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  • Slab / base support

    hi again,

    so Iím going to be likely making a home brew style formed WFO.
    ill attach a pic of my base thatís nearly complete. Excuse the mess.....
    What Iím stuck with is the base - Iím wanting to make a concrete work top.
    What im wondering is do I need lintels in or is reinforced cement strong enough ?
    that led me to this could I do half reinforced cement 3inch. 2inch vermiculite and cement for insulation?
    sound great in my head but so does my singing voice
    any suggestions ? Basesize is 1300x1300.
    Will I be able to lift this with a couple of people?

  • #2

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    • #3
      Usually some form work is used and 4Ē thick slab cast over it. The slab should contain steel reinforcing. You can probably get away with only 3Ē thick. The lrecommended underfloor thickness is 4Ē, again you can probably get away with only 3 but no less. Casting a few holes near the centre of the slab provides an exit for insulating slab moisture. Some UK builders have successfully used lintel blocks placed at close intervals instead of casting a concrete slab.
      Last edited by david s; 05-30-2018, 02:21 PM.
      Kindled with zeal and fired with passion.

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      • #4
        Would 4inch concrete and two inch vermiculite and concrete be a solution?
        so 6 in total. Was thinking for insulation. If itís not needed Iíll just do 4inch concrete.

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        • #5
          I am unclear on what you intend. If you are casting a slab to support the oven and intend to polish the exposed areas then three inches would be prudent depending on the support structure. If the slab is not supporting the oven thinner slabs are possible for many years we used terrazzo which is concrete made with marble chips, and these panels were up to two meters square and only forty millimetres or an inch and a half thick.

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          • #6
            Iím trying decide if I need to use lintels
            under a concrete slab or if a concrete slab will be enough alone. The slab would support the oven. As it stands
            my base is a hollow box the top is currently vacant.

            Ill post a pic that should sort of explain the top. I think itís standard most seem to do what Iíve done.

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            • #7
              There is little strength in a 5:1 vermicrete slab, only enough to prevent compression from the oven sitting on top of it. For supporting the whole thing you need a strong supporting slab of reinforced concrete min 3Ēí, but 4Ē better. Or you could use closely spaced lintel blocks. Between the supporting slab or lintel blocks and the oven floor and dome you need a min of 3Ē insulation, 4Ē better.
              Kindled with zeal and fired with passion.

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              • #8
                Originally posted by Flash15 View Post
                Iím trying decide if I need to use lintels
                under a concrete slab or if a concrete slab will be enough alone. The slab would support the oven. As it stands
                my base is a hollow box the top is currently vacant.

                Ill post a pic that should sort of explain the top. I think itís standard most seem to do what Iíve done.
                Nice looking stand there.
                It's 1300 x 1300. Presumably that is the outside measurement so the hole in the middle, assuming 1 brick wide all round, is a touch under 1100mm x 1100 mm.
                You should not need lintels for holding up that expanse of reinforced concrete.
                Lets say the oven you build on top weighs 500 kg. Evenly distributed, which of course it is not, it's not even 1 pound per square inch.
                Reinforced concrete should be fine without lintels.

                Then absolutely put a layer of insulation between your slab and your oven.
                I always harp on about this.
                If you decide, like my mate did for example, that your finished oven looks so damned cool that you don't want to cover it up with insulation that's fine.
                If you change your mind later, it's a piece of pie to insulate over the top.
                However, if you don't put the underfloor insulation in from the get-go, it is almost impossible to fit it later.

                Poms seem to be like Italians, with bricklaying and concreting imbedded in their DNA, so I'm probably telling Grandma how to suck eggs, but make sure your reinforcing is centre or below centre in your slab.
                The reason is that reinforcing works in tension, so if you think about your concrete bending under the load, you want the reo on the outside of the curve so that the load is trying to stretch it.
                Naturally the reinforcing should stop 2 inches short of the edge of the slab so the concrete protects it from corrosion.
                Last edited by wotavidone; 05-30-2018, 05:30 PM.

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                • #9
                  Thanks everyone appreaciate the help. And nice comment about the brick work. It was the hardest thing Iíve ever built so not sure about it in my DNA lol.

                  im taking from this - fire bricks as the base of the oven is t enough insulation? So would be insulationís then firebricks the. Oven on top?

                  whats best to use for insulation?

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                  • #10
                    • You need a strong concrete slab. Use reinforced concrete, it must support the whole oven. I personally think 3 inches with reinforcing mesh would be adequate.
                    • Put a layer of insulation such as low density vermiculite and cement mixture on top of the concrete slab. This is to stop heat from the cooking floor wicking away into your concrete slab. 4 inches of "vermicrete" would do it. About 5 parts vermiculite to 1 part cement. David s will confirm ratio, I hope.
                    • Put a layer of dense firebrick on top of the insulation. This will be the cooking floor.
                    • Build your dome/oven chamber on top of that. Use high density material such as dense (not insulating) firebrick or dense castable refractory.
                    • Insulate the whole lot over the top with something like low density vermiculite and cement mixture. About 10 parts vermiculite, 1 cement.
                    They say a picture is worth a thousand words:
                    Click image for larger version  Name:	oven layers.JPG Views:	2 Size:	43.8 KB ID:	405162
                    Attached Files
                    Last edited by wotavidone; 05-31-2018, 03:43 AM.

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                    • #11
                      Thatís some amazing details thanks very much.
                      there Isnít a insultating board I could use instead of the vermiculite? That would save me some time thatís all I am thinking.

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                      • #12
                        Eg 1 inch vermiculite board?

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                        • #13
                          Originally posted by Flash15 View Post
                          Thatís some amazing details thanks very much.
                          there Isnít a insultating board I could use instead of the vermiculite? That would save me some time thatís all I am thinking.
                          A quick search "calcium silicate board UK" made me quite envious. Seems like you are spoiled for choice with regard to calcium silicate insulating board.
                          Here in Oz, the closest supplier I could find was 1000 km away.
                          Find a supplier of calcium silicate board or maybe vermiculite board that you like the look of. Post the specs here.
                          Plenty of guys who've used it will be able to say whether the product is suitable.

                          While you are finding the cal sil underfloor insulation, search out ceramic fibre blanket for over dome insulation.

                          Vermiculite cement "vermicrete" = low tech, inexpensive, adequate insulation.
                          Calsil board underfloor and ceramic fibre over dome insulation = high tech, spendy, superb insulation.
                          Worth it if you have the quids.

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                          • #14
                            This stuff seems to have good strength and good insulation values at our operating temperatures.
                            http://www.dupreminerals.com/wp-cont...-datasheet.pdf

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                            • #15
                              Cheers guys. Looking at Skamotec 225.

                              Would be good to hear from someone who has used it or similar.

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