Investigating Stress-Induced Morphology in Sand–Rubber Mixtures Using X-ray CT

The increasing accumulation of End-of-Life Tyres (EoLT) and the associated disposal challenges highlight the urgent need for their recycling and reuse. One such reusing application could be incorporating EoLT as a partial replacement for sand in pure sand mixtures in geotechnical and transportation engineering applications. Sand–rubber mixtures offer some advantages owing to their superior flexibility and energy absorption capacity. The macroscopic response of these mixtures is intrinsically linked to their evolving microstructure or fabric under loading. Among various investigative techniques, X-ray micro-computed tomography (µCT) is widely used to examine internal fabric changes. Although X-ray µCT provides direct visualisation of particle-scale interactions, achieving accurate phase segmentation remains a significant challenge in these multi-phase mixtures. This study investigates the microstructural evolution of unbound sand–rubber mixtures subjected to one-dimensional compression loading using X-ray µCT imaging. The mixture contains equal volumetric proportions of sand and rubber. The surface area and volume distributions of these constituent particles were computed to capture phase-specific morphological transformations across a wide range of stress levels. To enhance segmentation accuracy, a customised image processing pipeline was implemented, incorporating conflict region identification and morphological operations like binary dilation. The particle size distribution (PSD) obtained from the image analysis corresponding to the initial (0 kPa) condition closely matched mechanical sieve analysis data, affirming the robustness of the imaging approach. The response of rubber and sand particles under increasing stress levels reveals contrasting behaviours. Rubber particles tend to deform and move closer to each other, eventually behaving like a unified mass. This is reflected by an increase in their surface area and volume distributions at higher stress levels. In contrast, sand particles also experience compression but undergo fragmentation at higher stresses (1280 kPa and 2560 kPa), leading to a reduction in their surface area and volume.