Flexural And Durability Performance Of RC Members With Cracks

On the base of the latest revision of our national Code for design of concrete structures (GB50010), this thesis deals with the subject of the durability of reinforced concrete (RC) elements. Taking account of the influence of loads, environments and structural materials, some research works, including the flexural properties of RC elements, the corrosion mechanism of reinforcement in cracked concretes, the cover cracking induced by steel corrosion and the evaluation and design method related to structural durability, have been performed. The conclusions obtained from this study can enrich and provide some references to the durability assessment and design in the revised code.The main research contents in this thesis are as follows:1. For RC flexural members, the development mechanism of transverse crack and its main influencing factors were analyzed. Then, the effect of transverse crack on the durability of RC structures in chloride aggressive environment was discussed on the following two issues. One issue is the influence of the cracks on the chloride ingress in concrete:and another issue is the effect of the cracks on the corrosion of reinforcement. Concrete cracking and delamination caused by corrosion were also discussed. The obtained conclusions can provide some references to the studies on the flexural and durability tests for RC members.2. The flexural experiment of RC beams reinforced with 500MPa steel bars was completed, and some properties including the mean crack space, the short-term maximum crack width and the deflection were analyzed under normal service load. The experimental results were compared with the data calculated by the formulas in GB50010 code. It is found that the formula of short-term maximum crack width proposed in GB50010 code isn't fit for the beams reinforced by 500MPa steel bars. Based on the techniques of mathematic statistics and probabilistic analysis, some suggestions were proposed for the revision of the code's formula.3. On the basis of the bond-slip effect, a numerical model to analyze the flexural properties of RC beams was built with MATLAB langrage. The model can take account of the tension-stiffening effect of concrete and the nonlinear properties of materials. The predictions got from the numerical model were compared with the experimental results. It is verified that the proposed model is feasible to predict the crack widths and deflections of RC flexural beams, which give an effective method to analyze the flexural properties of RC beams with transverse cracks.4. Taking the cover thickness and crack distribution state including crack width and spacing into account, eight RC beams under sustained cracking state were designed and tested with chloride drying-wet cycles. By the non-destructive detecting technique and destructive inspection, the influence of cover thickness, crack width and spacing on the reinforcement corrosion in concrete was discussed. It is verified that the ratio of mean crack width to cover thickness is effective in assessing the durability properties of cracked RC beams. The conclusions obtained from the test can provide some references to RC structural design.5. Based on the assumption of uniform corrosion of reinforcement, two models for predicting the time from corrosion initiation to cover cracking and its corresponding corrosion ratio of reinforcement were established according to the mechanics of elasticity and Faraday's Law. In the present models, the deformation property of corrosion products and the extent of corrosion product's ingress into expansive cracks were all considered and simplified. The comparison of the predicted results with experimental values shows that the proposed model is in good agreement with test results. Finally, the probabilistic property of the cover cracking time was analyzed with the Monte Carlo simulation and?2 test technique.6. The degradation process of RC structure in the chloride aggressive environment was introduced and three-level durability limit states, viz. corrosion state, cracking state and collapse state, were proposed. Using the probability method, the models for predicting the durability life of RC structures were given for different durability limit states. These probabilistic models were applied to a practical marine RC structure. Finally, the cover thicknesses for RC structures were optimized on the basis of the integrated consideration of durability, crack resistance and fire resistance. The results can give some reference to the durability design of RC structures.This research was supported by the project of National Natural Science Fund "Fundamental Research on Durability Design and Assessment of Concrete Structures in Chloride Environment" (50538070).863 program of National Hi-Tech Research Development "Durability Test Method and Assessment Technology of Important Coastal bridges" (2006AA04Z422), the National Science and Technology Supported Project "Research on inspection and assessment technology for existed buildings" (2006BAJ03A02). the science and technology project of Zhejiang Province "Key Anti-corrosion Techniques of Harbor and Costal Engineering and Development and Application of Supporting Facilities" (2006C13090), the agreement project of Chinese and Japanese (NSFC/JSPS) "Durability design theory of concrete structure based on Life-Cycle Cost Management Technique" (50811140088).