Mongo's 42" CT Build

[mongota]

Is your arch entirely of fire bricks? Or, did you use regular bricks for the end?

Chris, the arch is all firebrick.

[fhausback]

The stone looks great. Are you putting anything between the dome and the stone? sealers, etc...

I'm thinking of doing an igloo with a brick veneer for the outside, and am thinking hard about how to waterproof the thing.

[mongota]

fhausback,
I have two coats of Thoroseal on the stucco. Thoroseal is waterproof, but vapor permeable. I also have a vent pipe at the apex of the dome.
I had considered using a slip sheet between the stone and the stucco, but discarded that idea. I plan on adding a roof over everything.

[mongota]

I've had a version of this grinder for years. First one disappeared, this is its replacement. Usually use it for metal work. After mounting a 7-1/2" diamond blade that I usually use on a circular saw, it trims stone with ease. Didn't have to trim much, just a few to minimize mortar here and there.

I'll lay out the stone dry, then mark the heck out of it for location and orientation. This way, after I pull them down and mix the mortar, I'm not standing there with a stone in one hand a trowel loaded with mortar in the other, trying to remember where the stone is supposed to go!

Last pic shows the back side of the dome, roughly 3/4 done.

[mongota]

Stone veneer pretty much done. The back is done. The front, I need to cast a few decorative concrete items to trim out the entry, then I'll cover the vent arch with stone veneer.

[Mongo]

Well done! Those pilasters are impressive indeed. Great concrete work. While reading through the forum during my build I saw a few references to a 'Mongo'. Nice to meet you.

[mongota]

While reading through the forum during my build I saw a few references to a 'Mongo'. Nice to meet you.

lol, you too!

[mongota]

Time to cast some concrete. First, the form.

This piece will be cast in place, the entry shelf. I recycled the same PVC as I used for the edge detail on the dome slab, just had to cut them down a bit. Two pieces of PVC, one piece is a quarter round ripped from 2" Sch40 PVC pipe, the larger half round is ripped from 4" PVC. Easy to construct, and it gives something a little different in the world of concrete.

[mongota]

Took a section of hardware cloth and tapconed it to the hearth slab. Necessary? Probably not.

Calculations came out to a little over a cubic foot, so I mixed two 80lb bags of concrete mix and added colorant. On the photo caption I wrote 'bags of mortar". Nope. It's concrete.

Filled the form, vibrated it until I couldn't stand it any more, struck the surface with a wood float, and let it sit until the water bled off. Came back then and did a hard steel trowel.

Wrecked the form, gave the edges a light wire brushing, and I'll call this one done.

[mongota]

Next up, casting the front arch.

[mongota]

I switched gears a few times regarding the dimensions of the front arch. It's decorative.

Originally I was only going to clad the dome itself in stone. But when I decided to also clad the landing tunnel in stone, I increased the width of the arch's legs so its width would cover the edge of the stone veneer behind it. It's wider than I want it to be, and the shape is not exactly what I wanted, but it will serve its purpose.

I had a piece of 1x4 PVC trim, I resawed it into 5/16 x 4, they flexed nicely to accommodate the curves in the arch form.

I added a couple of pieces of #4 rebar, wired tied together.

The volume of the form came out to just over a cubic foot, so I used two bags of concrete mix and some colorant.

Beat the form to vibrate it a bit, after the water bled off I gave it a hard trowel and put it to bed for the night.

[mongota]

Got the cast arch out of the form the next day. Wire brushed the surface for a bit of rustic texture.

Lifted it up to see how it fit...fit was good, so I mixed up a small amount of mortar and wet set it.

Not sure if this will happen...But see the last photo with the red line under the arch. After this cast arch cures for a few more weeks, I might sink a couple of anchors and attach a steel plate to the underside of the "Keystone" area of the arch below top dead center. Gapped maybe 1/4" away from the arch. Perhaps to act as a heat shield of sorts. I'll shoot some temps the next time I fire it up and see how hot that area gets. It's mostly a concern during the initial fire, when the fire is built under the chimney.

[mongota]

Time to cast the chimney cap, this cap will cover the vent landing tunnel.

Cap is roughly 25" x 35", it's over 3" thick at the hole and slopes down a bit towards the perimeter of the form.

The hole is roughly 11-1/4" in diameter, large enough for the 8" insulated chimney pipe to pass through. The form for the hole is a couple of 11" diameter plywood discs, separated by a spacer, to make the cylinder about 3-1/4" tall. I wrapped it with a couple of layers of thin packing form, then wrapped that with a 3-1/4" wide strip of sheet metal. Those wrappings brought it to the desired 11-1/4" final diameter. The foam, being slightly compressible, will allow the plywood discs to be shimmied out after the concrete has cured. I'll leave the sheet metal sleeve that lines the chimney hole permanently in place.

Three sides of the form are lined with a half-round of PVC pipe. Same stuff recycled from the previous forms.

I overfilled the form, then vibrated it a bit. The vibrating results in the mix slumping and a bit of form overflow. Once the mix set up, I added a bit more concrete so I could establish the slight slope from the pipe hole towards the perimeter of the form.

This is the last of the three decorative pieces (shelf, arch, and cap) I needed to cast for the oven entry.

[mongota]

Chimney cap is in place, it's simply dry set for now.

I had to be careful getting it up there, as it was just pulled out of the form and the edges would be a bit delicate. But I hoofed it up there despite the thought that the Earth's gravitational pull on random chunks of cast concrete seems to have increased as I've gotten older.

Once up there, I notched out the stones under the rear corners so it would sit level, no rocking.

While I like the rustic nature of the voids in the bullnose, due to New England freeze/thaw cycles, I might fill them with a slurry while the concrete is still green.

Overall I'm happy with it.

Next task, veneer the sides of the vent arch with stone.

[SableSprings]

Mongo, this build has turned out fabulously. I love the way you used PVC for the molds & forms...all really eye catching. After seeing what you did with your entry and chimney cap, I'd definitely be going that way on my next build (or recommending it to a friend whose threatening to build soon...).