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 so

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-concrete-surfaces/flame-treatment-of-concrete-for-finishing-or-cleaning_o http://www.concreteconstruction.net/products/decorative-conc...