I believe what you are looking to do is a larger scale of what I typically map. That being said, I usually take a concrete texture and "dirty it up" with cracks, mud, debris (bricks, steel, or stone), or other sci-fi effects. In GIMP - I make a base layer and fill it with a concrete texture and then add layers on top of it to get the desired "dirty" effect. [I would normally provide a link to textures, but my computer crashed a few months back and I'm just now getting it back online.]

As has been mentioned before, if you give us a image of what you're trying to do then we can be more specific in how to get your desired look.