Tensile Fatigue Damage Characterization of Cement-Stabilized Aggregates Subjected to Multilevel Loads

Characterizing the fatigue behavior of cement-stabilized aggregates (CSAs) is essential to semirigid base asphalt pavement design. However, the relevant research is relatively limited and retains significant challenges. Therefore, fatigue tests subjected to multilevel loads were designed, and a mechanical damage evolution rule was proposed to describe the CSA's damage behavior better. Four-point bending fatigue tests were conducted following the designed loading steps composing different cyclic stress levels and frequency combinations. The damage evolution patterns in sequential and disorder loading cases, together with the plastic strain accumulation trends, were analyzed to uncover the factors influencing CSA's damage evolution. It was found that the general damage evolution of CSA exhibited a three-stage pattern, and it was affected by the cyclic stress level, loading sequence, and history of plastic and damage evolution. Remarkably, loading frequency appeared to have a negligible impact. Plastic strain and damage evolution demonstrated congruent evolution trends in most cases; however, they differed in disorderly loading. To address these, a new damage evolution rule was proposed based on the continuum damage mechanics and driven by the equivalent plastic strain rate. The damage dissipation rate was introduced to characterize the impact of effective stress level, and the damage variable and plastic strain path were also included in the proposed rule to reflect the effect of loading or damage history. The comparison results between the fitted and measured damage evolution curves validated the effectiveness in characterizing the fatigue damage behaviors of CSA when subjected to multilevel loads. Furthermore, the proposed damage evolution rule was also employed to model the damage curves of CSA's uniaxial and indirect tensile fatigue tests with single-level loads. The commendable agreement between the fitting and experimental results confirmed the validity of the proposed rule and showed its broad applicability under different fatigue loading forms.