Detectability of Subsurface Defects with Different Width-to-Depth Ratios in Concrete Structures Using Pulsed Thermography

The active thermography technique is one of the most effective nondestructive tests for evaluating subsurface delaminations in concrete structures. The limitation of this method, which has been studied for some time, is that the width of the smallest detectable defect should be at least two times larger than its depth. However, controversy on this matter remains for concrete material with largely uncertain homogeneity, although the development of the infrared (IR) detector technology improved the above-mentioned limitation. In this study, the pulsed thermography (PT) technique is therefore conducted in the laboratory to investigate the detectability of delaminations with the width-to-depth ratio (w2d) ranging from 1.0 to 7.9 by using a long IR wavelength detector with a focal plane array of 640 480 pixels. The study focuses on the w2d ratio lower than 2.0. A concrete specimen was made with 12 embedded simulated delaminations having different sizes and depths. The results showed that a combination of PT and pulsed phase thermography can be used to detect delaminations with a w2d ratio equal or greater than 1.25. In addition, the absolute contrast above the delamination increases with the higher w2d ratio, indicating that even for a relatively deep delamination, it is still detectable if a delamination is provided by appropriate heat energy and its size is sufficiently large. Finally, the study also indicates that as the amount of heating energy provided is increased, the greater accuracy in predicting the depth can be obtained.