**7. Conclusions**

In this study, an effort was made to consider the possibility of simultaneously using the results from the handheld IR camera (H-IRC) and the IR camera hanged on a UAV (UAV-IRC). From the results presented in this work under given experimental conditions and processing method, the following conclusions are obtained:

For delaminations at the depths of 4.0 cm or lesser, the larger the WTDRs of the delaminations, the higher is the temperature difference produced on the thermal image, which implies that there is a strong correlation between the size and detected depth of delamination.

The minimum WTDR of a delamination that can be detected by using H-IRC was determined in this study. Delaminations with WTDRs of 1.9 or larger located at depths of 4.0 cm or lesser from the structure surface may be identified during the daytime on a sunny day, whereas during the nighttime of a sunny day, only delaminations with WTDRs of 2.5 or larger can be discovered. However, if the delamination depth is 5.0 cm or higher and its WTDR is under 2.25, it may not be noticeable on the thermal images.

Passive IRT must be applied to scan the concrete bridge deck on a sunny day because adequate heat energy is provided that helps to extract accurately the delamination occurrence. Generally, the optimal time to inspect the concrete bridge deck is from 10:00 to 15:00 and from 19:30 to around 2:00, respectively, during the daytime and nighttime on a sunny day.

Although the absolute contrast produced in the case of UAV-IRC is slightly smaller than H-IRC, UAV-IRC also can be employed to detect delaminations with the depths equal to or less than 4.0 cm and WTDRs of 1.9 or larger during the daytime of a sunny day. Thus, it is proven that UAV-IRC in the passive IRT technique is a feasible solution that can be utilized separately or simultaneously with H-IRC to discover the delamination with a shorter inspection time compared to H-IRC in the concrete bridge deck.

**Author Contributions:** Conceptualization, V.H.M., Q.H.T. and J.H.; methodology, V.H.M., Q.H.T. and J.H.; software, V.H.M., Q.H.T. and N.S.D.; validation, V.H.M., Q.H.T., J.H. and D.H.; formal analysis, V.H.M.; resources, V.H.M., Q.H.T., N.S.D. and C.K.; data curation, V.H.M., N.S.D. and J.H.; writing-original draft preparation, V.H.M., N.S.D. and C.K.; writing-review & editing, J.H., V.H.M. and Q.H.T.; supervision, J.H. and Q.H.T.; project administration, D.H.; funding acquisition, D.H.

**Funding:** This research was funded by [Ministry of Land, Infrastructure and Transport, Korea] grant number [19RDRP-B076564-06] and [Ministry of Oceans and Fisheries, Korea] grant number [20180323].

**Acknowledgments:** This research was supported by a grant (19RDRP-B076564-06) from Regional Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government and is a part of the project (20180323) titled 'Development of Design Technology for Safe Harbor from Disasters', funded by the Ministry of Oceans and Fisheries, Korea.

**Conflicts of Interest:** The authors declare no conflict of interest.
