Coupling effects of strain rate and steel fiber content on fracture performance of ultra-high-performance fiber reinforced concrete

Current research on the strain rate effect on type I fracture performance in ultra-high performance fiber reinforced concrete (UHPFRC) remains relatively limited. This study aims to investigate the effects of strain rate (loading rates of 0.12, 1.2, 12, and 120 mm/min) and steel fiber content (1.0%, 1.5%, and 2.0% by volume) on the fracture properties of UHPFRC. Type I fracture tests were conducted on pre-cracked UHPFRC specimens subjected to three-point flexural loads. The results indicated that the nominal flexural strength of UHPFRC was increased with the increase in the loading rate. Specifically, for steel fiber contents of 1.0%, 1.5%, and 2.0%, the nominal flexural strength exhibited increases of 16.20%, 17.01%, and 10.69% as the loading rate was elevated from 0.12 mm/min to 120 mm/min. Furthermore, the increase in fiber content significantly enhanced the flexural and fracture properties of UHPFRC. As the loading rate increased from 0.12 mm/min to 120 mm/min, the fracture energy of UHPFRC with steel fiber contents of 1.0%, 1.5%, and 2.0% increased by 23.28%, 19.41%, and 18.98%, respectively. Additionally, with the increase of loading rate, the characteristic length of UHPFRC decreases gradually. Finally, a linear regression method was employed to establish a predictive formula for the nominal flexural strength and fracture energy of UHPFRC, considering the combined effects of strain rate and steel fiber content. This study will enrich the existing body of research on the strain rate effect on Type I fracture performance in UHPFRC.