The Thixotropy of WMA Binders and Its Influence on Flow and Fatigue Properties

Warm mix asphalt (WMA) additives have been increasingly adopted in recent years to achieve environmental benefits and improve construction-friendly paving practices. Most WMA additives significantly influence asphalt viscosity and flow behavior, which are closely related to the thixotropic behavior of asphalt binders. This study investigates the thixotropy of WMA binders and evaluates its effects on binder viscosity and fatigue resistance. Three representative WMA additives were examined: a wax-based additive (Sasobit), a surfactant additive (Evotherm), and a dilution-type additive [waste cooking oil (WCO)]. Their working mechanisms and microstructural characteristics were identified using Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). Thixotropy was quantified using the hysteresis loop test, while its influence on viscosity was characterized by the three interval thixotropy test (3ITT). In addition, a thixotropy-related phase associated with fatigue behavior was determined based on time sweep and strain sweep tests, and its correlation with fatigue life was analyzed using the linear amplitude sweep (LAS) test at different strain levels. The results indicate that thixotropy is closely associated with the internal structure of asphalt binders and exhibits a decreasing trend with increasing temperature. Above 100°C, all WMA binders investigated show negligible thixotropy, suggesting that thixotropy has a limited influence on mixture workability at temperatures higher than 100°C. At lower paving temperatures (i.e., 80°C), the Sasobit-modified binder exhibits the strongest thixotropy, resulting in the fastest and greatest reduction in viscosity under increasing shear rate, which may be beneficial for low-temperature compaction in the field. A strong correlation ( R 2 = 0.92 ) is observed between the thixotropy level (hysteresis loop area) and the thixotropy phase. WMA binders with stronger thixotropy exhibit longer fatigue lives at low strain levels (1%); however, this advantage diminishes rapidly with increasing strain level, and binders with high thixotropy show lower fatigue lives at high strain levels (10%).