Damage Mechanism Of Carbonate Crystallization In Shotcrete Shotcrete Of Dolomite Tunne

Concrete is a typical main material of buildings,bridges and tunnels,etc.It is often used in various environments（containing chloride salts,sulfates,carbonates,etc.）,and is susceptible to erosion and corrosion.Tunnel engineering is located in a complex underground environment,and its working conditions are often closely related to surrounding rock and groundwater.The research group’s previous research shows that dolomite contains carbonate or sulfate,and groundwater can easily form an erosive solution containing CO₃^2-or SO₄^2-after flowing through the dolomite.This erosion will affect the strength and durability of primary support and secondary lining concrete,shortening its service life.In this paper,combined with the engineering problem of concrete carbonate solution crystallization damage in dolomite tunnels,and based on the dry-wet cycle accelerated deterioration test,this paper explores the influence of carbonate solution on the durability of shotcrete.The main work and research results are as follows:（1）Carry out physical and mechanical tests of shotcrete under carbonate erosion,and find out the evolution law of the physical properties of shotcrete under different erosion ages.The mass and volume of the sample showed a trend of first increasing and then decreasing.There are many crystallization products in the early stage of ordinary shotcrete erosion,and the mass and volume changes significantly;mechanical tests show that the increase in crystallization products can temporarily increase the compressive strength and dynamic elastic modulus of shotcrete,and delay the damage cycle.However,the increase of reaction products will deepen the carbonation depth of shotcrete.（2）The CT tomographic images of sprayed concrete at various erosion ages were obtained through CT scanning tests,and the mesoscopic damage characteristics of shotcrete at various erosion ages were qualitatively analyzed based on the gray histogram technology;the CT tomographic images were enhanced and reduced with the help of image processing technology.Noise and threshold segmentation to obtain clear two-dimensional tomographic images and pore features.（3）Based on the three-dimensional reconstruction of CT images,a three-dimensional digital model of shotcrete was established,and the three-dimensional pore characteristics of shotcrete under various erosion ages were studied,and the variation laws of porosity,pore diameter,and pore fractal dimension were obtained,and a model based on Compressive strength and dynamic elastic modulus damage equations for pore features.The damage trend of the internal structure of shotcrete was explored from the mesoscopic point of view,which provided a reference for the durability evaluation of concrete.（4）X-ray diffraction analysis（XRD）and scanning electron microscopy（SEM）were carried out to reveal the corrosion products and microstructural damage characteristics of sprayed concrete under carbonate solution erosion,indicating that:under carbonate solution erosion,sprayed concrete crystallization products It is dominated by calcium carbonate crystals,and its microstructure gradually evolves from dense to loose and porous,accompanied by the continuous expansion of pores and micro-cracks;the damage in the early stage of carbonate solution erosion is mainly the dissolution and crystallization of gelatinous substances,while in the later stage of erosion,it is easy to Affected by the wet-dry cycle.Therefore,the erosion of carbonate solution will seriously affect the durability of shotcrete,which needs to be paid attention to in design and construction.