Properties Research And Mechanism Analysis Of Shotcrete In A Hot-dry Environment Of Hightemperature Geothermal Tunnel

With the development of tunnel construction toward longer,larger and deeper tunnels,thermal damage caused by high-temperature geotherm has become increasingly prominent.In these tunnels,hot-dry environments are ubiquitous.Due to the micro-structure and meso-structure degradation of cement-based material caused by too fast temperature rising and water loss,physical and chemical shrinkage of concrete,and non-uniform temperature effects of interface,the damage mechanisms of shotcrete performance are very complex.At present,there have been scarce studies concerning the properties of shotcrete used in hot-dry environments of high-temperature geotherm tunnels,which have failed to grasp the nature of concrete performance degradation thoroughly.In view of this,the damage mechanism and improvement measures for shotcrete properties are explored in this study by taking the shotcrete cement-based material and the shotcrete-rock bonding surface as the research objects.The study is carried out from four aspects:The microscopic pore structure and fracture performance of shotcrete,as well as the restrained shrinkage and interface fracture properties between shotcrete and rock.The main research contents are as follows:(1)Temperature grades of surrounding rock are identified based on the preliminary research combined with the temperature environment investigation of high-temperature geothermal tunnels,and a laboratory method for simulation of hot-dry environments is developed.The mix proportion of shotcrete for tunnel engineering is designed in accordance with specifications,and the test conditions are set using three parameters,i.e.surrounding rock temperature,base material modification component and fiber type.(2)By employing mechanical properties test and mercury intrusion porosimetry(MIP),the strength and pore structure parameters of concrete are obtained,and the influences of surrounding rock temperature on the concrete strength,porosity,pore size and pore size distribution are investigated under hot-dry environments.The performance degradation mechanism is analyzed through comparison with the standard curing environment.For a 100?hot-dry environment,the effects of silica fume and fiber materials on improving the mechanical properties and pore structure of concrete are explored.The fractal dimension of pore structure is calculated separately with the Menger sponge model and the fractal model based on thermodynamic relationship,which proves that the fractal model based on thermodynamic relationship is more suitable for solving the fractal dimension of concrete in hot-dry environments.The strength model considering the influence of porosity,pore size distribution and pore structure morphology is established,which can describe the relationship between strength and pore structure parameters.(3)The fracture performance of concrete is examined with a notched three-point bending beam,and the effects of surrounding rock temperature on the fracture parameters are obtained.It is found that fiber materials can significantly improve the fracture performance of concrete in hot-dry environments.By utilizing the double-K fracture model and the K_R-curve crack propagation criterion,the fracture energy,initial fracture toughness,unstable fracture toughness and crack extension resistance of concrete are calculated.Further,the expression for opening displacement on the fracture process zone is derived based on the Paris'displacement formula and the principle of linear superposition.Consequently,numerical simulation is performed on the whole process of concrete crack propagation,which is in good agreement with the test results.The correlations of fracture parameters with the ratio of initial crack length to beam height and specimen height are clarified.(4)Through fabrication of a testing device for rock-concrete restrained shrinkage,the relationship between restrained shrinkage deformation and age of concrete are investigated under hot-dry environment,as well as the inhibitory effects of steel fiber and special admixture on the shrinkage deformation.Meanwhile,the mechanical model of rock-concrete restrained shrinkage is established,and the interface restraining force and the stress and strain distributions on concrete section are calculated by using this model.(5)Wedge splitting specimen test is performed to examine the fracture performance of rock-concrete bonding surface,thereby analyzing the influence of surrounding rock temperature on the interface fracture performance.Water transfer printing technique is utilized to fabricate the digital speckle patterns,while digital image correlation technique is employed to obtain the whole field displacement value in the calculation area.The size of interface fracture process zone and the opening and sliding displacement of fracture area under arbitrary load are measured,and the evolutionary course of interface fracture process zone is quantitatively described.Finite element modeling of rock-concrete specimens is carried out based on the experimental data,and the initial fracture toughness of rock-concrete interface is calculated by the interaction integral method.Cohesive force distribution at fracture process zone is derived using genetic algorithm,based on which the unstable fracture toughness of rock-concrete interface is calculated.The variation of fracture toughness and mode ratio with temperature is clarified.(6)Rock-shotcrete slabs are formed by wet spraying technique,and the bond strengths of the specimens are determined in the standard curing and 100?hot-dry environments through core-drilling and drawing test,thereby verifying the the improvement effect of steel fiber and special admixture on bond strength in hot-dry environments.