# Experimental Study on Slender CFRP-Confined Circular RC Columns under Axial Compression
## 3 Experimental Results and Discussion
### CFRP Wrapped Specimens  
The failure of the CFRP-confined specimens was caused by the circumferential fracture of CFRP sheets. At loads close to the ultimate load, discontinuous crisp noises of fiber rupture were heard, which can be attributed to the debonding from the surface of the RC column. As the load increased, the debonding zone of the fiber increased, and the buckling of the steel bars intensified consequently. Specimens failed by the sudden rupture of the CFRP sheet accompanied by a loud noise. After failure of the specimens, the inner surface of CFRP sheet was stuck with concrete debris, and the tension fracture surface was serrated. Due to the circumferential constrain of FRP, the stirrups did not open or break in the failure area of the test specimens. When the slenderness ratio (\(\lambda\)) of the specimens was below 32, the rupture of CFRP cloth was observed at the mid length in a wide range. The longitudinal and hoop strain of the column reached the maximum with the full play of CFRP sheets. The specimens constrained by CFRP sheets exhibited higher load-carrying capacity and ductility than the unwrapped specimens. When the slenderness ratio (\(\lambda\)) of the specimens exceeded 32, at the initial stages of loading, a lateral deflection occurred at the mid height region of the columns, and exhibited a linear increase as the load increased. A nonlinear transition occurred at the load level of about 70% of the ultimate load. The lateral deflection increased sharply at loads very close to the ultimate value. When the columns failed, the longitudinal and circumferential strains of the column at the mid height region was relatively small, which indicated that the CFRP sheets did not give full play. Local compression failure was not initiated at the end of the specimens, which can be attributed to the measures of strengthening the column ends with multilayer CFRP sheets, which can effectively prevent the lack of local load capacity. The increasing slenderness reduced the ultimate bearing capacity of slender columns, and the effect on specimens wrapped with CFRP was more remarkable than that on unwrapped columns because the dominant failure mode for slender columns was buckling failure under peak load, as shown in Table 3. The ultimate bearing capacity of unwrapped specimens and CFRP wrapped specimens moderately decreased as slenderness increased, as indicated in Figure 3. For the circular RC columns with a slenderness ratio less than 60, CFRP hoop wraps can still effectively improve the axial load-carrying capacity. As the slenderness rose from 12 to 56, the load-carrying capacity of the CFRP wrapped specimens and unwrapped specimens decreased by approximately 27% and 19%, respectively. The test results show that the influence of the slenderness on the ultimate axial load of CFRP-confined slender columns is greater compared with short columns. The strengthening effect of CFRP hoop wraps on the load-carrying capacity of short columns is better than that of slender columns, as illustrated in Figure 3. For example, as the slenderness rose from 12 to 56, the increase of ultimate load of CFRP wrapped columns is reduced from 80% to 62% compared with unwrapped columns. To analyze the performance of slender CFRP-confined circular RC columns, the curves of load vs. deflection with different slenderness ratios were compared as shown in Figure 4. The load-deflection curve can be approximately divided into two stages. First stage is the elastic ascending stage, the slope of the first stage curves of all test columns is similar. With the increase of slenderness ratio, the columns become more unstable. When the slenderness ratio (\(\lambda\)) of the specimens exceeded 32, the lateral deflection of mid-height increased significantly at loads close to the ultimate value. It can be concluded that CFRP circumferential restraint cannot increase the flexural stiffness of the column. Figure 5 shows the load-strain curves of some columns. In the elastic stage, the strain increases with the increase of axial load.