How best to create a round mold for a concrete fire pit?

Question

I need to create a fire pit / fire bowl that sits on top of a gas connection. It's basically just a concrete tube that's 42" across and 16" tall with 2" - 3" walls. The top and bottom are both open and a fire ring pan will sit in the top opening.

I had thought that maybe I could find a piece of foam that's 48"x48"x24" and just cut out a circle that's 40" and then cut it again at 42" while securing the 36" - 38" circle back in the middle to make a mold.

The problem is that I can't find a piece of foam like that that's not hundreds of dollars on the internet. Does anyone else have a cost-effective way for me to build / form this mold so I can pour my concrete fire bowl? Build a larger foam pieces out of smaller ones? Spray foam? Something else?

Thanks!

Answer 1

You can purchase precured concrete cylinders used for drainage culverts or access tunnels. They're made in sections. Try searching for "precast concrete riser ring"

You might also get two cardboard sonotubes for the form: a larger one for the outer diameter and a smaller one for that inner.

You might get plywood and use a jigsaw to cut a circular frame from it. Two pieces of ply and 2x4s to set the height. Then use ramboard or thin plywood to make the inner and outer forms.

I suspect you're looking at a few hundred dollars for this.

Yet another alternative is to pour a base then lay brickwork in the circular shape you want.

Answer 2

• Cut out 3/4" plywood to make the inner and outer reinforcing rings. (1 pair of rings on the top and bottom of the cylinder, one pair at the mid-height.)

• Soak 1/8" plywood strips (16" wide) overnight, then bend to fit the rings to form the circular shape. Clamp the plywood boards and allow them to be air dry completely.

• Apply glue to the rings and re-attach the shaped thin plywood forms. (Note: do not over glue for ease of removal.)

You might need a few temporary spacers in between the thin plywood boards to keep the assembly in shape. Also, it might help if the ring is made into two segments and assembled using clamps or fasteners. The inner support can be a solid circular plate, and the outer support can be a square plate with a hole in the center.

Another helpful thing is to apply a bond breaker on the surface of the form boards. Pour the concrete slowly to not to overstress the form, and compacting the concrete thoroughly using a rod to avoid honeycomb. You can remove the outer support and form when your finger can't dent the concrete. Wait at least 72 hours before taking down the inner support and form, but do not wait too much longer. Apply cement patch immediately after removing the form. Water the structure daily to keep it in a moist state for at least 7 days, if not exposed to direct sunlight, 14 days otherwise.