Assessing Stress-Block Parameters in Designing Circular High-Strength Concrete Members Reinforced with FRP Bars

Several standards and guidelines allow the use of the equivalent rectangular stress block (ERSB) as a simple concept to accurately predict flexural strength with or without axial force. North American codes (ACI 318 and CSA A23.3) allow the use of the ERSB as an alternate approach to stress-strain relationships in predicting the strength of high-strength concrete (HSC) reinforced with steel bars (steel-HSC-RC). The strength of high-strength concrete specimens reinforced with fiber-reinforced polymer (FRP) bars (HSC-FRP-RC) could be predicted similarly to steel-RC specimens. This research represents an early attempt to investigate the applicability of several ERSBs proposed by different associations to predict the strength of HSC-FRP-RC specimens subjected to combined flexural and compression loads. An experimental database was assembled from 92 specimens studied by the authors and others. The test parameters included concrete strength, level of eccentricity, reinforcement type, glass fiber-reinforced polymer (GFRP) reinforcement ratio, and confinement configuration. Extensive analyses were integrated into the test results to investigate the impact of each test parameter on ERSB parameters. The accuracy and conservativeness of several models were assessed. Comparing the predictions for the tested FRP-RC specimens made with normal-strength concrete (NSC) and HSC revealed that the level of conservatism dramatically decreased for all models, leading to overestimations for some specimens. The American Concrete Institute model always had the lowest level of conservatism, which led to overestimations of some specimens for steel and FRP-RC specimens made with HSC among all the other models. Modified expressions of the ERSB given in ACI 440.1R and CSA S806 were developed. The results indicate good and safe correlation of predicted and measured strength values with reasonable levels of conservatism.