Structural fatigue response of porous ultra-thin asphalt overlays across concrete joints under coupled joint-opening and vertical disturbance

Reflective cracking in ultra-thin asphalt overlays results from repeated deformation of underlying pavement joints. This study investigated fatigue damage under coupled low-frequency horizontal and high-frequency vertical displacement loading using composite specimens comprising an asphalt overlay and two cement slabs with a reserved joint. Fatigue performance was evaluated through initial horizontal force, normalised force degradation, and fatigue life. Increasing horizontal displacement reduced fatigue life from 5765 to 336 cycles, while increasing vertical displacement reduced it from 1330 to 90 cycles. Overlay thickness increased initial force (R² = 0.9853) but had no significant effect on fatigue life under displacement-controlled loading. Crack sealing, anti-crack tape, and their combined application improved fatigue resistance, with the combined treatment achieving the longest fatigue life of 1467 cycles. These findings demonstrate that coupled horizontal and vertical displacement accelerates fatigue degradation, while overlay thickness and joint treatments affect initial resistance and fatigue development through different mechanisms.