Research On Concrete Detection And Imaging Monitoring Technology Based On ERT

During the service period of concrete structures,damage will be accumulate under the coupling of environmental erosion,material aging,load effects and man-made or natural abrupt effects.The safety and durability of the structure can not be guaranteed.Effective monitoring and early warning of the structure is the technical way to solve the problem.Electrical resistance tomography(ERT)is a detection technology based on the distribution of the conductive medium of the measured object which has the characteristics of low cost,non-invasiveness and visualization,and it has broad prospects in non-destructive detection of structures.However,the application of ERT technology is relatively difficult due to the poor conductor characteristics of concrete structures.In order to realize the application of ERT technology in concrete structures,this paper has made some researches from imaging algorithms,system construction,and concrete conductivity improvement.The main contents of this paper are as follows:(1)Determine the optimal algorithm for ERT.Based on the basic principles of ERT,the theoretical derivation of the mathematical physical model and the boundary conditions of ERT is performed.The solution of the forward problem is the basis of the inverse problem as also as the basis of image inversion imaging.Based on the COMSOL software,the forward problem is solved by finite element method.According to the sensitivity theory,the imaging pros and cons of linear back projection algorithm,sensitivity coefficient method,Landweber iterative algorithm,Newton-Raphson algorithm and conjugate gradient algorithm were investigated.Simulation experiments were conducted through COMSOL with MATLAB which showed that the imaging quality of the conjugate gradient algorithm is optimal.(2)Construct the ERT concrete test system.Based on the NI virtual modular instrument as well as the PXI platform,a test system suitable for signal acquisition of concrete structures was built.The test system used constant current as the excitation source and adopted the adjacent excitation mode to conduct current excitation on 16 embedded square electrodes evenly distributed in the concrete structure to obtain boundary voltage data for image reconstruction.(3)Improve the characteristics of concrete non-conducting conductors and improve the collection accuracy of the test system.Concrete materials have poor electrical conductivity.In order to optimize the electrical conductivity of the concrete test block,carbon fiber was added in concrete.In addition,orthogonal experiments were designed to study the effects of carbon fiber content,carbon fiber length and water-cement ratio on the conductivity of concrete incorporating carbon fibers,so as to prepare concrete test blocks containing carbon fiber with good electrical conductivity.According to the test results and comprehensive consideration of economic and other factors,water-cement ratio of 0.5,carbon fiber length of 7mm and carbon fiber content of 0.6% were designed as the test block mix ratio for the tests.(4)Research on imaging test of concrete ERT technology.Based on the ERT detection imaging system,imaging tests were performed on the internal damage of the concrete test block.,which was divided into four parts.First,through the study of the electrode shape,the results showed that the sensitive field generated by the square electrode was more uniform than the spherical electrode,so it has a greater contribution to the imaging quality.Second,the column,beam,and plate crack imaging experiments were designed.To investigate the effect of carbon fiber on image quality,a control test was designed for ordinary concrete and carbon fiber concrete.The results showed the conductivity of the concrete test block is improved after the addition of carbon fiber in concrete.The long-term monitoring of the damage status of the concrete structure can be ensured.At the same time,the imaging quality of the concrete test block was improved.And ERT technology can realize the monitoring of cracks in concrete structures.Once again,the thermal imaging test of concrete is designed.The center,unilateral and bilateral sides of the concrete test block were heated to observe the imaging effect of the temperature distribution in the concrete.The test results show that ERT can be applied to the thermal temperature change of the concrete structure.The monitoring can reflect the change state of the structure after heating.Finally,a multilayer electrode hole imaging test was designed.The test block was arranged at an equal interval to form a three-layer electrode array,and each layer was imaged separately.The test results showed that ERT can realize multi-layer electrodes damage imaging.