Molecular Dynamics Simulation Of Chloride Transport And Steel Corrosion Under The Coupling Of Chloride And Fatigue Load

The main cause of durability failure of reinforced concrete structures is steel corrosion,which follows the following process:The chloride transmit to the surface of steel bar in concrete,which leads to the corrosion of steel bar,the volume of corrosion products expands and extrudes concrete,and then produces the rust expansion force.With the passage of time,the rust expansion force changes with the corrosion rate and the content of rust products,which eventually leads to the peeling of the concrete protective layer,and cracks appear on the surface of the reinforced concrete structure.As the cracks appear,more chloride ions enter the concrete.Internally,the rust is further aggravated,eventually leading to structural durability failure.The speed and quantity of chloride transport in the nanopore will affect the corrosion process of the steel and the content of the corrosion products.The content,structure and micromechanical properties of the corrosion products will affect the rust expansion of the corrosion products.In the actual service process,the reinforced concrete members are subjected to various changes such as vehicles,pedestrians,wind loads,snow loads,etc.,and the durability of the concrete changes due to the coupling of these loads and chloride corrosion.Therefore,starting from the whole process of steel corrosion,it is of practical significance to study the microscopic transmission characteristics of chloride ions and the micromechanical properties of steel corrosion products under fatigue loading.In this paper,the molecular dynamics software is used to simulate the process of steel corrosion and cracking from the aspects of chloride ion transport under fatigue loading and micro-mechanical properties of corrosion products under fatigue loading.The main contents are as follows:(1)The chloride transport process was simulated on a microscopic scale:Concrete is a complex structure.Therefore,the C-S-H structure with cement paste is used as the micro-structure of concrete,and the Tobermorite 11?structure is chosen.And the reliability of the structure was confirmed by verification.The C-S-H pore structure with pore diameters of 1.5 nm and 3 nm was established,and the pores were filled with 7%NaCl solution.The 3nm pore structure was simulated by chloride ion transfer without load,and the 1.5nm pore structure was simulated under the condition of no load and fatigue load.It is concluded that the diffusion coefficient of the simulated water molecules is close to the experimental data,indicating that the simulation method is reliable;comparing the diffusion coefficients of chloride ions under fatigue loading and no load,it is found that the diffusion coefficient of chloride ions is far under fatigue loading.It is much larger than the unloaded chloride ion diffusion coefficient,indicating that the fatigue load can accelerate the transport of chloride ions.(2)We use modern testing methods such as X-ray diffraction(XRD)and scanning electron microscopy(SEM),the content of rust corrosion products and the microstructure of rust products under different fatigue load conditions were observed.The microstructure model of rust products was established based on the measured data.The molecular dynamics software LAMMPS was used to simulate the nanoindentation test and the rust expansion force was calculated.It is concluded that the coupling effect of chloride salt-fatigue load will lead to a large increase in the content of corrosive products,which will have a serious negative impact on the reinforced concrete structure.As the stress level of the fatigue load increases,the corrosion product of the steel bar also increases.With the increase of stress level,the coupling effect of chloride salt-fatigue load on the durability of concrete becomes more serious.It is found that with the increase of stress level,the elastic modulus of corrosion products increases and the expansion force increases gradually.Compared with the rust expansion force under chloride salt environment,the rust expansion force under the coupling action of chloride salt and fatigue load increased by 128.6%.The modulus of elasticity obtained by simulation is in the order of 10~2,which is larger than that of the corrosion products obtained by experiment.The reason is that the actual corrosion products are relatively loose and non-uniform structure.On the micro-scale,the corrosion products are dense homogeneous structure,and the modulus and hardness are obviously larger than the actual corrosion products.