A piezoelectric-based three-direction normal stress sensor for concrete structures

Piezoelectric-based sensors are low cost, highly stable, responsive, and changeable in shapes, allowing them to be easily embedded within concrete structures to perform various monitoring tasks. In this research, an in-built piezoelectric-based three-direction stress sensor is proposed, which could monitor the internal normal stresses in three mutually orthogonal directions. The sensor has a cubic shape with edges of about 25 mm. It consists of three PZT (lead zirconate titanate) patches, each of which is sandwiched between a steel base and a square steel plate with epoxy. To test the sensors’ characteristics, a finite element analysis was carried out to investigate stress transfer inside the sensor. Besides, a servo-hydraulic machine was used to conduct a cyclic loading test, delivering compressive stress up to 30 MPa on the three sensors. Both the numerical simulation and experiment results have shown that the cross-interference effect between different directions is negligible. Also observed was a high level of consistency in the sensitivity of measurements, regardless of the monitoring direction and sensor. With the advantages of a simple structure, low cross interference, and consistent sensitivity, the proposed three-direction normal stress sensor has a great potential to be mass produced and used in monitoring the health state of structural concrete.