Behaviour of Circularized and FRP Wrapped Hollow Core RC Columns under Different Loading Conditions

It is well-known that the efficiency of FRP confinement is higher for circular columns than for non-circular columns. Circularizing a square column by bonding concrete segments onto the sides of the square column and then wrapping the circularized column with FRP is considered an effective technique in strengthening square solid concrete columns. However, the applicability of circularization and FRP wrapping in strengthening square hollow columns has not been investigated in the available literature. This study investigates experimentally and theoretically the behaviour of circularized and FRP wrapped hollow concrete columns. The main experimental program was designed to investigate the behaviour of circularized and FRP wrapped hollow RC specimens under different loading conditions. A total of 20 square hollow RC specimens of 150 mm x 150 mm cross-section and 800 mm height with 50 mm x 50 mm central square hole were cast and tested. The specimens were divided into five groups of four columns. The specimens in the first group (N) were the square hollow RC specimens, which served as reference specimens. The corners of the specimens in the second group (RF) were rounded to 20 mm and wrapped with two layers of carbon FRP (CFRP). The specimens in the third group (CF) were circularized and wrapped with two layers of CFRP. The specimens in the fourth group (VCF) were bonded with vertical CFRP strap (on each side) then circularized and wrapped with two layers of CFRP. The specimens in the fifth group (HCF) were wrapped with one layer of CFRP then circularized and wrapped with two layers of CFRP. The influence of circularization, vertical CFRP straps, transverse CFRP wrapping and the load eccentricity (concentric axial loads, 25 mm and 50 mm eccentric axial loads and fourpoint bending) were investigated. The experimental results showed that circularization increased the strength and ductility of the hollow specimen under different loading conditions. Transverse wrapping with CFRP mainly improved the performance of the specimens under concentric axial loadings, while the longitudinal CFRP straps mainly improved the performance of the specimens under eccentric axial loading. A theoretical unified strength model was developed to predict the compressive strength for FRP confined circular and non-circular solid and hollow concrete. The model was developed by regression analysis of a large number of data collected from the available experimental results in the literature. The model was proposed based on the predefined selected criteria. The proposed model covered data with unconfined concrete compressive strengths between 6.2 MPa and 112 MPa and confined (confined with different FRP materials) concrete compressive strength between 18.5 MPa and 217.3 MPa. The accuracy of the proposed model was verified with a large number of 1154 experimental data collected from the available literature. Based on the developed model, a theoretical study was also conducted to investigate the axial stress-axial strain relationships and the axial load-bending moment interactions of the circularized and FRP wrapped hollow concrete specimens. The results of the theoretical model showed good agreement with the experimental results