Research on the graded assessment method for structural cracks in semi-rigid base pavements based on radar signals and deflection responses

Semi-rigid base asphalt pavements are prone to structural cracks propagating upward from the base course. Existing assessment methods fail to comprehensively quantify the full crack evolution process, making it hard to grade crack severity and prioritize maintenance across distress locations. To address this issue, this study combines Discrete Element Method (DEM) simulation, Ground Penetrating Radar (GPR) detection, and Falling Weight Deflectometer (FWD) testing to investigate the whole evolution process and graded assessment of structural cracks.Through PFC-GprMax joint modelling and field coring verification, a nine-stage evolution pattern of structural cracks is identified, covering initial base cracking, upward propagation to surface penetration, and subsequent branch cracking and interlayer damage. The response characteristics of radar time-domain variance (P4), skewness (P18), deflection range, and deflection curve area during crack deterioration are determined. On this basis, quantitative relationships between structural performance degradation and multi-source indicators are established. Three-level grading thresholds (mild, moderate, severe) are defined, and the Structural Cracking Grading (SCG) method is proposed. Field cases show that this method can characterize the relative severity of different distress locations, providing a quantitative basis for crack severity grading, maintenance prioritization, and rational resource allocation.