Creep Analysis of Axially Loaded Fiber Reinforced Polymer-Confined Concrete Columns

An analytical model is developed to study the time-dependent behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) and fiber-wrapped concrete columns (FWCC) under sustained axial loads. The model utilizes the double power law creep function for concrete in the framework of rate of flow method, and the linear viscoelastic creep model for FRP. It follows geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multiaxial state of stresses, creep Poissonʹs ratio, stress redistribution, variable creep stress history, and creep rupture. The model is verified against previous creep tests by the writers on FWCC and CFFT columns. It is then used to study the practical design parameters that may affect creep of FRP-confined concrete under service loads, or lead to creep rupture at high levels of sustained load. Creep of FWCC is shown to be close to that of sealed concrete of the same mix, as the effect of confinement on creep of concrete is not very significant. CFFT columns, on the other hand, creep much less than FWCC, mainly due to axial stress redistribution. As the stiffness of the tube increases relative to the concrete core, larger stress redistributions take place further reducing the creep. However, there is a threshold, beyond which, stiffer tubes would not significantly lower the creep of concrete. Creep rupture life expectancy of CFFT columns is shown to be quite acceptable.