Mechanics Of Chloride Ions Transportion In Concrete

The durability of concrete structures has attracted extraordinary concern from the field of civil engineering. Besides the cost of rebuilding and maintenance, it is also related to the protection of environment and the sustainable development of society. The durability of concrete structures in marine environment is focus of the concern for the serious corrosion of reinforcement in concrete caused by chloride penetration. The issues of the durability of concrete structures in chloride environment before corrosion initiation are in fact the issues of chloride ions penetrating into concrete. As a result, the transportation model of chloride into concrete is taken as the axes, four hotspots on durability of concrete structures in chloride environment are involved i.e. "Transportation model of chloride in unsaturated state", "Law of chloride penetrating distribution in dry-wet cycling zone", "Rapid chloride diffusion coefficient determining methods by electro-accelerated migration" and "Effect of material damage on the durability of concrete against chloride penetration". The content includes:(1) The transportation of chloride into concrete is in fact the mass transfer of charged particle through porous medium. The process of chloride transportation into concrete under complex condition can be divided into a series of basic physical and chemical processes based on the mechanics of driving forces. The occurrence conditions, mechanics, calculation models and the coupling relationship with other processes are studied based on the theories of electrochemistry, which can serve as a basis for calculation of the chloride penetration into concrete under complex situation;(2) The penetration model of chloride into concrete in dry-wet cycling zone is in fact the model coupling the infiltration flow of pore solution and the transportation of chloride ions e.g. diffusion coupling with convection. The nonlinear diffusion model of moisture in concrete is constructed taking into consideration the "Hysteresis" effect during the evaporation process of pore solution and the nonlinear relationship between the diffusivity of moisture and the saturation of pore as well. Based on the nonlinear diffusion model of moisture and the coupling relationship between diffusion and convection processes of chloride ions in concrete pores in unsaturated state, the transportation model of chloride ions into unsaturated concrete is achieved. The Crank-Nicolson Format of Finite Difference Method is engaged to realize the numerical calculation of the model above. The proving experiment shows that, the correlation between the results of experiment and model calculation is fairly well;(3) Parameter sensitivity analysis on the transportation model of chloride ions into unsaturated concrete shows that, the initial saturation of pore in concrete, evaporation rate of pore solution from surface and the dry-wet cycling regime are the three main factors affecting the transportation of chloride ions in unsaturated state. The law of chloride penetrating distribution along altitude is decided by the law of environment factors changing along altitude. The results of numerical simulation coupling with results detected in situ show that the penetration of chloride is firstly aggravated and then weakened along altitude from bottom to top. At a certain altitude, the penetration of chloride reaches the peak value, which should be the key spot of design and maintenance for durability. For the convenience of engineering application, the accumulated factor of surface chloride concentration and the decay factor of chloride diffusion coefficient are related to the core factor of environmental action: cyclic wetting time;(4) Series of rapid chloride diffusion coefficient determining methods are in fact the application of Nernst-Planck Function which coupling the diffusion and migration processes of chloride ions together. Mechanics analysis is performed on every method to discuss the merits and drawbacks. The results show that, the electro-osmosis should not be neglected in electro-migration test especially for high performance concrete. Correcting format for electro- osmosis of unsteady state electro-migration test is suggested, and the proving experiment is engaged to validate the correction;(5) Crack is the main cause of the acceleration of chloride penetration in damaged concrete. Analysis results on the effect of cracks on the chloride transportation show that, the width of transverse cracks w is an important factor affecting chloride transportation. Therefore, w/c is suggested to be engaged to limit the width of cracks, where c is the established depth of cover; and another important factor is the frequency of cracks, the penetration of chloride in the cracked concrete is aggravated as the frequency of cracks increases, especially in deep spot from surface of concrete; the cracked concrete will fail to protect reinforcement when the frequency reaches a certain threshold value. In addition, RCM (Rapid Chloride Migration test) is engaged to test the effect of micro-cracks caused by high temperature on the durability performance against chloride penetration in concrete. The experimental results give the chloride diffusion coefficients of different types of concrete exposed to different elevated high temperatures and the corresponding recovery factors caused by self-healing function of the micro-cracks.