Effect of microfibers and cenospheres on the interfacial performance between steel fiber and multi-scale fiber reinforced UHPC subjected to high temperatures

To enhance the ductility of traditional ultra-high-performance concrete (UHPC), a strategy involves incorporating multi-scale fibers into UHPC. This aims to restrict crack propagation at different scales, consequently enhancing the material's ductility . Despite these advancements, the mechanism governing the cracking resistance of the newly developed multi-scale fiber reinforced UHPC (MSFUHPC) subjected to elevated temperatures remains unclear, particularly the interfacial bonding properties between steel fibers and MSFUHPC. Therefore, eighteen distinct mixture proportions are employed to systematically investigate the impact of microfibers—specifically carbon fibers, calcium sulfate whiskers, and cenospheres on the interfacial bond properties of steel fiber-MSFUHPC subjected to various temperature levels by a single factor test in this study. Specifically, a novel fiber pullout test method is employed to accurately measure fiber displacement by using digital imaging correlation technology. In addition, electron computed tomography scans and scanning electron microscopes are utilized to analyze the influence of microscopic fibers on the internal composition of MSFUHPC. Experimental findings demonstrate that carbon fibers can enhance the interfacial bond strength between MSFUHPC and steel fibers, thereby improving the mechanical properties of MSFUHPC subjected to different temperatures. Meanwhile, adding calcium sulfate whiskers with a proper dosage can improve the compressive strength of MSFUHPC at room temperature, but exhibit contrasting effects at elevated temperatures. In addition, the blend of 1 % CSW and 0.4 % CF or 0.8 % CF yields the best mechanical properties and interfacial bond performance, which is more obvious after exposure to 400 °C. Besides, the toughness index can be increased by about 80 % and average pullout strength can be almost doubled in comparison with that of the control group at 400 °C. Although adding cenospheres slightly diminishes mechanical properties, it can mitigate the negative effect of the excessive fiber dosage on compressive strength . • A novel multi-scale fiber reinforced UHPC is developed. • Positive effects of multi-scale fibers on mechanical properties of MSFUHPC after exposure to high temperature. • Effect of microfibers on the interfacial bond strength of steel fiber-MSFUHPC.