Concerned About Rebar in Insulated Layer

[DaveDQ]

Hi all,

I just poured my insulating layer and have the grid of rebar in it as instructed. I noticed that the grid doesn't sit exactly in the center of the concrete slab. It's about 2/3 of the way up from the bottom of the slab, where it should be half way up. Should I be concerned about the integrity of the concrete and how the rebar will be holding it?

Thanks so much,

David

[james]

Re: Concerned About Rebar in Insulated Layer

I'm sure other builders will chime in, but I think you'll be fine!
Keep going,
James

[dmun]

Re: Concerned About Rebar in Insulated Layer

Rebar in the insulating concrete? Am I missing something? What's it reinforcing?

[james]

Re: Concerned About Rebar in Insulated Layer

Duh. I missed that part. I was assuming that you meant the structural slab -- I guess we read what we expect to see. :-)
James

[DaveDQ]

Re: Concerned About Rebar in Insulated Layer

Rebar in the insulating concrete? Am I missing something? What's it reinforcing?

One layer of vermiculite/portland cement and then a layer of concrete over that (that's what has the rebar in it.. It's the Allan Scott method. Sorry, I should have specified that.

[dmun]

Re: Concerned About Rebar in Insulated Layer

There's a lot of talk on concrete sites about where the steel should be in the slab for maximum strength, but as far as an oven is concerned, as long as the bars are well covered you should be OK. We're not building bridges here, after all.

[james]

Re: Concerned About Rebar in Insulated Layer

I know many of you have heard this before, but Dave, would you consider adding a new insulating layer between your concrete slab and the cooking floor. The Scott oven is designed for commercial bread baking and it has some shortcomings when baking pizza and doing other types of baking.

How are going to be using the oven?
James

[DaveDQ]

Re: Concerned About Rebar in Insulated Layer

This oven will be for my home cooking purposes. I'll be making bread and pizza, along with cooking some meals in it. I've read here before about the benefit of more insulation, what makes the Scott oven a compromised pizza oven?

[Jed]

Re: Concerned About Rebar in Insulated Layer

DaveDQ,

The insulation is the thermal barrier. Every thing on the fire side of the insulation will get hot.

And along those lines, you will have to heat up everything on the fire side of the insulation if you want to get the oven hot enough to cook pizza.

If you install your cooking floor on top of the structural slab which is all on top of the insulation, you will have to build a big enough fire to A) heat the dome, B) heat the cooking floor, and C) heat the structural slab. The bad news is there is enough thermal mass in the structural slab that it will take a lot more fire (wood and time) to get the cooking floor and the structural slab hot (wood and time = money). The good news is that once the cooking floor and slab are hot, they will hold the heat for a very long time. So if a person wanted to cook multiple batches of bread, heating the cooking floor and the structural slab would be an advantage. As it is, in a 36" Pompeii, the most bread I have cooked in one batch was close to ten pounds, and it takes our family quite awhile to go through that much bread, even when we give a bunch away, so I don't need to cook multiple batches of bread...

The decision most of us make is there is enough heat in the cooking floor bricks only to meet the needs of a back yard cook (see the Pompeii oven plans - free on this web site). My oven uses the standard fire brick over the insulating layer, all on top of the structural slab - I can be cooking pizza in two hours from starting the fire (others on this site report shorter times). After cooking pizza, I can cook one batch of bread, then a roast, then a slow cook item (beans or pork butt, etc.) over the next day and a half, all from the one firing of the oven. For my cooking I don't need the extra heat storage you will get if you insulate below the structural slab and have to heat it with every firing, and I don't want to spend any more time tending and feeding a fire to get the oven up to temperature.

So, long winded, but short story is that there is an advantage for most of us to insulate above the structural slab, below the fire brick.

Hope this helps,

JED

[DaveDQ]

Re: Concerned About Rebar in Insulated Layer

Jed, thanks for the insight. Very helpful. Unfortunately, it's too late to reverse the process so I'm going to have to settle with insulation below the slab. I read about your points before, and I read from those who built the traditional Scott oven. I decided to go with his process as no one mentioned the hassle of heating it up, and these ovens were being made primarily for pizza.

Nevertheless, I'm excited. I went out to my backyard this morning and everything has cured nicely. It's hard to believe I'm ready to lay the firebrick. Hopefully I can produce some of the same visually beauties I've seen on here.

Thanks again.

[james]

Re: Concerned About Rebar in Insulated Layer

Dave, if you wanted to, you could still put a layer of either vermiculite concrete or insulating board between your concrete slab and the brick cooking floor. I have a lot of hands-on experience with this (I've built and used two Scott ovens), and the problem with getting and keeping the floor hot is real. Something to think about.

You will enjoy your oven either way!
James

[DrakeRemoray]

Re: Concerned About Rebar in Insulated Layer

Dave, I am glad you are excited about your build. The shape of the Scott oven will not be a problem, but that big slab/heat sink could be a problem. Take the advice here and go ahead and insulate on top of your slab now. You can still bake a ton of bread. I have attached a picture of a 25 lb bake in my pompeii. Plenty of heat retention with a floor of bricks on the flat side and insulation underneath. You just don't need that extra mass (the slab) for home use.
See these threads for proof:
http://www.fornobravo.com/forum/f30/...cord-6502.html
http://www.fornobravo.com/forum/f25/...king-1134.html

If you are worried about height, use the insulation board that Forno Bravo sells. If height is not an issue (and a higher oven is generally easier to work in, check the height of most professional pizza ovens) then just pour 4" of vermiculite concrete on top of the slab.

I started out planning to use the Scott design and changed my mind...see my thread here:
http://www.fornobravo.com/forum/f6/m...orado-472.html

You are gonna love your oven!
Drake

[DaveDQ]

Re: Concerned About Rebar in Insulated Layer

Thanks for the replies. As I read this, I'm a bit upset with putting up that slab. The main reason I went with it was because I was worried about the structure, and though having that would hold up the actual oven better. I have this anxious fear that everything is going to collapse.

I will consider the insulating boards and then surround it with vermiculite. One question, when you say lay the bricks flat face, is this also done to retain heat better? I'm sure most of you have seen that his ovens have the upright bricks.

[Jed]

Re: Concerned About Rebar in Insulated Layer

Hey Dave,

The 'which way to place the floor bricks' question is one we all address. An American mantra is 'more is better', so we are drawn to the idea that standing the floor brick on the short side, will be the better solution.

Again, it is a builders choice question.

If you stand the brick on the short edge, the thickness of the cooking floor is now about 4 inches thick. If you set the brick on the wide side, the thickness of the floor is about 2 inches. The difference, in this application, is thermal mass. The thicker floor will take longer to heat, and it will hold more heat allowing for longer cooks.

Experience has shown that for the regular back yard cook, the 2 inch thick floor is good enough; it provides 'enough' thermal mass; that the 'more is better' mantra does not always produce the best solution to the problem....

Like I say, it is a builder choice.

Keep up the good work! It is always easier to improve results before the bricks are installed, so you are doing the right thing by asking questions!

JED

[roobqn]

Re: Concerned About Rebar in Insulated Layer

ok I am also in the planning phase and was going to use the Scott build, and find this thread interesting.

If the bricks are placed on their short side creating 4" floor would this not act as an insulation layer? Instead of vermiculite and 2" flat brick floor you would have 4 inches of firebrick.

Does the vermiculite have any heat retaining properties?

Good thing this forum is around, can avoid a lot of mistakes.