Mesostructure-informed balanced gradation design of rubberized asphalt concrete for dam-face seepage control: Skeleton optimization and multi-performance enhancement

Conventional gradation design for hydraulic asphalt concrete does not explicitly control coarse-aggregate skeleton formation and internal load-transfer paths, limiting the coordinated optimization of multiple performance requirements for dam facings. To address this limitation, a two-stage mesostructure-informed gradation design method integrating Particle Flow Code (PFC) simulation, X-ray computed tomography (CT) characterization, and systematic performance tests was developed. PFC was first used to optimal coarse-to-fine aggregate ratio based on particle packing and force-chain uniformity, followed by CT-based optimization of the internal coarse-aggregate fractions through contact characteristics, spatial uniformity, and compaction orientation. The results show that a 70:30 coarse-to-fine aggregate ratio achieves the densest packing and the most uniform force-chain distribution.Compared with the specification-recommended hydraulic asphalt concrete gradation, the optimized gradation reduces the strength contribution of mortar-mortar contacts by 21.62% and increases that of aggregate-mortar contacts by 65.63%, reconstructing the internal load transfer network from mortar-dominated bearing to aggregate-mortar collaborative bearing. A 3:1 mass ratio (G2 gradation) of 13.2–9.5 mm to 9.5–4.75 mm aggregates produced the most favorable mesostructure. The G2 gradation increased residual Marshall stability and low-temperature flexural failure strain by 3.75% and 4.56%, respectively, while reducing the 70 °C slope-flow value by 17.84%. These findings confirm that rationally designing mixture gradation based on quantifiable mesostructural parameters can provide a replicable reference for the proportion design of hydraulic asphalt mixtures.