Vent transition: castable inner arch?

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!

Steve

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