Long-term behavior of hybrid FRP-concrete beam-columns.

摘要

An extensive study was carried out into the long-term behavior of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT). The experimental study comprised of shrinkage of concrete core, bond strength at the interface of concrete and FRP, axial creep and creep recovery of CFFT columns, hollow FRP tubes and fiber-wrapped concrete cylinders (FWCC), and flexural creep, creep recovery, and creep rupture of CFFT beams. The creep analysis followed the rate of flow method and the double power law for concrete, and the Findley's power law function for FRP. The model adhered to geometric compatibility and static equilibrium, and considered the effects of sealed concrete, multi-axial state of stresses, creep Poisson's ratio, stress re-distribution, variable creep stress history, and creep rupture of the column. For flexural creep analysis, average stress-creep strain isochronous curves were used as non-linear constitutive relation. The creep models were verified against previous tests for bonded and unbonded concrete-filled steel tubes (CFST), and the experiments of the present study on FWCC and CFFT columns and beams. A parametric study was carried out for creep rupture and creep under service loads.; The study showed that concrete core in CFFT has negligible shrinkage strains, similar to that of sealed concrete. Bond strength at the interface of concrete and FRP is lower than that of steel and concrete, but it is still large enough to prevent concrete from shrinking away from the tube. The existing models are shown to grossly overestimate creep of hybrid columns. The creep behavior of FWCC and unbonded CFST is very close to that of sealed concrete of the same mix. The effect of confinement on creep of concrete is generally not very significant. Bonded CFST and CFFT columns creep much less than their unbonded counterparts, mainly due to axial stress re-distribution. As the stiffness of the tube increases relative to concrete core, a larger stress re-distribution takes place, and a much lower creep develops. There is a threshold, beyond which, stiffer tubes would not significantly lower creep of concrete. Creep behavior of CFFT beam-columns is much better than CFFT beams, due mainly to the presence of axial load in the section, as it tends to retard cracking of concrete and growth of its curvature. Creep rupture life expectancy of CFFT columns and beam-columns is shown to be within acceptable range.; Future research on this topic must focus on creep buckling of slender CFFT columns and applicability of the moment magnification method, effect of tension stiffening of concrete core on its performance, and effect of strain rate on the creep behavior of CFFT columns and beam-columns.