Painted concrete:
Ryan Duebber Architect: corrugated metal with corrugations at 1-3/4 centers peak to peak. They come in 24" sheets. Your local building material supplier should be able to find some for you if they don't have it in stock.[https://www.houzz.com/photos/duncan-avenue-basement-renovation-industrial-basement-cincinnati-phvw-vp~1780142[(https://www.houzz.com/photos/duncan-avenue-basement-renovation-industrial-basement-cincinnati-phvw-vp~1780142)
Q
egress window example
Q