Energy-based characterization of the fatigue crack density evolution of asphalt binders through controlled-stress fatigue testing

Fatigue crack behavior of asphalt binder plays an important influence on the fatigue performance of asphalt pavements. To evaluate the fatigue crack density evolution within the steady growth stage of #70 unmodified asphalt binder and SBS modified asphalt binder, the crack initiation and propagation were characterized based on Griffith theory and modified Paris’ law, respectively. Aiming to calculate the horizontal length of crack initiation (a), the surface free energy of those two asphalt binders were calculated by contact angle measured with Wilhelmy plate test. Also, the stress sweep tests and time sweep tests were performed using Dynamic Shear Rheometer (DSR) to obtain the mechanical parameters of asphalt binders for calculation of fatigue crack density (ϕ) and dissipated pseudo strain energy (DPSE). They were fitted by modified Paris’ law, and coefficients n and A were determined. Results show that a for two asphalt binders increase with the increasing stress level and SBS modified asphalt binder shows a larger crack size in crack initiation, which means that SBS modified asphalt binder shows a better crack initiation resistance compared with unmodified asphalt binder. The ϕ and DPSE within the steady growth stage follow the linear relationship, indicating that the unit DPSE causes the equal amount of increase in ϕ. In addition, the A and n are independent on the stress levels. They are fundamental material properties and there is a linear relationship between the log(A) and n. SBS modified asphalt binder processes the better fatigue crack resistance because of the smaller value of n.