Energy-based mechanistic approach for crack growth characterization of asphalt binder

The cohesive cracking within asphalt binders has a significant influence on fatigue cracking resistance of asphalt pavements. To more clearly understand the mechanism and accurately characterize the process of the cohesive cracking occurring within the asphalt binder, an energy-based mechanistic (EBM) approach is applied to determine crack length and the pseudo J-integral is adopted to calculate crack growth rate in this study. First, a critical strain level separating nonlinear viscoelasticity from damage is determined based on a statistical analysis approach, and the result indicates that 0.7% is a critical nonlinear strain level for unmodified asphalt binder. Then, the crack length of asphalt binders is derived based on a torque equilibrium equation and two energy balance equations, as well as crack length is measured and verified based on an image processing method. It is found that contact regions in cracked area of the asphalt binder are formed when performing a strain-controlled rotational shear load. The contact regions have two stages, which first increase to the peak and then decrease with the increase of loading time. Next, the crack growth rate is formulated based on the pseudo J-integral Paris' law equation considering nonlinear viscoelasticity. A linear relationship between the crack growth rate and the function of material properties (such as shear modulus, phase angle) in double logarithmic scales is proven and experimentally verified. In addition, the Paris' law parameters (n. and A) associated with crack growth rate are determined. Results show that they are independent on strain levels and temperatures. For example, six values of n of the unmodified asphalt binder are approximately equal to 1.10 at 5%, 6%, 7% strain levels when test temperatures are 15 degrees C and 20 degrees C. They are inherent material properties for the asphalt binder.