixtures using the dynamic mechanical analyzer

Fatigue cracking is one of the primary modes of distress in asphalt pavements that has an important economic impact. Fatigue resistance characterization of an asphalt mixture is a complex issue due to: (i) composite nature of the material, (ii) gradation of aggregate particles, (iii) variation of asphalt film thickness, (iv) air voids distributions, (v) asphalt binder nonlinear viscoelastic behavior, (vi) effects of binder oxidative aging as a function of time, and (vii) micro crack healing during rest periods. Different methods to assess fatigue cracking in asphalt materials are available in the literature. However, there is no methodology to characterize fatigue cracking behavior of asphalt materials that is independent of the mode of loading (controlled-strain or controlled-stress). The objective of this research is to develop a new methodology to characterize fatigue cracking of the fine aggregate matrix (FAM) portion of asphalt mixtures using dynamic mechanical analyses (DMA). This is accomplished through different, but related, approaches. The first approach relies on identifying the various mechanisms of energy dissipation during fatigue cracking that are manifested in: (i) nonlinear viscoelastic deformation, (ii) fracture, and (iii) permanent deformation. Energy indices were derived to quantify each of these energy dissipation mechanisms and to quantify fatigue cracking irrespective of the mode of loading. The first outcome of the approach is a fatigue damage parameter (crack growth index) that provides comparable results for a given material even when tested under different modes of loading and different load (strain or stress) amplitudes. The developed fatigue characterization method has a lower coefficient of variation when compared to conventional parameters (number of load cycles to failure or cumulative dissipated energy). The crack growth index parameter was also qualitatively and quantitatively compared to three dissipated energy methods available in the literature. The second outcome of this research is a constitutive model that can describe both asphalt mixtures? nonlinear viscoelastic response and fatigue damage in one formulation. Nonlinear viscoelastic as well as damage parameters were obtained for both modes of loading. This second approach has the advantage that the constitutive model can be implemented in a numerical framework to describe the response of asphalt mixtures under various boundary conditions.