Concrete-filled fibre-reinforced polymer tubes for axial and flexural structural members

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Fam, Amir Zakaria Yassa Hanna
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The use of fibre reinforced polymers, FRP, in new structures is still limited to few demonstration projects. Innovative hybrid systems such as the concrete-filled FRP tubes are effective in facing the great demand for piling, poles, highway overhead sign structures and bridge components. The new products have to withstand aggressive corrosive environments such as the splash zone in case of marine piles. The concrete-filled glass fibre reinforced polymer, GFRP; cylindrical tube system utilizes the best characteristics of the individual materials. The FRP tube provides lightweight permanent formwork as well as non-corrosive reinforcement for the concrete, which simplify construction and reduce erection time. The round tube also confines the concrete in compression and increases its strength and ductility, while the concrete core supports the tube and prevents premature local buckling failure. Research work related to the flexural behaviour of concrete-filled FRP tubes and their behaviour under axial compression loading conditions is, however, very limited to date. A two-phase experimental program was conducted at the University of Manitoba to examine the flexural behaviour of concrete-filled GFRP tubes and their behaviour under axial compression loads as short columns. Phase-I included eighteen simple beam tests conducted under four-point bending. The beams ranged from 1.07 to 10.4 meters in span and from 89 to 942 mm in diameter. The specimens included hollow and concrete-filled GFRP and steel tubes. Different cross-section configurations including totally and partially-filled GFRP tubes with central voids were considered. GFRP tubes of different laminate structures including filament-wound and pultruded tubes were included. Also GFRP tubes with different diameter-to-thickness ratios were also considered. Phase-II included twelve stub tests under axial compression. The stubs ranged from 100 to 219 mm in diameter. Different cross-section configurations including totally-and partially-filled GFRP tubes were considered. The partially filled GFRP tubes included different inner-to-outer diameter ratios. Different laminate structures included filament-wound and pultruded GFRP tubes. A concrete-filled steel tube was also tested for comparison. The thesis also presents two theoretical equilibrium/strain compatibility models, proposed to predict the behaviour of concrete-filled FRP tubes under axial compression as well as in flexure. The first model is a confinement model intended to predict the full stress-strain response of concrete confined by FRP shells, which is subjected to a variable confining pressure and also accounts for the concrete nonlinearity. The model can predict the behaviour of totally and partially filled FRP tubes, and also accounts for axially loaded concrete core only or axially loaded core and tube. The second model is capable of predicting the ful load-deformation response of concrete-filled FRP tubes in flexure using a layer-by-layer approach. The model accounts for the linear FRP material, the concrete nonlinearity in compression and the tension stiffening effect. The predictions using the theoretical models showed excellent agreement with the experimental results. The models were used in parametric studies to investigate the effect of wall thickness and laminate architecture of the tube and the inner void size. The effect of concrete filling on flexural behaviour of high and low stiffness tubes is also studied.