Dynamics of filler particles in asphalt mastic under thermal cycling: A Nano-CT study of migration and rotation

Asphalt mastic is a concentrated particle-filled viscoelastic composite whose microstructure critically influences asphalt pavement mechanical behavior. Conventional models treat asphalt mastic as a homogeneous continuum, inferring thermal-fatigue behavior primarily from bulk rheology and leaving thermally driven particle dynamics and microstructural instabilities poorly quantified. In this study, accelerated thermal cycling is combined with high-resolution 3D Nano-CT and Voronoi analysis to track size-dependent filler kinematics (migration and rotation) and the evolution of packing heterogeneity. The results reveal a hierarchical three-tier dispersion architecture: Filling-Matrix-Skeleton, in which fine particles preferentially occupy and fill interstitial spaces, intermediate particles dominate the load-transfer matrix, and coarse particles constitute a load-bearing skeleton. Thermal cycling destabilizes the initially quasi-uniform dispersion, inducing size-selective redistribution consistent with the reverse Brazil nut effect (RBNE). Specifically, the coarse fraction (>30 μm) exhibits a net downward migration of 43-165 μm, while fine particles preferentially enrich the upper region. Voronoi analysis quantifies this structural degradation, revealing that the coefficient of variation for coarse-particle packing surged from 63.3% to 135.4%, marking a transition from near-uniform packing to through-thickness stratification. The resulting “coarse-bottom, fine-top” architecture comprises a load-bearing lower skeleton and a fine-rich upper crowding zone with enhanced agglomeration. These observations support a gravity-thermal ratchet competition framework for migration in highly viscous media and provide particle-scale evidence to inform durability-oriented material selection and microstructure-guided asphalt mastic design.