This study looked at the performance requirements of repair materials for concrete structures in cold regions, systematically analyzing the effects of steel fiber dosage (0.7–2.1%), early-strength agent PRIORITY dosage (6–10%), and their coupling effects on the workability, interfacial bond strength, and freeze–thaw resistance of rapid-hardening ultra-high-performance concrete (UHPC). Through fluidity testing, bond interface failure analysis, freeze–thaw cycle testing, and pore analysis, the mechanism of steel fibers and early-strength agent on the multi-dimensional performance of fast-hardening UHPC was revealed. The results showed that when the steel fiber dosage exceeded 1.4%, the flowability was significantly reduced, while a PRIORITY dosage of 8% improved the flowability by 20.5% by enhancing the paste lubricity. Single addition of steel fibers decreased the interfacial bond strength, but compound addition of 8% PRIORITY offset the negative impact by optimizing the filling effect of hydration products. Under freeze–thaw cycles, excessive steel fibers (2.1%) exacerbated the mass loss (1.67%), whereas a PRIORITY dosage of 8% increased the retention rate of relative dynamic elastic modulus by 10–15%. Pore analysis shows that the synergistic effect of 1.4% steel fiber and 8% PRIORITY can reduce the number of pores, optimize the pore distribution, and make the structure denser. The study determined that the optimal compound mixing ratio was 1.4% steel fibers and 8% PRIORITY. This combination ensures construction fluidity while significantly improving the interfacial bond durability and freeze–thaw resistance, providing a theoretical basis for the design of concrete repair materials in cold regions.

The performance of concrete materials deteriorates under load fluctuation and long-term erosion from the external environment, leading to damage. In order to solve the problems of concrete materials needing repair, poor working performance and the low mechanical strength of ordinary concrete repair materials, this paper explores a series of experimental studies on the preparation of ultra-high performance concrete repair materials and solves the two technical problems of the low early strength and poor fluidity of ultra-high-performance concrete. The effects of different steel fibers and dosages of early strength agents on the working properties, mechanical properties and microstructure of fast-hardening UHPC are discussed. Finally, the mix ratio of fast-hardening UHPC with a 2 h compressive strength of up to 47 MPa and fluidity of up to 300 mm is obtained through the test, which is in line with practical applications in engineering and provides a reference for the design and analysis of fast-hardening UHPC repair materials.