Fiber Optic Condition Monitoring during a Full Scale Destructive Bridge Test

Abstract

Fiber optic sensors were used to monitor structural parameters during a destructive bridge test. A system composed of four different types of fiber optic sensors was used to demonstrate versatility and possible completeness for these type of measurements using fiber optic sensors. The fiber optic sensors were monitored continuously during the test using a single control unit. Local strain measurements on the reinforcing bars were conducted using fiber optic Bragg grating sensors. A new technique employing both current and temperature tuning of a DFB (Distributed Feedback) laser to interrogate gratings was developed, showing good correspondence with resistance strain gauges. This sensor conducted the only strain measurements during and after the failure of the bridge, since all expensive control electronics could be operated from a safe distance. A Bragg grating laser sensor system was also used, with a laser cavity of 40 m using standard connectors. The system was well behaved and the measurements corresponded well with resistance strain gauges. A fiber optic polarimetric sensor measured displacements over 2.5 m at the bridge surface. The measurements were comparable to conventional extensiometer measurements at the same location. Cracking of the bridge surface was monitored by studying reflection and transmission characteristics of optical fibers glued to the surface. Although less cracking occurred than was expected, both transmission loss and OTDR (Optical Time Domain Reflectometry) reflection measurements successfully detected cracking of the bridge surface. The most severe practical problem was unintentional fiber breaks caused by personnel not accustomed to using fiber optics.