Experimental Investigation of Cyclic Behavior of Concrete-Filled Fiber Reinforced Polymer Tubes

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) have in the last decade been used as girders, beam columns, and piles. The focus of research, however, has been exclusively on their monotonic behavior, with little or no attention to the implications of using CFFT in seismic regions. A total of six CFFT specimens were tested as simple span beam columns under constant axial loading and quasi-static reverse lateral loading in four point flexure. Three of the tubes were made using centrifuge (spin) casting with $12.7\mathrm{mm}$ thickness with the majority of the fibers in the longitudinal direction, whereas the other three were filament wound with $5\mathrm{mm}$ thickness and $\pm 55°$ fiber orientation. One specimen for each type of tube had no internal reinforcement, whereas the other two incorporated approximately 1.7 and 2.5% steel reinforcement ratios, respectively. The two types of tubes represented two different failure modes; a brittle compression failure for the thick tubes with the majority of the fibers in the longitudinal direction, and a ductile tension failure for the thin tubes with off-axis fibers. The study showed that CFFT can be designed with ductility behavior comparable to reinforced concrete members. Significant ductility can stem from the fiber architecture and interlaminar shear in the FRP tube. Moderate amounts of internal steel reinforcement in the range of 1–2% may further improve the cyclic behavior of CFFT.