I don't have any pics of them uploaded but one thing I'd recommend is trying just painting one of your gravel bases. I think you'll be surprised at how effective they can look.
For example, here's how I paint an urban base (Citadel paints):
Flock with anything...even sand works
Paint with 1:1 mix of Mechanicum Standard Grey or Adeptus Battlegrey and Shadow Grey or the Fang - gives you a blue-grey
Heavy drybrush of Fortress Grey
Drybrush Skull White
Even if you're just using sand the end result looks like pulverised concrete.
As for accessories, Gale Force 9 do a "concrete rubble mix" which looks good.
I have also used this:
http://www.darksphere.co.uk/p.php?p=30449&c=563
No picture on the site but it's a pack of plastic tubing, girders and other bits. I'm sure cheaper versions are available. One thing I did with a few models was having a short piece of plastic tubing sticking out form under some bits of rubble, or from a hole in
th egorund, to look lik ebroekn pipes.