Study on Performance Evolution of High Air-Content Hydraulic Concrete Under The Ultra-Low Temperature Freeze–Thaw and Load Coupling Action

In frigid regions with extremely cold temperatures, hydraulic projects are mostly subjected to the ultra-low temperature freeze–thaw and load coupling action (UFT-L), so the hydraulic concrete structure faces the challenges of insufficient frost durability. This paper investigates the evolution of typical high air-content hydraulic concrete (H-AHC) performance under the UFT-L, including durability, mechanical properties, and microstructure. Macro-performance test results show that the coupling action reduces the compressive strength and relative dynamic elasticity modulus (RDEM), increases the mass loss (ML), leads to a reduction in the integrity of the specimen and the emergence of the phenomenon of fracture failure, and ultimately leads to the inability to achieve the original frost resistance and strength of the design level. Microscopic test results show that UFT-L coupling action accelerates the degradation of the microscopic pore structure, as evidenced by the increase in average chord length, spacing factor, porosity, and percentage of >50 nm pores, as well as the decrease in the air-voids specific surface area. In addition, using the grey entropy correlation analysis, the correlation results between the macroscopic mechanical properties and microscopic pore characteristics are all >0.9, which achieves the quantitative characterization of the relationship between the mechanical properties and the microstructure after the coupling action of H-AHC. The research results can provide a theoretical basis for the durability design of typical high air-content hydraulic concrete structures in frigid regions.