Study of Factors Influencing Moisture Susceptibility of Warm-Mix Asphalt Using the Surface Free Energy Approach

The application of warm-mixing technology brings considerable economical and environment benefits by decreasing the mixing temperature during warm asphalt mixture (WMA) production. However, the possible water residue also generates concerns for moisture susceptibility. For deep investigation on the influencing factors and mechanisms of the moisture susceptibility of WMA, surface free energy (SFE) tests and laboratory tests are applied in this research. A novel indicator based on SFE, namely, effective adhesion work, is proposed to assess the asphalt–aggregate adhesion with different moisture contents. Then, given the mixing procedure of the dry-mixing method, an advanced three-phase model as a form of asphalt–aggregate-warm mixing additive is introduced, improving the conventional two-phase asphalt–aggregate model for better reflecting the separate addition of warm-mixing additives during mixing. Afterwards, the influence of aggregate type, asphalt type, aggregate moisture content, warm-mixing agent type, and the warm-mixing process on the moisture susceptibility of WMA is analyzed utilizing the models and indicators proposed. Finally, the validity of the SFE indicator is verified by comparing the calculation of effective adhesion work with freeze–thaw splitting test results. The results show that all of the above factors impact the moisture susceptibility of WMA by influencing the interfacial adhesion, with the effect of moisture content being the most significant. Meanwhile, effective adhesion work and the three-phase model brought out in this research are proven to be feasible to characterize the adhesion properties of WMA, offering theoretical support to the research on warm-mixing technology.