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Two moisture damage models based on major moisture failure mechanisms are proposed. The adhesion failure model was developed to analyze the adhesive fracture between asphalt and aggregate in the presence of water. Cohesive and adhesive fractures in an asphalt-aggregate system are directly related to the surface energy characteristics of asphalt and aggregate. The surface energy of adhesion with or without the presence of water can be calculated from the surface energies of asphalt and aggregate. A moisture diffusion model was developed based on the one-dimensional consolidation of soil and a gas adsorption model. The moisture diffusion model was used to obtain the moisture diffusion characteristics of asphalt binders, including the amount of moisture that can permeate a binder and the diffusivity of the binder. The amount of moisture that permeates a binder is identified as a key factor in the moisture damage. Finally, mechanics-based experiments conducted on asphalt mixtures validated the results from the adhesion failure and diffusion models.

The traditional state-surface approach to the study of unsaturated soil behavior is becoming much less popular these days, as it uses unique constitutive surfaces to represent unsaturated soil behavior. This approach is essentially a nonlinear elastic formation and cannot be used to explain complex stress-path dependency for unsaturated soils. In this paper, a modified state-surface approach (MSSA) is proposed to represent unsaturated soil behavior under isotropic stress conditions in which a conventional void-ratio state surface is considered to be made up of an elastic surface and a plastic hardening surface. The plastic hardening surface remains stationary at all times, whereas the elastic surface remains unchanged when the soil experiences elastic deformation and moves downward when there is plastic hardening occurrence. Using the MSSA, the loading–collapse (LC) and the suction increase (SI) yield curves in the Barcelona basic model (BBM) are derived. The prediction of three typical cases of soils under isotropic conditions and experimental results using the proposed approach confirmed its feasibility, simplicity, and potential for the study of unsaturated soil behavior.

The Hamburg wheel tracking test (HWTT) has been widely used as a standard laboratory test to evaluate the moisture susceptibility and rutting resistance of asphalt mixtures. The stripping infection point and the rut depth at a certain number of load cycles are two common parameters obtained from the test. Although these parameters have been widely adopted by several transportation agencies, the accuracy and variability in characterizing mixture properties of these parameters have been questioned. In this study, a novel method to analyze the HWTT results is introduced and three new parameters are proposed to measure the moisture susceptibility and rutting resistance of asphalt mixtures. The new parameters are compared against the current ones to assess their capability to discriminate between three types of asphalt mixtures with different performance results in the HWTT. Significant advantages in characterizing mixture resistance to stripping and rutting are demonstrated by the new parameters. In addition, the effect of antistripping additives and recycled materials on mixture performance in the HWTT is evaluated with mixtures from a field project in Texas. Test results for the new parameters show that the addition of antistripping additives improves the susceptibility of asphalt mixtures to moisture. Specifically, the use of lime is more beneficial for improving mixture performance than a liquid antistripping agent. Conversely, the addition of recycled materials provides mixtures with increased moisture susceptibility but improved rutting resistance.