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Old 08-21-2012, 09:02 AM
Peasant
 
Join Date: Sep 2011
Location: iowa
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Exclamation Anyone know about concrete and fire?

Well, one of the slabs we poured on either side of our oven was meant to be for a tuscan style grill. I had planned, after seeing it done on a cooking show) to simply burn my wood or charcoal directly on the concrete slab.

Unfortunately a close friend, who happens to be a concrete contractor, stopped by and mentioned that he wasn't sure it was a good idea. He says he's seen concrete explode from direct contact with fire and coal.

So my questions are:

1. Can anyone verify this?
2. If indeed it's the case, can we rectify the situation by simply adding layer of firebrick?

I'll never hear the end of this debacle! But I'd rather that than chance someone getting hurt.
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Old 08-21-2012, 09:33 AM
Peasant
 
Join Date: Aug 2012
Location: Michigan
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Default Re: Anyone know about concrete and fire?

The burning charcoal not only produces a strong alkaline, but the heat can also spall the concrete. When I built my side charcoal grill, we used all 2 5/8 firebrick.

I am a firefighter, and I have seen extreme heat inside commercial buildings destroy the concrete.
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Old 08-21-2012, 09:38 AM
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Default Re: Anyone know about concrete and fire?

The process that creates concrete begins to reverse itself at around 500 degrees. Basically that means it turns back to dust. It seldom has time to do so, however, as the aggregate in the concrete normally has a different thermal coefficient of expansion that causes explosive spalling.

So the short answer is no to fire directly on concrete, ever.

You can line it with firebrick and that will be fine for normal intermittent use, but extended use at high temps would eventually allow the heat to penetrate the firebrick and begin to heat the concrete. So for home use, sure, for restaurant use, no.
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Old 08-21-2012, 09:57 AM
Peasant
 
Join Date: Sep 2011
Location: iowa
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Default Re: Anyone know about concrete and fire?

Thanks to you both, I knew you'd chime in!

Thanks...........just what I expected!

Oh well.

Last edited by Abouna; 08-21-2012 at 12:36 PM.
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Old 08-21-2012, 12:37 PM
Laborer
 
Join Date: Jun 2012
Location: London, UK
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Default Re: Anyone know about concrete and fire?

You could put a layer of insulating material (vermicrete/calsil board) and then some firebricks on top...
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Old 08-21-2012, 12:53 PM
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Default Re: Anyone know about concrete and fire?

"The process that creates concrete begins to reverse itself at around 500 degrees."

The interesting thing is that the concrete starts to loose strength as it cools back down from about 500F. The reinforcing steel also begins to loose strength at about this temperate.

Structural concrete is usually condemned if it is ascertained that it suffered these high temperatures.
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Old 08-21-2012, 01:33 PM
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Default Re: Anyone know about concrete and fire?

Right, once the crystallization is lost it is permanent (unless you remelt and then grind the clinker again).
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Old 08-21-2012, 06:18 PM
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Default Re: Anyone know about concrete and fire?

Now that we have determined that you don't put fire on concrete, this seems a no brain er. Just put an insulation layer on top of the concrete, like the rest of us. Search this site for Perlite.
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Old 08-21-2012, 07:12 PM
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Default Re: Anyone know about concrete and fire?

This is for a grill, no need even for that, it shouldn't normally be used long enough for the backside of the firebrick to reach 500+ degrees.
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Old 08-21-2012, 08:01 PM
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Default Re: Anyone know about concrete and fire?

Right Tom, I was thinking this had something to do with an oven. I would avoid getting that concrete slab wet to avoid it possible exploding. Just a layer of firebrick should be plenty.
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