Design and Performance Enhancement of Steel Fiber–Reinforced Rubberized Concrete Based on Rubber Gradation Index

Rubberized concrete (RC) is a recognized, environmentally sustainable building material with functional benefits. However, when the rubber content exceeds 20%, the strength will deteriorate significantly (>30%); thus, the adoption rate of waste rubber is limited (≤10% aggregate replacement). To address this issue, this study proposes a new “gradation optimization—interface enhancement” strategy to alleviate the significant strength degradation commonly observed in high rubber content concrete (HRC). A continuous gradation design method of a rubber particle gradation index (G) is proposed, where waste rubber particles simultaneously replace coarse and fine aggregates, optimize filler density, and improve the performance of the interfacial transition zone. Steel fibers are introduced to further compensate for the strength loss through their crack-bridging mechanism. The apparent density, slump, compressive strength, axial compressive strength, and splitting tensile strength of steel fiber–reinforced rubber concrete (S-HRC) were evaluated by combining orthogonal experiments with statistical analysis. The synergistic incorporation of graded rubber particles and steel fibers significantly enhanced the mechanical properties of HRC. When the rubber content is 25% and the steel fiber content is 0.75%, the compressive strength reaches 54% of that of conventional concrete. The splitting tensile strength of the optimal mix ratio (S0.75-HRC20-c) is 3.4% higher than that of ordinary concrete. Axial compressive strength is more sensitive to interface defects, with an attenuation coefficient of –96, whereas the cubic compressive strength attenuation coefficient is –86. A multifactor mechanical property prediction model was established, confirming that the compensation efficiency of steel fibers given confined compression has increased by approximately 25%. This comprehensive strategy enables the production of structural-grade lightweight concrete with significantly improved tensile and compressive properties.