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

a School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, 150090, China
b CERIS, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, 1049-001, Portugal
c China Academy of Transportation Sciences, Beijing, 100029, China

10.1016/j.partic.2026.02.018

Volume 111, April 2026, Pages 274-292

Received 8 December 2025, Revised 25 January 2026, Accepted 14 February 2026, Available online 6 March 2026, Version of Record 13 March 2026.

E-mail: xuhn@hit.edu.cn

Highlights

• A hierarchical 3-tier dispersion architecture (Filling-Matrix-Skeleton) was identified in asphalt mastic by Nano-CT.

• Thermal cycling destabilizes the uniform three-tier dispersed microstructure of asphalt mastic.

• Thermal cycling triggers size-selective filler particles redistribution consistent with a reverse Brazil nut effect.

• Voronoi analysis quantifies the shift from near-uniform packing to stratified, agglomerated morphology.

• A thermal ratchet-gravity framework parameterized by oscillatory Péclet number captures stratification evolution.

Abstract

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.

Graphical abstract

Keywords

Asphalt mastic; Thermal cycling; Nano-CT; Filler particle migration; Voronoi analysis; Reverse Brazil nut effect

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