AXIAL COMPRESSION – BENDING INTERACTION BEHAVIOUR OF SQUARE RC COLUMNS STRENGTHENED USING HYBRID FRP COMPOSITES

Civil infrastructure industry is under constant pressure to upgrade the existing concrete structures, which have deteriorated due to corrosion. In a reinforced concrete (RC) bridge and building system, columns are the most important structural components. Thus, the performance of the entire building/bridge relies heavily on the performance of the columns. The majority of columns are non-circular in cross-section and are subjected to a combination of axial compression and bending loads. In this research, an innovative hybrid strengthening technique is developed for the strengthening of RC square columns using near-surface mounting (NSM) of carbon fibre reinforced polymer (CFRP) laminates and external bonding (EB) of CFRP fabrics. The overall objective of this study is to establish axial compression (P) – bending (M) interaction behaviour for square RC columns. The experimental program is carried out in two phases. Phase I involves testing undamaged RC columns and the phase II involves testing of severely damaged RC columns. In phase I, thirty-two RC columns are tested. Out of thirty-two specimens, eight specimens are tested under each loading series namely (i) pure axial compression (e/h=0), (ii) high axial compression with low bending (e/h=0.15), (iii) low axial compression with high bending (e/h=0.63) and (iv) pure bending (e/h=∞). For each loading combinations, four different specimen series are tested which includes (i) two control columns, (ii) two columns strengthened using NSM technique, (iii) two columns strengthened using EB technique and (iv) two columns strengthened using hybrid FRP technique. Phase II consists of testing eighteen columns. First, all the eighteen columns are severely damaged till failure in three different loading series (six specimens each) namely (i) pure compression (e/h=0), (ii) low axial compression with high bending (e/h=0.63) and (iii) pure bending (e/h=∞). After severe damage, all the specimens are repaired using high strength cement grout (HSCG). Out of six specimens, two specimens represent the control repaired with HSCG, two specimens repaired with HSCG and strengthened using NSM technique and two specimens repaired with HSCG and strengthened using hybrid FRP technique. Experimental results reveal that NSM strengthened RC members had significant strength improvement when subjected to predominant bending behaviour. However, the ductility was completely lost, and the NSM strengthened specimens underwent debonding failure. Strengthening with only EB technique was found to be effective under compression and showed only a marginal strength improvement under combined compression and bending loads due to strain gradient effect. Hybrid FRP strengthening significantly improved the strength, stiffness, and ductility of columns under all combinations of axial compression and bending loads. Moreover, hybrid strengthening was able to completely restore the peak strength and ductility of severely damaged RC columns. NSM strengthening restored the original capacity of severely damaged columns under eccentric compression and pure bending load combinations. The axial compression and bending (P-M) interaction diagram was developed analytically using the strain compatibility procedure for both undamaged and severely damaged RC columns. The analytical predictions exhibited a close correlation with the test results under all combinations of P and M. Also, a nonlinear three-dimensional finite element (FE) analyses are carried out using the software FEMAP and MASA. The validated FE modelling approach was used for performing a detailed parametric investigation for understanding the effect of concrete strength and different FRP reinforcement ratios. Parametric studies from the analytical and FE analyses indicated that the optimum hybrid CFRP ratio could be with a CFRP laminate ratio of 0.7% confined using two layers of CFRP wrapping.