Is it okay to build a fire pit with concrete?

Question

I want to put a small fire pit (3ft x 3ft) into backyard. I checked the stone options at home depot & lowes and just the material will cost around $250.

I was thinking about building a mold and pouring concrete into it instead of using stone. Cost would be less than $50 in this case.

Is it ok to use concrete instead stone in terms of high heat and fire? Should I put some sort of heat shield inside of the fire pit to protect the concrete?

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Update-1

After the suggestions made by @BobOClockand, and @Ecnerwal I decided to go with blocks (got from Lowe's and they are fire resistant) to the bottom I put lava rocks. I checked the outside surface of the blocks for temperature after a while and they were relatively cold, so I will cover the outside with natural stone for better looking.

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Update-2

I put lots of wood into fire pit for one last final test kept it burning for 4-5 hours. The heat was very intense and when I checked the next day, almost all the blocks were cracked.

Conclusion: Even fire resistant blocks need fire insulation with firebrick. I will replace the blocks and add firebricks inside.

Answer 1

You should line it with firebrick or (since you like molding and casting things) castable refractory. Edit: on second thought, stick to hard firebrick - most castable does not like to get wet. Regular concrete is prone to spalling & cracking in direct fire exposure.

Depending on the shape of the "pit" a 2" layer of sand can work, if it will stay put (ie, if the "pit" is rather shallow)

Answer 2

Concrete blocks should NEVER be used for firepits. They can explode and cause serious injury and/or death. Always use firebrick or cinderblock for fire pits.