Development and Application of an Ultra-Compact Mid-Infrared Hyperspectral Camera for Chloride Sensing in Concrete

In recent years, chloride-induced deterioration of reinforced concrete (RC) structures has become increasingly severe, necessitating periodic inspections to mitigate its progression. As a potential solution, a non-destructive inspection technique using an Imaging-Type Two-Dimensional Fourier Spectroscopy system [Fig.1], developed at Kagawa University, has been proposed. This system enables non-destructive, non-contact, and two-dimensional acquisition of mid-infrared spectra from the surface of structures, facilitating efficient monitoring. However, in the mid-infrared region, a reliable method for detecting chloride-induced deterioration has not yet been established, as the characteristic wavelengths associated with chloride ions fixed in cement remain unclear. Therefore, this study aims to establish a quantitative method for chloride ion detection in concrete using mid-infrared spectroscopy.

First, cement paste specimens with varying chloride ion concentrations were prepared and measured using both the Imaging-Type Two-Dimensional Fourier Spectroscopy system and an FT-IR spectrometer. The spectral peaks correlated with chloride ion concentration were then analyzed. As a result, a peak wavelength around 10.6 µm was identified as exhibiting a clear correlation with chloride ion concentration. This trend was consistently observed using both measurement instruments. Fig.2 shows the spectra after baseline correction at the peak wavelength. These findings suggest that the chloride ion concentration in concrete can be estimated by analyzing the spectral peak near 10.6 µm. This study contributes to the development of a non-destructive chloride detection method, which is expected to enhance the durability assessment of RC structures affected by chloride-induced deterioration.