Microstructural Investigation of Skeleton-Reinforced Thin Asphalt Overlay Using the Discrete Element Method

Premature skid resistance deterioration is a critical issue limiting the long-term performance of thin asphalt overlays. To elucidate the meso-scale degradation mechanisms, this study employs the Discrete Element Method (DEM) implemented in Particle Flow Code (PFC) to compare a conventional Stone Matrix Asphalt (SMA-10) mixture with an optimized skeleton-reinforced design, termed Optimized Gradation-10. The optimized gradation was developed by introducing supplementary sieve sizes of 5.6, 6.7, and 8.0 mm within the critical range of 4.75–9.5 mm following the V–S gradation framework. Cross-sectional images of actual mixtures were vectorized using Python (Version: 3.11.5) and MATLAB (Version: R2024a) to reconstruct irregular aggregate clump models that accurately capture particle morphology and spatial arrangement. Meso-scale parameters were calibrated and validated through uniaxial compression tests, and the evolution of contact number, contact force, and stress transmission was analyzed under 2.3 × 105 wheel load cycles. Compared with SMA-10, the optimized mixture increased effective aggregate contacts by 41.8%, enhanced stress transfer efficiency by 19.8%, and reduced rut depth by 10%. These findings confirm that synergistic gradation optimization through supplementary sieves and the V–S method markedly improves structural stability and deformation resistance, providing a meso-mechanical foundation for prolonging skid resistance in thin overlays.