Corrosion Monitoring of Steel Rebar by Long-Period Fiber Grating Sensors Coated With a Thin Fe-C Layer

In this study, the corrosion of steel rebar is monitored by distributed optical fiber gratings sensors deployed in parallel with the rebar. Long-period fiber gratings (LPFG) are coated with a thin Fe-C layer so that the corrosion of the Fe-C coating is correlated with the rebar corrosion. Light can propagate through the core and cladding of a LPFG sensor. The cladding mode of light propagation is significantly affected by the environmental refractive index of the sensor, resulting in a potential change of wavelength over time. The wavelength change is related to the degree of corrosion through laboratory calibration tests by placing both the LPFG and a reinforcing steel bar in the same corrosion environment. The wavelength change and the corrosion degree are quantified by an optical spectrum analyzer (OSA) and an electrochemical impedance spectroscope (EIS), respectively. Test results indicated that the Fe-C coated LPFG sensor can be applied to successfully monitor the onset of corrosion along the length of steel rebar. In practical applications, the change in wavelength of the LPFG sensor is first measured with the OSA and then converted to the degree of corrosion in iron coating through their calibrated relation. For long-term monitoring, LPFG sensors may be coated with a thin layer of stainless steel that can be corroded slowly over time and better correlated with the corrosion of the steel rebar.

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