Research on the long-term durability of concrete has continued due to its widespread application in construction. Freeze–thaw cycles significantly impact concrete durability, particularly in regions with harsh climates. While most studies focus on the material properties of concrete, limited research has addressed the performance degradation of reinforced concrete structures. This study investigates the freeze–thaw resistance of RC beams made with 35 MPa concrete, with particular emphasis on the influence of air content on flexural performance. RC beams were exposed to freeze–thaw cycles using the air freeze–thaw method and the ASTM C666/C666M-15 water freeze–thaw method. Their flexural behavior was evaluated through four-point bending tests. The results showed that low-air-content RC beams exhibited notable reductions in yield load and energy absorption capacity after freeze–thaw cycles, indicating decreased strength and ductility. Conversely, RC beams with appropriate or high air content exhibited minimal reductions, demonstrating superior freeze–thaw resistance. These findings underscore the importance of optimizing air content to enhance the durability of RC structures in harsh environmental conditions.

The aim of the research is to investigate thixotropy in high-fluidity normal-strength cement paste containing PVA and borax. Although high-fluidity concrete offers advantages such as reduced ease of placement and improved concrete quality, it also presents disadvantages, including increased formwork pressure and a higher risk of segregation. Thixotropy is the rheological property of securing the stability of fresh state cement-based materials, depending on the applied forces on them. Although many studies and techniques have suggested that clay can impart thixotropy to cement-based materials, the thixotropy imparted by clay is relatively weak, and recovery is slow. In this research, thixotropy was applied to cement-based materials using the cross-link between PVA and borax. Based on the tests conducted, the cross-link between PVA and borax remained effective within the cement-based materials, and thixotropic behavior was observed. Additionally, the setting time delay due to using borax was resolved by reduction slag. Finally, based on the optimal proportions of PVA, borax, and reduced slag for high-fluidity normal-strength cement paste with high thixotropy, the material was applied to simulate formwork gaps and evaluate its leakage-reducing performance. The high-fluidity cement mortar showed progressive performance in reducing formwork leakage, particularly with PVA and borax, due to its high thixotropy. Based on the research results, it is possible to impart thixotropic behavior to cement-based materials using PVA and borax, and to safely use high-fluidity cement-based materials.