*4.3. Experimental Condidion*

The environmental conditions on the experimental days are summarized in Table 4. The ambient temperature, relative humidity, and wind speed are calculated as the average of the values for every three hours. Generally, the weather condition is sunny on day 1, day 3, and day 5 while day 2 is rainy and day 4 is cloudy. According to the ASTM Standards, the inspection should not be conducted when the wind speed is higher than 24 km/h [51]. The maximum wind speed on experimental days was 3.2 km/h, so all experiments were carried out under appropriate weather conditions in terms of wind speed.

As recommended in the ASTM D4788-03 Standards, the sunshine should be included because the temperature difference of 0.5 ◦C within a delaminated area and its neighborhood can be obtained after at least 3 h of direct sunshine [51]. Based on the data listed in Table 4, three experiment cycles including days 1, 3, and 5 are in the range while days 3 and 4 are out of the scope recommended by the ASTM Standards [51]. The non-sunny days selected in this study are for developing a comprehensive system for concrete bridge deck inspection with the combination of H-IRC and UAV-IRC under different weather conditions.

In 2009, Washer et al. stated that the ambient temperature change is an important factor affecting the inspection outcome in passive IRT [30]. The positive ambient temperature change leads to the positive surface temperature difference above a delamination and its surrounding while it demonstrates a negative trend under the effect of negative ambient change. In addition, relative humidity change is another important factor that significantly impacts the surface temperature difference [30]. Thus, the ambient temperature and relative humidity during the test are presented in detail in Table 4 as well as Figure 10. This data is necessary for the discussion of the results as well as for the application of the study's propositions to real structures.

The changes in relative humidity and ambient temperature during a sunny day (day 1) and rainy day (day 2) are shown in Figure 10a,b, respectively. In these figures, the red line represents relative humidity while the blue line is for ambient temperature.


**Table 4.** Weather conditions during the experiments.

**Figure 10.** Ambient temperature and relative humidity: (**a**) day 1 (sunny day); (**b**) day 2 (rainy day).

On the sunny day, it can be observed that relative humidity (*RH*) is inversely related to the air temperature (*Te*); if temperature increases, the relative humidity decreases and vice versa as shown in Figure 10a. Overall, the temperature tends to go up from 6:00 and reaches the maximum value around 14:00 before falling down and towards the smallest value at 4:00 in the next day. After that, there is a small rise in the temperature from 4:00 to 6:00. In contrast, during the time between 6:00 and 14:00, the relative humidity descends dramatically. Thereafter, the relative humidity increases rapidly from 14:00 to nighttime (until 4:00). During the rainy day (Figure 10b), the tendency of both ambient temperature and relative humidity is not observed clearly. The temperature and humidity lines fluctuate significantly and there is no correlation between these two variables.
