Monotonic and Cyclic Axial Compressive Behavior of CFRP-Confined Rectangular RC Columns

The majority of experimental and analytical investigations on the stress-strain behavior of rectangular concrete columns externally confined with carbon fiber–reinforced polymer (CFRP) composites have concentrated largely on unreinforced concrete columns of small size. However, research investigating the influence of larger cross-sectional size and internal steel reinforcement on the stress-strain behavior of CFRP-confined rectangular RC columns under axial compression loading has been limited. Therefore, the first part of this paper aims to present the results of experimental tests on 28 larger-sized rectangular plain and reinforced concrete (RC) columns confined with CFRP wraps. The primary test parameters were (1) cross-sectional size and aspect ratio; (2) number of CFRP composite layers; (3) volumetric ratio of hoop steel reinforcement; and (4) nature of loading (i.e., monotonic and cyclic). The experiments showed that the stress-strain curves of the monotonically loaded CFRP-confined columns exhibited a softening second branch followed by an ascending or almost flat third branch. All the confined columns experienced a significant enhancement in axial strain but only a slight enhancement in axial strength. An important finding of the study was that the internal longitudinal and hoop steel reinforcement influenced the shapes of the axial stress-strain envelope curves, the unloading and reloading paths, and the plastic strain values. A model was finally proposed for predicting the ultimate strength of CFRP-confined rectangular RC columns. The performance of the proposed model was validated against the results of current and existing experiments.