Steel Corrosion Monitoring in Concrete Slab Using a Long-Period Fiber Grating Assembly

Abstract Steel corrosion in concrete structures is a slow process that usually takes several years to a degree that induces concrete cracking, and the corrosion amount or the penetration depth is hard to detect due to the presence of concrete cover. Long period fiber grating (LPFG) couples incident light from propagating core mode to co-propagating cladding mode, producing a series of attenuation bands of transmission spectrum. The resonant wavelength of the spectral attenuation band is closely related to the effective refractive index of the medium surrounding the fiber. As the chemicals in the surrounding medium change, the resonant wavelength is shifted. Therefore, it is widely used for physical, chemical and biochemical sensing. In this study, a corrosion sensing assembly to monitor steel bar corrosion in concrete slab based on the LPFG sensing principle is proposed and its performance is experimentally investigated. The assembly was made by encasing the grating part of a LPFG in a steel straw with an inside diameter around 450 μm, which was made of rebar steel. Five wall thicknesses were considered including 400 μm, 800 μm, 1000 μm, 1200 μm and 1500 μm. The corrosion sensing assemblies were embedded in concrete slab and then immersed in salt solution. Accelerated corrosion tests were performed periodically and the corrosion rates at different time intervals were measured with linear polarization resistance (LPR). The change in the resonant wavelength over time was recorded using an optical spectrum analyzer (OSA). After corrosion test, all the steel straws were taken out of the concrete slab, and visual observation was performed. A linear relationship between the corrosion attack depth in concrete slab and the test time was established.