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.
INTRODUCTION
Corrosion of reinforcement steel is one of the main causes of premature deterioration in reinforced concrete (RC) structures. It can induce concrete cover cracking, impair the steel-concrete bond action, reduce the mechanical properties of steel bars, and consequently affects the serviceability and the safety of RC structures.1-4 According to an inspection data released by the Federal Highway Administration in 2015, approximately 9.6% of the nation's bridges are structurally deficient because of corroded steel or steel reinforcement. The indirect cost due to traffic delays and lost productivity, were estimated to be as high as 10 times that of direct corrosion costs.5 To restore the structures and make them in daily function, maintenance work is performed periodically. The economic impact of deteriorating RC structures can be minimized through the optimization of maintenance strategies. The implementation and evaluation of these maintenance strategies need some quantitative data such as inspection interval, repair threshold, maintenance technique and repair efficiency.6,7 For corrosion- induced structural deterioration, the corrosion rate or penetration depth is one of the most important input parameters for the design of these strategies.
