Made a start on some crumbling walls.
I have been trying to establish an alternative pipeline to software such as substance designer/painter using cycles.
So the wall segments, were high poly modelled first. Materials created in Blender cycles, using previously made maps and generated detail such as the cracks. Then use cycles to bake all the materials to the low poly models. Any basic alterations such as brightness/contrast etc can just be done in-line with cycles. You can however still make any significant changes in Gimp etc if needed like you normally might do. You can also use Blenders texture painter to paint to any of the maps you have loaded in.
This allows you to create some fairly complex materials with cycles nodes and then bake out your final diffuse/spec/normal etc on to the low poly model.
It’s a bit ropey as the materials wont quite look the same if you are using in a PBR engine, but you just need to keep that in mind when you are working. Just focus on the out maps instead of how exactly the material looks like in Blender.
The elements in the photo below are a final result of this process. Displayed in UE4. The wall segments aren’t entirely seamless, that’s just laziness and it’s a test batch. With some further editing in Gimp/PS this could be solved. In a filled environment, your’e less likely to spot any seams anyway.