Stress-strain behavior and design-oriented model for FRP spiral strip-confined concrete

Partial fiber-reinforced polymer (FRP) wrapping has become an attractive strengthening technique for reinforced concrete (RC) columns. Apart from FRP full wrapping strengthening technique, FRP partial wrapping with FRP spiral strips leads to some other advantages such as better fire resistance. However, existing study on FRP spiral strip is limited and the confinement mechanism of FRP spiral strip wrapped concrete columns remains unclear. To this end, axial compression tests on 28 concrete columns wrapped with FRP spiral strips are conducted and the test results are presented in this paper. Results show that the ultimate axial stress and the slope of the post peak linear branch of the axial stress-strain curve are significantly affected by the proposed confinement effectiveness factor. The desired confinement effectiveness factors for specimens can be achieved by varying the key confinement parameters (i.e., spiral angle, width and layers of FRP strips). It is found that the increases in FRP strip width and thickness lead to enhancements in both ultimate axial stress and strain of the FRP spiral strip-confined concrete, whereas a decrease in the spiral angle has a more pronounced effect on the ultimate axial stress enhancement than the ultimate axial strain enhancement. A new design-oriented stress-strain model is proposed for FRP spiral strip-confined concrete based on the new test database assembled in this study. The predictions corroborate reasonably well with the test results, demonstrating the accuracy and robustness of the proposed model.