Saturation evolution induced by suction effect in asphalt mixtures during air-water two-phase seepage

The unsaturated service state of asphalt pavements caused by air-water two-phase retention and flow is the intrinsic cause of moisture damage and durability deterioration of the asphalt mixture. However, the mechanism by which matric suction, as one of the key driving forces, affects the retention and flow behavior of unsaturated asphalt mixtures has not been understood. Hence, the objectives of this study were to (1) investigate the saturation evolution induced by the suction effect in asphalt mixtures during air-water two-phase seepage and (2) clarify the characteristic relationship between equilibrium saturation and matric suction. Three different types of asphalt mixture were produced in the laboratory. An air-driven water replacement experiment with a controlled suction gradient was conducted to describe the evolution of water saturation induced by different matric suction conditions. The results showed that, at a specific suction level, the evolution of water saturation exhibited two stages: a rapid drainage phase and a dynamic stabilization phase. On this basis, a sensitivity analysis using the Friedman test revealed that the constituent materials (binder and aggregate types) contributed slightly to desaturation, while the void structure characteristic was the main factor influencing desaturation in asphalt mixtures. In a stable unsaturated state, the curve of equilibrium saturation and suction indicated that high matric suction meant low saturation and water retention capacity of the asphalt mixture. For the same matric suction, the densely graded asphalt mixtures exhibited a higher overall water retention capacity than the open-graded friction courses. For the same mixture types, the water retention capacity at high void contents exhibited a more significant sensitivity than that at a low void content. These results contribute to reasonably evaluating the unsaturated hydraulic characteristics in asphalt mixtures, and these characteristics indirectly guide the selection of asphalt pavement materials and structures in areas with frequent dry-wet cycles and freeze-thaw cycles, as well as the measures to prevent moisture damage.