Research Of Multi-ion Transport Mechanism And Ionic Reaction Behavior In The Concrete Under The Electric Field

In the reinforced concrete structure(RCS),which is exposed to coastal environments in the long term,the adverse phenomenon such as steel corrosion,concrete cracking,reduction in strength and etc.will happen owing to the invasion of harmful media such as chloride ions,sulfate ions,and magnesium ions.Among the above-mentioned phenomenon,the steel corrosion is the most important and universal factors which cause the loss of RCS durability.Inhibiting the steel corrosion is beneficial for improving the structural durability.Through the effect of electric field,migrating harmful media out of the RCS,or promoting the cathodic polarization of the steel bar to decrease the velocity of anodic reaction(oxidation reaction of iron),the steel corrosion can be effectively inhibited.Therefore,in this paper,based on the mechanism of multi-ion transportation and reaction behavior,a model about multi-ion transportation and reaction in the concrete under the electric field is putting forward,and it will be used to explore the influencing factor of multi-ion transportation and reaction behavior.The research work in this paper can be concluded as:1)Investigating the diffusion coefficient of multi ions in the concrete.Exploring the multi-ion diffusion coefficient based on Nernst-Einstein equation and the relationship between ionic concentration and electrical conductivity,and putting forward the multi-ion diffusion coefficient model;exploring the ionic diffusion coefficient in the two-phase composite based on the multi-phase composite theory;comprehensively considering the influence of multi-ion concentration and components of concrete,putting forward a predicting model of multi-ion diffusion coefficient in the concrete based on multi-phase composite theory,and using the experimental data to verify the predicting results of this model.The results show that,compared with the traditionally ionic diffusion coefficient model,this model can directly predict the diffusion coefficient in the concrete based on ionic species and concentration,and the predicting result is more rational.2)Investigating the mechanism of multi-ion transportation in the concrete under the electric field.Based on the mechanism of ionic diffusion,electro-migration and electroosmosis,establishing the multi-ion transportation model in the concrete under the electric field;through comparing with the experimental data,investigating the difference of ionic transportation in the constant potential,electric neutrality and Gauss’ s law models;through comparing ionic transportation amount,electric flux and power consumption,investigating the difference of ionic transportation in the concrete under the electric field with current and boundary as boundary;based on electric double layer theory and the relationship between dielectric constant and electric field frequency,putting forward a multi-ion transportation model considering electric field frequency and investigating the influence of electric field frequency on the ionic transportation.The results show that,the modeling results of electroneutraility and Gauss’ s law model are similar and conforms the experimental result better,which differs from that of constant potential model.The ionic transportation amount has no relation with the boundary type of electric field,it depends on the electric flux in the concrete and power consumption.The electroosmotic velocity of ion has obvious change as the electric field frequency increases,while the diffusion and electro-migration velocity of ion have nearly no change.3)Investigating the ionic reaction behavior in the concrete under the electric field.Based on the mechanism of hydrogen evolution reaction on the steel,investigating the influence of potential distribution on the velocity of hydrogen evolution reaction and hydrogen embrittlement risk of steel bar;based on the mechanism of electrochemical reaction on the anode,investigating the influence of potential distribution on the type and velocity of anodic reaction;through the electrochemical chloride removal(ECE),mercury injection and chloride ion titration experiments,investigating the spatial distribution of ionic concentration and porosity in the concrete as time goes on,and using the experimental data to amend the ionic transportation model.The results show that,as the water to cement ratio increases or diameter of steel bar decreases,the risk of hydrogen embrittlement of steel bar induced by hydrogen evolution reaction increases.On the anode made of active metal,the electrode reaction only includes oxidizing reaction of metal;on the inert anode,the electrode reaction consumes hydroxyl ions,but the consumption rate is lower than the production rate of hydrogen evolution reaction on the steel bar.Porosity evolution in concrete is time-and spatial-dependent,the ionic transportation considering porosity evolution can improve the accuracy of modeling results.4)Investigating the influence of spatial distribution of electrode on the ionic transportation and reaction behavior in the concrete.Establishing the multi-ion transportation and reaction model in the concrete under the electric field,and using it to investigate the influence of spatial distribution of steel bar on the ionic transportation in the concrete;exploring the influence of spatial distribution of inert anode on the transportation of corrosion media;exploring the influence of spatial distribution of anode made of active metal on the velocity of anodic reaction on the steel bar.The results show that,the influence of stirrup on the ionic transportation in the concrete has dual character,it can accelerate the ionic transportation in the concrete cover while delay the ionic transportation in the region enclosed by the stirrup.For increasing the emigration efficiency of chloride ions,the position of inert anode should be determined by the chloride ions distribution.For decreasing the velocity of anodic reaction and corrosion current on the steel bar,when the total mass of active metal anode is same,reducing the space of anode can improve more protection effect compared with enlarging the size of anode.5)Based on protection effect and work efficiency of system,through the numerical modeling results of multi-ion transportation and reaction model,designing the parameters of electrochemical protection and rehabilitation systems.For the sacrificial anode cathodic protection(SACP)system,putting forward a parameter,ration of current produced by anode corrosion and corrosion current on the steel bar without SACP,to predict the effect of SACP system,and determine the proper parameter to generate tradeoff between effect and efficiency of the system.For the SACP system,the results show that ratio of current produced by anode corrosion used to inhibit steel corrosion current is less than 30%;increasing ration of current produced by anode corrosion and corrosion current on the steel bar,the protection effect of system increases with work efficiency decreasing,applying current produced by anode corrosion which is 5.0-6.5 times steel corrosion current can generate tradeoff between effect and efficiency of system,and it can also avoid the hydrogen embrittlement of steel bar.