Enhanced Mechanical Behavior of Rubberized Concrete with Circular FRP–Steel Double-Skin Confinement

Abstract The accumulation of waste tire rubber in landfills, water bodies, and its open burning significantly contributes to environmental pollution and public health risks, indicating the need for effective mitigation strategies. Use of waste rubber as a partial replacement for natural aggregates in concrete, i.e., rubberized concrete (RuC), has potential for ecofriendly construction. However, RuC, in comparison to normal concrete, exhibits a major reduction in mechanical properties and shows a marginal increase in ductility, energy absorption, and impact resistance, thereby limiting its suitability for structural applications. This study investigated the behavior of fiber-reinforced polymer (FRP) confined rubberized-concrete (FRuC) sandwiched between outer FRP and inner steel skins in a double-skin tubular column (DSTC) configuration, a system that effectively makes RuC suitable for structural applications. Experiments were performed on 12 cylinders and 12 circular rubberized-concrete DSTCs (i.e., RuDSTCs) having variations in rubber replacement ( R v ) of fine aggregates by volume (0%–30%) and FRP-tube thicknesses (4 and 6 mm). The FRuC axial stress–strain/hoop–strain curves, failure modes, ductility, and strength characteristics were examined and compared with RuC under compressive loading. Test results showed that at R v = 30 % , the axial strength capacity of unconfined and confined concrete decreased by approximately 43% and 28%, respectively. However, confinement provided a strength gain of approximately 26% relative to the unconfined strength at the same rubber content. The confined-to-unconfined concrete strength ratio increased from 3.59 to 4.49 (25% increase) for R v of 0% and 30%, respectively. Hence, RuC in an FRP–steel double-skin configuration provides a sustainable concrete solution with notable strength recovery, making it suitable for demanding structural applications. Finally, a modified empirical equation is proposed for estimating the confined concrete strength ( f c r c ) of FRuC in RuDSTC, having exclusive parameters accounting for the effects of the R v for fine aggregates in concrete. A comparison of predicted versus experimental results showed excellent agreement and accuracy of the proposed empirical formulation.