Erosion Of Chloride In Reinforced Concrete Structures-Migration Mechanism Of Multi-field Coupling And Active Electric Corrosion Control

In recent decades,a large number of siginificant reinforced concrete?RC?engineerings have been,are being or will be built.Their durablity must affect the development of the national economy and threaten the life satety of the people.Chloride-induced corrosion is one of the major factors in degrading the durability of RC structures.Considering that the corrosion of RC structures is a series of electrochemical reactions,electrochemical corrosion control techniques are the most effective and direct ways to improve the service life of RC structures.Tranditional electrochemical corrosion control techniques include cathodic protection?CP?,electrochemical chloride extraction?ECE?and electrochemical realkalization?ER?.Since CP originats in petroleum,chemical industry,shipbuilding,etc.,it arrests the corrosion of RC structure only from the thermodynamics based on the corrosion control in liquid erosion medium,and does not consider the dynamic factors according to the corrosion control in multiphase and porous concrete media,and thus,this leads to a low effectency.In addition,ECE and ER retard the corrosion only from the dynamic factors,and their theoretical models lack profound basic theory in achieving the corrosion control for RC structures.Therefore,the paper aims to solve key‘bottleneck'problems in the chloride erosion and the corrosion control for RC structures.By employing the theoretical analysis,numerical simulation and experimental verification,the chloride erosion based on the physicochemical essence of the transport and the active electric corrosion control have conducted.The specifical research is as follows:?1?Based on surface complexation reactions and dissolution/precipitation reactions,the thermodynamic model was established to investigate the essential physicochemical interactions between pore solution and cement hydrates.The effects of chloride concentration,temperature,and saturation degree on the physicochemical interactions were discussed in detail using the resulting thermodynamic model.The published experimental results indicate that the resulting thermodynamic model can accurately reflect the adsorption capacity of cement hydrates for chloride ions.The results show that the physical adsorption of C-S-H for chloride firstly occurs with the ingress of chloride,and then,the chemical adsorption of AFm occurs.Further,the relationship between the free and bound chloride ions was presented.Additionally,the physicochemical interactions between pore solution and cement hydrates were discussed under the combined action of carbonization and the ingress of chloride.It is found that the CO₂ penetration causes the gradual dissolutions of CH,AFm and C-S-H,which further affects the adsorption of C-S-H for chloride ion.Then,the relationship between the free and bound chloride ions at different carbonization degrees was established.?2?Based on the theory of the species transport in porous materials,the developed thermodynamic model was couple into Nernst-Planck equations using a non-iterative sequential split operator approach,and the convective action of water transport was taken into account simultaneously.Thus,the theoretical model of the ionic species transport in concrete materials was established.Then,a COMSOL-PHREEQC interface was built using MATLAB language,and this could achieve a 3D numerical simulation of the ionic species transport in saturated or unsaturated concrete materals.In this numerical model,the spatial and temporal distributions of ionic concentration in pore solution,the content of cement hydrates and the saturation degree can be obtained,and the changing trend of the porosity can be presented.In addition,the published experimental results were used to validate the accuracy of the developed numerical model.?3?From the thermodynamic insight of the corrosion control,the optimitzation method of the electric field to control the corrosion of RC structures was established based on the numerical model of the distribution of electrical field.The potential of the reinforcing steel and the service life of electrically driven corrosion control system were constrained by the protection potential and the life expectancy of the protected RC structure,respectively;the location and the area of anode overlay,and the amplitude of the output voltage of electrically driven corrosion control system were optimized to achieve the minimization of the total cost of electrically driven corrosion control system.The results indicate that the power consumption,the anode materials and the total costs after the numerical optimization have a significant decrease.?4?The corrosion control electric field thrown into was coupled into the developed transport model to establish the theroical model of the active electric corrosion control,and the electrode kinetic reactions were taken into account simultaneously.In this numerical model,the spatial and temporal distributions of the ionic concentrations in the pore solution,the content of cement hydrates and the saturation degree,the changing trend of the porosity and the local current densitis at the steel/concrete interface can be obtained.Moreover,the experimental results confirmed the accuracy of the numerical model.The results indicate that the developed theroical model can prevent the ingress of chloride ion,improves the pH value and the content of CH adjacent to the steel bar.?5?Two types of reduced-scale bridge pier were prepared,and the active corrosion control technique was applied to fully verfy the reliability of the established theroical model and provide a paradigm for the corrosion control of pratical RC structures.The results of the local current densities and the potential distribution of the steel indicate that the optimized active corrosion control system can sufficiently protect the reinforced steel,and avoid hydrogen embrittlement.In addition,the numerical and experimental results of the ionic concentration in the pore solution indicate that the active electric corrosion control technique can arrest the penetration of chloride ion into the concrete from the external environment,and contribute to the removal of chloride from the chloride-contaminated concrete protective layer.