[frahs]

I know with plastics, "flame treatment" is a common way to prepare surfaces for bonding (with epoxy).

I could imagine that maybe flame-treating concrete has some benefits for tunnels. Probably gets rid of any oils on the surface.

I'm totally just guessing here. It could also just be an inside joke or a something else, but Googling shows that it might be a thing:

http://www.concreteconstruction.net/products/decorative-conc...