Axial behavior of circular CFFT long columns internally reinforced with steel or carbon and glass FRP longitudinal bars

This paper presents the test results of an experimental study aimed at investigating the behavior of concrete-filled fiber-reinforced-polymer (FRP) tube (CFFT) long columns internally reinforced with longitudinal steel or carbon and glass FRP bars tested under axial compression loading. A total of ten reinforced concrete (RC) and CFFT columns measuring 1900-mm in height and 213-mm in diameter were constructed and tested until failure. The test parameters were: (1) internal reinforcement type and amount; (2) GFRP tube thicknesses; and (3) nature of axial loading (i.e. monotonic and cyclic). The experimental results showed that the GFRP-reinforced CFFT columns had comparable ultimate axial strength and strain capacities compare to their counterparts reinforced with steel bars. As expected, an increase in the FRP tube thickness (or stiffness) resulted in an increase in the strength and strain enhancement ratios. The results also indicated that the residual plastic strain of FRP-reinforced CFFT columns is linearly related to the envelope unloading strain, and this relationship is not influenced significantly by the FRP confinement level but strongly influenced by the internal reinforcement amount and type, particularly when the envelope unloading strain (>0.0035). The presented study showed the applicability of exclusively reinforcing the CFFT columns with FRP bars and subjected to axial compression load. However, further experimental investigations on the axial cyclic behavior of CFFT columns internally reinforced with FRP bars are required to establish such key relationships.