Axially-compressed Behavior of Carbon-Frp Strengthening CFT Short Columns Having Circumferential Void Defects

To clarify the viability of externally bonding carbon-fiber-reinforced polymer (FRP) for strengthening the structural integrity of concrete-filled steel tube (CFT) short columns having circumferential void defects, this study comprehensively explores the strengthening effectiveness of various wrapping parameters (i.e., carbon-FRP strength, wrapping layers, and wrapping ratio) on the axially-compressed CFT short columns with various void ratios, material strengths, and geometric parameters by numerical method. Detailed mechanical responses, encompassing failure patterns, load-shortening (N-Δ) curves and stress distribution, are delved to reveal the strengthening mechanisms. Finally, strength prediction formulas for these composite columns are proposed rooted in the ring extension and local force transmission theories. Results show that the presence of initial voids delays the occurrence of steel-concrete contact stress (p) and weakens the p-value at peak load, thus reducing the axial resistance of column. Particularly when the void ratio exceeds 0.2%, the longitudinal strain when the concrete contacts with outer tube is beyond the concrete's peak compressive strain, thus the shape of the N-Δ curve is obviously changed, exhibiting two peaks. Externally bonding carbon-FRP strips can obviously improve the peak load of the column with a void ratio ≤ 0.2%. If the void ratio exceeds 0.2%, externally bonding carbon-FRP strips can also improve the column's second peak load, even exceeding the strength of pure CFT column, while it has no effect on the first load. The formulas proposed herein have a satisfactory accuracy based on the tested validation, which are expected to guide the design of strengthening scheme in engineering.