The last time we had to replace our AC unit, the AC man had to put down a new (bigger) concrete pad. I kept the old one (about 30" x 30") and stuck it just off the porch in the yard. This is what I use my smoker on. So far, when I have heated up the chimney on it, it has only discolored the concrete. I'm not saying that it wont crack or explode it, I just think that somehow I have been lucky. For the most part I just set the chimney in the dirt (what can I say, it is dry in Texas and water is expensive).
Jack
Jack