Study On Deterioration And Evolution Of Bearing Capacity Of Lining In Mountain Tunnels Under Sulfate Corrosion

The erosion and corrosion of tunnel linings in sulfate environments involves corrosion-induced deterioration under complex multifactor coupling actions,including ion diffusion,chemical reactions,and mechanical damage.These “corrosion factors”involve not only a simple “addition” process or pure “multiplication” but also an interaction and mutual amplification process.Currently,the erosion and corrosion of lining structures under sulfate corrosion environments and their bearing capacity deterioration–dissipation constitute a interesting research subject.This topic has important theoretical and engineering significance for large-scale complex tunnel engineering construction in sulfate corrosion environments.Thus,on the basis of the Chongqing General Natural Science Fund Project entitled “Study on the Disaster-Inducing Effect of Multi-Field Coupling Action in a Tunnel at a Gypsum Salt Belt”(Project No.cstc2020jcyj-msxm X0963),we explored the deterioration and damage behaviours as well as the bearing capacity of tunnel linings under corrosive environments by combining experimental study,theoretical study,and numerical analysis.These approaches were applied from the perspective of the coupling action between the corrosion induced by harmful ions to lining concrete and various environmental factors.The main study contents and results are presented as follows:(1)A high-adaptability,corrosion-stress-drying,and wetting cycle coupling testing device was designed.The sulfate ion concentration distributions in the specimens were tested through a laboratory test,and their transport laws under multifactor actions were systematically studied.Regarding the influencing factors,the external environmental factors mainly included the loading level,ion concentration,ion type,and drying and wetting cycle,while the water–cement ratio was considered as the main material factor.The sulfate ion concentration on the concrete surface was acquired through the polynomial fitting of sulfate ion concentration distribution data,and the time-varying laws of surface sulfate ion concentration and the influences of various factors were analysed.The results revealed that the surface sulfate ion concentration increased rapidly in the initial corrosion phase and tended to be steady in the latter phase.In addition,the time-varying laws could be well reflected by an exponential function.Accordingly,a time-varying model of the surface sulfate ion concentration was proposed,and the time-varying boundary conditions for the ion transport equation were established.(2)The micromorphological evolution inside concrete specimens under corrosive action and the growth characteristics of the main corrosion products were both observed through micromeasurements.The corrosion products and their growth forms were found to be consistent under a sulfate single-salt corrosion environment and a sulfate–chloride composite corrosion environment,indicating that the corrosion mechanism of sulfate was not changed by chloride ions.However,the structure exposed to the sulfate–chloride composite environment was affected by more corrosion products,which were particularly averse to reinforced concrete structures.(3)The macroproperty degradation laws of concrete and reinforced concrete specimens under corrosive action were explored through macromeasurements,with an emphasis on the mechanical characteristics of bent reinforced concrete specimens under corrosive action.The results indicated that the eccentric load accelerated the bearing capacity degradation of reinforced concrete eccentric loading columns under a corrosive environment,especially under a high load level.Furthermore,the presence of microcracks inside the concrete and corrosive action formed a mutually amplified load–transport–deterioration coupling effect,thus accelerating the deterioration of structural performance.(4)An unsteady sulfate ion diffusion model considering external load and damage evolution was proposed based on mathematical and mechanical analysis methods.The model was improved on the basis of the existing static diffusion–reaction theory.Moreover,the time-varying model of sulfate ion concentration on the concrete surface was introduced to replace static boundary conditions.Meanwhile,the concept of the“equivalent diffusion coefficient” was abandoned,and a spatiotemporal model of the diffusion coefficient was proposed considering the spatial and temporal differences of the diffusion coefficient in the diffusion domain.Next,the load,temperature,cement ratio,solution concentration,internal strain of expansion and damage evolution were coupled through the diffusion coefficient function and the time-varying model of boundary ion concentration.On the basis of the proposed unsteady sulfate ion transport model,we further considered the sulfate–chloride coupling action to construct a sulfate radical–chloride ion coupling transport model,after which the ion transport characteristics under their joint action were theoretically depicted.The results showed that the chloride ions repressed the sulfate ion transport within the whole corrosion period,but the sulfate ions influenced the chloride ion transport differently.Specifically,the sulfide ions inhibited chloride ion transport in the initial phase and then accelerated their diffusion due to expansion and cracking in the latter phase.This cracking–accelerating effect was more evident at the concrete surface layer,which accelerated steel bar corrosion.(5)A prediction model was proposed for the resistance evolution of tunnel linings in a corrosive environment by combining damage mechanics theory and ion transport models.This model contained three submodels: a concrete strength degradation model under corrosive action,a concrete-reinforcement adhesion degradation model,and a residual reinforcement area model.Then,calculation formulas for the bearing capacity of the tunnel lining under a corrosive environment was further derived,and a “bridge”between the diffusion–reaction process of microcorrosion ions and the macromechanical property evolution of the tunnel lining was built.The results indicate that for the reinforced concrete lining,the attenuation of the bearing capacity triggered by the corrosion in the tensile zone was more serious than that in the compressive zone.Under the same corrosion degree,the level of bearing capacity degradation of the side wall and arch springing is higher than that of the apex of the arch.Furthermore,under corrosive action,the tunnel lining is more susceptible to large eccentric compressive failure.