Permeability Monitoring Of Cement-based Materials Based On Electrical Resistivity Tomography

Concrete structure is one of the most important building structures,with the development of society,people pay more attention to the safety of building structure.Leakage is one of the main reasons that the durability of concrete structure is difficult to be guaranteed,so it is very important to monitor the leakage of concrete structure.Electrical resistivity tomography(ERT)is a new non-destructive monitoring technology,which has the advantages of low cost,visualization,non-invasive,non radiation and convenient operation and has a wide application prospect in the leakage monitoring of concrete structures.However,there are still some problems restricting the development of ERT technology and its application in engineering.For example: 1)at present,the research of ERT sensitive field mostly uses two-dimensional model,and seldom uses three-dimensional model for analysis.2)There is a lack of analysis of SNR,measurement accuracy and other indicators when optimizing the ERT sensor system and determining the optimal excitation mode.3)The application of ERT technology in the detection of concrete structure is mostly the study of damage monitoring,but lack of application analysis of penetration monitoring.In view of the above problems,on the basis of previous research,this paper takes the application of ERT technology in the leakage monitoring of concrete structures as the engineering background,and conducts a series of research on ERT technology according to the simulation results and test data.The main work of this paper is as follows:(1)Theoretical analysis and Simulation Research of each image reconstruction algorithm.The imaging principle of each algorithm is analyzed systematically,and the simulation models with different complexity are established based on COMSOL software.The simulation imaging of the models is carried out by using each algorithm through COMSOL with MATLAB.The results show that: in each algorithm,the conjugate gradient algorithm can best restore the actual situation of each simulation model.(2)The limitation analysis of two-dimensional model of ERT technology.Through the establishment of two-dimensional and three-dimensional simulation models of ERT sensor,the quantitative study of "edge effect" shows that the distribution range of ERT sensitive field is far larger than the spatial range of electrode height,so the assumption that "edge effect" does not exist in two-dimensional model is not reasonable;meanwhile,the use of two-dimensional simulation will lead to serious energy loss,so there is a large error in the measurement results of two-dimensional simulation,The results of 3D simulation are closer to the measured data.(3)Study on the "soft field" property of ERT three-dimensional sensitive field.From the perspective of potential and voltage data,the influence of the location,content and conductivity of discrete medium on the distribution of ERT sensitive field is analyzed in detail,and the law of "soft field" characteristics of ERT three-dimensional sensitive field is obtained:the sensitive field of ERT technology is not uniformly distributed even in the uniform field,the sensitivity near the excitation electrode is the highest,and the farther away from the excitation electrode,the lower the sensitivity;the higher the content of the discrete medium,the greater the absolute value of the position sensitivity,and the greater the difference with the conductivity of the continuous medium,the greater the impact on the potential and voltage data,that is,the greater the impact on the sensitive field distribution.(4)Determination of the optimal incentive mode of ERT system.Through the establishment of different models,this paper makes a detailed analysis of the incentive modes of ERT system.The results show that: relative incentive mode has little effective data compared with other incentive modes,so relative incentive mode should be avoided as much as possible;adjacent incentive mode can obtain the most effective data,which is most beneficial to the solution of inverse problem;quasi diagonal incentive mode is the optimal incentive mode.(5)The optimal design of three-dimensional ERT sensor.The influence of electrode arrangement,electrode number,electrode shape,electrode size and other factors on the measurement effect of ERT system is studied based on the uniformity of sensitive field distribution,effective data amount,signal-to-noise ratio,measurement accuracy and sensitivity,the results show that the measurement accuracy of the electrode array system is much higher than that of the electrode array system,and the imaging effect is much better than that of the electrode array system;in addition to the amount of effective data,other performance indicators are worse with the increase of the number of electrodes,so the8-electrode,12 electrode and 16 electrode system are the most suitable imaging systems;theoverall performance of the rectangular electrode is better than that of the point electrode,so using rectangular electrode is more conducive to image reconstruction;increasing the height or width of the electrode can improve the uneven distribution of the sensitive field,but it will also bring the disadvantage of increasing the distribution range of the sensitive field or the bypass effect;Increasing the aspect ratio of electrode can improve the measurement accuracy of ERT system.Finally,through a large number of experiments,the optimal electrode size width is 0.6L,and the aspect ratio is 2.(6)The ability of ERT technology to monitor the permeability of cement-based materials was explored.Based on the optimized electrical resistivity tomography system,the permeability of water in cement-based materials was imaged and observed in different periods of time.The experimental results show that ERT technology can not only monitor the location,size and shape of the water source inside the cement-based materials,but also determine the area of water erosion inside the materials.At the same time,based on Richards equation,the permeability simulation analysis of the same cement-based material test block is carried out,which is basically consistent with the test results,which proves the accuracy of the test results.