Research On Fatigue Life Prediction And Crack Fractal Of Concrete Bridges Based On The Destructive Experiments Of Full-scale Components

There are many highway concrete bridges in China,most of them have been in service for a long time.The center of work gradually shifts from construction to the maintenance.The maintenance work of concrete bridges in service can be divided into two parts :(1)evaluation of the service status of concrete bridges at the current moment;(2)long-term performance prediction of concrete bridges.Concrete bridges bear the long-term erosion of vehicle load and environmental load.Due to the rising vehicle load and the implementation of design concept of making full use of material properties,the fatigue of the concrete bridge is becoming more and more outstanding,which affects the normal service condition and its expected service life,which is an inevitable and important factor in the maintenance work.In view of this,the static and fatigue experiments on full-scale concrete beams were carried out,and fatigue life prediction and crack fractal characteristics of concrete bridges were studied,so as to assist the performance evaluation and management policy formulation of such structures.The main research contents and conclusions are as follows:(1)Several static and fatigue experiments on the dismantled members of a concrete beam bridge in service were carried out.Firstly,the appearance and technical condition of the test beams were detected,and the performance tests of main materials were conducted.The experiment beams were in good technical condition.The process of experiments was introduced detailedly.The development law of each mechanical property index was demonstrated.The basic S-N curve was provided for the fatigue life prediction of concrete bridges.And the crack database was accumulated for subsequent study on fracture fractal characteristics.(2)Taking the data from weigh in motion as the data source,a multi-lane random traffic flow simulation method based on unsupervised classification algorithm “K-means++”,Markov Chain Monte Carlo simulation method and Copula theory was established.An eight-lane expressway was used for case analysis,laying the foundation for subsequent vehicle loading.The case analysis showed that the simulated traffic flow considering the correlation of vehicle sequences can accurately reflect the real structure of the traffic flow,and the distribution of total weight and full wheelbase considering the correlation of axle load and wheelbase were closer to the real distribution than the traditional method.(3)According to the time-varying characteristics of load and resistance of concrete bridges,the time-varying vehicle load model,road roughness model and steel corrosion model were established,which lays the foundation for the fatigue life prediction.Firstly,the statistical data of vehicle load in some areas of China were collected,the parameters of the growth model of traffic volume were estimated,and the evolution model of traffic structure was put forward.Then the evolution model of road roughness was derived based on the decay law of international roughness index.Finally,according to the environment of the concrete bridge,the carbonization model and the chloride erosion model were established respectively.The corrosion rate of the carbonization model was smaller than that of the chloride erosion model.(4)The classical fatigue damage accumulation criteria were reviewed,the P-M criterion was selected as the basic criterion.The S-N curve of the damaged material was obtained by considering the influence of the fatigue damage of each stage load on the next stage,and the improved fatigue damage accumulation algorithm was proposed.The algorithm was verified by two fatigue experiments of concrete beams.Finally,the prediction method of fatigue life of concrete bridges was established by integrating the model of time-varying load and corrosion of reinforcement,and a bridge in service as a case to analyze.The results showed that the prediction accuracy of the improved cumulative fatigue damage algorithm was higher than that of the conventional algorithm.The method in this paper fully considers the time-varying factors of the whole life cycle of concrete bridges,and the prediction results were more reasonable.(5)The application process and applicability of multifractal method for cracks on concrete beam bridges were studied.Then the fractal characteristics were analyzed based on the experimental data,the function relations between fractal dimension and the mechanical behavior were fitted.The development model of fractal dimension of concrete bridge cracks was established.A rapid evaluation method of service status of concrete bridges based on the fractal characteristics of cracks was proposed,and the validity and convenience of the idea were verified by the analysis of a real bridge.