Surface crack detection by flash thermography on concrete surface

The life of a structure can be extended by early detection of cracks with appropriate repairs. In reality, structural failure is always accompanied by the formation of cracks and early detection of cracks is essential in extending structure life and preventing structural failure(1). Therefore, there is a huge amount of research on monitoring surface crack development in concrete structures, especially in infrastructures such as pavements and bridges. The studies are mainly focused on embedding optical sensors(1), measuring electrical potential by electro-conductive paint(2), or image enhancement techniques for post-processing(3),(4). Surprisingly, until now, the common practice of surface crack detection is still by visual inspection(5). This technique is subject to the inspectors’ experiences and the fact that it is based on their visual ability makes the detection inefficient and erroneous(3). Surface crack detection by this means is not sufficiently scientific. Therefore, this study proposes to apply short duration (~3ms) pulsed thermography, more specifically named flash thermography (FT), for surface crack detection on concrete structures. The technique has been widely used in the aerospace industry(6),(7) in recent years. No pre-installation of sensors is required in FT. It is a fast, invasive and full-field detection technique which enables a permanent record. The minimum width at which a crack is considered to be significant depends on the function of the structure members. In general, for interior members a crack width of 0.3 mm is claimed to be significant while for exterior members the figure is 0.15 mm(8). More specifically, according to the technical guideline made by the Buildings Department in Hong Kong, for cracks noted in structural elements such as beams, slabs, columns and walls with crack widths less than 1 mm, no detailed investigation is required and they are classified as severity index 2(5). The follow-up actions and the corresponding severity index on different building defects are listed in Table 1. Therefore, the range of crack width of interest in this study is from 0.15 mm to 1 mm. A cement panel with major cracks with widths of 0.5-1 mm and micro-cracks having widths of 0.1-0.5 mm are inspected and the results are presented. Moreover, this paper provides a comparison of the effectiveness of two traditional crack detection techniques, Sobel and Canny, and one proposed method, the sheared image subtraction method, and the results are also presented.