Question
I built this above ground fire pit that is round and bar height.  The pit is made in three sections.  I took a two commercial size pots...one for a trash can and one for a flower pot.  Both have a reddish type concrete material, that you would see in the South West.

The trash can, I turned up side down to cover the gas pipe and the planter I placed on top of the trash can...both sit flush.

Then I had the masonry company make me a table out of the same color concrete, but with the same size hole has the round planter.  The table sat on top of the planter...very cool.

I placed the the gas rink and the stuff to make the gas turn yellow instead of blue and placed fire place lava rock all over the inside of the planter.  It worked great.

However, one day, do to the heat of the gas, the concrete table broke on one side.  Oh, it still hold together, But, I don't use it as much any more in fear that it will crack through and both halves will fall on some ones food.

So, how to I keep the new table from cracking?  Should i have lined the inside of the concrete planter with something to help defuse the heat?  Even though the planter is fine...the table is ready the concern.  Should I use something other than concrete and will hold up to the gas heat?

Thanks for your help in advance.
Rob

Answer
Rob:
On the issue with your fire pit's concrete support table, without specific details of the concrete construction and design I am taking an educated guess here...but the main issue with your fire pit table construction is likely in the design of the concrete table. Heat causes expansion in almost all materials. Concrete when it expands can easily crack if it is not properly reinforced, does not have expansion and contraction joints, is not thick enough or is made of a bad mix design. The first thing you can do it to try and find a way to reflect most of the heat and therefore reduce the amount of heat expansion in your fire pit table. You can do this by applying a reflective material like a shiny metal lining or you can insulate the concrete with a  veneer of fire rated brick, lava rock or fire rated tile.  Second you can design your concrete with the proper control joints. These are breaks in the concrete, just like in a driveway or sidewalk, that allow for the concrete to expand and contract. You fill these joints with color matched caulking. If your table is only a few feet wide then these are not necessary. Also you can ensure the proper thickness, probably at least 3" to be safe, 4" to be safer. Finally, you can add reinforcing such as glass fibers, welded wire fabric or #3 (1/2" rebar tied and joined at 12" on center each way (a grid). Be sure all steel is at least 2" from the finished edge of any concrete.
The last thing is making sure you have a good mix in the concrete with the right amount of water and aggregates to Portland cement. If your contractor made the mix too wet or let it dry and cure too fast, it could have less strength. The less water generally the stronger the concrete. And the slower the cure, also the stronger it will be.
When pouring concrete be sure to seal it properly and keep it moist for a couple of weeks to slow the curing process.

Finally, be careful in construction; a heavy bowl on a concrete table may have cracked the concrete and it just became visible over time.

So Rob, I hope this helps, its the best I can do without pictures and a full understanding of how you constructed this fire pit.

Best of Luck

Please see my personal blog on landscaping, etc.
http://www.seanjmurphy.com

Also check out his articles on fire pits I wrote a month ago.
http://amenityarchitects.com/category/fireamenities/firepits
http://amenityarchitects.com/287
http://amenityarchitects.com/fire-pits


Also see this related article:
http://www.paloform.com/blog-paloform/bid/41713/Do-Concrete-Fireplace-Mantels-or...


Thanks for your interest,

Sean J Murphy, LA, ISA, LEED AP