Experimental assessment of CFST beam-columns strengthened with CFRP grid-reinforced ultra-high-strength engineered cementitious composites

Due to their favorable mechanical properties, concrete-filled steel tubular (CFST) members are widely used in infrastructure. However, their long-term durability may be compromised by environmental factors such as seawater erosion and chemical corrosion. This study therefore proposes an approach for strengthening CFST members using ultra-high-strength engineering cementitious composites (UHS-ECC) reinforced with carbon fiber-reinforced polymer (CFRP) grids. The behavior of 14 circular CFST members is investigated experimentally under lateral monotonic loading with sustained axial compression. UHS-ECC, with a compressive strength of 150 MPa, tensile strength of 10.3 MPa, and tensile strain of 3.5 %, is used to enhance the performance of several of the CFST members. The test parameters include the number of CFRP grid layers (0–3), the UHS-ECC thickness (20–50 mm), and the axial compression ratio (0–0.6). Additional tests on beam-columns strengthened with conventional ECC and normal-strength concrete (NSC), as well as an un-strengthened specimen, are also included for comparison purposes. The failure modes, load-displacement curves, bending stiffness, ductility, and strain development are examined in detail. The results indicate that CFRP grids significantly improve the UHS-ECC confinement leading to crack mitigation and enhanced load-bearing capacity. It is shown that strengthening with CFRP-grid-reinforced UHS-ECC effectively reduces the lateral expansion of the steel tube and enhances the composite action, particularly with the deployment of thicker UHS-ECC layers. The lateral load-bearing capacity is found to increase by 66–80 % using 1–3 CFRP layers, with over a 30 % enhancement in ductility. It is also shown that the use of thicker UHS-ECC layers can increase the lateral load capacity by over 2.4 times but can, in contrast, result in a nearly 40 % reduction in ductility. Comparison with other strengthening approaches shows that UHS-ECC significantly outperforms both ECC and NSC in terms of enhancement of bending stiffness and lateral load capacity. Significant ductility and energy absorption can also be achieved by members strengthened with CFRP-grid-reinforced UHS-ECC through careful consideration of the number and thickness of the layers. Based on the findings, a simplified design model is proposed for predicting the compression-bending capacity of CFRP-grid-reinforced UHS-ECC-strengthened CFST members. The findings demonstrate that the CFRP grid-reinforced UHS-ECC strengthening method offers a practical solution for CFST members, combining ultra-high compressive strength with exceptional tensile performance and ductility. Its superior crack resistance and energy absorption make it suitable for retrofitting existing structural elements such as building columns and bridge piers in demanding service conditions.