Analytical investigation of long-term behavior of normal concrete filled UHPC tube composite column

The long-term creep and shrinkage behavior of a recently proposed normal concrete (NC) filled ultra-high-performance concrete (UHPC) tube (abbreviated as NFUT) composite column has been rarely investigated. The present study aims to propose an analytical solution for calculating the restrained stress and initial axial stress of NC core based on elastic mechanics knowledge. The nominal creep coefficient or strain of the NFUT composite column is analytically calculated as well, based on the age-adjusted effective modulus method. The perfect bond between the UHPC and NC is assumed and the shrinkage of the UHPC tube subjected to steam curing is negligible during the calculation. Moreover, a finite element model is established, and the numerical restrained stress and initial axial stress are compared with the analytical ones. Also, the effect of the Poisson’s ratio and reinforcement ratio in the UHPC tube on these stresses is investigated. The analytical nominal creep coefficients or strains of the specimens are compared with the test ones and the previous numerical parametric analysis results. It is found that the numerical restrained stress and initial axial stress show good agreement with the analytical ones with errors smaller than 10 % The restrained stress factor and initial axial stress are linearly proportional with the Poisson’s ratio difference between the UHPC and NC. Also, the analytical nominal creep coefficients generally show good agreement with the test ones, and the errors between the analytical and the numerical final strain predictions are within 1 %, which demonstrates the applicability and accuracy of the age-adjusted effective modulus method based nominal creep coefficient calculation.