Durability of Manufactured-Sand-Reinforced Concrete and Its Wet Joint Prepared in Plateau Environment under Corrosion, Freeze–Thaw Cycle and Their Coupling Effect

In order to explore the durability of manufactured-sand-reinforced concrete and its wet joint in a plateau environment, an integrally formed (IF) specimen and a wet joint specimen with punched interface (PI) made up of manufactured sand concrete were prepared in the simulated plateau environment. On the one hand, the accelerated corrosion test on IF and PI specimens was conducted to investigate their durability under corrosion. On the other hand, the freeze–thaw cycle test was carried out on IF and PI specimens to evaluate their frost resistance. Subsequently, the accelerated corrosion test was continuously performed on IF and PI specimens subjected to freeze–thaw cycles. The development of surface cracks, the corrosion morphology of internal steel bars, the actual corrosion rate, the section loss of corroded steel bars and the degradation of mechanical properties of steel bars after corrosion were analyzed. Moreover, the microstructural changes of specimens after different freeze–thaw cycles and corrosion degrees were observed. The results show that during single corrosion, the development of crack width of concrete, the increase in actual corrosion rates of steel bars and the degradation of mechanical properties of steel bars for IF and PI specimens before the theoretical corrosion rate of 6% were relatively slow, and once the theoretical corrosion rate exceeded 6%, these began to accelerate. The development of concrete cracks and the distribution of crack width are affected by wet joints. Compared with IF specimens, the average and maximum longitudinal crack widths of PI specimens increase by 0–22.54% and 12.16–21.95% for different freeze–thaw cycles, respectively. The frost resistance of the PI specimen decreases due to the existence of a wet joint. After freeze–thaw cycles numbering 50, the difference in frost resistance between IF and PI specimens obviously increased. Compared with IF specimens, the nominal yield strength, nominal ultimate strength and elongation of PI specimens after freeze–thaw cycles numbering 25~100 and corrosion with the theoretical corrosion rate of 6% decreased by 5.56–9.11%, 4.74–6.73% and 23.08–28.72%, respectively. The combined effect of freeze–thaw cycle and corrosion has a great influence on the ductility of steel bars.