Internal damage evaluation and remaining fatigue life prediction of asphalt pavements in highway widening projects

The longitudinal joint zone in highway widening projects constitutes a persistent structural vulnerability where subsurface damage preferentially accumulates beneath an apparently intact surface. Conventional Pavement Condition Index (PCI)-based assessment captures functional degradation but fails to resolve internal cracking states, creating a diagnostic blind spot where high PCI ratings coexist with substantial subsurface damage at widening joints. This study establishes an integrated approach combining field detection, targeted coring, laboratory characterization, and mechanistic modeling to quantitatively evaluate internal damage and predict remaining fatigue life. A constitutive model was formulated based on viscoelastic continuum damage theory and pseudo J-integral fracture mechanics, with damage density defined as the ratio of cracked area to total cross-sectional area. Field investigations were conducted on the G60 Shanghai–Kunming Expressway widening section (K304–K306), with a representative transverse cross-section selected for targeted coring across the joint. Laboratory tests including dynamic modulus and indirect tensile (IDT) fatigue characterized material degradation and calibrated damage evolution laws for joint-zone mixtures. Key findings are: (1) 3D-GPR reveals significant internal damage with cracks propagating upward from the joint interface between the existing and the widening pavement structures; however, diagnostic blind spots persist where severe internal cracking precedes surficial manifestation; (2) materials adjacent to the joint exhibit a dual nature: embrittlement in the AC-25 binder course, evidenced by high stiffness concurrent with low fatigue life, and reduced fatigue resistance in SMA-13 and AC-20 specimens, manifested by earlier onset of stable crack propagation; (3) damage density and remaining fatigue life exhibit sharp transverse gradients across the joint zone, with the base course serving as the critical layer: remaining life at the joint edge is merely 0.07 years under 2025 design-lane ESALs.