Performance of Hybrid Crumb Rubber–Steel Fiber Concrete Incorporating Silica Fume and Metakaolin

Abstract

This study developed hybrid concrete incorporating crumb rubber (CR), micro-steel fibers (STF), silica fume (SF), and metakaolin (MK) to balance strength, stiffness, deformability, and energy absorption. Twenty-six mixtures were produced using 5–20% CR as a volumetric replacement for river sand, 0.5–2.0% STF by concrete volume, and 10% SF and 10% MK, each by cement mass. Workability, compressive strength at 7–96 days, splitting tensile strength, flexural response, modulus of elasticity, compressive and flexural toughness, and SEM–EDX characteristics were evaluated. The combined SF–MK addition increased the 28-day compressive strength from 70.43 to 84.20 MPa, an improvement of 19.55% over the plain control. Increasing STF enhanced strength, crack resistance, and post-cracking energy absorption, whereas CR reduced strength and stiffness but improved deformability. The mixture containing 1.5% STF and 10% CR achieved a balanced response, with a 28-day cube compressive strength of 77.65 MPa and flexural toughness of 80.71 N·m. The highest hybrid flexural toughness, 103.04 N·m, was obtained with 2.0% STF and 10% CR. SEM–EDX observations supported the mechanical trends. The novelty lies in demonstrating a combined contribution whereby STF and the SF–MK blend substantially compensate for CR-induced mechanical losses while preserving deformability, yielding structural-grade concrete with improved energy-absorption capacity.