Axial Load Testing of Hybrid Concrete Columns Consisting of UHPFRC Tube and Normal-Strength Concrete Core

To investigate the axial load behavior of hybrid concrete columns consisting of an ultra-high performance fiber-reinforced concrete tube (20 mm thick, 92.6 MPa or 143.3 MPa) and normal-strength concrete core (28.2 MPa), concentric axial load tests were performed for five column specimens. The UHPFRC tube can function as a form during core-filling and as a cover having high performance at service and ultimate load conditions. Axial loading was applied to the core only, and the tube was indirectly loaded by bearing of transverse threaded bars. Test parameters included the volume fraction of steel fibers, volumetric ratio of transverse bars, and usage of wire-mesh in the tube. Test results showed that structural performance such as initial stiffness, peak load, displacement ductility, and energy absorption capacity varied greatly according to each test parameter. During the course of axial loading applied on the core only, the axial load behavior of the hybrid concrete columns generally corresponded to the full composite action at the initial loading stage and then changed to that of non-composite action (core only) after the failure of the threaded bars.