**1. Introduction**

In bridge structures, compared to other components, defects occur more frequently in the concrete deck because it is directly subjected to traffic loads [1,2]. Among the various defects, delamination often develops in the concrete cover due to the corrosion effect of steel bars in reinforced concrete [2,3]. Delamination is one of the most dangerous deteriorations because it is usually invisible under visual inspection and can lead to potential spalls (Figure 1). Therefore, delaminations need to be detected as accurately as possible to maintain normal working conditions of the bridge structure.

**Figure 1.** Locations of delaminations in concrete structure.

To investigate the health of a concrete bridge deck in terms of delamination, conventional methods such as chain dragging and hammer sounding are widely used, coupled with modern non-destructive techniques (NDTs) to enhance the accuracy and quality of the inspection. The NDTs that are currently used in concrete bridge inspections include infrared thermography (IRT), ground penetrating radar, ultrasonic surface waves, impact echo, and electrical resistivity [3–13]. In the IRT method, there are two different approaches: passive and active [14]. In active IRT, several minutes to a few hours should be spent to inspect just small regions of structures that are not exposed directly to the sun because the structure surface must be heated by an artificial heat source [15–27]. On the other hand, passive IRT has been proven to be the preferred method to inspect structure components exposed directly to the sunlight [4,12,13,28–31]. The information related to potential delaminations can be extracted accurately without using any extra artificial heat source. Therefore, the concrete bridge deck is scanned more rapidly and simply using passive IRT compared to the active approach.

According to the American Association of State Highway and Transportation Officials (AASHTO)-Manual for Bridge Element Inspection (2015), the condition of the concrete bridge deck is divided into four states from one to four based on the level of the severity of the damages as shown in Figure 2 [32]. If delaminations/spalls occur inside the concrete bridge deck, it means that it does not belong to the condition state 1 (good status). The condition states 2 (fair status) and 3 (poor status) correspond to the areas of moderate and severe delaminations/spalls, respectively. In the condition state 2, the delamination depth (*d*) is smaller than 2.54 cm or diameter (*w*) is less than 15.24 cm, while *d* is greater than 2.54 cm or *w* is larger than 15.24 cm in the condition state 3. When a structure is in the condition state 4 (severe status), load reduction, bridge closure, or replacement with a new structure should be performed. It is found that both the size and depth of delamination are critical indicators used to classify the health condition of concrete bridge deck.

**Figure 2.** Classification of concrete bridge deck conditions required by American Association of State Highway and Transportation Officials (AASHTO) 2015.

Moreover, although the effect of the size and depth of the delamination on the detectability of delaminations has been studied in several works, it still remains to be a controversial issue. A shallower delamination with a small size sometimes may not be detected, but a deeper delamination with a larger size may be identified. Thus, in consideration of the practical application for concrete bridge deck inspection, the employment of the passive IRT technique with various width-to-depth ratios (WTDRs, the ratios between the size and depth of delaminations) is investigated in detail in this work. This implies that the effect of the size and depth of delamination is not considered independently, like previous works, but is studied using the WTDR [1,29,30].

In addition, as the environment strongly affects the detectability of defects in passive IRT, suggesting a suitable inspection time is mentioned in previous researches [12,13,29,30,33,34]. However, this topic might not be studied fully, especially for defects with small size or WTDR, it therefore is one of the main tasks in the present work. In addition, the applicability of unmanned aerial vehicles (UAVs) is studied for both quantitative as well as qualitative assessment of the concrete bridge deck conditions. The UAV mounting an IR camera (UAV-IRC) is used alongside the handheld IR camera (H-IRC). The UAV-IRC is more suitable for places where the H-IRC might not be used owing to heavy and high-speed traffic. The results of the UAV-IRC would be verified by those from the H-IRC. This study determines the limits and applicability of passive IRT in detecting delamination with different WTRDs in concrete bridge deck. Hence, a comprehensive system for bridge inspection combining H-IRC and UAV is expected to be developed.

### **2. Literature Review**

Regarding the detectability of subsurface defects in concrete structures using passive IRT techniques, the main concerns growing in recent years are the effect of delamination parameters like the size, depth, and thickness, effect of the type of the infrared detector, impact of weather conditions, and effective inspection time, as well as the minimum threshold of the surface temperature difference used to define a defect as detectable or undetectable [1,29,30,35,36].

The detected depth of delaminations has been studied by researchers like Yehia et al. in 2007, Farrag et al. in 2016, Sultan et al. in 2017, and Hiasa et al. in 2018 [1,13,36,37]. Detected and undetected depths of delaminations with various sizes are depicted in Figure 3 as summarized from previous works [1,2,13,29,30,34–39]. It is noted that the results shown in Figure 3 were based on normal concrete specimens, using an uncooled IR camera, and monitored via the passive IRT approach. In Figure 3, the blue filled-in circle represents undetectable delaminations while the red unfilled-in diamond stands for detectable delaminations.

**Figure 3.** Detected and undetected delaminations summarized from the literature.

It can be observed in Figure 3 that despite having the same WTDR, delaminations could be detected or undetected. The reason is that the experiments were performed in different environmental conditions, and the parameters of the artificial delaminations, the dimensions of concrete specimens and the specification of IR detector are not the same. For example, in the case of delaminations with the same WTDR of 4.0 shown as "Case 1" in Figure 3, one delamination could be detected while other one was not identified. The same phenomenon can be observed as pointed out in "Case 2" (WTDR = 4.8) and "Case 3" (WTDR = 2.0). Thus, the conclusions given in the previous works regarding the detected depth and size of delaminations still remain a controversial topic. Hence, further studies need to be conducted, especially in the case of heterogeneous material, such as concrete. In this work, WTDR is considered instead of the depth or size separately unlike in previous studies.

The most effective time to conduct the concrete inspection (i.e., either daytime or nighttime) proposed in the above-mentioned studies is also inconsistent. Washer et al. in 2009 and 2010, conducted tests on a concrete specimen for approximately six months on the field [29,30]. During the test, every 10 min, the surface temperature of the specimen was monitored by an IR camera. Washer et al. recommended that the optimum times of the day for identifying delaminations at depths of 25, 51, 76, and 127 mm are after sunrise 5 h and 40 min, 6 h, 7 h and 9 h, respectively [29,30]. However, in 2018, Hiasa et al. proposed different times to perform the delamination inspection in concrete structures based on the results of the field test on four concrete specimens in which an IR camera was utilized to capture the surface temperature of the specimen from 7:00 am to 12:00 am [1]. They concluded that delaminations at depths of 5.1 and 7.6 cm could not be detected. It was suggested that delaminations at depths of 1.3 and 2.5 cm can be detected at any time except during the interchange periods of 09:00–9:30 and 15:00–16:00. The authors stated that the inspections should be conducted during nighttime because the available time during nighttime is longer than that during daytime, and the probability of the misdetection of the delaminations may be reduced.

Furthermore, Yehia et al. (2007), Kee et al. (2012), and Gucunski et al. (2013) gave their own recommendations about the available inspection time [12,13,34]. Yehia mentioned that during the period from 10:00 to noon, delaminations with depths equal to or smaller than 5.1 cm were shown more clearly than from noon until 3 pm [13]. Focusing on deep delaminations of 6.35 and 15.24 cm, Kee et al. stated that delaminations could be detected clearly at 45 min after sunrise during the cooling cycle or after sunrise 7 h and 45 min while no delaminations were shown at 3 h and 45 min after sunrise [34]. Gucunski et al. recommended that 40 min after sunrise is a better time than around noon for the delamination inspection [12]. As reviewed above, the proposed times are completely different and this can be explained by the effects of different environmental conditions, delamination characteristics, concrete specimen dimensions, infrared detectors, and experiment setup. Hence, the determination of feasible inspection time should be studied further, especially in the case of small WTDR delamination in order to reduce the misdetection problem in the concrete bridge inspection.

In passive IRT, the IR camera is normally kept in hand or mounted on a car to scan the concrete bridge deck. In these cases, although the lane closure is not required, the vehicle speed on the bridge is reduced and it is inconvenient in certain aspects. Recently, some researchers have utilized UAV-IRC to capture thermal images in order to conduct bridge inspection [40–44]. In 2017, Omar et al. used UAV-IRC to detect defects in a concrete bridge [40]. In their study, the severity of the delaminated area in the concrete bridge deck was indicated by a condition map based on their proposed objective thresholds. Khan et al. in 2015 used the UAV-IRC to collect thermal images of a mock-up bridge. As a result, potential defects in the concrete bridge deck were indicated [41]. In addition, the effect of technical data of IR camera, UAV and the overlap of individual images were considered by Vasterling and Meyer in 2013 and Gillins et al. in 2016 [43,44]. They stated that the uncooled sensor IR camera with 8–14 μm of bandwidth is a good choice for the UAV-IRC because of its lightweight. The flight altitude should be selected based on the camera resolution and separated images should be taken with a minimum overlap of 50%. In all works mentioned above, the effects of WTDRs as well as the ranges of the WTDRs in which delaminations may be detected by using UAV-IRC have not been presented. These matters will be clarified in this study and the applicability of UAV-IRC is examined by comparing its results with those of experiments from H-IRC.

In the present work, the experiments were carried out on a concrete slab embedded artificial delaminations with various WTDRs using passive IRT. Two IR detectors with different specifications are used including an H-IRC and a UAV-IRC. To develop a comprehensive system in inspecting the bridge using both H-IRC and UAV, the aims of this study is to: (1) Examine the effect of WTDRs on the detectability of delaminations in a concrete specimen; (2) Determine the minimum WTDR of a delamination that can be detected during the daytime and nighttime; (3) Propose a suitable time for concrete bridge deck inspection using passive IRT technique; (4) Evaluate the applicability of UAV-IRC on delamination detection in concrete bridge decks.
