Axial compressive behavior of circular RC stub columns jacketed by UHPC and UHPFRC

This study investigated the axial compressive behavior of circular reinforced concrete (RC) stub columns jacketed by ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHPFRC). Eighteen composite columns were tested under compressive loadings on the entire section and only the RC section, while three control RC columns were tested under axial compression. Loading on the entire section exhibited a higher maximum compressive load and better expansion restraint than loading on the RC section. Adding 1 % and 2 % steel fibers by volume significantly increased the maximum compressive load and ductility index. However, the difference between 1 % and 2 % fiber content was not substantial. Maximum compressive load when loading on the entire section surpassed that when loading on the RC section, increasing by 75.6 %, 76.18 %, and 76.78 % for 0 %, 1 %, and 2 % steel fiber volume, respectively. Loading on the RC section led to faster cracking and failure, resulting in more tensile stress and lateral strain in the UHPC and UHPFRC jackets. Conversely, loading on the entire section maintained the expansion restraint even after reaching the maximum load. Columns with higher steel fiber volume demonstrated a greater enhancement in strength and ductility. Furthermore, a finite element model (FEM) using ABAQUS software was built to identify the experiments. Material models for UHPC, UHPFRC, and NSC core were proposed. The comparison between FEM and the test results shows that the FEM can predict accurately the behavior of tested columns. Based on developed FEM, a parametric study with the change of two variables, including the thickness and the compressive strength of the UHPC and UHPFRC layers, was conducted. The increase in the thickness of the UHPC or UHPFRC layer led to a remarkable increase in the maximum compressive load and ductility, indicating the importance of the thickness of the UHPC and UHPFRC jackets in restraining the expansion of the RC core. However, increasing the compressive strength of UHPC and UHPFRC had minimal impact on the maximum compressive load and ductility. Derived from these findings, UHPC and UHPFRC can be considered alternative material to provide lateral confinement to RC columns in addition to the traditional materials, indicating the potential application in strengthening or repairing damaged RC columns