Axial Load–Moment Interaction Diagram of Circular Concrete Columns Reinforced with CFRP Bars and Spirals: Experimental and Theoretical Investigations

North America’s current design codes and guidelines allow the use of fiber–reinforced polymer (FRP) bars as the primary reinforcement in concrete structures and provide design recommendations for using these bars. Because of a lack of experimental data, however, FRP bars have not been recommended for resisting compression stresses as longitudinal reinforcement in columns or compression reinforcement in flexural elements. This paper presents test results of an experimental program to investigate the structural performance of 10 full-scale circular concrete columns reinforced with carbon fiber–reinforced polymer (CFRP) bars and spirals subjected to combined axial compression loads and bending moments. The test variables include different eccentricity-to-diameter ratios and two types of reinforcement (CFRP and steel). The test results show that the CFRP- and steel-reinforced concrete columns behaved similarly up to their peak loads. The failure of the test specimens under different levels of eccentricity was not triggered by rupture of the CFRP bars in the tension side, but rather it was attributed to gradual concrete crushing, followed by bar crushing on the compression side. Based on the test results, a detailed sectional analysis and plane section analysis were then conducted. Furthermore, a comprehensive parametric investigation was performed to generate numerous nominal axial force–bending moment ($P\text{-}M$) interaction diagrams. The experimental and analytical results are discussed and compared.