Estimating Depth of Neuralization in Concrete Using Nearinfrared Spectroscopy with Hyperspectral Camera

Abstract

Carbonation of concrete is one of the deterioration mechanisms observed in reinforced concrete structures, such as manholes, utility conduits, and tunnels. Since carbonation leads to the corrosion of embedded reinforcing steel, regular inspection is necessary. However, current inspection methods rely on destructive testing through core sampling, which incurs significant costs, highlighting the need for simpler and more efficient approaches. Despite numerous studies on inspection techniques for concrete deterioration, a reliable non-destructive method for evaluating concrete carbonation has not yet been established. To address this issue, this study proposes a non-destructive method for evaluating carbonation depth using a hyperspectral camera (HSC) and near-infrared spectral analysis. In the proposed method, HSC is used to acquire absorbance spectra in the near-infrared wavelength range from the concrete surface, and the spectra are analyzed to evaluate changes in concrete components associated with carbonation. During carbonation, calcium hydroxide present in the concrete reacts with carbon dioxide in the air to form calcium carbonate, and this reaction progresses inward from the surface layer of the concrete. This study investigates the correlation between near-infrared spectra from the concrete surface and carbonation depth. Concrete samples with carbonation depths ranging from 0 to 30 mm were prepared using an accelerated carbonation chamber. Each sample was imaged using an HSC, and spectral analysis was conducted. It was found that absorbance at 2092 nm, which is associated with calcium hydroxide consumed during carbonation, showed a correlation with the progression of carbonation. These findings indicate the potential of the proposed method for estimating carbonation depth on the basis of changes in near-infrared spectra from concrete surface obtain from HSC.