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Portable System Design for Post-tensioned Prestressed Ducts Based on Coplanar Capacitive Sensor
Keywords: post-tensioned prestressed ducts, grouting quality, coplanar capacitive sensor
Post-tensioned prestressed ducts are widely used in construction of bridges in China. A portable system to test grouting quality for post-tensioned prestressed ducts is presented. As a kind of NDT (non-destructive testing) technology, coplanar capacitive sensing method is used in both theoretical study areas and engineering applications and shows the potential of testing grouting quality. With semi-analytical equation for capacitive proximity sensor in our previous work, the optimal parameters of sensor are determined. Three capacitance measurement methods are compared and capacitance to digital converter (CDC) is choosed to design measurement circuit. The system consists four parts: data acquisition unit-coplanar capacitive sensor; data conversion unit- capacitance measurement circuit based on AD7745; system control unit- control circuit based on the PIC18F45K20; data display unit-1602LCD. According to the approximation theory, the system verification test is designed by measuring capacitance of different thickness of liquid on glass surface. The capacitance is measured by both our system and impedance analyzer, the measured values are compared with the simulated results by COMSOL, data processing and system error evaluation are also done. Experiments show that the average relative error of the system is 7.3%, which is lower than the impedance analyzers, and the resolution is 0.1mm. In addition, the inspection system volume (except the coplanar capacitive sensor) is 100mm80mm30mm. The experimental results illustrate that the system can effectively distinguish the stratification of grouting in the ducts, and the test results are reliable and accurate.
Nan Li, associate professor
Beijing University of Technology
Beijing, Beijing

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