Ultra-high-performance concrete (UHPC) is an advanced cementitious material with high strength and excellent durability. UHPC presents a new class of concrete developed in recent decades for its exceptional properties of strength and durability. Specifically, UHPC can achieve very high compressive strength (over 150Mpa), tensile (over 8 MPa) strength and strain capacity (0.5%), when steel fibers are incorporated. At present, practical scenarios for this type of concrete demands that it achieves even better tensile performance. To this end, multiple studies have been carried to with the goal being to improve the tensile performance of UHPC, particularly by adjusting the UHPC matrix components, interfacial transition zones and fiber properties. Regardless, steel fiber pullout properties and their influence on the UHPC matrix have not yet been fully investigated nor developed. Alternatively, various inquests have been proposed; particularly researching on the influence of cryogenic temperatures on the physical properties of normal strength concrete, following which enormous increases of compressive and tensile strength have been reported due to frozen and hardened moisture in the concrete.
Despite there being numerous publications reporting on various properties of UHPC, there has been no published study investigating the influence of geometrical deformation and inclination of steel fibers and impact loading rate on the entire fiber pulling out process from the UHPC matrix, at cryogenic conditions. To bridge this gap, researchers from the Department of Architectural Engineering at Hanyang University in the Republic of Korea: Min-Jae Kim (PhD candidate) and Professor Doo-Yeol Yoo investigated the effect of impact loading condition on the pullout property of steel fibers from ultra-high-performance concrete (UHPC) under various temperatures. Specifically, they focused on assessing the effects of the loading rate and cryogenic condition on the pullout behavior of various steel fibers from the UHPC matrix by considering their inclination angles and geometries. Their work is currently published in the research journal, Construction and Building Materials.
In their approach, bond–slip curves, bond strengths, slip capacities, and normalized pullout energy were analyzed, and the factors that increased according to the impact loading rate and cryogenic temperature were evaluated. In fact, for better understanding of the influences on the fiber pullout properties and for more reliable data analysis, optical micrographs on the fiber–matrix interfaces and fiber exit were used.
The authors reported that the steel fibers mostly had positive values of dynamic increasing factor (DIF) for the average bond strength at ambient temperature, but these were significantly reduced when the fibers were geometrically deformed, inclined, or tested under cryogenic temperature. Additionally, slip capacities were seen to generally decrease under the influence of the impact loading condition, and this effect was more severe at cryogenic temperature.
In summary, the study investigated the effects of impact loading rate, cryogenic temperature, geometrical deformation, and inclination of steel fibers in UHPC on fiber pullout behavior. The team reported that there was no obvious impact loading rate effect on the probabilities of fiber and UHPC matrix damages at both ambient and cryogenic temperatures. Overall, it was seen that the DIF values for pullout energies normally decreased or even became negative when the fibers were inclined, geometrically deformed, or tested at cryogenic temperature. In a statement to Advances in Engineering, Professor Doo-Yeol Yoo highlighted that their work presented requisite insights for better UHPC concrete.
Min-Jae Kim, Doo-Yeol Yoo. Cryogenic pullout behavior of steel fibers from ultra-high-performance concrete under impact loading. Construction and Building Materials, volume 239 (2020) 117852.