Evaluation Of Durability Of Concrete Structures Based On Finite Element Method

With the advent of Portland cement, human began to use concrete to construct buildings in 1824. Nowadays, reinforced concrete structure has been widely used, is still the first choice form in the civil engineering structural design. However, the damaget of concrete structure is a serious problem because of the concretes’natural aging and has brought enormous economic losses. Therefore, strengthening the durability of concrete structures, forecasting and extending the structural service life, has become a very important realistic topic and duty in front of us.In 1991, Professor Metha in the Second International Conference on Durability of Concrete pointed out:"the reasons for the current concrete structure destroyed by order of importance are:steel bar corrosion, freeze injury, physical chemistry function.", while the chloride ion penetration of concrete and the concretes carbonization are the main factors causing steel corrosion.This paper summarized the existing research literature, analyzed the failure mechanism of the concrete structure under the conditions of the freezing and thawing, carbonization, high temperature and the chloride ion corrosion, researched the mechanical properties of the aging concretes and carried out the research on the structure life from the structure level. The results of this study are as follows:(1)The paper summarizes the failure mechanism of the freezing and thawing concretes and has analyzed the influencing factors which destroy the concrete. Through the analysis of the existing test data, this paper deduces a formula, reflecting the relationship among the cohesion c, internal friction angleφvalue and concrete natural freeze-thaw cycle times N, uniaxial tensile strength f(?) and uniaxial compressive strength fc0.This paper also deduces a formula, reflecting the relationship among the elastic modulus E, poisson’s ratioμand concrete natural freeze-thaw cycle times N, and also gets a formula between stress/strain and concrete natural freeze-thaw cycle times N, which reflects the changeable rule of the material performance of the freezing and thawing concretes. Finally, the FEM is applied to analyze the life of the concrete structure and the life of a reinforced concrete beam is successfully predicted.(2)The paper summarizes the concretes carbonization influencing factor. According to the existing empirical datum, the paper fits the carbonization concrete stress-strain relations curve formula, infers formula between the concrete cohesion c and internal friction angleφunder the condition of the carbonization, the concrete cohesion c and internal friction angleφunder the non-carbonization condition, the single axle tensile strength f(?) and the single axle compressive strength fc0 under the non-carbonization condition, reflecting the changeable rule of the material performance of the carbonized concrete. And the paper also summarizes the mechanical properties of the carbonized and non-carbonized concretes and the steel bar corrosion rule under the carbonized condition, has successfully simulated the steel bar corrosion expansive action, and establishes life-span analysis process of the carbonized concrete structure based on the FEM. Through this method, the life of a reinforced concrete beam has been successfully predicted.(3)The paper deduces a formula reflecting the relationships among the cohesion c, internal friction angleφand the temperature, summarizes the changeable rules of the material performance of the ordinary concrete under/after the high temperature conditions and the high-strength concrete after the high temperature conditions, simulates the temperature distribution in concrete structures under the high temperature conditions. Finally, the FEM is applied to realize the thermo stability function of the ordinary concrete under/after the high temperature conditions and the high-strength concrete after the high temperature in concrete structures.(4)The paper summarizes the reinforcement corrosion mechanism under the condition of the corrosion of the chloride ion and analyzes the chloride ion penetration model. Connecting the steel corrosion rate and the chloride ion environment around the reinforced concrete and based on the FEM, the paper sets up the analysis process of the life of the reinforced concrete structures. And according to this method, the life of a reinforced concrete beam has been successfully predicted under the condition of the chloride ion penetration.