Rough contact mechanics of particles with a crushable coating: Experimental investigation of recycled concrete aggregate

Many granular particles have a heterogeneous structure in which a softer coating overlies a distinct core, yet most contact mechanics frameworks treat them as homogeneous elastic–plastic solids. This paper examines how a crushable coating modifies the contact response of recycled concrete aggregate (RCA) and assesses classical contact models. RCA particles from three parent materials — cement mortar, basalt and red brick — were tested using inter-particle loading and laser scanning confocal microscopy, with dilute-HCl washing used to substantially remove the accessible cement mortar coating. In normal loading, the first-loading contact stiffness is approximately one order of magnitude lower than the Hertz prediction. The Greenwood roughness parameter (α = 0.7–2.0) indicates non-negligible roughness effects; however, its correction reduces the model–measurement discrepancy only at small displacements. Thus, asperity crushing and the coating's plastic compaction/crushing remain important during first loading. Initial tangential stiffness increases after washing for cement and basalt but decreases for red brick. Friction behaves differently: the coating unifies the friction coefficient to 0.60–0.70, whereas washing causes divergence — unchanged for basalt but markedly higher for red brick — indicating that stiffness and friction are governed by different mechanisms. The Mindlin–Deresiewicz model significantly overestimates tangential stiffness. Surface characterisation reveals the coating's dual roles that unify these observations: softening over dense parent materials and filling porous red brick. These findings support a two-stage shell–core compliance interpretation not captured by homogeneous contact frameworks: coating-related deformation strongly influences first loading, whereas the parent material becomes more prominent during reloading.