Effectiveness of CFRP Strengthening in Improving Cyclic Compression Response of Slender RC Columns

The effectiveness of fiber-reinforced polymer (FRP) based strengthening on the monotonic compression response of reinforced concrete (RC) short columns has been studied extensively. Effectiveness of FRP strengthening in improving cyclic compression response of slender RC circular and square columns is investigated. A total number of 24 slender RC columns of 1,200 mm height were cast in two groups of circular and square columns. Specimens of each group had four unstrengthened and eight strengthened columns. Two schemes were followed to strengthen the columns. In the first scheme of strengthening, one layer of carbon FRP (CFRP) sheet was attached to the columns, maintaining CFRP fibers in the transverse direction. In the other strengthening scheme, however, the first two layers of CFRP sheets were externally bonded to the column, keeping the fibers along the height of the column, and then one layer of CFRP sheet was wrapped over the longitudinal sheets with its fibers in the transverse direction. Specimens were tested under eccentric monotonic and cyclic compression loadings. The cyclic compression involved several loading-unloading compression cycles before failure. Test results showed that CFRP strengthening of slender columns is equally effective in cyclic compression as in monotonic compression. A simple formulation is also presented to derive load-moment (P-M) interaction diagrams for FRP-strengthened columns. The prediction complies well with the experiments. The CFRP confinement of RC columns decreases the slenderness limit of the columns and can be reasonably predicted by the recommended equation. This reduction is more in CFRP-confined circular columns than square columns.