Axial stress-strain model for square concrete columns internally confined with GFRP hoops

Good modelling of the axial compressive stress-strain behaviour of confined reinforced-concrete (RC) columns is necessary for the structural analysis and design to assess their strength and ductility capacities. Thus, this paper presents closed-form expressions for predicting the stress-strain response of square concrete columns confined by internal glass-fibre-reinforced polymer (GFRP) hoops and subjected to axial compression loading. The stress-strain proposal is based on test results from the available studies and is mainly composed of a polynomial first branch (pre-peak zone) and a linear second branch (post-peak zone). To ensure a sufficient level of confinement, the thresholds of lightly and heavily confined concrete have been determined based on the proposed model and the compiled database. The model also considers the effects of key parameters such as volumetric ratio, size, configuration and spacing of hoop reinforcement, as well as longitudinal bar type (i.e. steel as opposed to GFRP). Existing models for steel-confined concrete and for concrete in circular cross-section confined by FRP hoops did not accurately predict the test results. Contrary to that, comparisons of the test results with the predictions of the proposed model revealed the model to describe well the stress-strain behaviour of GFRP RC columns with different arrangements of hoops.