Hearth Design Philosophy

[james]

This is a sticky posting that describes the logic behind the Forno Bravo hearth design.

I think of the hearth and cooking floor assembly as serving three functions:

1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.

2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.

[stuart]

Originally Posted by james

2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.

James can you address these two points again now that the use of Super Isol is being recommended for insulation in lieu of the vermiculite/portland mix? I'm curious how one would incorporate an Island hearth design with a Fornor Bravo oven using Super Isol.

[dmun]

If you wanted a concrete slug for thermal mass under your brick floor, you could make a ring out of the insulating material (which would keep the mass where you needed it) and fill it with concrete, using the ring to level it.

It seems the current thinking is that the additional mass in not necessary.

[stuart]

Yes I understand the current thinking , but for sake of discussion this Easter morning...would you then have a full layer of the Super Isol beneath the slug and surrounding insulation?

[dmun]

Yes, you absolutely need insulation under the island hearth. Otherwise all the heat you need to cook pizza will bleed into the support slab and be lost. The original pompeii had insulation under the support slab, which went out to the side of the enclosure. It was found that this caused too much heat loss, hence the decision to switch the two. Whether you use insulation board rather than vermiculite concrete is mostly about having room for the extra thickness for insulation. I think the two both work well.

[Les]

While we are on the subject of hearth design. I poured mine yesterday with concrete. I plan on using the superisol product so I did not use any vermiculite. About three hours after the pour and finish, I noticed hair line cracks that seem to follow the pattern of the rebar underneath. Do you guys think that this should be a concern in regard to strength? I've poured many yards of concrete and never seen this before. The weather conditions were perfect for the pour.

Any insight would be appreciated.

Thanks,

Les...

[CanuckJim]

Les,

Small hairlines shouldn't be a problem, but get yourself some burlap, cover the slab with it, wet it and cover it with a tarp to reduce shrinkage as the concrete cures. Keep it wet for a week. What sort of ratio did you use in your mix? Also, was it straight Portland, Type S, Type N, or something else?Sounds like you might have mixed it on the dry side.

Jim

[Les]

Thanks Jim,

I sprayed it down immediately and covered it with tarps. Prior to covering, I took a rag and smoothed out the cracks so I could keep an eye on their progress. As near as I can tell they have not reappeared (as of yesterday). I bought the concrete pre mixed from United Rental – 5 sack. I filled most of my cores using my mixer – that’s why I decided to take the easy way on the hearth pour.

Thanks again,

Les...

[CanuckJim]

Les,

Sounds like the cracks were hairline and superficial. This sometimes happens, depending on humidity levels during the mix and the pour. I think you'll be fine. Good job on keeping it wet.

Cheers,
Jim

[Unofornaio]

Concrete pre mixed from United Rental – 5 sack.

In my experience this stuff from the rental yards is usually crap at best. But it is a VERY GOOD solution to mixing by hand and I would recommend it. What I suggest is for those of you that get the "spin buggy" the little tow-able spinning mixer, the day before get a 94# bag of Portland and have it ready, after you get the mixer home spin it up (make it go faster) and add the whole bag a shovel full at a time (adjust water content if necessary).
The problem with most of the rental concrete is the base mix is usually lacking cement and its mostly pea gravel which is easier to finish but doesn't have the strength of a 3/4 mix. In addition there is usually A LOT of sand and not much rock which even with proper rebar makes the slab weaker than it should be. When we pump a pea-gravel mix we always use 6 sack the extra cement makes up for the rock and the higher water content.
Checking the yards dry base mix (sand and gravel) ahead of time is also a good thing to do if its mostly sand thats what you are gonna get. Like wise you can also add a couple of bags (or more if its that bad) of rock into the mixer and mix it up. This is all much less hassle than saying "its crap I'm gonna mix it my self"...
Les- I'm not saying thats what your issue was I just thought Id add this advice in this thread. Ive had that happen to me as recently as 2 months ago. We did a patio for a lady and where the rebar was it was going off like crazy no idea course redi-mix is so bad now who knows what was reacting with what.it worked out though cause it was colored and when we troweled in the color it all went fine...very scary though to see that