| With the massive construction of underground infrastructure in China,the development and utilization of large-scale and multi-functional underground space in coastal megacities will inevitably reach 40m～100m.The groundwater in deep coastal underground spaces is deeply affected by the ocean,with high concentrations of chloride ions.Meanwhile,rail transportation will become the main mode of transportation to connect each node,facing serious risks of stray current leakage.Therefore,identifying the transport behavior of chloride ions under high water pressure coupled with stray current effects and revealing the related degradation mechanisms are considered to be one of the main problems that need to be addressed in the future durability design of deep-buried underground structures.The mechanism of chloride ion transport under the action of coupled high hydraulic pressure and stray current,and the deterioration mechanism of reinforced concrete in deep environment are considered as one of the urgent problems to be solved in the durability design of deep underground structures in the future.Firstly,the degradatio n test equipment simulating deep environment was developed.Based on the self-developed equipment,the microstructure evolution mechanism and chloride migration law of concrete in deep environment were revealed.The molecular scale mechanism of chloride ion transport was revealed through molecular dynamics simulation,and the experimental results were supplemented and verified.On this basis,the “ITZ effect induced by high water pressure and stray current” was analyzed.A fluid-solid-chemical coupled mass transfer model of concrete considering phase assemblages and microstructure evolution was established.Considering the particularity of deep buried environment,the deterioration mechanism between corroded reinforcement and concrete was revealed,and an instantaneous conductivity method for evaluating the chloride diffusion coefficient of reinforced concrete in deep buried underground engineering was proposed.Further,the bond deterioration model of corroded reinforcement was established.The research results can provide a reference for the durability design of the deep-buried rail transit project.The main research contents and results in this thesis are shown as follows:(1)The corrosion system simulating deep environment with independent intellectual property rights was used for experimental research(Chinese invention patent: CN112986124B).The laws of microstructure evolution and phase evolution of concrete under the coexistence of stray current and high hydraulic pressure,as well as the mechanical mechanism of crack propagation,were revealed by a series of factorial tests.The research results show ed that the stray current and hydraulic pressure changed the microstructure of the internal pores of concrete,resulting in higher specific surface area,large r porosity,larger pore size,and transition to ink-bottle shaped pores.The specific surface area and volume of micropores were significantly increased,while the volume ratio of mesopores to micropores was reduced.In addition,the study found that the diffraction peak intensity of Friedel salt decreased with the increase of the stray current density,indicating that Friedel’s salt would be decomposed under the stray current environment,while calcite,ettringite and unhydrated cement were difficult to be electrochemically decomposed.The calcium leaching depth represented by the reduction of calcium hydroxide content was basically consistent with the chloride ion migration depth.The development of dominant microcracks in concrete reaction surface was observed and studied,and the action mode of “local fracturing” effect induced by high water pressure was revealed for the first time,thus the mechanical action framework of “local fracturing” was established.Further analysis showed that the weakening of elastic modulus was mainly due to phase decomposition caused by stray current,and the “local fracturing” of pores on concrete surface caused by hydraulic pressure was the main reason for new cracks.The study also showed that the evolution of microstructure would affect the transport of chloride ions.Under the coexistence of high hydraulic pressure and stray current,chloride ions mainly migrated along the pores.The main reason was that the desorption of free chloride caused by electric field and phase decomposition could not recover the adsorption site and maintain the free state.(2)The transport mechanism of chloride solution was studied based on molecular dynamics(MD)simulation,which further verified and supplemented the experimental results.Under different electric fields,the interaction between water molecules,chloride ions and C-S-H nanopore surface was studied at molecular scale.The structure and molecular dynamics dynamic properties of water and chloride ions under different electric fields were analyzed.The following conclusions can be drawn: the electric field made the water molecules change the layered structure,resulting in density stratification and obvious orientation preference.With the increase of electric field intensity,the number of water molecular layers in the confined nanopore decreased.The bond length of O-H and the dipole angle of H-O-H changed significantly.In addition,the electric field changed the type of H bond,and further affected the distribution of core-shell structure.In the environment of larger electric field,water molecules have stronger order and more concentrated distribution.In the environment without electric field,chloride ion shows extremely low permeability,and shows a non-electric field induced adsorption state due to the increase of nominal radius and strong chemical correlation of ion clusters.With the increase of electric field strength,the hydrated chloride ion “cage” and ion cluster are destroyed,and the chloride ion permeability is significantly increased.(3)The “ITZ effect” induced by high water pressure and stray currents was revealed to have an impact on chloride ion migration.The significant increase in thickness of ITZ and porosity caused by stray currents is the fundamental reason for the huge differences in chloride penetration depth and effective diffusion coefficient among different aggregate gradation specimens.The permeability front was measured by silver nitrate spray method,and the effective diffusion coefficient of chloride ions was calculated,thus the contribution of ITZ and cement matrix to chloride ion migration was obtained.The ITZ porosity distribution was determined by concentric expansion method with overflow criterion,and the ITZ thickness was quantitatively characterized by microhardness.The results showed that the closer to the aggregate,the greater the porosity,and the porosity distribution could be described by“sigmoid” function.The thickness of ITZ in contact with solution increased significantly,which was attributed to the stronger calcium leaching of ITZ.When stray current existed,the diffusion coefficient of chloride ion in ITZ was tens to hundreds of times that of cement matrix.(4)A fluid-solid-chemical coupled mass transport model for concrete considering phase assemblages and microstructure evolution was established.which solved the limitation that the interaction between concrete and environmental solution could not be considered in traditional model.The reliability of the model was verified by accelerated chloride migration test.The developed fluid-solid-chemical coupled mass transfer model was a complete scheme.The grid solution of partial differential equations,governing system,chemical thermodynamic equilibrium,phase assemblages and microstructure evolution calculation methods were fully considered.The calculation based on this model showed that the specific supplementary cementitious material(SCM)could improve the chlorine resistance of concrete.The cement with low reactivity with environmental solution could obtain smaller total porosity and smaller pore size,thus obtaining smaller chloride ion transport depth.(5)The mechanism of bond deterioration between rusty reinforcement and concrete caused by chloride corrosion induced by high hydraulic pressure coupled stray current was revealed.The strength degradation law of ITZ between corroded steel bars and concrete was revealed by microhardness tests.The molecular structure characteristics and the evolution of chemical bonds and functional groups of deteriorated concrete at the ITZ site were revealed by FTIR.In addition,CT scanning was used to construct local rough bodies,Vickers hardness was used to calculate ITZ mechanical parameters,and a bond mechanical model was established.Based on the analysis of the evolution of the electrical conductivity of the unit during the degradation process,an instantaneous conductivity evaluation method for the effective diffusion coefficient of chloride ions in deep environments was proposed,and the physical relationship between the instantaneous conductivity and the effective diffusion coefficient of chloride ions,as well as the internal microstructure of concrete,was derived.The results show that the instantaneous conductivity can reflect the microstructure characteristics of concrete pores(the “path” of chloride diffusion).The established bond stress degradation model and instantaneous conductivity evaluation method provide a reference for building a resilient deep underground space. |
