Effect Of Carbonization Resistance Of Shotcrete Based On Initial Damage

Shotcrete is a material that is sprayed to the sprayed surface at a fast speed by a conduit under high pressure and instantly compacted and formed,which is widely used in tunnel support because of its short setting time and high early strength.Due to the complex material composition,rapid hydration rate,unfavorable construction environment and other factors lead to different degrees of initial damage defects in the process of shotcrete forming,resulting in increased permeability and accelerated CO₂ diffusion rate,which accelerates the carbonation of the lining structure with shotcrete as the main body and seriously threatens the safety of the structure.Therefore,it is important to study the development law and microscopic mechanism of the anti-carbonation performance of shotcrete under the action of initial damage to improve the durability,service life and safe operation of tunnel lining structure.In this paper,we take shotcrete as the research object,consider the force characteristics of tunnel,prefabricate initial damage to specimens by universal testing machine,define the initial damage degree by ultrasonic wave velocity,and quantitatively analyze the effect of initial damage degree,quick-setting agent and water-cement ratio on the carbonation resistance performance of shotcrete.Scanning electron microscopy（SEM）,energy spectrum analysis（EDS）and X-ray diffraction test（XRD）were used to test the microscopic morphology and material composition of shotcrete.Based on a large amount of experimental data,the deterioration mechanism of the anti-carbonation performance of shotcrete was analyzed,and a prediction model of the carbonation depth of shotcrete based on the initial damage was constructed.The main conclusions are as follows:（1）With the increase of initial damage,the carbonation depth of shotcrete increases and is higher than the carbonation depth of molded concrete under the same conditions.The carbonation depth of shotcrete increases with the increase of quick-setting agent,and the trend of decreasing and then increasing with the increase of water-cement ratio.（2）Based on Fick’s law and a large amount of experimental data,the initial damage degree influence coefficient K_D=1+γD and molding method influence coefficient K_C were introduced on the basis of X=at^b form carbonation model,and the carbonation depth prediction model of shotcrete was fitted by Origin software:X_D=K₀K_DK_Ct^0.5,and the relative life:t₁/t₂=1/（K_D）².The larger the initial damage degree,the greater the carbonation depth of shotcrete and the smaller the relative carbonation life.（3）The composition and content of substances in each group of specimens were analyzed by SEM and XRD test methods,and it was found that the main hydration products were Ca（OH）₂ and C-S-H gel,and the carbonation products were mainly Ca CO₃,and the amount of quick setting agent admixture and water-cement ratio would affect the hydration rate but not change the type of hydration products.The greater the initial damage degree,the more pores and microcracks inside the shotcrete,i.e.,the looser the microstructure,but the carbonation produces Ca CO₃ to fill the pore structure,and the overall compactness is slightly improved compared with that before carbonation.