Lateral Strain-To-Axial Strain Model for Concrete-Filled FRP Tube Columns Incorporating Interface Gap and Prestressed Confinement

The use of fiber-reinforced polymer (FRP) composites as external confinement in the manufacture of high-performance concrete-filled FRP tube (CFFT) columns has recently gained significant research attention. Accurate prediction of the lateral strain-to-axial strain relationship is of vital importance in predicting member behavior under various loading conditions, as the confinement pressure created by the FRP confining shell on the concrete core depends on the lateral expansion of concrete. A recent review of the literature revealed that while lateral-to-axial strain models exist for FRP-confined concrete, a model that is applicable to CFFT columns with prestressed FRP tubes or an interface gap caused by radial shrinkage of concrete is not yet available. To address this research gap, this study presents an accurate lateral-to-axial strain model based on the experimental observations from two test databases. Analyses of the experimental databases that consisted of 24 specimens manufactured with FRP-to-concrete interface gap and a further 23 specimens prepared with lateral prestress is presented and discussed. Based on close examination of the ultimate conditions of the column specimens and hoop strain development on the FRP confining shell, expressions to predict strain-reduction factors and lateral-to-axial strain relationships are proposed. The comparison of the proposed model predictions with the experimental test results of specimens prepared with an interface gap or prestressed FRP tubes shows good agreement, with average absolute errors (AAE) below 11% in all predictions.