Fiber Optic Sensor Based Corrosion Assessment in Reinforced Concrete Members

In this 50-minute lecture, the fundamental concepts of fiber optic sensors for both distributed and point corrosion measurements are reviewed. For the distributed monitoring of a line bridge component such as steel reinforced girders, Brillouin scattering and fiber Bragg gratings (FBG) can be coupled to measure both temperature and radial strain as an indirect indicator of corrosion process. For the point monitoring of steel structures, long period fiber gratings (LPFG) are specially designed for a direct measurement of mass loss or the loss in cross sectional area of the component. In particular, a Fe-C coated LPFG sensor is introduced for corrosion induced mass loss measurement when Fe-C materials are comparative to the parent steel component to be monitored. The sensing system operates on the principle of LPFG that is responsive to not only thermal and mechanical deformation, but also the change in refractive index of any medium surrounding the optical fiber. Fabrication process of the LPFG is demonstrated through the CO2 laser aided fiber grating system. To enable mass loss measurement, a low pressure chemical vapor deposition (LPCVD) system is introduced to synthesize a graphene/silver nanowire composite film as flexible transparent electrode for the electroplating of a thin Fe-C layer on the curve surface of a LPFG sensor. An integrated sensing package is illustrated for corrosion monitoring and simultaneous strain and temperature measurement. Two bare LPFGs, three Fe-C coated LPFG sensors are multiplexed and deployed inside three miniature, coaxial steel tubes to measure critical mass losses through the penetration of tube walls and their corresponding corrosion rates in the life cycle of an instrumented steel component. The integrated package can be utilized for in-situ deterioration detection in reinforced concrete and steel structures. Assisted by a permanent magnet in pipeline monitoring, both FBG and LPFG sensors are combined with an extrinsic Fabry-Perot interferometer (EFPI) to measure both internal and external thickness reductions without impacting the operation of the pipeline.