Barrel Vault Design in NC

[Ron-NC]

After laying out the red clay bricks, a 50/50 sand / fireclay mix was used to fill in the gaps of the red clay bricks - probably not necessary given what was done next. However it did give me a chance to work with the sand / fireclay mix as a leveling agent which will be used later. The lesson learned was to make this mix wetter and use fine silica sand so it spreads easier and the need to work quickly as it dries out fast.

I decided it would be too difficult to level out the uneven red clay bricks with sand / fireclay so instead used home brew effectively adding another 3/4" of mass on top of the red clay bricks. After reading many posts, I went with a 5:1:1.5:.5 mix (sand:fire clay: lime: portland) using slightly less portland and slightly more lime. The sand is a medium 25 grade silica sand. Water was added to make the mix wet enough to make a smooth finish which should make laying out the firebricks mush easier. Less portland was used as I was not concerned with the stickiness or quicker set up that Portland would afford, and as many have stated, portland cannot take the high heat as it will break down. So using a bit more lime in place of the portland should give the necessary bond of the mix. Using a drill and a drywall mixer attachment made quick work of mixing up two batches in a large bucket. This should work well when I get to the brick laying phase. I will cover up the layer with a wet blanket to keep things moist to allow a slow cure - probably about two weeks.

Forgot to mention in a prior post, I am cutting the bricks with a hand held circular saw and 7" diamond masonry blade. The bricks were soaked in water before cutting to reduce dust. This worked surprisingly well and the blade is still going strong after 30 or so diagonal cuts.

In the attached pic, the forms (7 1/2" height) were pulled from the hearth slab and re-used to set around the floor, and leveled. This form was used as the guide to smooth the 3/4" layer. They will be removed today as they served no other purpose.

[Ron-NC]

I left out some of the details. Fire clay bricks were temporarily set around the red clay brick to help facilitate creating the smooth home brew layer. These will be removed, leveled and re-set later once everything is dry. The fire clay bricks were wrapped in plastic so they do not stick to the home brew.

[david s]

I've only just reread this thread. Is there a particular reason that you want such thick walls and floor? Two layers of brick will take ages to heat through. This is fine for an oven that's designed to cook bread because the temperature required is not very high and if it were a commercial oven it would never really cool down if fired every day. But if it's a domestic oven that's used infrequently an enormous amount of energy is required to heat all that thermal mass as well as the additional time required to get it to temperature. If you want to do pizza then the outer layer of bricks will be sucking heat from the inner layer so achieving higher temperatures will be more difficult. Normally the bricks are cut in half giving you a dome thickness of around 4" and floor bricks 3" thick laid flat. Pre-cast ovens which don't require a wide mortar join are typically only 2" thick. I presume you have researched this aspect and have taken this into consideration. You'll have a great bakers oven for your community with your present design.

[Ron-NC]

Hi David, yes this will be primarily a bakers oven for bread and roasted items. I do hope it will also function well as a pizza oven. The total mass thickness will be about 5" in the floor and walls, but hopefully it will not take to long to heat up to 800 degrees F. I am also hoping the 5" + of insulation will perform well with this additional mass.
ron

[Ron-NC]

The homebrew leveling layer, 3/4" thick has now set rock hard and the second layer of Fireclay bricks, in a herringbone pattern, was set on top of the 3/4" homebrew layer. At David's suggestion I used a thin dry layer of 50/50 sand (fine 50 grade) and fireclay as a final leveling agent. Very little of the dry material was needed as the homebrew surface was already quite level. I did not take a picture of this step but the brick layout pattern was posted earlier as part of the dry fit of the herringbone pattern.

[Ron-NC]

Now on to the oven... The perlcrete insulation surrounding the floor insulation has dried rock hard over the past four weeks. It seems the floor insulation of Ceramic Fiberboard (1") and Rockwool Board (4") are quite stable, supporting the two layers and bricks.

The vault form was set on small pieces of wood which should be easy to knock out when removal is necessary. I am now using the homebrew mix at 1:1:1:4. The silica sand used is 50/50 medium (25 grade) and fine (50 grade). The variation in grade will provide a better / robust mortar especially for the thicker areas.

The base of the vault sits on three layers of fireclay bricks. The third top layer was notched with an L shape which is something I saw Tscar do in his barrel vault. I liked this idea best as it was easily done with my circular saw (no angled cut required). If you look closely at the picture you will see the notch where the first layer of firebick was set. The bricks were mortared in bond (staggered) to achieve a more solid vault. Unfortunately I forgot to put down a piece of plastic on the oven floor so I will have some additional cleanup due to my poor mason skills.

A picture was also included of my mixing area. A large buck was used to mix small batches. Each unit of 1 equaled 64 oz. The amount of water was between 25 - 30% of the dry volume. The mortar was easier to mix with the drill / paddle when the water was closer to 30% and was easier to spread.

The vault is open on the front and back. The backwall will be dry stacked while the front will contain an arch forming the door.

[Ron-NC]

The first vault layer of fireclay bricks was let dry for two days and then the second layer of red clay bricks was set on top of the first layer. A lot more mortar was used in the second layer. First a thin layer of mortar was placed on the first layer so the second layer could be bedded (set) into this thin homebrew layer. This is important to make sure the two brick layers have no air spaces between the two layers to maximize the heat transfer from one brick layer to the next. After the second layer of red clay bricks was set in place, it was covered with a 1/2" layer of home brew bringing the thickness of the vault up to 5" which is about the same as the oven floor thickness.

[Ron-NC]

The second layer was then left for two days to dry and the buttresses were then formed. This is a step unique to barrel vault ovens to make sure the walls do not move outward due to the force of the vault on the three base layers of bricks below the vault.

Four buttresses were formed with dimensions 18" high by 5" deep by 41/2" wide. Each buttress is reinforced with 1/2" rebar drilled 2" into the slab and then epoxied into the holes. A 2" X 2" angle iron bar was then set behind the buttresses, against the vault wall which should prevent any future movement of the vault. In order to insulate the buttresses, 2" of ceramic fiber were cut to the shape of each buttress and sit between the oven wall and angle iron / buttresses. If pressure does exert outward from the vault weight, it will compress against the insulation which should be ok since the ceramic fiber board has good compression ratings. Rockwool board insulation was not used for this application as the compression rating is not sufficient for any potential vault movements. Given the semi circular shape of the vault, I do not expect too much outward pressure. It is possible the buttresses were not necessary, but better safe than sorry.

Hopefully the oven will not suffer too much from the lesser amount of insulation (2" of ceramic fiber board) vs. 5" + for most of the oven . The buttresses were made with high strength concrete - same as used for the slab.

[david s]

In kiln construction a vaulted oven should have its end walls built under the vault rather than beside it. Because the expanding vault tends to push the walls out, they need to be braced if they are built beside the vault. Kilns are fired to around double the temperature of an oven, leading to a greater degree of expansion, but the same principle applies. The beauty of a domed oven is that it does not require this buttressing or bracing.

[Ron-NC]

Thanks David,
Given the design where the hearth has a softer insulation material under the floor bricks, I cannot place the heavy back wall under the dome. Any idea how much of a risk I have with the back wall moving (falling)? If I dry stack the back wall, I was thinking it would move with any expansion of the vault. This pre-supposes that expansion will occur uniformly around the perimeter of the vault -not sure if this is a correct assumption.

Is there any other way to allow for expansion in this circumstance? Should I leave a 1/4 - 1/2 inch gap between the vault and the back wall?

[Ron-NC]

Lots of progress last week so I am catching up with the updates. The front inner arch was built using pie shaped wedges between full bricks. Since I am using a circular saw, the cuts are not very precise but it worked out ok. However, the inside of the arch had many small gaps between the pie wedge / brick pieces, so I decided to cover the entire area with a couple thick coats of home brew. I am hoping the thick coat of home brew seated into the various gaps will hold during the high temperatures. If it does start to crack and fall out, I will go back and chip out the home brew as it does no affect the structural integrity of the arch. A pic was also included of the flooring which cleaned up nicely,

[Ron-NC]

I do have a question about the the vault walls. As can be seen from the photos, I have some gaps in the walls, and am wondering if I should fill them in with home brew. Would appreciate any input on if this if it is necessary from a functional standpoint, to prevent any future issues. I am not concerned from an aesthetic standpoint.

[Ron-NC]

Next, I moved to the back wall. I am taking david's advise and building the back wall inside the vault walls to account for expansion, which could have pushed out the back wall if built flush with the vault. Each brick at the end of each row was cut to fit as close a possible to the vault arch. The bricks were then removed and laid out on my work surface to for easy access during installation. The first row was mortared to get things level (sort of level that is) and all remaining layers were dry fitted. A thick coat of home brew was then applied around the perimeter to fill in any gaps which occurred due to less than precise cuts to fit the pieces in.

I do have another question about the back wall gaps. The bricks are dry fitted so there are small gaps where the bricks sit on top of each other. Should I put a coating of home brew on the back to cover the gaps? I would think the small gaps would have no impact since the smoke will be pulled to the front toward the chimney. Just want to make sure I don't miss something here. Seven inches of Roxul insulation will be applied behind the back wall so I don't think any heat will escape.

[Ron-NC]

With the back wall in, I decided to move next to the chimney arch. A rough layout is presented here to show what I am thinking about doing. I have an 8 inch inside diameter triple insulated stove pipe to be placed on the inner arch edge, and the chimney arch. The second pic has an orange box drawn in to show where the stove pipe will go. The plan is to use clay bricks for most of the arches, and then pour a home brew stove pipe insert (essentially a very large keystone), into which the stove pipe will rest. This should allow the heavy stove pipe to rest on a more stable platform than using bricks alone. I am thinking if I were to build the entire chimney arch out of bricks, the arch would need to be wider to accommodate the stove pipe which has a 10 inch outside diameter. Any thoughts on this approach would be appreciated.

Another departure is the heat break. As can be seen in the picture, there will be a 2 inch heat break between the inner / chimney archs filled in with Roxul insulation, capped with some stainless steel sheets wrapped around the insulation. This is another reason I am thinking of pouring a chimney insert, as my approach is using an 8 inch wide chimney arch rather than the more traditional 12 - 13.5 inch chimney arch. The total wide of the chimney area is:

8 inch chimney arch
2 inch insulation gap
2 inch inner arch flat area
1 inch chimney insert additional width
or 13 available inches for the chimney insert where the chimney will sit. As noted, the insert will extend out 1 inch (maybe 1 1/2 inches) to add a bit of thickness to the insert unit.

I will provide pics of the form once put in place, but if anyone notices anything that is problematic, let me know.

[Ron-NC]

Rain day today so I'll finish catching up my progress. Here are a number of pics of how the insulation heat break was constructed. I scavenged some stainless steel off an old grill and cut it to shape to wrap around a piece of Roxul Insulation which is two inches thick. This insulation piece will sit next to the hearth bricks. In order to make a good fit a piece of wood was cut to the shape of the insulation and the metal then pounded around the wood. Since this was used metal, I cleaned it with Bar Keepers Friend, mixing a paste, applying the paste to the metal, and lots of scrubbing. It cleaned up nicely and this will no doubt be done annually to keep the rust at bay.

The last pic shows the location of the heat break and two cut to size one inch thick pieces of ceramic fiber on either side of the hearth heat break. I would have preferred to use fiber ceramic all around as it is more rigid, but ran out of material.

This approach of wrapping metal around insulation will be followed for the remainder oh the heat break pieces between the chimney and inner arches.