Effect of fiber angles on normal- and high-strength concrete-filled fiber-reinforced polymer tubes under monotonic axial compression

Concrete-filled fiber-reinforced polymer tubes are a novel form of composite columns, which are particularly attractive for structural members in harsh environments and seismic regions due to their corrosion resistance and ductile behavior. Over the past two decades, many studies have been conducted on concrete-filled fiber-reinforced polymer tubes under axial compression, and many stress-strain models have been proposed. However, existing studies mainly focused on concrete-filled fiber-reinforced polymer tubes with only hoop fibers. In order to investigate the effect of fiber angles (i.e. the fiber angle between the fiber orientation and the longitudinal axis of fiber-reinforced polymer tube), this study conducted axial compression tests of 42 concrete-filled fiber-reinforced polymer tubes with +/- 80 degrees, +/- 60 degrees, or +/- 45 degrees fiber angles. These concrete-filled fiber-reinforced polymer tubes were constructed using normal-strength concrete or high-strength concrete. Fiber-reinforced polymer tube thickness was also investigated as an important parameter. In order to clarify the effect of fiber angles on the properties of fiber-reinforced polymer tubes, axial compression tests on 15 short fiber-reinforced polymer tubes and tensile split-disk tests on 75 fiber-reinforced polymer rings were conducted. Experimental results indicate that fiber angles had significant influences on the hoop properties of fiber-reinforced polymer tube; the confinement effect of fiber-reinforced polymer tube and the peak stress of the confined concrete decreased with the decrease of the absolute value of fiber angles, while the ultimate strain of the confined concrete increased with the decrease of the absolute value of fiber angles. Two existing stress-strain models, which were developed mainly on test results of concrete confined by fiber-reinforced polymer tubes with only hoop fibers, are capable of providing reasonably accurate predictions for concrete-filled fiber-reinforced polymer tubes with +/- 80 degrees and +/- 60 degrees fiber angles, but it underestimates the ultimate axial strain of concrete-filled fiber-reinforced polymer tubes with +/- 45 degrees fiber angles.