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pdxsds 12-27-2012 10:47 PM

Vent transition: castable inner arch?
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Hi all,

Thanks to the many who have made this my favorite place on the web. The wealth of knowledge and input is amazing. I've been reading here for quite a while and got enough confidence to move ahead on a 31" pompeii style oven, on a 48W x 50L hearth stand. (Space is limited). The stand is done, the concrete hearth poured and cured, the 2" ceramic fiber board insulation is down, and the firebrick floor is laid. I still need to cut and fill some edges of the floor, but need some advice on the inner arch.

After reading so many descriptions of the difficulty in getting a good transition from dome to inner arch, and hoping to avoid a lot of fancy cuts, I'm planning to cast the inner arch from refractory concrete, adding 1% stainless steel fibers. To make the most of this, I want to cast the arch with triangular legs, as you'll see in the attached sketchup. Someone else here has done a castable arch - see pics, can't remember who it was. My idea is to create a similar form, but cut it out of foam in order to create the angle for the triangular legs. Any other ideas?

However I've discovered that the angle necessary to do this with a 10"H x 18"W opening will result in the normal circumference of the oven not hitting exactly against the arch. In the sketchup, I've shown 2 options. On the right, I fit the oven bricks right against the cast arch, which means some sharper angles to get the curve of the oven back on track. On the left, I kept the bricks on the course for a true sphere, but that means I'll have to fill the gap between the arch and the first brick with mortar. Which one is best in your opinion?

The dimensions of the cast arch legs are 2.5" (front) x 3.75" (deep) x 4.5" (on the diagonal). Does this seem structurally sound to you? Would it be advisable to add a piece of 1/4" rebar to the castable for strength?

Thanks for your thoughts!


shuboyje 12-28-2012 09:26 AM

Re: Vent transition: castable inner arch?

What you are looking to do is pretty similar to my method.

david s 12-28-2012 01:48 PM

Re: Vent transition: castable inner arch?
It is easier to shape the form you want with sand and then trowel the castable over it. Skip the rebar reinforcing and go to 2% stainless needles.

david s 12-28-2012 02:26 PM

Re: Vent transition: castable inner arch?
Here are some pics of my cast vent.

pdxsds 12-28-2012 08:06 PM

Re: Vent transition: castable inner arch?
Great advice, David, thank you. I hadn't thought about making a sand form for the arch. Thanks for the link to your cast vent also. Looking at your photo gallery - did you cast your whole oven?

david s 12-29-2012 12:55 AM

Re: Vent transition: castable inner arch?
Yes, I did.

pdxsds 02-08-2013 05:21 PM

Re: Vent transition: castable inner arch?
6 Attachment(s)
Well, an update is due. Two jobs and 3 kids under the age of 6 make this a long term project. But with the help of my father in law, we were able to figure out a form that would hold the sand at an angle and trowel the castable on top.

The form is a sheet of plywood on the bottom, then the outer shape of the arch cut out of melamine board. The inner arch is cut from styrofoam, with veneer layer on top to reach the right height. The walls are very thin flexible paper-backed veneer. I then packed wet sand into the outer side of the form to ensure a 4.5" diagonal line from outer top edge to inner bottom edge.

Then I mixed the Sparcast 30, 0.5gal water for 25lbs dry concrete mix, plus 0.5 lb stainless steel needles. This I troweled on top of the sand form and packed down. Looking back, I should have troweled smaller amounts on and packed each layer down, because the final result is not ideal as you'll see in the photos. The sharp inner angle looks like it was too tight for the consistency of the castable aggregate, so it crumbled badly.

The arch is strong to vertical compression, despite the voids and crumbled edges. The question now though, is whether it can be salvaged by filling the voids with castable, or if it is likely to fail and I should start over.

Any thoughts?


david s 02-08-2013 07:49 PM

Re: Vent transition: castable inner arch?
They are pretty serious voids. I think you should start again, but it's your call. Did you vibrate the mould? An easy way to do this is to use an orbital sander without the sandpaper in it. Just hold the sander against the sides of the mould. Small voids can be filled using the following method. Sieve a small quantity of the castable refractory to remove the large aggregate. Then make a paste of the finer stuff and water, then rub this mixture into any small voids. You need to do this operation before the casting has dried properly, say a day after casting and straight after removal from the form.

pdxsds 02-09-2013 10:08 PM

Re: Vent transition: castable inner arch?
Thanks David. I didn't vibrate the mold, partly because I was worried that the sand form inside would dislodge and lose its shape. Is that unlikely? Should I put a layer of plastic film on the sand to keep it from mixing into the concrete when vibrated?

One other idea I had was to slightly increase the water content for better flowability, and cut back on the SS needle content since they seemed to clump together quite a bit on the first try.

david s 02-09-2013 11:05 PM

Re: Vent transition: castable inner arch?
I'm sorry, you won't be able to vibrate a sand mould. I was thinking of a hard mould.when using a sand mould I use wet newspaper in small pieces over the sand mould. It prevents sand sticking to the castable and sucking moisture from it. If you make the mixture too runny it won't stand up vertically. Sprinkle the needles over the surface as you are mixing it to prevent clumping.just be aware of not creating voids as you place the mixture, I'm sure the second attempt you'll gain from the first and get it ok. The castable is quite thixotropic ie. it becomes more fluid on agitation. Take a handful of the mixture, Mind the needles, jiggle it and slap it with your trowel and you'll discover it becomes more sloppy. Don't mix any more than abou 12kg at a time , the stuff goes off pretty fast.

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