| Concrete has become one of the most widely used engineering materials because of its advantages of low cost and high strength.The working stress of concrete,as an index to reflect the stress state of the structure,is very important for the safety evaluation of the concrete structure.Piezoelectric materials are widely used in the field of structural health monitoring because of their advantages of fast response and low price.At present,many researches have been done on the monitoring of damage and crack,early strength development and impact load of concrete structures by using piezoelectric plates,but there are few reports on monitoring the working stress of concrete.In this paper,the piezoelectric plate was used to fabricate the concrete working stress sensor.The change of the signal of the sensor under the action of stress and temperature was studied,the relationship between the signal index and the stress was established,and the sensor was embedded in the concrete to calculate the internal working stress of the concrete.The feasibility of using piezoelectric principle to monitor the working stress of concrete was verified.Specific studies include:In order to improve the performance of the piezoelectric plate,the piezoelectric plate was encapsulated,and the encapsulated piezoelectric plate was embedded in the cement paste to make a concrete working stress sensor.The change of the signal of the cement paste sensor within 28 days was monitored.The results show that using copper foil tape packaging can effectively improve the electromagnetic shielding performance of piezoelectric plates,and epoxy resin packaging can effectively improve the waterproof and stability of piezoelectric plate.In the process of monitoring the change of the signal in the cement paste sensor curing 28.Both the wave velocity and the peak frequency in the fixed frequency band show the law of rapid and late growth,which is consistent with the increasing rule of cement strength.In order to study the effect of stress on the signal of the sensor,the cement paste sensor was loaded with a pressure tester,and the change of the signal under various stresses was analyzed,and the relationship between the signal index and the stress was established.These include relative wave velocity,relative peak area,Pearson correlation coefficient and conductance root mean square index.The results show that the relative wave velocity is not sensitive to the change of stress.The relative peak area is sensitive to the change of stress.Pearson correlation coefficient and conductance root mean square index do not change obviously at low stress.In order to study the effect of temperature on the sensor signal,the cement paste sensor was placed in the thermostat box at different temperatures,and the sensor was actively monitored.The results show that with the increase of temperature,the pulse velocity decreases gradually,the peak frequency in 15 kHz~25 kHz band decreases,the peak area decreases,the Pearson correlation coefficient decreases gradually.Conductance root mean square index and admittance root mean square index increased gradually.The cement paste sensor was embedded in the concrete short column,and the change of the sensor signal in the concrete short column under load was analyzed,and the stress of the concrete short column was calculated by using the established index and stress relation.The results show that the signal direct wave of cement paste sensor will not change obviously before and after embedding because the direct wave propagates mainly inside the cement paste,and is little affected by the change of the boundary.By using the relative peak area as an index to calculate the stress,the average relative error will be small er by using the relative Pearson correlation coefficient.When the theoretical stress is greater than 15 MPa,the average error of the calculated stress can be controlled to about 10% by using the relative peak area.Therefore,the piezoelectric sensor can be used to monitor the working stress of concrete structure. |
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