Study on the dynamic fracture mechanical properties of ultra high-performance concrete under the influence of steel fiber content

Ultra-high performance concrete (UHPC) is characterized by its ultra-high strength and durability, making it suitable for use in large-span bridges, super high-rise buildings, and other special structures. These types of structures often have high requirements for crack control, making the exploration of UHPC's fracture mechanics properties of significant importance. Therefore, this paper sets up four steel fiber contents (0 %, 0.6 %, 1.2 % and 1.8 %) and four strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s), totaling 16 working conditions. The hydraulic servo system combined with digital image correlation (DIC) technology was used to conduct fracture mechanics tests on UHPC three-point bending beams. The test results show that the unstable fracture load of UHPC increases with the increase of strain rate and steel fiber content, and increasing the steel fiber content can enhance the strain rate effect of UHPC to a certain extent. Additionally, combined with the J-criterion model, this study proposed a dynamic criterion for the unstable fracture load of UHPC considering the influence of steel fiber content. The initial fracture toughness, unstable fracture toughness, and fracture energy of UHPC are significantly more affected by steel fiber content than by strain rate. Through DIC technology, the analysis of UHPC crack propagation shows that UHPC without steel fibers experiences rapid crack expansion in the Pre-90 % to Peak stage, with less influence from the strain rate. However, UHPC with steel fibers exhibits a more uniform crack growth rate, with the crack emergence phase gradually lagging as the strain rate increases. For UHPC without steel fibers, the crack opening displacement (COD) at the unstable fracture load point surges suddenly, while the COD development in UHPC with steel fibers is slower. At lower strain rates, an increase in steel fiber content leads to an earlier sudden increase phase in UHPC's COD; at higher strain rates, the influence of steel fiber content on the sudden increase phase of UHPC's COD is more scattered. The research results of this paper provide a theoretical basis for the calculation and analysis of the dynamic response of UHPC structures under seismic loads. •This study explores the effects of strain rate and fiber content on the fracture parameters of UHPC.•This study proposes a fracture criterion for UHPC based on the J-criterion.•This study explores the development law of cracks and the mechanism of crack development t