Barrel Vault Design in NC

[Ron-NC]

Before the chimney arch is built, a base was constructed with solid clay bricks. Next the arch form will be constructed in such a way as to also accommodate the poured concrete discussed earlier. The home brew mix will be 1:1:1:4 -slightly more rich in cement than the 1:1:1:5 mix I have usually been using.

A side view has been included to better see how the heat break insulation fits into the chimney arch.

[Ron-NC]

Quite a bit of time has been spent trying to figure out the chimney arch. Since I am pouring the keystone / chimney base, I am a bit concerned with how thick the front / back cement wall should be to make sure it supports the heavy chimney. I just ordered the chimney anchor plate which measures 11.25" x 11.25. The anchor plate will be bolted to the poured cement base. I would like to minimize thickness of the front / back cement wall as much as possible as any additional thickness will extend beyond the lower bricks. I am looking for any advise as to whether or not I need to extend the front / back cement wall beyond the anchor plate measurement of 11.25 inches. The attached pics show what I am thinking. The drawing below has an assumed cement width of 12", yielding a 2 inch overhang, but if this could be reduced to just beyond the anchor plate dimension, I think it may look nicer. I am particularly looking to hear from anyone who has used home brew and what you think the minimum thickness should be for acceptable strength.

[david s]

One of the problems in accommodating a an anchor plate is that it ends up drastically increasing the depth of your entry which makes the oven more difficult to work. Having a deep tunnel to work past before getting anywhere near the oven door is a drawback for front flued ovens. You may be able to trim down the anchor plate to help reduce this. My solution is a little different, I've set the flue pipe slightly behind the oven door which ends up making the depth of the entry the same as the width of the flue pipe. Pic attached to show my explanation. I don't use an anchor plate either, my flu fits into the hole in the flue gallery. I think you should use castable refractory rather than home-brew to make your casting.

[Ron-NC]

thanks - I like the idea of setting the chimney on the oven inner arch, especially since it saves space toward the front of the base. The anchor plate should be here in a couple days so I can evaluate it's use then. It appears from the specs that the anchor plate is only slightly bigger then the chimney itself - 11.25" plate vs. 10" chimney, so using the plate should not add much to the depth. The loss of depth may be more a function of how thick the cement around the plate needs to be. How much castable was required for the above assembly you made?

[Ron-NC]

David, forgot to ask, what type of material did you use to coat the inside of the chimney base. Thanks again.

[david s]

I've really pushed the boundaries with my flue gallery design because I want to keep the weight down as much as possible. In my design the flue gallery sits directly onto the dome, so I don't want a heavy heat sink there. My heat break/expansion joint is between the flue gallery and the outer decorative arch.
The flue gallery weighs only 10 kgs, but is rich with stainless steel reinforcing. The buttresses on the sides provide extra strength allowing the rest of it to be thinner and therefore lighter. I haven't had any failures . I coat the inside with black tile grout, cosmetic only.
I think one of the most important things is to have some volume at the base of the pipe. i.e. the flue gallery should be like an inverted funnel. This is essay to do when casting but quite difficult to form with brick.
Also, make the hole that the pipe fits into a slightly loose fit. You don't want the expanding steel pipe placing undue stress on the surrounding refractory as it will expand first being more conductive.

[Ron-NC]

The form was built for the chimney arch and the bricks mortared in place. My lack of masonry skills again kicked in as the arch is a bit crooked. Hopefully using two bricks side by side should be enough to support the chimney structure, avoiding any further buttressing. The arch thickness measures 8" X 8". I will let this sit for three days before proceeding with the poured keystone / chimney insert.

[Ron-NC]

While the chimney arch is curing, the form for the chimney insert was built. One inch of insulation was placed between the concrete and inner arch as part of the heat break for the front of the oven. A cardboard tube (not shown) will be cut and set in the form for the flue area. A chimney anchor plate will then sit on top of the poured keystone / chimney insert. Stainless steel bolts will also be inserted into the cement to firmly attach the anchor plate to the form. The thickness of the cement will be 1 1/2" at the thinnest point in the front and back and getting much thicker as you move to the sides. The concrete mix will be 1:1:1:4 with stainless steel needles mixed in for reinforcement. A small amount of Polyester cement fibers will also be mixed in. Some time has to spent today reinforcing the form in advance of the pour.

[Ron-NC]

Here is a pic of the poured chimney insert. The cardboard tube was a bit more toward the front than what I had envisioned; hopefully there is enough thickness in the front to avoid any issues. The concrete will cure for a full week before I move the forms, as the weather has turned cool, so the cure will be slow. While the cure is happening, in three days the insulation will be placed on the vault.

[Ron-NC]

The insulation used is standard Roxul wall insulation from any big box store. The material has some firmness but is still flexible enough to form around the vault. This worked well in a barrel vault application with a combination of vertical walls and curves. A utility knife was used to score a line for measuring the cut pieces and a thin kitchen serrated blade knife used to saw through the material, which made a nice clean edge - very easy to work with.

[Ron-NC]

The insulation was placed in two layers, alternating horizontal and vertical direction to better cover the seems. Random pieces were cut and stuffed into places as necessary to fill all voids.

[Ron-NC]

Chicken wire was then used to cover the insulation using tapcon screws and wire to pull it tight. I don't think I had the correct drill bit for the screws as it was nearly impossible to drill the holes into the slab concrete, but I did find some alternative areas in the base to get some leverage to pull the wire tight. There is a bit more to do today to finish the front and some additional chicken wire will be used in the back to better accept the stucco on this vertical surface.

I have about 4 cubic ft. of perlite leftover which will be used today, in a 7:1 ratio applied over the vault area only to smooth out the vault shape and provide a base for the stucco finish. I may first use a 10:1 ratio to fill in any voids in the insulation area before a richer, thin coat is used over the vault surface. I would probably skip this entire step if not for having some leftover Perlite, as the Roxul insulation is plenty thick at seven inches total around most areas (except for the front which will be thinner in some spots due to the chimney).

The front and back will stay open when the oven is slowly cured a week from now.

I am thinking about placing some sort of steam vent behind the chimney but not sure if this is necessary or how yet to do that. Any advice would be great.

[UtahBeehiver]

A vent would be good, Roxul (rockwool) is highly water absorbent so a place for the steam to vent will help keep pressure under the stucco from building up and hopefully from cracking.Gulf and I used hydraulic casing vents.

[Ron-NC]

Hi - does a "hydraulic casing vent" go by another name? I did a search for this but only came up with references to venting in the oil and gas industry - thanks

[Ron-NC]

I found what looks like the same thing called a breather cap. If I cannot find a stainless model I will go with plastic - steel stuff rusts away around here - thanks
Ron