Roughly 1 year ago I built a firepit out of home depot high strength concrete in a 4" ring approx 2' diameter. I lined this with firebrick inside, pool coping on top, tile outside. Every time I fire it up I get a crack that splits open in 1 spot. As the pit gets hotter the crack gets wider, as it cools the crack closes, this wreaks havoc on the mortar, concrete, grout. Any suggestions how to stop this?