Use of advanced thermographic signal processing techniques to improve the subsurface damage detection in concrete columns under varying solar exposure

Abstract

Infrared Thermography (IRT) is a Nondestructive Testing (NDT) method that can complement the concrete infrastructure condition assessment in a fast and contactless manner. When applied to large structures in outdoor areas, the heat source is usually the Sun, which is dynamic and varies through the days, months, and year. Solar irradiation is vulnerable to changes in environmental conditions, which affects the upcoming IRT measurements. Besides that, vertical elements have multiple locations and orientations, where the solar exposure varies according to the solar cycle. Consequently, column faces can experience reduced energy flow, where low or inexistent thermal contrast restrains the detection of existing subsurface damages. In this study, three signal processing techniques, named Principal Component Thermography (PCT), Pulsed Phase Thermography (PPT), and Partial Least Square Thermography (PLST), were applied to thermograms sequences acquired from a concrete element under varying solar exposure. One reinforced concrete column was constructed with ten simulated subsurface defects positioned in the Northeast, Southeast, Northwest, and Southwest faces. This column was inspected hourly through different days of summer and winter periods. It was demonstrated the difference between the signature contrast registered in thermograms acquired from faces exposed to small and large periods of solar irradiation. The preliminary results of using thermographic signal processing techniques verified the possibility of increasing the signal-to-noise ratio and thermal contrast in elements under unfavorable solar exposure. In addition, the research explored the use of different image sequence intervals on the performance of the signal processing techniques.