Mesoscopic Erosion Mechanism And Shear Properties Of Shotcrete Materials Under Sulfate-containing Environments

Sulfate attack and shear failure are the common types of distress appeared on tunnel shotcrete structures.Among them,sulfate-caused damage on shotcrete is a long-term and complex process involving chemical,physical and mechanical aspects and macro-,meso-and micro-scales.How to effectively reveal the associated mechanism of sulfate attack on shotcrete,and to shed light on the time-dependent evolution of its shear resistance under sulfate rich environments are of great significance in science.This study,relied on the National Natural Science Foundation of China(no.51578091),mainly investigated the mechanism of sulfate attack on tunnel shotcrete and the evolution of its shear resistance under sulfate rich environments,using the methods of“dry-spray”,indoor physical and mechanical tests,sulfate attack test and multiscale detection techniques,in combination with the related knowledge on material mechanics,material science,geotechnical engineering,micro-dynamics and the theory of pore.The main research contents and results are as follows:(1)Through in-site and indoor tests,we obtained the fundamental physical and mechanical properties of standard cubic specimens of tunnel shotcrete,and their shear properties in terms of the effects of water-to-binder ratio(w/b)and normal stress.Results showed that w/b and normal stress significantly affected the shear properties of shotcrete specimens.For example,a higher normal stress condition(?_N?4 MPa)could induce the shear failure of specimens,but a lower normal stress(?_N<4 MPa)mainly caused the sliding failure.(2)The long-term evolution of mass and uniaxial compression strength of shotcrete specimens were obtained by the long-term exposure test of sulfate attack.Results show that the mass and strength of shotcrete under different exposure conditions have different evolution characteristics over time.Compared with the chemical erosion condition,the mass growth of shotcrete specimens in the early stage is small and the mass loss in the later stage is large;For the strength loss,the ultimate strength loss of shotcrete specimens under physical erosion condition is less than that under chemical erosion condition.(3)Based on the analysis of the microscopic pore structure characteristics of shotcrete materials and the classical pore classification method,a new pore classification method of cement based multi-porous materials is proposed.Based on this method,the characteristics of different mesoscopic pore grades in sprayed concrete(pore and pore throat)were analyzed.Results show that the higher the water-to-binder ratio is,the higher the total porosity of shotcrete specimen is,and the smaller the volume of uninjurious pores(gel pores and transition pores),and the larger the volume of detrimental pores(capillary pores)of the three grades are.(4)Based on micro-dynamics,theory of pore,material science and micro-scanning analysis,this study discussed the distribution characteristics of sulfate transfer mechanism,the reaction principle and crystallization rule in shotcrete,and expounded the mesoscopic mechanism of sulfate attack.Results showed that the mechanism of sulfate attack was significantly different under different exposure conditions.Exposure-I mainly caused chemical sulfate attack(CSA),the mesoscopic mechanism of CSA was a long-term process where the sulfate attack products were produced in mesoscopic detrimental pores and smaller pore throats(PT-I and PT-II)at the early stage(Filling effect)and the reactions between sulfate ions and cement slurry consumed cementing materials at the middle and late stages(Hollowed effect);Exposure-II mainly caused physical sulfate attack(PSA),the mesoscopic mechanism of CSA was a long-term process where the sulfate attack products were produced in larger detrimental pores(DP-III)at the early stage(Filling effect)and the cracking was induced by crystallization pressure at the middle and late stages(Cracking effect).(5)The time-term evolution of the shear characteristics of shotcrete specimens was studied by direct shear test at different sulfate exposure periods.Results showed that sulfate attack significantly affected the time-dependent evolution of shear strength components,failure criteria and failure modes of shotcrete specimens in response to shear stress-shear displacement curves and coulomb shear strength parameters.With the increase of exposure time,the nominal cohesion decreased linearly,while the nominal internal friction angle decreased slowly and then accelerated,the adhesion component showed a linear decreasing trend,and the dilatancy rate gradually decreased,while the friction component showed a slight increase first and then gradually decreased.Sulfate attack may lead to the degradation of shear strength criterion of shotcrete specimens from bi-linear Mohr-Coulomb(M-C)criterion before exposure to linear M-C criterion after exposure,and the failure mode after exposure was mainly sliding failure.(6)Based on shear test data,combined with damage mechanics theory and statistical mathematical model,a shear model of tunnel shotcrete considering sulfate attack damage was established.Based on test data,the damage mechanics theory and mathematical statistical model,the shear model of shotcrete considering the effect of sulfate-caused damage could well predict the basic shear performance of shotcrete under sulfate attack conditions,and the model error is within a controllable range.