## 2 Related Works  
Pulsed thermography is a well-known NDT to detect defects in concrete structures. In this method, the surface of the structure is heated up by external heat sources, such as infrared radiators, halogen lamps, or flash lamps [23; 24]. The surface temperature of structure in both heating time and cooling time is analyzed to determine and characterize delamination in LPT [25]. The absolute contrast is usually used to detect delamination, while the depth of defects is quantitatively computed by using the linear relationship between the depth square and observation time [16]. Section 5.1 will discuss in detail the definition of absolute contrast and observation time. Several studies relating the above-mentioned methods are presented as follows. In 2002, [PERSON] et al. [26] carried out an experimental test on two concrete specimens using three infrared radiators with 7200 watts of total energy. Eight artificial delaminations with sizes of 20 cm \(\times\) 20 cm \(\times\) 10 cm and 10 cm \(\times\) 10 cm \(\times\) 10 cm by polystyrene were implemented inside the specimen. The radiators were moved vertically on a line array with a fixed distance of 15 cm, and they concluded that a big delamination at a depth of 10 cm (with WTDR of 2.0) could be identified. In addition, using impulse thermography to detect voids in four concrete slabs with dimensions of 100 cm \(\times\) 100 cm \(\times\) 30 cm was conducted by [PERSON] et al. in 2007 [17]. Defects were simulated by the inclusion of polystyrene cuboids or integration of gas concrete parts with sizes of 10 cm \(\times\) 10 cm \(\times\) 5 cm. The defect depths were 6 and 10 cm, and the heating duration varied between 5 and 30 min. One, two, and three layers of reinforcement mats were embedded inside a specimen. This work was studied on the influence of concrete age, pore content, aggregate type, and reinforcement density on the detectability of shallow voids. In all cases, the void at a depth of 6 cm was detected (with WTDR of about 1.67). The results showed that the reinforcement density has only a slight influence on the detectability of subsurface defects. Furthermore, in 2008, [PERSON] et al. [19] carried out empirical research using an IR camera NEC TH7102 (Tokyo, Japan) with minimum detectable temperature difference of 0.08 \({}^{\circ}\)C. Four halogen lamps were used to produce uniform heat flux. The authors concluded that it was difficult to identify the defect with the smallest size (5 cm \(\times\) 5 cm) at the largest cover thickness of 3 cm. The applicability and limits of void and delamination detection in concrete structures using infrared thermography was performed by [PERSON] et al. in 2015 [18]. They conducted experiments of 51 artificially defects with sizes from 1.2 to 10 cm located at depths from 0.5 to 12.5 cm in concrete, using active infrared thermography. The heating time was from 5 to 30 minutes. The study mentioned that it was possible to detect defects with the ratio between depth-and-size \(\leq\) 0.9 and 1.3, when using the thermal contrast and phase contrast methods, respectively. A 3-dimensional numerical simulation of the thermal transfer occurring in concrete specimens containing defects was also performed. The relationship between the depth of defect and maximum time, as well as maximum frequency, was determined. Based on that, the depth of defects in concrete structures could be predicted. In addition, the research works of [PERSON] in 2015 [3] and [PERSON] in 2013 [6] showed that all five delaminations placed deeper than 5.72 cm (2.25 in), or with a WTDR \(<\) 1.33, were undetectable. In these studies, a 1500 watt infrared heater placed at 1.1 m and a FLIR SC640 camera (FLIR, USA) located at 1.7 m from the specimen surface were used. All artificial delaminations had the same sizes of 7.62 cm \(\times\) 7.62 cm \(\times\) 0.95 cm or 3 in \(\times\) 3 in \(\times\) 3/8 in. In the IRT field, the SNR has been demonstrated as a highly effective criterion in judging the detectability of defects.