7W D Friebel,Dr lng J Krieger.Quality Assurance and Assessing the State of Road Tunnels using non-destructive Test Methods.Quality Assurance,2000.
8W. -D. Friebel, Dr.-lng. J. Krieger. Quality Assurance and Assessing the State of Road Tunnels using non-destructive Test Methods. Quality Assurance,2000,31-34.