Nano-engineering steel fiber for UHPC: Implication for varying cryogenic and elevated exposure

The interfacial bonding between steel fibers and Ultra-High Performance Concrete (UHPC) matrix is pivotal for the mechanical properties in extreme environments. Herein, a surface nanoengineering approach using the sol-gel method was reported to enhance the resistance of fiber bonding to varying cryogenic and elevated exposures (−170 °C ∼ 200 °C). Additionally, the interfacial bonding and failure of the steel fiber-matrix interface were evaluated by in-situ acoustic emission (AE) monitoring and a series of microscopic characterizations. The result indicated that the deposition of a nano-SiO₂ coating with a film structure thickness of approximately 150 nm on fiber surface could be achieved. The coating exhibited excellent cryogenic resistance but inferior elevated resistance, as elevated temperatures caused the coating to crack and corrode. After modification, the interfacial bonding stability exposed to thermal variations was enhanced. At ambient temperature, the bond strength of the modified fibers increased by 44.68 % compared to before modification, while that increased by 1.54 %–13.49 % in a single thermal-variations cycle compared to the ambient modified group. Interface enhancement arises from imbalances in nanocoating thermal stability, moisture phase changes, properties of three-phase interface transition zone, and thermal expansion coefficient disparities. Those findings provide new insight into the attempts to improve the mechanical and durability properties of concrete under extreme temperature environments.