Experimental Study On Dynamic Stress Monitoring With Embedded Piezoceramic For Concrete Structures

The mechanical behavior of concrete structures under dynamic loads that oughtto be well understood is crucial to monitoring and evaluating the performance ofstructures. Nowadays, in the experiments of measuring the concrete stress, most of theresearchers obtain the results indirectly by strain gauges mainly pasted on the surfaceof concrete specimens. As a result, the accurate stress distribution of the internalstructures can hardly be reflected. So it is becoming increasingly important to developan effective and inexpensive sensor that can directly measure the inner stress ofstructures, especially those under dynamic loads. Recently, with the rapiddevelopment of sensing technology, monitoring the mechanical behavior of concretestructures using intelligent materials has become a vital study in the fields of civilengineering. Apparently, that piezoelectric ceramic(PZT) has been widely applied tomonitor the behavior of structures is due to its excellent properties, such as speedyresponse, widely frequency bands, linearity, easy to manufacture, low price, as wellas its unique sensing and actuation characteristic. The technology to measure theinner stress of structures is crucial to the study of dynamic mechanical behavior ofconcrete structures. In this thesis, a novel embedded stress sensor based on thepiezoelectric effect has been presented to measure the inner stress of concretestructures under dynamic loads.(1) From the previous research, the embedded PZT-based stress sensor has someweakness, such as individual differences and it was harder to be installed because ofits smooth surface. By some improvements, now the sensor has the waterproofproperty and the sensitive stability, also, it can be easily installed in the concretestructures.(2) Using a small dropping hammer machine and a servo actuator, the sensitivityof PZT-based stress sensor under the dynamic (impact, sine, square wave, irregular)loads had been calibrated. The experiment results show that the sensor possesses gooddynamic responsibility, and the sensitivity of sensor would not change with externalshape, loading conditions, loading frequency and loading strength in the measurableranges. Moreover, the individual difference is little. The linear property of PZT-basedstress sensors lays a foundation of the measurement of concrete impact stress.(3) The PZT-based stress sensors embedded in the inner part of the plain concrete cylinder specimens were applied in the study of concrete dynamicconstitutive tests so that the time-history of stress in the different heights could bemeasured. It was presented that the stress was uniformly distributed in the cylinderspecimens without cracking while the stress of the specimens from top to bottomdeclined in the damage of the specimens. The dynamic stress-strain curve of theconcrete cylinder specimen was derived through the measurement of the strain gauge.It is proved that the PZT-based stress sensors have effectiveness on monitoring thestress of concrete structures. Based on the stress time-history analysis of differentsensors, the stress wave velocity of the specimens has been analysed and comparedwith the theory.(4) The PZT-based stress sensors which were installed in reinforced concretebeams were used to measure internal stress. The results showed that the internal stressof reinforced concrete members could be qualitatively, reflected by the PZT-basedstress sensors. And it provided a useful basement for the quantitative measurement ofreinforced concrete members in the future.