Effect of rubber particles on compressive strength and deformation properties of concrete: Experimental investigation and DEM simulations

To meet the requirements of flexible load-bearing in underground engineering, the incorporation of rubber and fibers into concrete combined with the optimization of aggregate gradation to enhance the deformability and energy absorption capacity of gob-side backfill bodies is of great research significance. In this study, 0.5% polypropylene fibers (PPF) and 1% steel fibers (SF) were added to normal concrete, while 10-mesh and 20-mesh rubber particles were used to partially replace fine aggregates at different replacement ratios to produce Hybrid fiber-reinforced rubber concrete (HFRC). Uniaxial compression tests, digital image correlation (DIC) monitoring, scanning electron microscope (SEM) observation, and discrete element method (DEM) simulations were conducted to systematically investigate the mechanical behaviors, crack propagation characteristics, and energy evolution mechanisms of HFRC. The results indicate that with increasing rubber content, the compressive strength, elastic modulus, and Poisson’s ratio gradually decrease, whereas the peak longitudinal strain first increases and then decreases. HFRC incorporating 10-mesh rubber particles exhibits superior overall mechanical performance, which is characterized by narrower cracks and more significant fiber pull-out and bridging effects, with a threshold replacement ratio of approximately 30%. When the rubber content reaches 40%, significant mechanical deterioration occurs for both rubber particle sizes, and the particle-size effect becomes negligible. DEM analyses further reveal that 10-mesh rubber HFRC maintains better matrix integrity during the elastic stage but undergoes accelerated force-chain attenuation after peak stress. At peak strength, the input and dissipated energies of HFRC are approximately 4.7–4.9 and 2.2–2.3 times those of normal concrete, respectively, demonstrating a markedly enhanced energy absorption capacity. These findings provide theoretical insights into the deformation and energy dissipation mechanisms of HFRC and offer practical guidance for the design and application of deformable backfill materials in flexible support systems for underground engineering.