Monotonic and cyclic axial strengths of rectangular concrete columns circularized and retrofitted with low CFRP confinement

Abstract

This study experimentally and analytically investigates the ultimate axial strengths of carbon fibre reinforced polymer (CFRP) confined circularized rectangular concrete columns under monotonic and cyclic compression. 12 concrete columns were divided into four groups. The first group was a control group. Columns of the second, third, and fourth groups were circularized to a diameter of 267 mm and then wrapped with low CFRP confinement configurations F1, F2, and F3 of 0.33, 0.67, and 1.0 layers, respectively. Of each group, the first column was monotonically loaded, while the other two columns were subjected to cyclic loadings. The results indicated that the control specimens experienced brittle failure with low ultimate lateral strains and inclined failure surfaces. In contrast, both circularization and CFRP wrapping provide a favourable condition to significantly increase the lateral strain capacity up to 300 times that of the control columns. For partly wrapped columns of groups F1 and F2, concrete failure initiated at the unwrapped region, which caused local stress concentration on CFRP, leading to failure of CFRP and then failure of columns. For fully wrapped specimens of group F3, the failure of columns occurred at a point and then progressively developed to a larger area of CFRP. Compared with control columns, the combination of circularization and CFRP retrofitting increased the ultimate strengths of concrete column groups F1, F2, and F3 by 267%, 304%, and 394%, and the ultimate lateral strain by 744.2, 628.5, and 367.7 times, respectively. This is attributed to the mutually beneficial interaction among the CFRP wrapping, circularized concrete, and original concrete. Due to its mitigation of the local failure, cyclic loading slightly increased the axial strength, while it decreased the ultimate lateral strain of CFRP confined circularized concrete columns compared with monotonic loading. The available Lam and Teng model reasonably predicts the ultimate axial strength of CFRP confined circularized concrete columns.