Damage Detection Of Meso-Scale Concrete Structures Based On Time Reversal Method

A specific Time Reversal Imaging Method is proposed in this thesis to detect defects in concrete structures.In order to detect the damage the same scale as the aggregate and eliminate the drawbacks of modeling concrete as isotropic and homogeneous material,a meso-scale concrete model is introduced in this dissertation.As the concrete is a composite material composed by cement,aggregate,water and concrete admixtures,the Monte Carlo random model and the aggregate grading curve of real concrete samples are introduced for designing this finite element model which is finally realized in Matlab.Then the damaged model is analyzed by employing a self-adapted Time Reversed Model to achieve the ultrasonic wave field simulation.Meanwhile,a wavelet transform(WT)image method is proposed which has been verified by experiments.The results reveal the conclusion that the TRM is applicable for damage detection in complex concrete structures and the energy peak image can eliminate noise and improve the accuracy of defect imaging.In this dissertation,the main steps used for imaging the meso-concrete TRM are as follows:1)Meso-concrete model was built by aggregate grading of real concrete experimental samples,then this model was applied in Comsol Multiphysics including defect simulating,meshing and transducers establishment.2)Time Reversed Method(TRM)was amended to detect concrete structures.This imaging method contains two steps: the first is the forward detection.A series of reflected echo signals with damage information are obtained in this section.These received signals can be reversed in Matlab to serve as the actuating signals in the next Time Reversal process;the second step is to image the damage location via interfering the wave-fronts actuated by different transducers to illustrate the peaks of waveform amplitudes.By determining the interfering wave peak time,obtaining the original wave field of that moment,we can form the wavelet transformed energy field,and then complete the damage imaging of the concrete model with defect.In the Time Reversal process,we introduced the efficient elastic parameters as the same geometric dimension of the original meso-sacle concrete model to locate thedamage effectively.Moreover,we discussed the equivalent elastic parameters and the damage sizes to verify the robustness of this method which is applicable in monitoring and evaluating the damage in concrete structures.3)Finally,a through hole in concrete sample was casted to verify the accuracy of this TRM image method.By simulate incident ultrasonic signal and get the received signals from defect,then these signals were applied in efficient elastic parameters to complete TR process.Wavelet Transform(WT)was used to eliminate noises and achieve the image of Time-peak which shows the efficiency and accuracy of the proposed TRM in detecting concrete structures.