1. U.S. Department of Transportation Federal Highway Administration, “National Tunnel Inventory [database],” 2020 Retrieved from https://www.fhwa.dot.gov/bridge/inspection/tunnel/inventory.cfm
2. U.S. Department of Transportation Federal Highway Administration, “Specifications for the National Tunnel Inventory,” Federal Highway Administration. FHWA-HIF-15-006, 2015
3. Cheng, C., Sansalone, M.: The impact-echo response of concrete plates containing delaminations: numerical, experimental and field studies. Mater. Struct. 26, 274–285 (1993)
4. Zhu, J., Popovics, J.S.: Imaging concrete structures using air-coupled impact-echo. J. Eng. Mech. 133(6), 628–640 (2007)
5. Pristov, E., Dalton, W., Likins, G.: Measurement of concrete thickness and detection of defects using ultrasound methods. In: Proceedings of the fifth highway geophysics–NDE conference: Charlotte, NC (pp. 295–301) Dec 2008