Damage detection of reinforced concrete beams with novel distributed crack/strain sensors

Coaxial cables are used as distributed sensors to detect cracks or measure strains in reinforced concrete (RC) structures with the electrical time domain reflectometry (ETDR). An emphasis was placed on the development and validation of a type of novel cable sensors. The new sensor was designed based on the change in topology of its outer conductor under strain conditions instead of the change in geometry of a conventional cable sensor. Finite difference time domain (FDTM) models of four types of cables were established to understand the inter-relationship among various design parameters. They were calibrated with a standard displacement transducer in nine tension tests and then mounted near the surface of six RC beams of 0.91-m long to validate their sensitivity and performance. The bending test results of RC beams are in general agreement with those from the calibration test of cables. Both indicate that the proposed sensors are over 15-80 times more sensitive than sensors based on commercial coaxial cables. The test results of the beams also show that the measured reflection coefficient correlates well with the measured crack width. Overall, the new cables have a sensitivity of 160-750 milli rho per unit crack width in mm and a spatial resolution of 50 mm.