Time-dependent behavior of fiber-reinforced polymer-confined concrete columns under axial loads

An extensive investigation into the long-term behavior of fiber reinforced polymer (FRP)-confined concrete columns was carried out, Shrinkage, interface bond, creep, creep recovery, static, and reserved strength of a number of concrete-filled FRP tubes (CFFTs) and fiber-wrapped concrete columns (FWCCs) were measured. The study showed the shrinkage of the concrete core in CFFT columns to be quite negligible. Bond strength at the interface of the concrete core and FRP tube was found to be lower than that in steel tubes, but still large enough to counteract the axial shrinkage of the concrete core. The effect of lateral confinement on the creep of the concrete core was found not to be as significant as sealing of the concrete core and the stress redistribution that takes place between the concrete and the FRP tube in the a-vial direction. As the stiffness of the tube increases relative to that of the concrete core, a larger stress redistribution can occur over time, yielding a lower creep coefficient. The ACI 209 model was shown to overestimate creep of FRP-confined concrete. The creep coefficients for CFFTs may be as low as 22% of those recommended by ACI 209 for an equivalent sealed concrete. The difference between ACI 209 and test results of the FWCC specimens, however, is not as significant. While no strength degradation was observed for FWCC specimens, FRP tubes showed approximately 30% strength degradation.