Bond between circular UHPFRC jackets for columns and concrete core at different temperatures

The bond-slip behavior at the interface between ultra-high-performance fiber-reinforced-concrete (UHPFRC) circular tubes and the normal strength concrete (NSC) inner core is investigated using 44 push-out tests. This system simulates new construction comprising NSC-filled UHPFRC tubes with a smooth inner surface, or retrofitting NSC columns using UHPFRC jackets applied to the roughened NSC surface. The UHPFRC tube was supported at the bottom while the inner NSC core was loaded from above. The study explored thermal effects on bond at a nominal −20°C to + 40 °C range, including steady and thermal cycles, as well as NSC compressive strength ( f c ’ ) (20–47 MPa). Different fiber types (steel and POM) and hoop reinforcement ratios (0–0.82%) were explored for UHPFRC. The study showed that peak bond strength ( τ u ) increased at low temperatures, by a factor of 2 at −19°C, and reduced at high temperature, by 10% at + 42 °C, due to the higher thermal coefficient of UHPFRC relative to NSC. However, residual bond strength was not much affected. A thermal cycling history increased τ u by 14% at room temperature. Increasing f c ’ of the core 2.4 times increased τ u by 54%. POM fibers reduced τ u by 72%, compared to steel fibers. Adding a 0.82% UHPFRC reinforcement increased τ u by 10% only. Surface roughness for retrofitting application enhanced bond significantly to the point that UHPFRC jackets crushed at the base, but bond stress at failure increased with roughness level, reaching up to 2 times τ u of the smooth interface. Retrofitting jacket with POM fibers resulted in an NSC core cohesive bond failure at τ u of 2.6 times τ u of the smooth interface. Bond strength models were developed.