Master of Engineering (Research)
School of Civil, Mining & Environmental Engineering
Lei, Xu, Shape modification of square reinforced concrete columns confined with fibre-reinforced polymer and steel straps, Master of Engineering (Research) thesis, School of Civil, Mining & Environmental Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3537
Fibre Reinforced Polymer (FRP) has been demonstrated as an excellent material to strengthen existing structures. However, the confinement efficiency of FRP-confined square columns is relatively low compared to circular columns due to stress concentration at the sharp corners. This study presents a new technique aimed to maximise the efficiency of FRP confinement of square concrete columns. This new method is circularisation by using segmental circular concrete covers which changes the cross section of the column from a square to a circle. Moreover, steel straps were used as an alternative confining material and incorporated in the study to evaluate their performance.
Sixteen square reinforced concrete columns were cast and tested. Four specimens were used as a reference group; four specimens were rounded 20 mm at each corner and wrapped with three layers of CFRP simulating the conventional methods; the remaining eight specimens were bonded with four pieces of segmental circular concrete covers simulating the proposed technique, after which four specimens were wrapped with three layers of CFRP and the other four were confined with steel straps. From each group, one column was tested under concentric loading, one at 15 mm eccentricity, one at 25 mm eccentricity and the last specimen was tested under flexural loading. The failure mechanisms of each specimen were analysed and the load –deflection diagrams and axial load -bending moment diagrams were generated and evaluated.
Followed by the experimental program, a theoretical study was carried out. An analytical model was first introduced to evaluate the stress -strain relationships of the unconfined and confined concrete columns. After that, a process was proposed to calculate the ultimate axial load and bending moment of the columns at different eccentricities and generate the theoretical axial load -bending moment diagrams. The theoretical values were then compared with the experimental data and good agreement was found.
Results of the experimental program and the theoretical study showed that circularisation significantly increases the efficiency of the FRP confinement compared to conventional methods which round the sharp corners. FRP-confined circularised columns exhibited higher ultimate load but lower ductility compared to normal FRP-confined columns. The bond between the segmental circular concrete covers and the original columns was found to be reliable and the modified columns can be treated as complete circular columns. It is therefore concluded that the circularisation process is viable and effective in maximising the confining efficiency of FRP-confined square concrete columns. Meanwhile, steel straps had been demonstrated as an alternate confining material by providing moderate increase in ductility. Therefore, steel straps can be used for ductility strengthening.