Pure concrete hollow cylinder

Question

My intention is to build a hollow concrete cylinder for a fire pit like this:

There are a few people who have done this by using steel sheets bent in to shape with supports between, but I would prefer to use just pure concrete.

The only trace I can find of this having been done is that picture from Pinterest, but there's no link to the build, so I'm reaching out to you guys. My idea is:

• 2 Concrete column formers (I think they're called sonotube in the US), 1 approx 600mm diameter x 500mm length and 1 approx 400mm x 500mm length
• Place smaller one in bigger one
• Pour concrete
• Peel off the column formers
• Cook food, drink beer, hi five.

A few questions that the UK suppliers can't answer:

• Is 200mm thickness enough to support its own weight and contain a fire?
• Can I actually use column formers in reverse (i.e. the inside one)? I'm not as concerned about the finish on the inside, but do want a perfectly round shape
• Is there a simpler way to achieve this shape?

Answer 1

The simple way to "achieve that shape" is, of course, to buy it precast, which is almost certainly what the picture is - depending on size, it's either a section of well-casing, drainage pipe, or a manhole extension.

@BrownRedHawk is correct that it may (indeed, probably will) destructively deteriorate if used as direct fire containment - the interior should be lined with firebrick or castable refractory cement. The picture is clearly of the first time the thing in the picture has been fired, as it's remarkably clean as well as being intact - it probably has not held up well to the use, but pictures of that won't likely be forthcoming.

It will also burn relatively horribly unless some air inlets are made near the bottom.

Answer 2

The item you have pictured appears to be a precast fire ring. Several companies manufacture them specifically for fire pits.

Depending on pricing and availability, it might be a lot easier, and possibly even cheaper to purchase one instead of trying to create your own. Since these are specially made for the purpose of being a fire pit, the manufacturers would design and build them to specifically withstand a lot of heat. They may also have some kind of a warranty that they won't disintegrate after the first fire.

To make a successful fire pit out of concrete, you would need to find out what kind of concrete mixes are the best for high heat applications. Here is a site that has a formula for making refractory cement.

Another thing you want to watch out for is air pockets in the cement. Commercial cement companies have an assortment of concrete vibrators which are designed to eliminate air pockets. An air pocket in the form would be devastating and could be potentially harmful if it exploded. You would also want to install a rebar mesh cage inside the form and suspend it from the sides and bottom a couple of inches. Without the rebar, the whole thing would be very brittle and could break apart if you tried to move it.

Answer 3

I realize this is quite a late reply, but in case it is still relevant: My dad has had one of these for ages, it's a small piece of concrete sewer pipe. The RCP seems to have held up quite well to both the environment and heat in spite of us living in an area with a good bit of freeze/thaw cycling. If you want to see the current condition or anything I can get some pictures when I get a chance.

Answer 4

what you can consider rather than sono tubes is hardboard or plywood. I've formed raised circular garden beds using 3/4 inch plywood. Cut the plywood to height and length. Next cut (a tablesaw helps alot) a series of saw kerfs spaced about 1 inch apart and 1/2 inch in depth. What I did to make the actual forming of a arc easier was to cut several cleats from 2x stock shaped to the rounded profile I desired. I then wetted the plywood surface opposite the saw kerfs. This allowed the wood fibers to loosen and become slightly flexible. Using the cleats as a template slowly press plywood down until it conforms to cleat radi. Secure with screws, moisten again and let dry. I'd also suggest using refractory cement or regular premixed concrete with perilite added. Construct a 3/8 inch rebar frame to the rough shape of the bowl. Pour cement slowly, tamping and vibrating air bubbles from the mix. Add as little water as possible. Just enough so that when you squeeze a handful it does'nt fall apart. A drier mix will cure stronger , but will be more difficult to work.