Sustainable self-compacting steel-fibre rubcrete: mechanical response and deep beam behaviour

Abstract This study experimentally evaluates sustainable self-compacting steel fibre-reinforced rubcrete (SCSFRR), incorporating recycled crumb rubber (up to 20% replacement of fine aggregates) and steel fibres (up to 0.75% by volume). Phase 1 assessed flowability/workability (slump flow, V-funnel, L-box, U-box) and hardened properties (compressive/splitting/flexural strengths, density, elastic modulus, brittleness index, and stress–strain behaviour). Phase 2 evaluated deep-beam performance under static and cyclic loading, focusing on load–deflection, shear ductility, and energy absorption. An exploratory analysis showed strong monotonic trends between compressive strength and density, tensile strength, flexural strength, and elastic modulus (R 2 ≈ 0.90–0.93). However, the limited dataset precludes design-level correlations. A novel shear ductility index is introduced to quantify improvements in deformation capacity due to crumb rubber and steel fibres. Results indicated that crumb rubber significantly enhanced ductility and energy absorption but reduced mechanical strengths, which steel fibres effectively mitigated. An optimal blend comprising 10% crumb rubber and 0.50% steel fibres achieved the best balance, increasing shear ductility by ≈ 22% and nearly doubling energy absorption while maintaining comparable strength to conventional concrete. A cradle-to-gate life-cycle assessment showed that replacing 20% of sand with rubber reduced embodied CO₂ per performance index by up to 42%. These findings indicate that SCSFRR can enable sustainable, resilient structural applications, particularly under seismic, cyclic, and low-rate dynamic loading.