An analytical and experimental investigation of concrete-filled fiber-reinforced plastics (FRP) tubes

Hybrid construction with concrete and fiber composites results in systems with pseudo-ductile characteristics and high stiffness and strength. One such application is demonstrated in concrete-filled tubes (CFTs) made of fiber reinforced plastics (FRP). The FRP tube provides the necessary longitudinal and hoop reinforcement, as well as a permanent form work for the concrete core. In return, the concrete core contributes to the overall stability and stiffness of the composite system. The tube consists of axial and hoop fibers in the form of filament-wound angle plies or axial-normal wound plies. The tube also includes internal ribs as shear connectors for beam-column applications. The present study is an analytical and experimental investigation of concrete-filled FRP tubes (CFFT) as structural beam-columns.; Experimental results of a series of 6" x 12" circular CFFTs subjected to uniaxial compression were utilized towards the development of a new analytical confinement model. The tested cylinders were composed of a filament-wound fiberglass/polyester tube filled with normal weight medium strength concrete. No bond was provided between FRP and concrete. A mathematical model was developed to generate the complete bilinear stress-strain response of FRP-confined concrete in both axial and lateral directions. The parameters of the model are directly related to the material properties of the FRP tube and the concrete core. The predicted stress-strain curves compared favorably with the available experimental data on uniaxial compression tests of FRP-encased concrete. The model was also shown to provide good estimate for fiber-wrapped columns tested by other investigators.; The beam-column behavior of CFFTs was studied by a series of 7" x 7" x 52" square CFFTs that were tested under various combinations of axial-flexural loads. The results of these tests formed the experimental force-moment interaction diagram for square CFFTs. This interaction diagram was compared with theoretical interaction diagrams of conventional RC sections with variable reinforcement ratios. The comparison revealed a superior load-carrying capacity for the CFFTs. Also, it was shown that the ductility of CFFT beam-columns is comparable to corresponding RC sections in compression control region of the interaction diagram. The results of the beam-column tests were also used to verify the applicability of the beam theory for the design of square bonded CFFTs. Finally, some design issues were discussed for both circular and square CFFTs subjected to either axial or axial-flexural loads.