Hybrid-reinforced concrete columns incorporating direct-wound rectangular CFRP ties: Numerical investigation and confinement model

Fiber-reinforced polymers (FRPs) are being increasingly applied as reinforcement in new concrete structures, particularly in aggressive environments, owing to their better mechanical and durability properties and ease of installation. However, the brittle failure of FRP bars, combined with the inadequate confinement of conventional pultruded FRP bar ties, caused by fiber kinking at bends and bond-slip at overlaps, limits the structural performance of columns. To overcome these drawbacks, this study explores a novel hybrid-reinforced concrete (HRC) column that employs steel bars as primary reinforcement in combination with in-situ direct-wound rectangular CFRP strip ties, offering superior confinement compared with conventional FRP bar ties. A numerical model was developed using Abaqus and validated against test results, achieving predictions within 10% of measured values. Subsequently, a detailed parametric study was conducted to evaluate the effects of concrete compressive strength, tie spacing, tie cross-sectional area, tie width, and load eccentricity on the axial behavior of HRC columns. The results showed that increasing tie spacing from 70 to 160 mm reduced peak load and confinement efficiency by 8 and 36%, respectively, while increasing the tie cross-sectional area from 15 to 105 mm ² enhanced the confinement efficiency by about 32%. Moreover, increasing the tie width from 15 to 30 mm had a negligible effect on confined strength, whereas further increasing the width from 30 to 50 mm enhanced the confined strength by up to 5.5%. Higher concrete strength increased the axial load capacity but reduced confinement effectiveness because of the relatively brittle response of high-strength concrete. Finally, building upon these findings, analytical equations were proposed to calculate the confined concrete strength of HRC columns.