Influence of Expansive Agent on the Mechanical Properties of Ultra-High-Strength Concrete Filled Steel Tube

Ultra-high-strength concrete filled steel tube (UHSCFST) specimen is a composite formed by filling steel tube with ultra-high-strength concrete (UHSC) and it has a great load-bearing capacity. However, the shrinkage of UHSC is relatively high compared to normal concrete and hence UHSC tends to debond from the tube in UHSCFST. UHSC and the steel tube are only “mechanically” combined together, making it difficult to realize their joint work and hindering the high performance of UHSCFST. In this study, 5%, 10% and 15% UEA expansive agents are added to UHSC to reduce the shrinkage. The influences of UEA dosage on the compressive strengths of UHSC and UHSCFST, as well as the bond strength between the tube and the concrete are analysed. The experimental results show that adding 10% UEA leads to only 3.2% increase of the UHSC compressive strength but 17.3% increase of the UHSCFST compressive strength thanks to the combined effects of the expensive agent and the tube confinement. Moreover, 15% UEA results in the highest pushout force, which is 29.1% higher compared to that without UEA. The study confirms that adding expansive agent contributes to improving the co-working properties of the steel tube and UHSC in UHSCFST.