Durability Study For Reinforced Concrete Bridges Exposed In Marine Environment

Sufficient attention has been paid to the durability of reinforced concrete bridge structures all over the word. During the long-term service of bridge, material aging and structure impairment will occur due to environment (corrosion, temperature, humidity variation) and working conditions (load, fatigue of material and structure). As structure impairments accumulate, these can result in deterioration of service performance, degradation of bearing capacity and durability for bridge, and so may influence structure safety. Especially in marine environment, reinforced concrete bridge structures, in which concrete subjected to chloride ingress, resulting in corrosion of reinforcing bars, induced early deterioration and loss their durability. This is a main problem in engineering practice. Based on the project of Study of Anti-corrosion technology and Durability evaluation method for bridge structure in marine environment, this thesis has finished the following aspects:1. A series of concrete specimens in a flexural loaded state was exposed in a container filled with 3.5% sodium chloride solutions for long-term tide cycle, the effect of stress state of concrete in flexural loading to chloride ingress level was estimated and analyzed. The results show there is a relationship between chloride diffusion coefficient and stress state of concrete. The diffusion speed of chloride in concrete was accelerated and the chloride concentration increases in tensile stress state. In contrast, the diffusion speed of chloride in concrete was degraded and the chloride concentration decreases in compression stress state. A diffusion coefficient with sustained load and a time-dependent diffusion coefficient are calculated based on the second Fick's law.2. Chloride threshold concentration of rebar depassivation are discussed and analyzed. According to the inspection results of existing reinforced concrete bridges, chloride ion concentration and chloride diffusion coefficient are established by optimization method, based on mathematical model of chloride penetration which is deduced on the basis of the Fick's Second Law. In addition, time-varying probabilistic model of rebar depassivation in chloride environment is established by considering the chloride threshold concentration as a constant value with exposure position and treating the thickness of concrete cover and the uncertainty coefficient of computation mode as random variables, chloride apparent concentration and diffusion coefficient on the certain location of the same bridge as constantvalues. Moreover, taking an actual reinforced concrete bridge for example, probabilities of rebar depassivation are estimated in the different years.3. The mechanism of concrete carbonation and influencing factor, as well as the model of carbonation depth of reinforced concrete structures are discussed and analyzed. According to the inspection results of existing reinforced concrete bridges, the mathematical model of carbonation depth based on concrete strength is established by regression analysis of concrete carbonation coefficient and strength. A time-varying probabilistic stochastic model of concrete carbonation developed to the surface of the reinforcement is established by treating concrete strength, the thickness of concrete cover and the uncertainty coefficient of computation mode as random variables, the carbonation depth of concrete structures as a stochastic process with time.4. According to characteristic of reinforced concrete bridge and requirement of durability comprehensive evaluation, bridge structure system is divided to the whole structure, substructures and component, and the influence factors of durability are analyzed. Moreover, the fuzzy comprehensive evaluation model for bridge structure is established. All the index weights are determined by adopting the uncertain analytic hierarchy method, the consistency approximation and error theory of the interval number judgement matrix, as well group judgement and set-valued statistics. Durability of bridge structure are assessed by using fuzzy comprehensive evaluation method, and Durability grade of bridge is delimitated by using asymmetric closeness degree method.