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Question about isolated hearth slab - Forno Bravo Forum: The Wood-Fired Oven Community



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Question about isolated hearth slab

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  • Question about isolated hearth slab

    My other "getting started question:"

    I appreciate the purpose of isolating the hearth slab from the surroundings as shown in the pdf that James posted in the Yahoo group. However, my impression was that vermiculite/cement mix isn't very strong. In the pdf, the vermiculite/cement appears to be holding the weight of the entire assembly since the regular concrete dosn't extend over the block walls. Is this the case, or is a stack of blocks under the center of the thermal slab recommended?


  • #2
    I've been wondering, too.

    I've also wondered about that same thing: how much will the hearth settle over time as the huge weight of the bricks and finish materials compress the vermiculite-based cement?

    I may go ahead and build a hanging hearth (a la Alan Scott) unless someone can give me some clear reasons not to expect the bottom layer of the hearth sandwich to compress.


    • #3
      There are two separe things going on here.

      1. If you pour your insulating hearth in two layers, with rebar in the concrete layer on top and the vermiculite concerete layer below, there is no risk that the vermiculite layer will compress. Jim did the vermiculite concrete compression numbers a while back, and I will find them, and post them here. I have done the hanging hearth described in the breadbuilders, and it's my opinion that it is a lot of work for no gain. I believe the idea behind hanging the hearth on the rebar was so that you could easily lift it with a forklift and move it.

      2. The second thing happening is that Jim has created drawings for a hearth where the concrete layer (thermal layer) is surrounded on the sides and below by vermiculite concrete. The idea is that having vermiculite concrete on the side of the thermal layer will block heat from escaping your oven out the sides. I think there are some questions on how the rebar works with that hearth, and how the hearth safely spans the stand opening.

      Let's call that the Island Hearth design.

      Perhaps Jim could jump in on this one.

      Pizza Ovens
      Outdoor Fireplaces


      • #4
        I was after point #2. I could understand compression being negligible with one uniform slab upon another. If the floor is independent of the dome (dome set around the floor) and/or the floor and dome are independent of the outer finish (as is accomplished by an insulating blanket even on an igloo, I think), then it may not even matter except for any loss of insulation.

        So the question in my mind was how well the vermiculite concrete works as reinforced concrete (compressive strength and adherence to the rebar) or how well the center slab will stay suspended as a hanging floor, with stress concentrated on the vermiculite concrete where the rebar lies.

        If there is concern, perhaps alternatives are (1) a stack of blocks at the center (at the expense of space/aesthetics underneath) or (2) a compromise consisting of a few inches of regular concrete around each length of rebar?

        These are just ideas. I'll await advice from those who know more! Thanks.


        • #5

          Hey, Jim, we're waiting....


          • #6
            I dunno...

            Alan, as I understand the rebar always resides within the solid concrete slab where it is very stable and solid. However, this solid slab would rest upon a much softer layer of insulating cement (vermiculite and cement).

            The weight of the concrete alone is very sizable. The weight of the brick alone is enough to stress any structure. Together with whatever structure stands above and around the oven dome, this is a lot of mass to support with a lightweight, highly-compressable mixture. Unless I'm missing something, I have to believe that this will compress considerably leaving us with what would amount to a dense cusion between the tops of the block walls and the solid slab.

            I'm not worried about structural failure, as the strength of the slab is more than adequate. I just wonder if this "island" is going to sink, leaving us without the supposed benefits of isolating the heat-retaining mass.

            But, heck, I dunno. I certainly don't mind being wrong. I just just don't want to find out I'm right after building it as planned and putting it into use.

            So Jiii-iiim, com out and plaaa-aaay!


            • #7

              Remember that people have day jobs


              • #8
                Just teasin'.

                I'm checking in between clients.

                aka: PizzaMan


                • #9
                  Here is the information on compression and vermiculite concrete that Jim did for the "standard" hearth desgin. I think it shows that the vermiculite layer will never get "squashed" under the weight of the oven.

                  I'm going to add a few thoughts on the "island" hearth next.



                  Actually I just did the calculation for this. If one creates a 5 1/2"
                  form (using 2x6 boards) around the top of the block walls and then
                  fills the first 2" with "standard" (1:6) vermiculite concrete followed
                  by a 3 1/2" layer of bag mix concrete you get both insulation &
                  thermal mass. The 2" of vermiculite concrete give you the insulation
                  effect of 40" of concrete (ASTM standards).

                  Of course, vermiculite concrete is weaker than regular concrete in
                  terms of compressive load (between 140 & 170 psi vs 2000-4000 psi). It
                  does however flex better without cracking when subject to tensile
                  stress. That's one reason you'll find it used in swimming pools. That
                  still leaves the issue of the crushing weight of the oven & thermal mass.

                  Taking a conservative 150psi for vermiculite concrete, that runs to
                  about 21,000psf -- a 64"x72" hearth will have a 30 sq ft load area (or
                  assuming you're really conservative and want to use point pressure on
                  the walls without regard to the overall spread -- 15 sq ft of wall top
                  surface). That means the vermiculite layer is capable of supporting in
                  compression over 150 tons. I don't think anyone has suggested that's
                  what everything skyward of their insulation hearth layer will weigh.
                  Pizza Ovens
                  Outdoor Fireplaces


                  • #10
                    Thinking behind the Island Hearth

                    There are two ideas behind the Island hearth. First, that you want vermiculite on the sides of the thermal layer, and second, that 3 1/2" of concrete in the thermal layer is overkill (it rarely gets fully hot).

                    While we wait to hear Jim's input, I can add a few thoughts.

                    What if the thermal layer of the hearth (standard concrete) were 2" thick, and embedded with 1/2" rebar, in both directions, on 12" centers and that the layer was large enough to rest on the concrete block stand. The blocks are 8", so it would be possible to rest the thermal layer on the stand and still leave 4" on the sides for vermiculite concrete.

                    Would the rebar become a problem in the 2" thermal layer with contraction and expansion? Would a 2" concrete layer ever sag in the middle?

                    Pizza Ovens
                    Outdoor Fireplaces


                    • #11
                      Another idea

                      Maybe the approach shown on the attached pdf would provide more security, at the expense of more concrete and more work:

                      I've shown a reinforced slab of regular concrete spanning the foundation. On top of that is the island slab within an insulating concrete surround.

                      The entire oven chamber rests on the thermal mass island, which in turn rests upon the insulating concrete, with loads pretty uniformly distributed. The exterior finish (I'm contemplating masonry so I've shown a first brick/stone on each side) rests on solid material down to the foundation. The space between chamber and exterior is filled with vermiculite, which is added as one proceeds with construction of the exterior finish wall.

                      I would propose (but I don't have the experience to tell whether it matters) that there be a slip plane between the oven chamber opening and the chimey proper (which is part of the exterior), so that the oven chamber and outer finish can move independently with temperature changes.

                      Comments/criticisms appreciated!
                      Attached Files


                      • #12
                        Verified compression load calculations

                        I agree with the compression load calculations, and went one step further:

                        For a round hearth sitting on an insulated slab,

                        Compressive strength, psi 150
                        Hearth diameter, in 42
                        Hearth area, in^2 1385
                        Max load if uniform, lb 207816

                        so "it'll be OK" seems like an understatement.

                        What if the loading isn't uniform? I assumed that all the load of the oven chamber rests upon a ring of width equal to the thickness of the walls. So for a 42" hearth, the ring is 42" ID and 52" OD:

                        Hearth wall thickness, in 5
                        Area under wall, in^2 738
                        Max load on area under wall, lb 110741

                        which is still plenty.

                        What if the entire weight of the oven rests upon the rebar, which in turn rests upon the insulated concrete, as in the orginal island slab design? This assumes that the insulated concrete provides no support other than compression beneath the rebar, like a hanging hearth. I assumed three pieces of rebar in each direction through the thermal slab, so 12 lengths in all, extending 6" outside the thermal slab:

                        Extension of rebar into insulated concrete, in. 6
                        Rebar width, in 0.5
                        Pieces rebar 12
                        Area of rebar, in 36
                        Max load, lb 5400

                        This seems close but probably OK. In reality, the insulating mix should give some support underneath. I haven't tried to account for bending load on the rebar; this is probably OK if the outer dimension of the island slab is close to the inner dimension of the foundation.


                        • #13
                          might as well get my two cents in, just so no one tries to hang an oven on a layer of vermiculite and cement....

                          first, the shear strength of the vermiculite/cement layer is bound to be much less than the compressive strength. for concrete, the shear strength is only about 5% of the compressive strength.

                          second, if my experience is any guide, i found it fairly easy to "overwater" my vermiculite/cement mixture. i was using a concrete mixer but still the water tended to "wash" the cement to the bottom of the mixing barrel. despite my best efforts i was not able to get a consistent distribution of cement throughout the vermiculite. thus leading to the conclusion that the insulating layer will probably have areas of varying strength.
                          my site for our pompeii and tandoor ovens


                          • #14

                            Looking at your PDF, that would definitely do it. The bottom concrete layer holds everything up and encases the rebar, and the Island and vermiculite layers are strictly for oven perforamance. Not much more cost, but more effort.

                            Pizza Ovens
                            Outdoor Fireplaces


                            • #15
                              Thanks for taking a look. Any thoughts on thickness? I have this rule of thumb in my head that anything concrete needs to be 3" minimum thickness (around 5x the largest aggregate) but I have no technical basis for it.

                              So I was thinking 4" for the support layer, 2" insulating, 3" thermal mass.