Author Topic: Does this sound dangerous?  (Read 2803 times)

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Offline jduchon

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Does this sound dangerous?
« on: June 21, 2012, 11:52:05 AM »
I have been watching the crew build a pizza oven for me and I noticed something odd with the base.

They poured a 2" slab 4' x 5'of concrete with rebar on top of a concrete block structure as a base to build the oven.  They then built a dome oven out of fire bricks.

Just now I noticed that they left a sheet of plywood under the slab that was part of the form they build to pour the slab.

Is there danger of this plywood catching on fire?  It has a 2 inches of concrete and about 2 inches of fire brick on top of it.

Any thoughts are appreciated.

JD



 


Offline Jet_deck

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Re: Does this sound dangerous?
« Reply #1 on: June 21, 2012, 11:55:50 AM »
I would love to see some pictures of your oven.
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Offline jduchon

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Re: Does this sound dangerous?
« Reply #2 on: June 21, 2012, 11:57:18 AM »
Here is the picture.  It is a dual purpose oven, bread and pizza's.

Offline Jet_deck

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Re: Does this sound dangerous?
« Reply #3 on: June 21, 2012, 11:58:42 AM »
Very nice.  I'm sure one of the wfo guru's will let you know for sure about the plywood.
Her mind is Tiffany-twisted, she got the Mercedes bends

Offline jduchon

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Re: Does this sound dangerous?
« Reply #4 on: June 21, 2012, 12:00:36 PM »
Here is early on.

Offline Jet_deck

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Re: Does this sound dangerous?
« Reply #5 on: June 21, 2012, 12:04:15 PM »
May I ask what they are charging you for that project?
Her mind is Tiffany-twisted, she got the Mercedes bends

Offline jduchon

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Re: Does this sound dangerous?
« Reply #6 on: June 21, 2012, 12:10:53 PM »
It is a barter deal.  A retired mason is a neighbor.  It will be a shared use deal.

Wine and materials.

JD

scott123

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Re: Does this sound dangerous?
« Reply #7 on: June 21, 2012, 12:11:55 PM »
If the concrete is just plain concrete, then that's going to be a problem.  If it's perlcrete, though, it probably would be okay- although, off the top of my head, I'm not that familiar with perlcrete's r value.

This doesn't happen much, but I think this question might be better suited for Forno Bravo.


Offline TXCraig1

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Re: Does this sound dangerous?
« Reply #8 on: June 21, 2012, 12:18:09 PM »
I don't know if it could burn or not. I tend to doubt it. There isn't going to be much oxygen where the heat contacts the wood. It might char though if you run the oven long enough?

I think the construction you described is unusual. I believe there is usually a 4" layer of insulating concrete (5:1 vermiculite:cement) between the structural concrete and the fire bricks. I'd be more worried that the structural concrete could be damaged by the heat of the oven due to the lack of insulation under the fire bricks. The lack of insulating concrete is what makes me think there might be a risk of the wood charring or maybe burning at some point?

I think the structutal concrete is usually thicker than 2" also - more like 3.5". Maybe for anoven your size 2" is fine; I don't know. It looks too weak to me.

I would ask about the lack of insulating concrete. I don't remember ever seeing any brick or cast WFO that didn't have either insulating concrete or ceramic insulating board between the structural concrete and the fire bricks.

Going on what little I know, I'd have him rebuild it right.

Craig
Pizza is not bread.

Offline shuboyje

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Re: Does this sound dangerous?
« Reply #9 on: June 21, 2012, 12:20:07 PM »
I really wish you would have posted sooner.  The plywood is a concern if it can't be removed, but it is not as big of a issue as the design problems I see.  Your oven lacks insulation.  The concrete slab and stand are going to suck heat away very quickly.  The vent also looks to be very oversized and located inside the oven.  This will cause a lot of heat to be lost out the top.  Put the two together and I think you are going to have some serious issues if you don't make some changes.  From what I can currently see I don't see this oven functioning for either of your uses.  It won't get hot enough for pizza and will not retain enough heat for bread.
-Jeff


Offline TXCraig1

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Re: Does this sound dangerous?
« Reply #10 on: June 21, 2012, 12:21:33 PM »
If the concrete is just plain concrete, then that's going to be a problem.  If it's perlcrete, though, it probably would be okay- although, off the top of my head, I'm not that familiar with perlcrete's r value.


If the 2"concrete is mixed with perlite or vermiculite, it may be able to handle the heat, but I don't think it would have the structural strength you would need. That's why ovens are typically built with a 3.5" layer of reinforced structural concrete with a 4" layer of insulated concrete on top of that, and the fire bricks on top of that.

CL
Pizza is not bread.

Offline Jet_deck

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Re: Does this sound dangerous?
« Reply #11 on: June 21, 2012, 12:25:41 PM »
I'm just going to say that I doubt the plywood will catch fire.  On my oven with the floor at 900, I measured the bottom of the firebrick and it was 550 or so.  So the additional 2" of concrete in your situation  will act as a radiator dissapating heat.  Maybe down to 400 or so.  I think that the plywood will degrade over a period of years from the heat, but I don't see it bursting into flames.

Is the base 2" concrete tied to the block stand in any way?
Her mind is Tiffany-twisted, she got the Mercedes bends

Offline TXCraig1

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Re: Does this sound dangerous?
« Reply #12 on: June 21, 2012, 12:26:24 PM »
You might want to look through this.

http://www.scribd.com/doc/17598908/Pompeii-Oven-Plans
Pizza is not bread.

scott123

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Re: Does this sound dangerous?
« Reply #13 on: June 21, 2012, 12:30:44 PM »
Nevermind my comments about perlcrete, I just re-read the original post and see that it's concrete.

Yes, this is dangerous. You don't put concrete directly under firebrick. Concrete isn't durable when heated to high temps.  The heat will eventually cause the concrete to fail, and, when it does, the oven will collapse.
« Last Edit: June 21, 2012, 12:40:33 PM by scott123 »

Offline jduchon

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Re: Does this sound dangerous?
« Reply #14 on: June 21, 2012, 12:37:26 PM »
I measured the base, it is 4 inches not two of high psi reinforced concrete.     The chimney is mostly for looks.  There is a small 1" x 2" opening near the front of the oven that channels into the chimney.  I suppose the plywood can just be pulled off for safety purposes.  The base is mortared to the stand plus metal angle brackets screwed to it.

The missing insulation between the base and the fire brick bottom is an issue.

Thanks for the info.

scott123

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Re: Does this sound dangerous?
« Reply #15 on: June 21, 2012, 12:58:48 PM »
JD, if your firebrick hearth isn't mortared, you can pull up those bricks and put a couple of inches of insulation underneath them. 

Offline Tscarborough

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Re: Does this sound dangerous?
« Reply #16 on: June 21, 2012, 01:06:27 PM »
The plywood is no problem but the lack of insulation under the firebrick means the oven will perform poorly and consume massive amounts of wood.

Offline jduchon

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Re: Does this sound dangerous?
« Reply #17 on: June 21, 2012, 01:37:47 PM »
Great idea,

Only the first row of the floor was mortared.  We will lift the fire bricks and pack a layer of insulation under them.  The height of the oven could be reduced some anyhow.

Is verm-crete or perl-crete the only option for an under firebrick base insulation?

Offline TXCraig1

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Re: Does this sound dangerous?
« Reply #18 on: June 21, 2012, 02:41:33 PM »
Great idea,

Only the first row of the floor was mortared.  We will lift the fire bricks and pack a layer of insulation under them.  The height of the oven could be reduced some anyhow.

Is verm-crete or perl-crete the only option for an under firebrick base insulation?

This is what many ovens use for the floor under the fire bricks: http://www.fornobravo.com/store/FB-Board-36242.html

You should probably insualte the dome too, under the stucco: http://www.fornobravo.com/store/FB-Blanket-50.html

CL
Pizza is not bread.

Offline TXCraig1

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Re: Does this sound dangerous?
« Reply #19 on: June 21, 2012, 02:42:46 PM »
Does your chimney come out of the dome, or does it come from the landing and roll over the dome?
Pizza is not bread.