Two-Stage Fatigue Life Decoupling and Reflective Crack Retardation Mechanisms of Interlayer Systems in Rigid–Flexible Composite Beams

For rigid-flexible composite pavement structures, traditional macroscopic assessment of crack-resistant interlayers is typically based on total fatigue life alone, without differentiating between crack propagation through the rigid substrate and crack arrest behavior at the modulus-mismatched interface. To fill this research gap, the present work characterizes the stage-specific crack retardation mechanisms of three distinct interlayer systems. Pre-notched composite beam specimens with dimensions of 300 mm × 100 mm × 100 mm were fabricated, each composed of a 50 mm C40 concrete base layer, a spray-applied interlayer coating (epoxy emulsified asphalt [EEA], SBS-modified asphalt, or rubberized asphalt), and a 50 mm SMA-13 asphalt mixture overlay. Static and cyclic three-point bending tests were conducted on these specimens across a range of test conditions: temperatures from −10 °C to 45 °C, loading frequencies of 1–15 Hz, and stress levels of 0.3–0.5. Using a combined approach of load–deflection hysteretic energy dissipation analysis and a cyclic J-integral (∆J) modified Paris law, the total fatigue life was resolved into two separate stages: crack propagation in the concrete base (N_c) and crack propagation across the interface into the overlay (N_a). The test results identify optimal interlayer binder dosages of 0.8 kg/m² for both EEA and SBS-modified asphalt, and 2.0 kg/m² for rubberized asphalt. EEA exhibited the highest peak static load capacity of 12.6 kN, but was followed by abrupt brittle fracture once the peak load was exceeded. Rubberized asphalt, by contrast, retained full structural integrity even at large mid-span deflections up to 7 mm. According to the two-stage fatigue life analysis, the crack propagation life in the concrete substrate (N_c ≈ 35,000 cycles) is entirely unaffected by the type of interlayer material. All divergence in anti-cracking performance occurs after the crack reaches the interface: rubberized asphalt prolonged N_a to more than 120,000 cycles, with a total cumulative dissipated energy of roughly 6000 J. These findings verify that flexible interlayer coatings suppress reflective cracks mainly via large-deformation deformation coordination and hysteretic energy dissipation at the post-interface stage, instead of altering the fracture behavior of the underlying rigid base layer.