You can build a concrete fire pit over the weekend if you have the right skills and materials. It will be the best centerpiece for the outdoor entertaining. You need to be careful while building the concrete fire pits and fireplaces. If the pit is exposed to too much heat, moisture which may be trapped inside may expand and it may lead to explosion. You may add lava rock at the bottom and fire bricks inside so that the heat is not in direct contact with a concrete. In case you live in a place where it is too cold, you may use a sonotube footing on the four corners.
To start working on your concrete fire pit, you should first get the concrete mix which is a commercial-grade and blend it with the gravel, stone, cement and sand that have been designed to be used where high strength is needed. Get the fire bricks from local masonry supply; you do not have to worry about its color because it will turn black from the soot after few days. The fire brick is heat-resistant and it protects the concrete so that it does not crack.
If you cannot get the mortar, you can make yours. You can mix the cement, sand and mortar clay to make your own mortar. Get a pre-cut rebar of different inches to use for the verticals. The vertical bar should be put inside the gravel. The rebar frame is used to keep all the layers of concrete bonded one to another. Use gavel for foundation layer. Use bucket of lava rock to get the masonry supply yard. The lava rock is resistant to the heat and it is not going to shatter or to crack. You should also get the items for the formwork. You can have the deck screwed together so that you may make panels for a formwork. Get also L-brackets to use at the corners.