This study investigates three methods of strengthening existing reinforced square concrete columns under different loading conditions. Four groups of sixteen reinforced concrete square columns were made from normal-strength concrete. Reinforcement was kept at minimum ratio, simulating columns that need retrofitting. Columns of the first group were reference columns (Group N), while the corners of the second group columns (Group RF) were rounded and wrapped with three layers of carbon-fiber-reinforced polymers (CFRPs). The sides of the columns of the third group (Group CF) were bonded with four pieces of concrete with a segmental circular shape, thus changing the cross section of the column from a square to a circle before each column was wrapped with three layers of CFRP. The columns of the last group (fourth) were modified as the third group to result in a circular cross section, but were confined with steel straps. Results from the study showed that all confinement methods increased the capacity and ductility of columns. In particular, segmental circular concrete covers dramatically reduced the stress concentration at the corners and increased confinement efficacy. The interaction (P–M) diagrams of experimental results and theoretical analysis all confirmed high performance of groups RF and CF.
This study investigates the behavior and failure modes of fiber-reinforced polymer (FRP) confined concrete wrapped with different FRP schemes, including fully wrapped, partially wrapped, and nonuniformly-wrapped concrete cylinders. By using the same amount of FRP, this study proposes a new wrapping scheme that provides a higher compressive strength and strain for FRP-confined concrete, in comparison with conventional fully wrapping schemes. A total of 33 specimens were cast and tested, with three of these specimens acting as reference specimens and the remaining specimens wrapped with different types of FRP (CFRP and GFRP) by different wrapping schemes. For specimens that belong to the descending branch type, the partially-wrapped specimens had a lower compressive strength but a higher axial strain as compared to the corresponding fully-wrapped specimens. In addition, the nonuniformly-wrapped specimens achieved both a higher compressive strength and axial strain in comparison with the fully-wrapped specimens. Furthermore, the partially-wrapping scheme changes the failure modes of the specimens and the angle of the failure surface. A new equation that can be used to predict the axial strain of concrete cylinders wrapped partially with FRP is proposed.
