Hyperspectral Reflectance for Determination of Steel Rebar Corrosion and Cl− Concentration

In this study, a new method is proposed to determine chloride ion (Cl- ) concentration and steel rebar corrosion from hyperspectral spectroscopy. Three groups of mortar cubes with water-to-cement (w/c) ratios of 0.5, 0.6, and 0.7 were subjected to rapid corrosion testing in 3.5 wt% NaCl solution to accelerate the transport of chloride ions. Embedded along the centerline of each mortar cube was a steel rebar that corroded when the Cl- accumulation around it exceeded a critical/threshold concentration. Open circuit potential was measured to characterize the corrosion possibility of steel rebar. Mortar surfaces were scanned with a hyperspectral camera in the infrared range (1000 nm - 2400 nm), and the reflectance intensity at 2258 nm wavelength was extracted to characterize Friedel's salt. The possibility of steel corrosion was experimentally shown to increase with the characteristic reflectance intensity that in turn decreases linearly with the diffusion depth at a given corrosion state. For each type of mortar cubes with a constant w/c ratio, the characteristic reflectance intensity linearly increases with the Cl- content up to 0.8 wt%. Therefore, the corrosion status of steel rebar and Cl- concentration can be predicted based on the combined information from the reflectance intensity on the mortar surface and the relation between reflectance and total chloride content.