Please, please, please tell me you don’t have a fire pit.Concrete has tiny air pockets trapped inside, add heat and simple physics will tell you the results. Bits of hot concrete flying through the air at high speed. If your instant on a fire pit buy one. DO NOT use any galvanized metal if you make your own as it can give off some rather nasty gasses when heated.
If done well they look great a shit one and you look like a nomadic caravan dweller.