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Old 08-03-2011, 07:04 AM
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Default Make the hearth thinner with more support?

In the instructions the hearth is poured onto the base and plywood supports and is 3 1/2" thick, with the rebar in it.

Can I make the support hearth thinner if there is more support from the base, ie if the base not only goes around the edge, but also comes across the middle from both sides, so like a square with a cross in it?

If so how much thinner could I go?
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Old 08-03-2011, 07:38 AM
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Default Re: Make the hearth thinner with more support?

Rebar needs to be kept a certain distance inside the concrete, to prevent corrosion, expansion, and concrete failure. I think the 3 1/2 inch recommendation is about the minimum. If for some reason you need a thinner support, you may need to go with a structural steel slab, which is a lot more money, even at scrapyard prices.
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Old 08-03-2011, 08:01 AM
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Default Re: Make the hearth thinner with more support?

Thanks. What about using thinner rebar, as the structural support requirements are surely less if it isn't spanning as great a distance...
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Old 08-03-2011, 09:58 AM
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Default Re: Make the hearth thinner with more support?

They make a welded grid to use in place of rebar, but I can't imagine that's it's going to reduce your thickness requirement much. Three and a half inches isn't thick by concrete standards. Why do you need a thinner slab?
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Old 08-03-2011, 10:24 AM
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Default Re: Make the hearth thinner with more support?

You can use a GFRC product to get down to about 1-1/2", but 3" is pretty much minimum for standard reinforced concrete, regardless of the underpinning support.

GFRC (Glass Fiber Reinforced Concrete) is not typically something that a DYI would want to mess with, although it could certainly be done.
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Old 08-03-2011, 11:35 AM
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Default Re: Make the hearth thinner with more support?

my main aim is minimizing weight, given my build location, which can be seen in my other question (Building platform on existing wall)
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Old 08-03-2011, 01:15 PM
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Default Re: Make the hearth thinner with more support?

If you make your hearth thinner the hearth temperature will not rebound as fast when you load bread or pizza onto the hearth and you may get bread/pizzas with bottom crusts lighter than you want. You may also find you have to rake coals out onto the hearth more often to recharge the hearth. Heat available from the hearth (and dome) is a function of mass and temperature. Put simply. making it thinner reduces the mass so you have less heat available. While for pizza the flames will do some charging of the hearth when the hearth is bare, when a pizza is loaded it will not - may or may not be a problem. For baking you will probably find your optimal oven temperature is hotter than those with thicker hearths. Not likely a big deal for roasts but could be pretty bad for bread.

From looking at your photos I doubt that weight is an issue. Waterbeds weigh more than an oven... Do you know how the pilings are made?

Good Luck!
Jay
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Old 08-03-2011, 01:51 PM
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Default Re: Make the hearth thinner with more support?

You could lay the concrete slab on this.
Im sure you can get similar stuff.
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Old 08-04-2011, 09:17 PM
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Default Re: Make the hearth thinner with more support?

As dmun notes, 3 1/2 inches is about the practical minimum, even with 3/8 inch rebar and careful placement.

Note that WWF (welded wire fabric or "mesh") is not a substitute for rebar in suspended structural slabs. WWF is primarily used for crack control in non structural at grade slabs, such as driveways and patios.

Last edited by Neil2; 08-04-2011 at 09:20 PM.
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