Abstract: In order to explore the composite salt erosion performance of shotcrete with initial damage defects,long-term natural erosion tests on shotcrete with initial damage were carried out by partial immersion and full immersion methods.The effects of initial damage,immersion method and erosion salt type on free chloride ion concentration,macro performance and microstructure of shotcrete were systematically studied,and the influencing mechanism was analyzed.And the erosion damage evolution model of shotcrete with initial damage was established.The results show that the initial damage has different effects on the deterioration process of shotcrete subjected to salt erosion.The greater the initial damage degree,the higher the concentration of free chloride ion at the same test depth,the more obvious the change of mass and relative dynamic modulus during the erosion process,and the more serious the corrosion degree of the specimen.The initial damage below 0.14 has little effect on the deterioration process of shotcrete damage,while the initial damage exceeding 0.21 significantly accelerates the deterioration rate of corroded shotcrete.After 180 days of single salt and compound salt partial immersion erosion,the mass of the specimens with initial damage of 0.30 decreased by 4.55% and 3.82%,respectively,and the relative dynamic modulus decreased to 0.39 and 0.53,respectively.Obvious damage cracking occurred on the surface of the specimen,and block shedding appeared at the corners.While the initial damage degree is the same,the damage degree of partial immersion and single sulfate attack on shotcrete is higher than that of full immersion and compound salt attack.Under the premise that the erosion damage factor is expressed as an exponential function of the erosion age,the obtained damage model curves of shotcrete are in good agreement with the experimental values,and the fitting correlation coefficients are all over 0.97.SEM-EDS and XRD analysis showed that the distribution of ettringite crystals in the initial damage specimens after salt erosion were densely,and the height of crystal diffraction peak was significantly higher than that of the reference group,indicating that the initial damage increased the intrusion channel of salt solution.Friedel salt crystal diffraction peak appears in XRD pattern of shotcrete under compound salt attack,and the ettringite crystal diffraction peak is significantly lower than that in single sulfate attack group,indicating that the presence of Cl^- in compound salt solution slows down the diffusion rate of SO₄^2-.