Monitoring The Performance Of Cement-based Materials Based On Embedded Ultrasonic Piezoelectric Transducers

Concrete structure is one of the main structural forms of the current building.However,during its service life,it is affected by environmental erosion,material aging and long-term effects of load,which will lead to structural damage accumulation and resistance attenuation,durability and safety are reduced.It is easy to ca use catastrophic accidents.Therefore,it is extremely important to conduct health monitoring and damage warning of concrete structures.As a dynamic non-destructive testing technology,ultrasonic testing can ensure the structural integrity while monitoring the quality of concrete online.Previous ultrasonic monitoring is mostly based on longitudinal wave,but longitudinal wave can propagate in solid,liquid and gas,and the accuracy of monitoring results is greatly reduced when there is liquid in damaged structure.Shear wave propagates by shear elasticity of medium,fluid can not propagate shear wave,so it can overcome the shortcoming that longitudinal wave is affected by liquid,and monitor the structure accurately.And the shear wave velocity is low and its attenuation is small.Using it for ultrasonic monitoring can narrow the blind area of the monitoring side and increase the monitoring distance.Therefore,the research of S-wave monitoring technology has important engineering application value.Based on this,a 2-2 type piezoelectric composite sensor is fabricated,which can sense longitudinal and shear waves.PZT-5 piezoelectric ceramic sensor is also fabricated,which can sense longitudinal wave.Combined with commercial transverse wave sensor,the performance of the sensor was tested,and sensors were applied to the performance monitoring of cement-based materials.The specific contents of the study are as follows:(1)Preparation of a piezoelectric sensor.Taking PZT piezoelectric ceramics as the functional body and ordinary Portland cement as the matrix,the 2-2 piezoelectric composite sensor and the PZT-5 piezoelectric ceramic sensor were prepared and the performance of the two sensors was tested.The results show that the resonant frequencies of the two sensors are between 100 kHz and 150 kHz,which meets the monitoring requirements.When the excitation frequency is set to 100 kHz,the amplitude of the received wave is the highest,which is the best monitoring frequency.The wave shape is clear and the first wave is obvious,which is suitable for signal monitoring.(2)Determination of the transmission and reception characteristics of the two types of sensors and the type of generated waves.In the ultrasonic standard test,the time for the longitudinal wave to pass the calibration rod is 21 us.In the test,the PZT-5 piezoelectric ceramic sensor is directly coupled to the two ends of the ultrasonic calibration rod by Vaseline.The 2-2-type piezoelectric composite sensor is pressed and shielded by the multilayer copper foil and coupled to both ends of the ultrasonic calibration rod to test wave propagation time.The results show that the PZT-5 piezoelectric ceramic sensor can generate longitudinal waves and the 2-2 piezoelectric composite sensor can generate transverse waves with good transceiving performance and can be used for ultrasonic monitoring.(3)Monitoring the performance of cement-based materials.The test is divided into two parts.First,the 2-2 piezoelectric composite sensor,the pzt-5 piezoelectric ceramic sensor and the commercial transverse wave sensor are buried inside the cement slurry to monitor the hydration process.The results show that all three sensors can better monitor the cement hydration process.The 2-2 piezoelectric composite sensor can receive both longitudinal and transverse waves and the longitudinal and transverse waves are separated.Secondly,the 2-2piezoelectric composite sensor and the pzt-5 piezoelectric ceramic sensor are embedded in the concrete block.The test is carried out before and after the concrete block is saturated with water and before the concrete crack is filled with water.The results show that the longitudinal wave propagation is affected by water.The impact is large,the damage monitoring results are not accurate and the transverse waves are basically unaffected,so the monitoring results are accurate.In summary,the 2-2 piezoelectric composite sensor can be used for the performance monitoring of cement-based materials and the monitoring of concrete damage is more advantageo us when the liquid is present.