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Structural Slab for new WFO - Forno Bravo Forum: The Wood-Fired Oven Community



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Structural Slab for new WFO

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  • Structural Slab for new WFO

    I do not know how many people have done this, but I am making my structural slab indoors for later transport to its final resting place when the weather cooperates.

    Made a metal framework from 75x75x6mm angle, used a wooden framework and plywood below, and additional forms on the perimeter to make a slab--total thickness 240mm (9 7/16). It also has a blockout for the under hearth insulation as well as thickness of firebrick floor. Slab: 75mm, 100mm insul, 65mm firebrick. Actually poured the slab: DIY mixture. The concrete company delivery price was unbelievable. I was able to get their chart and calculated batch quantities to mix .25m3 concrete in 8 small batches from bulk materials. From their chart what I am getting is an almost 5000psi mixture. They call it JIS3308.

    All went well with the mix, but now I have a block of concrete weighing around 580 kg sitting on the floor of my garage!

    In a different thread, I mentioned attempting to source things locally but having trouble. I found fiberblanket (ceramic fiber) and the board (2" thickness) but am blown away by its price. Also there are minimum order or box quantities--I would only need 3 pcs- 2'x3'x2" thick but they only want to sell 4. Each piece costs around $135! That makes hearth insulation more expensive than everything I have done so far! Today, I purchased vermiculite & perlite from a local agriculture center. The pericrete/vermiculite insulation will only run about $39 total for the area of the hearth/landing @ 100 mm thickness.

    On a different thread, community members said that you should not imbed the insulation in the slab because it might create a bathtub effect. Would there be anything wrong with drilling a hole in the center of the slab beneath the insulation to act as a drain (if) any water actually migrated there?

    A few more questions, then I'll begin this thread.
    -Is there any advantage to the ceramic fiber over pericrete besides higher insulation value for less thickness?
    -Can anyone give an exact number for compressive strength of ceramic fiber board? For pericrete 5:1:1 mix?

    Finally, I have browsed this forum and it will take months to begin to absorb all the information available. It is mind boggling to see all the talent and creative work that is displayed. Thank you Forno Bravo for making this site possible.

  • #2
    Re: Structural Slab for new WFO

    Progress with the WFO.
    Today was a nice day so I was able to mix the pericrete outside and then put the insulative materials into the base (located indoors). I tried both ways of mixing the pericrete: first making a slurry of portland cement & water, then mixing in the dry--5:1:1.

    Also, first mixing the dry materials and then adding the water.

    I found that I got more uniform results with the dry mix first then adding the water. The slurry method made it difficult to get all the ingredients blend smoothly.

    All my mixing was done by hand but used large rubber mixing buckets. These are 65 liter and I was able to tip them to stir all the bottom contents together. Also found it useful to split my batch in half - mixing in separate buckets, then mixing the buckets together... A little like you how you might mix a big batch of paint to get a consistent color. (there is a word for that but it slips my memory)

    When calculating the volume of materials necessary, it ended up taking about 20% more "dry" materials to get the required mixed pericrete. Still, my original calculations: 1-100 liter bag of perlite; 1-60 liter bag of vermiculite; 2-25kg bags of portland cement did the job with just a little of everything left over!

    Oh! forgot to mention- blended vermiculite & perlite...Someone said somewhere that mixing made a more workable mix. I don't know but this was easy working with --so it must be true!

    As mentioned before, the structural slab had a recess cast into it to accomodate the pericrete. I did not strip the forms from the outside and used the top of the form to screed the pericrete level. Actually the screed was made to hang down 65mm--so the finish pericrete is still below the top of the form, allowing for the hearth brick to be installed (flush) with top as a complete product.

    The finish surface is pool table level, I hope the bricks arrive uniform thickness!

    Next project will be cutting the firebrick for the oven floor. A lot of discussion about what is best??? Who knows? My plan right now is to cut all the brick for the oven floor and landing to fit inside my castings. Then wrap the primeter of the firebrick with cardboard and cast a ring (using castable refractory) for the dome to rest on. I had originally intended on setting the dome on top of the bricks but changed my mind after reading many threads! This way, the dome-door opening height ratio are still O.K. Also, can change out oven floor brick if they become damaged.

    So everything is wrapped in plastic with damp towels on top of the slab. Am putting it to sleep so the pericrete has a chance to set up slowly..


    • #3
      Re: Structural Slab for new WFO

      Pictures, we need pictures.
      The English language was invented by people who couldnt spell.

      My Build.



      • #4
        Re: Structural Slab for new WFO

        Do you have a plan of how to move it?
        Kindled with zeal and fired with passion.


        • #5
          Re: Structural Slab for new WFO

          Hello fellow citizens,
          I am stupid as far as pictures go. How do you get a photo from the attachment button onto a reply to thread?
          Attached Files


          • #6
            Re: Structural Slab for new WFO

            I don't know what I did but that is the structural base after placing the pericrete today.


            • #7
              Re: Structural Slab for new WFO

              Hello David S!
              The slab has an angle around the perimeter. It also has holes bored with nuts welded inside for attaching different things--like legs or near future some casters. I plan on using a larger angle, maybe 90x90x6mm-bolt the casters to that and then bolt the casters to the slab. That way, I can move it around the garage at least. When further along, it has to go outdoors for firing to set the castable etc. Then there has to be some kind of a run-way to get it past the garage doors (like a level playing field).

              When everything is complete- there are several neighbors within a km or so that I can beg the use of their forklift. Lots of farmers have them for moving their supplies. At least that is the plan now.

              Now that there is one photo out, maybe I can get some others to work.


              • #8
                Re: Structural Slab for new WFO

                Hello Brickie in oz,
                I'll try a few more photos. Name:  e1d4994eee90d411afcf6a3ed3a4c5b0.jpg
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                • #9
                  Re: Structural Slab for new WFO

                  Description of photos:
                  top to bottom-
                  a) 75x75x6mm angle frame- with 10mm rebar; 100 o.c. x 150 o.c.
                  b) formwork for blockout for pericrete/hearth firebrick
                  c) concrete placed in form completed
                  d) inside forms stripped next morning- then covered w/plastic to let concrete cure.


                  • #10
                    Re: Structural Slab for new WFO

                    Another short note: If you look at the photos closely, you will see some wires coiled up. They are stainless wires attached to the rebar. If I want to tie the dome down, they should assist. In the future no need to try to drill anchors into the slab etc and the wire does not rust or interact with refractory mortars.
                    They are in the way now! But so far, manageable.


                    • #11
                      Re: Structural Slab for new WFO

                      Originally posted by mikku View Post

                      allowing for the hearth brick to be installed (flush) with top as a complete product.
                      All your heat will leak out sideways through the concrete if the hearth bricks touch the concrete.
                      The English language was invented by people who couldnt spell.

                      My Build.



                      • #12
                        Re: Structural Slab for new WFO

                        I am sorry I do not describe things more accurately.
                        What I was referring to was the relationship of the height of the materials.
                        If you look at the oven base as viewed from above:

                        -oven inside diameter 750mm
                        with wall thickness of 80mm
                        -dome outside diameter 910mm
                        -size of block out in structural base 1110mm
                        This allows for the perimeter of the dome to have a 100mm ring of insulation.

                        The new scenerio that I described for the brick floor-

                        -brick cut to inside dimension (750 minus ??5mm)
                        cast refractory ring (inside dimension 750mm), ring width (same as firebrick width 114mm); outside dimension (978mm)
                        -size of block out remains same at 1110mm
                        Allowance for perimeter insulation to refractory 66mm (still better than 2 1/2")

                        So that should be OK, if I did not miss something else.

                        Reasoning for casting refractory ring around firebrick:
                        -Should tend to hold bricks in position better than a second ring of brick.
                        -$$$, I have an extra bag of ACG castable ($31)non-returnable vs the cost of the extra firebrick ($78) (which I don't have); just watching the pennies!

                        Thanks for checking out the details of this build, your thoughts are really appreciated!



                        • #13
                          Re: Structural Slab for new WFO

                          Today I became a "mobile structural slab!".
                          Searched a few of the home centers for short steel stock but came back empty handed. There is a fabricator of steel buildings not too far away so I stopped there and asked the "Shacho" -"boss man in Japanese" if he had any 90x90x6mm off cuts? He said not that size but said to look in the "trash box", there should be some larger-100x100x6 stock.

                          No luck there--then he pointed to where they were sheering angle for one of their projects. Wa La! There was a stack of pieces about 500mm long! I picked out 4, asked the price...he said "dozo". Just take them!

                          Went home, layed out holes--drilled and assembled casters to angle! Spent the rest of the afternoon jockeying my "heavy mass"-- (the structural base)! Usually when referring to my "heavy mass" it is "ME!" ha ha

                          this sucker is really going to be heavy when the firebrick and dome get added.
                          Now it is about the same as pushing a VW bug around!

                          Couple of photos for progress--next week firebrick cutting, bricks should show up on Monday sometime.

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                          Now, if I get busy with something else, I can roll the assembly in the corner and wait for some slack time again to pick up this project! Moving this mass outdoors is a "forklift project" for sure!
                          Last edited by mikku; 02-02-2013, 05:01 AM.


                          • #14
                            Re: Structural Slab for new WFO

                            Now that I can get these photos to load, I'll show the preparation work to cast the dome sections.

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                            1) dome half- with formed oven opening.
                            2) rear dome half
                            3) inner form for transition
                            4) transition to chimney support view
                            5) 3 pcs after castable poured.

                            All the formwork takes time, but a cutting challenge for trapezium dome (much easier with wood than tackling the same in brick)... At least in my opinion. Actual casting-mixing etc only took a couple of hours. A real fun project so far!


                            • #15
                              Re: Structural Slab for new WFO

                              Wow, that's really nice work. And very well thought out. Are you hoping to make more castings using the moulds? I found that wooden moulds do not last too well no matter how well you seal them and moved to fiberglass which holds up much better.
                              Kindled with zeal and fired with passion.