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Concrete for the hearth slab. - Forno Bravo Forum: The Wood-Fired Oven Community

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You will notice a new forum at the top of the main page called, "Ask Me Anything". This forum will be used for live one hour "Ask Me Anything" (AMA) sessions hosted by people who are knowledgeable in different areas pertaining to wood fired ovens. How it works:
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Concrete for the hearth slab.

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  • Concrete for the hearth slab.

    Hi all,

    Iím new to the forum and am building a Wood fired oven. My question is about the floating concrete slab and the fire bricks.

    What concrete should be used? Refractory or ordinary concrete for the floating slab?

    Iíve been told that the fire bricks provide little insulation and that the heat transfer to the concrete through the fire bricks would make the aggregate in the concrete explode. Is there any truth to this?

    Also where should the perlite/vermiculite layer go? Above or below the concrete slab. A lot of differing opinions here!

    Thanks in advance.

  • #2
    Re: Concrete for the hearth slab.

    Do you mean the "suspended slab" ? This is the reinforced structural slab on top of your wall which supports the insulation layer, then the hearth layer.


    This is normal portland cement concrete. If mixing your own - go with a 1:5 (portland/aggregate) mix.

    Normal Portland based concrete starts to give up its internal bonding if heated past and then back thought 650 Fahrenheit or so.
    You need some insulating layer between the suspended slab and the hearth bricks. This layer is usually a 1:5 or 1:6 portland/ vermiculite mix about 4 inches deep. Some use a thinner layer of vermiculete and add some other type of insulating material.
    Last edited by Neil2; 08-14-2009, 09:51 AM.

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    • #3
      Re: Concrete for the hearth slab.

      The best answer I can give you is download the free ebook from the store and read it. IT will answer these questions and most others you will have. It's a great text with lots of pictures.

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      • #4
        Re: Concrete for the hearth slab.

        Also where should the perlite/vermiculite layer go? Above or below the concrete slab. A lot of differing opinions here!
        There are no differing opinions on this issue. It's fully settled, that having to heat up your entire support slab to oven temperatures, insulation underneath or not, means you'll be burning entire forests to get your oven up to bread baking temps, and you'll find it difficult at best to achieve pizza temperatures.

        Insulate! Insulate! Insulate!
        My geodesic oven project: part 1, part 2

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        • #5
          Re: Concrete for the hearth slab.

          Thanks lads I appreciate your input. Just the answers I was looking for but with so much information on the web showing the insulation going on below the slab and also stating that heating up the suspended slab takes no time at all goes against all common sense. I thought I was missing something.

          I have seen aggregate explode at relatively low temperatures and could not understand how some builders are laying their fire bricks directly on the concrete slab with no insulation.

          Thank you all once again. I’ll download the book.
          Last edited by I burnt my fingers; 08-14-2009, 12:48 AM.

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          • #6
            Re: Concrete for the hearth slab.

            Originally posted by I burnt my fingers View Post
            I have seen aggregate explode at relatively low temperatures and could not understand how some builders are laying their fire bricks directly on the concrete slab with no insulation.
            Those builders are the ones that come here and ask "Why isn't my oven working?"

            Thank you all once again. Iíll download the book.
            Here's the link to Forno Bravo's Pompeii page and eBook download:
            Brick Oven Plans | Build an Italian Brick Oven

            Be sure to keep us up to date with your project!
            Ken H. - Kentucky
            42" Pompeii

            Pompeii Oven Construction Video Updated!

            Oven Thread ... Enclosure Thread
            Cost Spreadsheet ... Picasa Web Album

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            • #7
              Re: Concrete for the hearth slab.

              Originally posted by Ken524 View Post
              Those builders are the ones that come here and ask "Why isn't my oven working?"



              Here's the link to Forno Bravo's Pompeii page and eBook download:
              Brick Oven Plans | Build an Italian Brick Oven

              Be sure to keep us up to date with your project!
              I downloaded the plans and I should have done it sooner. They answer all my questions. Excellent stuff.

              I'm building a big bugger and am finnishing the shuttering for the concrete pour next week. Here's a pic. Stand measures 10'x8'.


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