Code-specified early delamination detection and quantification in a RC bridge deck: passive vs. active infrared thermography

Delamination in reinforced concrete (RC) bridge decks can degrade the serviceability of entire bridges, leading to concrete spalling and steel rebar corrosion and eventually becoming a safety concern. Drone-based infrared thermography (IRT) offers a promising tool for rapid assessment of bridge deck delamination compared to labor-intensive coring and visual inspection methods. However, the performance of passive IRT in detecting the delamination of RC bridge decks at its minimum depth and size (i.e., spall 25 mm or less deep or 150 mm or less in diameter) stipulated under a ‘fair’ condition state in the 2019 AASHTO Manual for Bridge Element Inspection has not been verified adequately. In this study, four RC slabs of identical design were cast with embedded thin foam sheets to simulate a wide range of delamination in thickness, size, spacing, and depth. Together, the four slabs form a representative RC deck of a mark-up bridge. Controllable indoor active IRT tests of individual slabs were conducted to detect and quantify the foams that serve as a ground truth for the performance of drone-based passive IRT for deck delamination detection on the mark-up bridge as the embedded foams may be displaced during concrete slab casting and the slab support is altered during erection. Statistical analysis was carried out on the thermal contrasts of both passive and active IRT tests on the four slabs to investigate the effects of delamination geometry and embedment depth. Both the active and passive IRT methods proved successful in localizing delamination and identifying its equivalent thicknesses of as low as 1.63 mm and a size (150 mm in length or 25 mm in depth) corresponding to the ‘fair’ condition state in the AASHTO Manual for Bridge Element Inspection.