Durability deterioration and corrosion mechanism of shotcrete in salt lake environment

Salt lake is widely distributed in western China. The high concentrations of chlorine salt, sulfate, and magnesium salt in the soil and groundwater are the major causes leading to the deterioration of lining shotcrete and the corrosion of reinforcement. In order to systematically study the deterioration rules and mechanism of the durability of shotcrete lining in salt lake corrosion environment, the dry-wet alternate method was employed to simulate the corrosion of tunnel lining, and the durability experiment of shotcrete was carried out with 5%Na2SO4+5%MgSO4+3.5%NaCl mixed solution. Relative dynamic elastic modulus, mass change and relative compressive strength were measured. After experiencing corrosion, mineral composition and microstructure of corrosion products, pore structure and ion content were tested by using XRD, TG-DSC, SEM & EDX, liner traverse method and electrochemical method. Acid-soluble sulfate content may obviously increase with the increase of the dry-wet cycles, and water-soluble chloride and sodium content slowly increased. However, the pH value of water-soluble calcium and concrete may decrease. The corrosion process of shotcrete is composed of three stages, including the formation stage of brucite, gypsum and ettringite, the stage of C-S-H decomposition and thaumasite formation, and the formation stage of M-S-H. Eventually, under the coactions of swelling stress and crystallization pressure from thaumasite, brucite, gypsum, ettringite and crystallized salt, the amount of pores and air content in shotcrete were increased, so that the formation of netty cracks could result in the quick deterioration of shotcrete in physical and mechanical properties. The microcracks of mould concrete quickly propagated under the crystallization pressure of salt and truned to macrocracks after being connected with pores. Steel fibers can remarkably alleviate the internal stress of shotcrete, and hence can improve the corrosion resistance of shotcrete. © 2019, Editorial Office of China Civil Engineering Journal. All right reserved.