IIRC James wappel does some amazing bases with polymer clay.
You could potenially do some nutty stuff with it like embedding wire mesh to make rebar. so you roll out a square and break up parts of it after its been baked.
Alternativly to be way cheaper and faster, you can buy linolium tiles pretty cheap and most home stores (at least in America) that can be broken up to look like realistic slabs.
it can also be scribbed with the back of a crap exacto to make stress relief lines and cracks.
You can buy hobby scale bricks. i think O scale looks fine, HO scale is a little small
IIRC,
evergreen plastics makes basicly every scaled steel beams possible like i beams C channels, roof panels and the like. as well as diamond plate.
it gets pricey but eh. there are plenty of other manufactures so shop around for hobby scale plastics.