*5.1. Surface Temperature*

The temperature of the concrete specimen surface is analyzed focusing on the back face with delaminations at depths equal to or less than 4 cm during both sunny day (day 1) and rainy day (day 2). It should be noted that in the experiment, sunrise appeared at 5:30 and sunset was at 18:40 on day 1. However, the sun started heating the specimen from 6:45 and stopped heating at 17:35 owing to the shadow of trees and buildings near the experiment site as shown in Figure 7.

The thermal images of the back face at 11:00 and 20:30 during the sunny day (day 1) are shown in Figure 11a,b, respectively. There are five images shown in each of the Figure 11a,b. This is because in the experiment one thermal image was captured for the entire specimen surface, and then four individual thermal images were recorded for four areas of specimen as shown by the red dashed rectangular. The reason in capturing four individual images is to reduce the temperature range of the thermal image for avoiding the misdetection of delamination that may occur if only one thermal image was recorded. There is a gap between the surface temperature above a delaminated area (*Tde*) and its surrounding (*Tso*). Particularly, *Tde* is higher than *Tso* owing to the effect of daytime heating (Figure 11a) while *Tde* becomes cooler than *Tso* during the nighttime (Figure 11b). Based on the temperature difference between *Tde* and *Tso*, delaminations can be observed on the thermal images during both the daytime and nighttime. In this section, the surface temperatures above a defect (*Tde*) and its surrounding (*Tso*) calculated as the mean value of surface temperatures within "delaminated area" and "sound area" respectively as presented in Section 5.2. The time when the temperature on the surface above a delamination changes from hotter to cooler in relation to its neighborhood or vice versa is called "interchange period". The interchange period can be observed more clearly in consideration of the absolute contrast that is discussed in detail in Section 5.2.

**Figure 11.** The thermal image: (**a**) at 11:00; (**b**) at 20:30.

Figure 12 shows the surface temperature (*Tde*) within delaminated areas on day 1 and day 2. Figure 12a–c plot *Tde* values during day 1 (the sunny day) for delaminations with depths of 2, 3, and 4 cm, respectively. It should be noted that each graph depicts delaminations that have the same depth but different sizes. Certain key phenomena can be indicated as follows:

First, all *Tde* value lines have the same tendency. There is a small increase of *Tde* in the first 2 h from 6:00 before it increases considerably and reaches the peak area around noon. Around 13:30, the sky became cloudy, which causes a significant decrease in the surface temperature, as mentioned by Hiasa et al. [1]. After noon, the surface temperature rapidly drops until 22:00; however, from 22:00 on the previous day to 5:30 the next day the *Tde* lines go down with a smaller slope. Then, the surface temperature increases slightly from 5:30 to 6:00 the next day.

Secondly, the effect of the delamination size on the surface temperature of the specimen during daytime and nighttime can be seen clearly. Small figures are added on the graphs in Figure 12 to zoom out the difference between the *Tde* value lines during daytime and nighttime. Overall, with the same depth, a larger delamination has a higher surface temperature than a smaller one during daytime while it becomes smaller during nighttime. For example, at 14:00, the surface temperatures are 51.66, 50.83, 50.72, and 48.81 ◦C corresponding to delaminations B-D1, B-D2, B-D3, and B-D4, as indicated in Figure 12. For the same set of delaminations, the observed surface temperatures are 27.00, 27.64, 28.37, and 28.90 ◦C at 20:30. The reason for this phenomenon is the effect of the trapped heat volume. During daytime, the trapped heat volume is developed above delamination under the heat energy from the sun. However, at the same depth, in comparison to a larger delamination, the trapped heat volume of a smaller delamination has a smaller intensity and diffuses more significantly in all directions leading to the decrease in the surface temperature [45]. During nighttime, the heat energy is radiated back into the air from the ground; thus, the volume of trapped heat is located under the delamination. Therefore, a higher surface temperature above a smaller delamination compared to a larger one is observed during nighttime.

The surface temperature above the delaminations with different sizes at a depth of 2 cm during the rainy day is presented in Figure 12d. There is no tendency in the surface temperature change during daytime and nighttime compared to the sunny day. During the rainy day, a slightly higher surface temperature can be produced in the case of a larger delamination, but it is quite difficult to observe this as the difference is small. For instance, at 12:00 the surface temperature above delamination B-D1 (21.66 ◦C) has a slightly higher value (0.01 ◦C) than delamination B-D2 (21.65 ◦C).

**Figure 12.** Surface temperature above delaminations: (**a**) at 2 cm depth on day 1; (**b**) at 3 cm depth on day 1; (**c**) at 4 cm depth on day 1; (**d**) at 2 cm depth on day 2.
