Synergistic strengthening of RC beams under torsion using NSM GSWR and steel bars: Experimental and numerical investigation

Torsional failure in reinforced concrete (RC) members is often sudden and brittle because of the complex three-dimensional (3D) stress state involved. Conventional strengthening techniques employing closed stirrups may face practical limitations in fully confining beams along all sides, especially under torsion where critical stresses develop on all sides. To address this, a novel strengthening technique was proposed utilizing galvanized steel wire rope (GSWR), a soft and flexible material, which was then recovered with glass fiber-reinforced self-consolidating concrete (GFSCC) as a sustainable, corrosion-resistant, and flowable protective layer. The GSWR was implemented using a near-surface mounted (NSM) approach, offering both sustainability advantages (via galvanization) and ease of installation, especially for wrapping around 3D beam geometries. This study experimentally evaluated the effectiveness of GSWR-based NSM strengthening in improving torsional performance. A series of nine full-scale normal concrete beams was tested under isolated torsional moment to clearly study and evaluate the gains of key factors: wrapping orientation (vertical, inclined, and combined), reinforcement type (GSWR and/or steel bars), and reinforcement ratios ranging from 0.15 % to 1.43 % . The results revealed that GSWR significantly upgraded torsional capacity, stiffness, and energy absorption, while also delaying crack initiation and controlling brittle failure risks. Beams with vertically wrapped GSWR at a ratio larger than 0.29 % exhibited a 95 % increase in torsional strength over unstrengthened defective beams and 58 % over the control beam. The most pronounced enhancement occurred in beams with 60° inclined GSWR wrapping, showing a 141 % increase in ultimate capacity. Furthermore, GSWR confinement of horizontal steel bars had a synergistic impact, outperforming each reinforcement strategy employed independently. To supplement the experiments, a robust 3D finite element model (FEM) was developed, capturing the plastic torsional response and enabling a parametric analysis of the proposed configurations. The outcomes confirmed the practicality and effectiveness of GSWR-NSM regimes for retrofitting RC members under torsional moment.