Long-term creep behavior of NC filled UHPC tube composite column

Abstract

Because normal concrete (NC) filled ultra-high-performance concrete (UHPC) tube (NFUT) composite column can avoid additional formwork, save construction time, and cost, and increase bearing load capacity, it shows great potential in the application of engineering structures. This study aims to investigate long-term creep behavior of the NFUT composite column. The creep test was conducted on the NC column, UHPC tube, and composite column with different axial loads, which were applied by the steel strands with a double-tensioned technology to reduce the prestress loss and achieve the creep loading. Also, a numerical model was established for reproducing the test results, and several assumptions (such as perfect bond between the UHPC tube and NC core, and computing models for material creep and shrinkage) were adopted in the numerical model. It is found that the nominal creep coefficient of the NFUT composite column after 433-day loading is 36% of that of the NC column due to the contribution of the UHPC tube. Although slight differences can be observed in the local region between the numerical and test nominal creep coefficient curves probably due to the varying temperature and relative humidity, they generally show good agreement. The validated numerical model was used for parametric studies. The effect of the longitudinal reinforcement ratio, UHPC area ratio, loading age, and NC core stress level on the long-term deformation and stress redistribution was discussed. As the reinforcement and UHPC area ratios increase, the elastic and total strains decrease, and their decreasing degrees gradually become small. Moreover, they show significant effect on the UHPC and NC stress changes due to the stress redistribution (i.e., UHPC and NC stress changes decrease as the two factors increase). Also, properly increasing loading age is beneficial for decreasing the creep deformation and the stress level does not affect the long-term total strain development trend.