Hyperspectral Imaging and Data Analytics for Civil Infrastructure Inspection

In this 50-minute lecture, the fundamental concept of hyperspectral imaging is reviewed. Each hyperspectral image represents a narrow, contiguous wavelength range of an electromagnetic spectrum, which could be indicative of a chemical substance. All the images in the spectral range of a hyperspectral camera are combined to form a three-dimensional hyperspectral data cube with two spatial dimensions and one spectral dimension. A hyperspectral cube can be sampled in four ways: spatial scanning, spectral scanning, snapshot imaging, and spatio-spectral scanning. This presentation will be focused on spatial scanning from a line-scan camera. The feasibility of horizontal imaging from a synchronized hyperspectral camera and LiDAR scanner system will be explored first. The hyperspectral images are then applied into several infrastructural inspections: concrete roadway condition assessment, fresh mortar strength evaluation, chloride concentration determination in reinforced concrete, steel reinforcing bar and steel plate corrosion, and surface plant stress monitoring as an indication of gas leakage from embedded pipelines. As an example, mortar samples with a water-to-cement (w/c) ratio of 0.4-0.7 were cast and scanned during curing. Reflectance data at a wavelength range of 1920 nm to 1980 nm, associated with the O-H chemical bond, were averaged to classify different mortar types with a Support Vector Machine (SVM) algorithm and predict their compressive strength from a regression equation. After baseline and bias corrections, 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 chloride ion Cl- concentration up to 0.8 wt.%.