Flexural toughness and stress-strain relationship of different types of steel fiber reinforced shotcrete in high geothermal environment

The shape of steel fiber plays a pivotal role in determining the mechanical properties of concrete. In this paper, the impacts of end-hooked, wavy and ultra-fine steel fibers (all with a doping amount of 1.0 %) on the compressive strength, flexural strength, flexural toughness, and strain-strain relationship of shotcrete under curing conditions of 20 degrees C, 40 degrees C, and 60 degrees C are studied. Furthermore, the flexural toughness of steel fiber shotcrete is evaluated using the energy absorption method, and a constitutive model for steel fiber shotcrete was established based on meso-damage mechanics theory. The results indicate that curing temperatures below 40 degrees C are conducive to enhancing the compressive strength, flexural strength, and flexural toughness of steel fiber shotcrete, while curing temperatures above 60 degrees C are unfavorable for the development of strength and toughness in steel fiber shotcrete. Regardless of the type of steel fiber, its incorporation can alleviate the negative impact of high-temperature curing on the strength and toughness of shotcrete. Under high-temperature curing conditions, the influence of end-hooked steel fibers on the strength of shotcrete is superior to that of corrugated and ultrafine steel fibers, while ultrafine steel fibers are beneficial for improving the flexural toughness of shotcrete. The slope, peak stress, and peak strain of the stress-strain curves of shotcrete cured at 40 degrees C and 60 degrees C with the addition of the three types of steel fibers are greater than those of concrete without steel fibers. Furthermore, the descending segment of the curve is wider and smoother, with a larger surrounding area, demonstrating good toughness, deformability, and ductility. The end-hooked steel fibers and ultrafine steel fibers exhibit superior performance in enhancing the toughness and ductility of shotcrete under high-temperature curing conditions compared to wavy steel fibers. Based on meso-damage theory, the impacts of curing temperature and steel fiber types on the constitutive model of shotcrete under axial compression were established, and the model parameters were discussed. The research results can provide theoretical support for the durability design of tunnel linings in high ground temperature environments.