The Embedded Acoustic Emission Sensor And Its Application In The Monitoring Of Concrete Freeze-Thaw Damage

In the cold regions of high latitude and high altitude,freeze-thaw damage is common in concrete structure.Freeze-thaw damage not only reduces the mechanical properties of concrete structure,but also shortens its service life,and even poses a great threat to the safety of people’s lives and properties.Therefore,it is necessary to monitor the operation of concrete structure to ensure its safe service.The current detection methods not only require manual and quantitative detection,but also difficult to provide effective information for the entire monitoring process of the concrete structure,in particular,it is difficult to realize the location judgment of the freeze-thaw damage,and it cannot reflect the real situation of the change of the concrete structure defect,resulting in the waste of human and material resources.In this paper,through the preparation of embedded acoustic emission sensors,utilizing acoustic emission technology to monitor the real-time freeze-thaw damage of concrete materials,using traditional characterization methods and acoustic emission analysis methods to analyze the freeze-thaw damage evolution of beams,acoustic emission characteristic parameters are used to characterize the freeze-thaw damage of concrete,verify the feasibility of acoustic emission technology in concrete freeze-thaw damage monitoring,and provide a new method for practical engineering applications.The main achievements of this paper are as follows:（1）The influence of three kinds of backing layers on the external acoustic emission sensor was studied.Compared with the reference sensors,the sensors had the advantages of excellent receiving performance,wide frequency response range,high accuracy and sensitivity.In the simulation test,the sensor containing tungsten powder/epoxy backing layer had strong signal attenuation ability and the widest frequency response range,which was more suitable for real-time and dynamic health monitoring and damage assessment of structures in noisy environment.（2）The embedded acoustic emission sensor was composed with 2-2 piezoelectric composite material as the core component and epoxy blended cement as the encapsulation material.The results showed that,the relative permittivityε_r,the piezoelectric strain constant d₃₃ and the electrical conductivity decreased with the decrease of the functional phase in the piezoelectric composite,while the piezoelectric voltage constant g₃₃ increased linearly.The research on the acoustic and mechanical properties of the packaging materials found that the acoustic impedance and the electrical resistance of the packaging materials increased with the increase of the cement quality,while the strength increased first and then decreased,and the elastic modulus was the opposite;The acoustic emission sensors with different PZT volume fraction were tested.It is found that the bandwidth of the sensor was widened by reducing the PZT volume fraction in the 2-2 type piezoelectric composite under the premise of the same piezoelectric element size,but at the expense of sensitivity;When the volume fraction of PZT in the 2-2 type piezoelectric composite was between 50%and 87.5%,the acoustic emission sensors containing different PZT volume fraction could effectively receive the acoustic emission signals and noise signals,which were suitable for monitoring concrete;When the volume fraction of PZT was determined to be 75%,the integrated performance of the embedded acoustic emission sensor was the best,and the sensor was used to monitor the concrete freeze-thaw test;（3）The results showed that the environmental noise of the freeze-thaw circulator is generally below 50dB,and the acoustic emission sensor had excellent waterproof,insulation and mechanical properties,which was suitable for the freeze-thaw damage monitoring.With the increase of the freeze-thaw times,the mass loss first decreases and then increases,and the relative dynamic modulus of elasticity and the maximum peak of the ultrasonic wave form increase;Compared with the external sensor,the embedded sensor had better coupling effect with the concrete and more abundant receiving signals;The concrete freeze-thaw was divided into four stages:microcrack germination stage,microcrack growth stage,microcrack decay stage and microcrack stability stage by combining the acoustic emission characteristic parameters and the freeze-thaw damage mechanism.The characteristics of large crack damage were determined by correlation parameter analysis,and the b value of acoustic emission was calculated by the least square method,with a fitting degree of more than 99.7%,and utilized to characterize the evolution process of freeze-thaw damage;Acoustic emission source location studies had found that the distribution law of freeze-thaw damage points is extremely random,and as the number of freeze-thaw cycles increased,the frequency of damage points decreased.