Study On Properties Of Shotcrete Subjected To Composite Salt Erosion Considering Initial Damage

Shotcrete is an important part of tunnel initial support,and its service environment is complicated.Salt erosion is one of the important factors causing its damage and failure.The internal porosity of shotcrete is much higher than that of ordinary concrete,and it is easy to be affected by improper construction of structure,load deformation and other factors,resulting in different degrees of initial damage defects,which will lead to the acceleration of salt penetration and greatly reduce its durability.Therefore,considering the impact of initial damage,it is more in line with the engineering practice to carry out research on durability of shotcrete under salt erosion,which can provide theoretical reference for durability prediction of shotconcrete materials under complex environment.In this paper,based on practical projects and considering the damage caused by different factors to shotcrete,the durability deterioration of shotcrete with initial damage under composite salt environment is studied on the basis of sulfate and chlorine salt environment.Saturated water absorption rate,mass change rate and relative dynamic elastic modulus were used to characterize the erosion deterioration law of shotcrete.The influences of initial damage degree,curing age,soaking method and erosion salt type on salt erosion deterioration of shotcrete were analyzed.The chloride ion concentration of specimens at different depths was measured,and the chloride ion penetration model of shotcrete based on grey correlation was established.Combined with the SEM-EDS analysis and XRD test technology,the damage mechanism of initial damage shotcrete subjected to salt erosion was revealed.The main conclusions are as follows:(1)When the initial damage degree exceeds 0.21,the chloride ion concentration in specimens at 30 and 150 days reaches 0.712 and 0.892,respectively,which is significantly higher than 0.613 and 0.786 of specimens with 0 damage degree,indicating that the existence of initial damage reduces the salt-erosion resistance of specimens.(2)The corrosion degree of the half-soaked specimen is more serious than that of the full-soaked specimen.Due to the capillary action,ions in the salt solution are largely gathered in the upper part of the specimen,resulting in damage and cracking of the upper part.(3)The results of chloride ion penetration model based on grey correlation show that the correlation degree of three factors on chloride ion permeability of shotcrete from high to low is:soaking method>initial damage degree>curing age.(4)Hydration product AFm combined with sulfate to form expanded ettringite is the main reason for the deterioration of the durability of specimens subjected to sulfate attack.The small volume of Cl~-in the composite salt solution has a fast diffusion rate and combines with the hydration products to form Friedel salt,which inhibits the formation of expanded ettringite and improves the durability of the test piece under sulfate attack environment.(5)The microscopic analysis results show that the larger the initial damage degree is,the more serious the crack is caused by salt erosion,and the amount of erosion products is roughly proportional to the initial damage degree.In the semi-soaking mode,there are many internal pores and loose structure,which are seriously eroded by salt solution,and obvious penetrating cracks appear inside.