Multi-Length-Scale Investigation of the Fatigue Behavior of Bituminous Composites: Experimental Approach

The fatigue behavior of bituminous composites was investigated at different length scales—bitumen, mastic, mortar, and asphalt mixture. The stress sweep test, as the initial test, and the dynamic modulus test, as the primary fatigue test, were conducted on the bituminous composites at three specific stress levels and two loading temperatures of 10°C and 25°C. The fatigue results reveal crucial parameters such as structural integrity (C), damage state (S), and average rate of total released pseudostrain energy (GR) based on the simplified viscoelastic continuum damage. Moreover, novel fatigue indices, namely the fatigue number (FN) and secondary stage gradient (SSG), are introduced using piecewise functions. The study reveals that during the initial fatigue stage, upper scales’ C-curve degrades rapidly due to factors such as thixotropy and self-heating. The power magnitudes of the C-S equation are 0.23, 0.32, 0.54, and 0.63 for bitumen, mastic, mortar, and the mixture, respectively. This indicates an escalated rate of fatigue cracking at upper scales in the second and third stages. This escalation is attributed to a stiffer matrix and higher inclusion volume, resulting in increased stress concentration through upscaling. Furthermore, the equation describing GR-fatigue life indicates a shift toward more brittle failure as scale increases from bitumen to the mixture. The multi-length-scale assessment demonstrates the precise predictive capability of the proposed criteria, FN and SSG, for the second and third stages of fatigue, respectively, whereas a 50% reduction in initial modulus may not offer reliable responses. These experimental findings provide valuable insights for interpreting the fatigue behavior of bituminous composites in numerical simulations in future studies.