Design Value Method For Durability Design And Assessment Of Concrete Structure In General Atmospheric And Chloride Environment

The construction industry is the pillar industry of the national economy,and its sustained and healthy developments play an important role in economic growth,urban and rural construction and the improvement of people’s livelihood.However,due to the durability damage in corrosive environment,building structures usually fail ahead of reaching their service life.Hence,the effective durability control of planned and existing structures is increasingly becoming an important part of structural reliability engineering.Structural durability control mainly depends on design,construction,usage,assessment,repair and other links,in which the first and the fourth one are the most difficult and crucial parts,for the reason that these two links are directly related to the damage law and control method.In order to realize the basic national policy of resource conservation and sustainable development,it is of great practical and economic significance to establish the durability design method of planned structures and the durability assessment method of existing structures.At present,some progress has been made in the research on durability design and assessment methods of concrete structures at home and abroad.The research orientation transforms gradually from qualitative research to qualitative combined with quantitative research,and from fixed value research to probability research.However,the analysis process of the probability research needs to master the probability characteristics of basic variables and involves complex reliability calculation,which lead the research still in the preliminary stage.Hereby,it is necessary to develop durability design and assessment methods which not only consider the probability characteristics of basic variables but also do not increase the computational complexity.In view of this,considering the characteristics of probabilistic design of structural durability,this paper established the design value method of structural durability based on the theory of structural reliability and the improved design value method.Subsequently,the practical design expressions of durability in general atmospheric environment and chloride environment were proposed.And then,based on the characteristics of basic variables and belief inference method of existing structures,the design value method was further extended to evaluate the durability of existing structures.Finally,the durability assessment method with certain theoretical and practical significance was promoted.The main work is as follows:(1)The reliability control accuracy of the improved design value method which is suitable for structural safety and serviceability design was verified.Afterwards,the characteristics of continuity and internal correlation in the former and the later service life were considered,by using the assurance rate adjustment coefficient and reduction coefficient of the design value of the previous service life,the differences of the reliability index and sensitivity coefficient among different functions were eliminated.Thus,the design value method of structural durability design was put forward.The proposed method established the functional relationship between the design value of the basic variables and its probability characteristics,target reliability index and design service life,and expresses the benchmark design value of the basic variable and partial coefficient in the form of fixed value.The application process is more flexible and accurate and does not need complex reliability calculation and accords with the design habit.(2)Aiming at the common distribution types of basic variables in design,the calculation expressions of benchmark design values and partial coefficients of basic variables related to target reliability index and probability characteristics were established.In view of the small samples in statistical analysis,the Bayesian approach based on Jeffreys prior was proposed.The test databases of concrete compressive strength,benchmark chloride ion diffusion coefficient and chloride concentration on concrete surface were collected and established based on DEEDS platform.The probability models of the above variables were established by combining large sample,small sample statistical analysis and Kolmogorov-Smirnov test.By using the integral average method,the time-varying chloride ion concentration on the concrete surface was converted into the equivalent surface chloride concentration during the service life,and the modified probability characteristic of this variable was obtained.As a result,the error caused by the assumption of constant boundary condition in the prediction model was reduced.(3)With regards to concrete structures in general atmospheric environment and chloride environment,the erosion model corresponding to the durability limit state and related test data were adopted to calculated the model uncertainty coefficient of the prediction model,and the implicit reliability index of each model was calibrated correspondingly.According to the average value of the implicit reliability indexes of all the erosion model,the target reliability index corresponding to each stage was determined combined with the analogy method,and then the final function was optimized by incorporating authority and generality of the erosion model,the target reliability index and the calculation results of the uncertainty coefficient of the model.The dominating variable of each probability model was determined by the sensitivity coefficient iterative algorithm,and the reduction coefficients of the design value in the three-stage expressions were calculated according to the number of basic variables and dominating variables.The variation coefficients of the three-stage service life were calculated based on the error transfer formula,and the guarantee rate adjustment coefficients considering the change of target reliability index and sensitivity coefficient were further solved.Finally,based on the design value method of durability design,the practical expressions of durability design in different stages of general atmospheric environment and chloride environment were established.(4)Considering the difference of the probability characteristics of the basic variables between the deigning and assessment procedure of structural durability,the probabilistic and the belief analysis method were used to establish the belief analysis model of existing structures.Based on this,the overall characteristics of random and existing things with comprehensive consideration of statistical,spatial and measurement uncertainty were inferred.Then,the design values of the basic variables of general atmospheric environment and chloride environment were inferred.Through the inferred design value of basic variables,the classification threshold of durability assessment was quantified in the form of reliability index,and the corresponding grade of durability was given.Finally,the durability assessment of concrete structures based on probability and belief method was realized,which improved the credibility of the assessment results.