| It has been proved that the resistance of the member of reinforced concrete structure is continuously changing with its service-time. The result of this fact is that the reliability of the structure is changing with the service-time, that is, the reliability is dynamic. Recently, a lot of research regarding the durability and dynamic has been made in the nation and abroad. Because commonly the degeneration of the resistance begins with the corrosion of the steel in the concrete, it is very important for durability research to count the beginning corrosion time of the steel in the concrete.There is a lot of chloride in ocean environment which is a main reason of the steel corrosion, so we must predict thickness of chlorine ion in order to count the beginning corrosion time of the steel under sea environment. But current calculating formula is unsuitable not only to counting the beginning corrosion time of the steel, but also to practically apllying because its hypotheses is much too ideally and the influence of section shape. So , based on previous research achievement and the knowledge regarding molecular movement and calculus and numerical value analysis, practical calculating method is set up in this paper:1. Traditional model is transformed and its anti-model is set up, which makes the calculation of beginning corrosion time of steel very easy;2. Based on the first PICK diffusion law, mathematical model of two-dimension diffusion of Cl- is set up and based on this, one-dimension diffusion model of Cl- of circular section;3. Differential arithmetic is utilized to set up the numerical value arithmetic of the diffusion model of circular and rectangular section; at the same time, the stability of the numerical value arithmetic is analyzed;4. Modifying coefficient is used to modify the traditional model;5. Many factors regarding the modifying coefficients of circular and rectangular section are analyzed:(1) It is theoretically proved that the diffusion coefficient of Cl- in concrete is dependent of modifying coefficients;(2) Through analysis of the results of numerical value calculation, it is discovered that the relation between the modifying coefficients and the thickness of protection layer of concrete is linear, from which the function expression of these two variables is gain; at the same time, the influence on modifying coefficient of the radius of circular section and the thickness of Cl- on the surface of concrete; based onthese , practical calculating method of beginning corrosion time of steel of circular section is set up;(3) To rectangular section, the diffusion of CF in concrete is two-dimension diffusion. In this paper , it is proved that the influence area of two-dimension diffusion begins with the corners of the rectangular section and gradually diffuse to the middle of the section, and the size of the influence area of two-dimension diffusion depends on the product of the diffusion coefficient and time of Cl-. Thus, the conclusion is drawn : when the diffusion coefficient of CF is invariable and the diffusion time is under a certain range, the beginning corrosion time of the steel in the corner of the rectangular section is independent of the size of rectangular section;(4) Through the numerical value calculation of rectangular section, the law is found of the influence on modifying coefficient of rectangular section of the thickness of protection layer of concrete and diameter of the steel and the thickness of CF on the surface of concrete, based on which practical calculating method of beginning corrosion time of the steel in the corner of rectangular section is set up.
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