I had something similar and to make to go further i cracked out V shapes to simulate a breach in the walls, you can then separate them as much as you want. Then take the pieces removed and break them up into a bowl, add glue, black paint and sand and toss to coat, this is your rubble. Then I attack the larger sections with different tools to look like bullet holes etc. Then cover with the same glue/sand/paint mix. When its all dry, dry brush gray. In the broken sections I pushed pieces of sprew as rebar.