Research On Propagation Properties Of Stress Wave In Concrete Based On Time-reversal Method

In recent years,the wave-based concrete structural health monitoring has attracted much attention due to its wide monitoring range,low construction cost,the feasibility of real-time and active monitoring.Due that concrete is a multiphase composite cementitious material which includes cement,sand and aggregates,the propagation of stress wave in concrete is very complex,and the reflection,scattering and wave packet aliasing are too serious,which leads to the analysis and interpretation of the stress wave are very difficult and the large-scale application of wave-based concrete structural health monitoring is limited.The time-reversal method has been widely used in structural health monitoring(SHM)due to the self-adapting focusing property without prior knowledge.In this thesis,the wave method and time-reversal theory are used in concrete medium and the propagation properties of stress wave in concrete medium are systematically studied.In addition,the wave method based on time-reversal theory is introduced to monitor the grouting compactness of post-tensioning tendon duct,and the good results are obtained.Hence,the application value of wave method based on time reversal theory is proved in practical engineering problems.Specifically,the main contents and results are listed as follows:(1)The propagation properties of stress wave and the applicability of time-reversal method in concrete medium are systematically studied.During the experiment,the piezoceramic transducers were embedded in the concrete material,and the dual sensing and actuating functions of piezoceramic transducers were used,hence,the piezoceramic transducers were used as the actuator and sensor to motivate or detect the stress wave.The selection principles of the excitation signal,the compute of stress wave propagation velocity,and the relationships between stress wave velocity and frequency in concrete medium are respectively studied based on the experimental phenomena.In addition,the acoustic reciprocity of stress wave propagation,the applicability of time-reversal method,the stability of the output signals and the relationships between the input signals and the output signals in concrete medium are also systematically studied.(2)The stress wave absorption attenuation law in concrete medium is studied based on Rayleigh damping theory and the self-adapting property of time-reversal method,and the criterion of absorption attenuation of stress wave in concrete is established.The Rayleigh damping model and wave equation are used to derived the absorption attenuation law of stress wave in concrete medium,and the formula for computing the absorption attenuation coefficient of stress wave is proposed,which indicates that the absorption attenuation coefficient of the stress wave in concrete is directly proportional to the square of the stress wave frequency when the damping ratio is small.In addition,the related experiments have been carried out and the mass damping coefficient ? and stiffness damping coefficient ? of Rayleigh damping model are respectively obtained based on the experiment results.In order to verify the accuracy of the mass damping coefficient ? and the stiffness damping coefficient ?,the related verification work is carried out and good results are obtain,hence,the validity of the stress wave attenuation model is proved.(3)A novel and effective method based on wave method to monitor the grouting compactness of post-tensioning tendon duct is proposed,and experiment verifications are carried out.In the proposed method,one PZT ring with the advantage of uniform actuation along all radial directions installed on the prestressed reinforcement is used as an actuator to generate stress wave and the other distributive PZT patches bonded on the outer surface of the tendon duct are applied as sensors to detect the propagated stress wave.In addition,the monitoring principle of grouting compactness of post-tensioning tendon duct enabled time-reversal method is proposed,and the results are verified by the experiments.The experimental results show that the grouting quality in the tendon duct can be estimated by monitoring the peak value changes of the time-reversal focused signal.