Application Of Radial Piezoelectric Ultrasonic Sensor In Concrete Detection

In the last few decades, with the growing of Chinese economy, its construction industry has been growing rapidly. Concrete material is not only widely used in the field of construction, but also widely used in the construction of roads and bridges. The quality of concrete is directly related to the quality of the engineering. Engineering structure is affected by various factors, which leads to be aged. Safety and durability of concrete has become a serious problem. So it is of great significance to detect concrete structure. Due to the advantages of piezoelectric ceramics with fast response, wide frequency band, low cost, which are widely used in the detection of civil engineering structure. In addition, piezoelectric ceramic have functions of actuating and sensing, which a lot of researches have been conducted on ultrasonic testing of concrete structures that based on piezoelectric ceramic. In the paper, a new radial piezoelectric ultrasonic sensor was fabricated based on the piezoelectric effect, which was applied in the detection of concrete structure. Concrete strength and internal defect was tested by ultrasonic testing technology. The research work includes as follows:(1) A new radial piezoelectric ultrasonic sensor was studied for detecting of concrete based on the characteristics of piezoelectric ceramic. Emission type sensor was based on PZT-41 piezoelectric ceramic tube as the sensing element, and receiving type sensor was based on PZT-52 piezoelectric ceramic tube as the sensing element. The sensors were packaged by matching layer and backing layer. The performances of radial piezoelectric ultrasonic sensor were researched, including matching layer thickness, linearity, sensitivity and the directivity. The results show that the amplitude of head wave is 1.083 V when the matching layer thickness 2.5 mm, which indicates the signal that sensor received is the best and the sensitivity of sensor is the highest. In addition, when the excitation frequency of the sensor is 50 kHz, the linearity of radial piezoelectric ultrasonic sensor is the best, the linearity correlation coefficient of sensors is similar, and the sensitivity is the highest. Finally, it is found that the transducer can achieve 360° in the radial direction of transmitting ultrasonic and the ultrasonic signal is similar along all radial directions.(2) Different strength grade of concrete were fabricated. Emission type radial piezoelectric ultrasonic sensors were embedded in the center of concrete. The ultrasonic testing technology was adopted to continuously detect the strength of concrete cubes for 28 days, in order to study the relationship between the acoustic parameters and concrete strength, which characterizes the strength of concrete. The results show that the velocity of the sound will gradually increase with the increase of curing age, then tends to be stable. At the same time, the frequency shifts to the right, which shows frequency tends to increase, and the amplitude of frequency is gradually increasing. By fitting the curves of velocity and strength, the regression equation is obtained, and the linear correlation coefficient R is relatively high, which indicates that the velocity of the sound and strength show good linear correlation. The strength is greater, the velocity of the sound is greater.(3) Three point bending test was carried out on the concrete beam with MTS fatigue testing machine, which studied the relationship between the receiving time, amplitude and frequency of the receiving ultrasonic signal and concrete cracks. The study shows that receiving wave appears the phenomenon of irregular waveform, and continuous several wave packets are appeared with the increase of load. The receiving time occurs to delay, and the amplitude tends to increase at the first stage and then decrease. When the concrete beam has large cracks, the frequency and the amplitude all decrease rapidly. Also, RMSD is used as damage index to quantify damage. RMSD is larger than 0.8. The change of amplitude is obvious. Therefore, the ultrasonic parameters of amplitude, dominant frequency's amplitude and RMSD can be used as characteristic parameters to assess the damage of concrete.The void was prefabricated in the concrete. Study on changes in the amplitude and frequency of ultrasonic signal with the testing depth, which determines the position of the void. The result indicates that the peak amplitude of the received ultrasonic wave is 7.9 mV, the frequency is 39.67 kHz when the depth was 12 cm. It obtains by the statistical method. The concrete contains void at the depth of 12 cm, which corresponds to the depth of the actual void. Therefore, the radial piezoelectric ultrasonic sensor can be used for detection of concrete void.