The adhesion-friction transition mechanism in asphalt pavement interlayers

Interlayer bonding plays a critical role in the structural performance and durability of asphalt pavements, yet the fundamental mechanisms governing its behavior under mixed-mode loading remain insufficiently understood. This study investigates the adhesion–friction transition mechanism in dense-graded asphalt pavement interlayers through a comprehensive experimental program incorporating shear-compression and shear-tension tests across multiple temperatures, stress levels, and surface morphologies. Three double-layered systems (AC-13/AC-20, AC-13/AC-16, AC-16/AC-20) were evaluated using custom-designed loading devices capable of applying combined shear, normal, and tensile stresses. High-resolution 3D interface morphology was quantified using four representative parameters, with the interlocking coefficient (IC) identified as the most effective descriptor of mechanical interlock. Results reveal a pronounced temperature-driven shift in dominant failure mechanisms: adhesion governs interlayer resistance at low temperatures, whereas friction and aggregate interlock increasingly control behavior as the binder softens. A quantitative adhesion–friction contribution decomposition model, developed using the Mohr–Coulomb framework, successfully separated and characterized these mechanisms. The adhesion contribution for the AC-13/AC-20 system decreased from 76.6% at 5°C to 26.8% at 50°C, demonstrating a clear transition from adhesion-controlled to friction-controlled failure. Under shear-tension loading, interlayer performance remained adhesion-dominated, with horizontal shear stress acting as a temperature-dependent pre-damage factor and surface morphology having limited influence. Statistical analysis (three-way ANOVA) verified the significance of temperature, normal stress, and IC in shear behavior, while tensile behavior was governed almost exclusively by temperature and pre-shear effects. Based on these mechanistic insights, a conceptual interlayer performance zoning framework is proposed, offering guidance for mechanism-driven, climate- and stress-condition-specific interlayer design.