Fabrication And Properties Of Cement Based Piezoelectric Sensor And Its Application Research In Civil Engineering Fields

Cement based piezoelectric composite is a kind of newly developed piezoelectric material with superior piezoelectric properties, high electromechanical coupling coefficient and good compatibility with concrete materials in mechanical and acoustic etc aspects, which has potential application prospect in health monitoring of civil engineering structure. According to the characteristics of civil engineering structure, piezoelectric ceramic, cement based and cement/polymer based (mixtures of cement, epoxy and solidified agent) piezoelectric composites were used as sensing elements to fabricate cement based piezoelectric sensors, respectively. In this research, the fabricated piezoelectric sensors were used to conduct the experiment tests and theoretical analysis as follows.PZT piezoelectric ceramic was used as functional component, Portland cement and cement/polymer were used as matrix materials to fabricate 2-2 type and 1-3 type cement based, cement/polymer based piezoelectric composite using cutting-and-filling technique. The researches indicate that comparing with piezoelectric ceramic, the piezoelectric voltage constant g33 and the thickness electromechanical coupling coefficient Kt of the piezoelectric composites increase greatly, while the acoustic impedance Z and mechanical quality factor Qm decrease obviously. Therefore, it can be concluded that the piezoelectric sensors with superior receving sensitivity, broad band and good acoustic impedance compatibility with concrete can be tailored to meet the requirements of civil engineering.Piezoelectric ceramic and piezoelectric composite were used as sensing elements, cement/polymer was used as packaging materials to fabricate kinds of piezoelectric sensors, respectively. The researches indicate that the piezoelectric sensors that fabricated using cement/polymer as packing materials have the common properties such as high mechanical strength, good durability, strong dynamic output ability and high reliability. Furthermore, the piezoelectric sensors using piezoelectric composites as sensing elements also have the properties of low acoustic impedance and broad band, which can be used effectively to monitor the cement hydration reaction process and dynamic vibrations of civil structures. The piezoelectric sensors using piezoelectric ceramic as sensing elements are more suitable for civil engineering structure monitoring based on electromechanical impedance technique. The mechanical impedance variation of the structures can be reflected by the electromechanical impedance variation of piezoelectric sensors that coupled with the structures. Therefore, the structure damage state can be detected using this method. Besides, the piezoelectric traffic sensors using piezoelectric ceramic as sensing elements also have the good linearity, good output response and repeatability, and high strength etc, which make it more suitable for traffic monitoring.The embedded piezoelectric ceramic and piezoelectric composite were used as emitter and receiver to monitor the cement hydration reaction process. Two kinds of different methods, that is, ultrasonic wave transmitting method and electromechanical impedance technique, were adopted in the research, respectively. The researchs indicate that the ultrasonic waves received by the piezoelectric composite sensors have the characteristics of large head/secondary wave ratio, obvious head wave and single dominant frequency etc. Among all the piezoelectric sensors, the piezoelectric composite sensors using cement/polymer as matrix materials have the best receiving ability. The cement hydration reaction variation in different stages can be obtained by analyzing the wave shape and frequency spectra of the receiving waves. In the initial 8 h of hydration reaction, the wave shape curves of the receiving wave are very weak and the amplitudes are not obvious, while the dominant frequency is clear. At approximately the final setting time, both the wave shape curve and dominant frequency show the obvious mutation phenomenon. After 8 h of hydration reaction, the starting point of head wave moves forward obviously and the amplitudes of the head wave also increase significantly, while the dominant frequency is independent of the hydration reaction time. After cement hydration reaction for 24 h, the wave shapes and parameters of the receiving wave tend gradually to be steady with increasing the hydration time.According to the variation of ultrasonic wave parameters received by the piezoelectric sensors, such as dominant frequency, amplitude of dominant frequency, wave speed and amplitude of head wave, the cement hydration reaction in 24 h can be defined to three periods, that is, induction period (about 0 h-5 h), acceleration period (about 5 h-10 h) and attenuation period (about 10 h-24 h). In the acceleration period, there will be an obvious transition point in the chart of wave parameters, which basically corresponds to the final setting time of cement.Besides, the cement hydration reaction process can also be reflected by the electromechanical impedance spectra of piezoelectric sensors embedded in the cement paste, and the similar variation regularity can also be obtained. Therefore, it is feasible to monitor the cement hydration reaction based on the electromechanical impedance technique of piezoelectric sensors.Based on electromechanical impedance technology, nondestructive detection on the little concrete structures was performed. The influences of temperature variation, loading and crack damage on electromechanical impedance spectra of affixed and embedded piezoelectric sensors were studied. The results indicate that temperature variation has great influence on the electromechanical impedance of the PZT piezoelectric sensor that coupled with structures, which can result in the drift of resonant frequency and the variation of resonant peak value. Therefore, the PZT piezoelectric sensors can be used as a kind of temperature sensor to monitor the environmental temperature variation. External loading also has great influence on the electromechanical impedance of the embedded PZT piezoelectric sensors. The variation of structural mechanical impedance can be reflected by the electromechanical impedance spectra of PZT piezoelectric sensors when the mechanical loading is applied to the structure. Especially when the structure generates damage due to the strong loading effect, the electromechanical impedance spectra of PZT piezoelectric sensors will appear obvious mutation.The structure damage induced by cracks, especially for the structural initial damage, can also be well reflected by both the affixed and embedded PZT piezoelectric sensors. The electromechanical impedance spectra were analyzed employing the mathematics statistical method. The variation regularity of electromechanical impedance of PZT piezoelectric sensors with kinds of influencing factors can be visualized and effectively observed by a kind of damage quantitative index. Therefore, the the apperance and development of the structure damage can be obtained using this method.The fabricated piezoelectric traffic sensors were embedded in the road. A set of piezoelectric sensor system, including cement based piezoelectric sensor, small charge amplifier, multichannel data collecting device and monitoring software was established to monitor the people flow, vehicle speed and loading. The results indicate that the amounts of people who passed through the sensing section as well as the population density can be calculated by the pulse response amounts of the piezoelectric sensors. The instantaneous vehicle speed can also be accurately calculated by the sensors distance and the time interval between two pulse responses, so the over-speed phenomena can be identified using this method. Besides, the dynamic loading information of vehicles can be calculatedd in terms of the output pulse voltage peak value or areas enclosed by the output pulse peaks, respectively. When the road was level and the vehicles passed through it at uniform speed, the vehicle loading error calculated by the linear fitting formulation is less than 5%. Therefore, the wight in motion of the vehicles can be realized and the overweight phenomenon can also be controlled effectively using this kind of traffic sensors. Besides, the cement based piezoelectric composite sensor can also be used to monitor the critical regions of bridges due to its superior compatibility with concrete structures.