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Help please: A huge hole right thru my cladding - Forno Bravo Forum: The Wood-Fired Oven Community


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Help please: A huge hole right thru my cladding

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  • Help please: A huge hole right thru my cladding

    Hi All,

    So i removed the formwork on the side of my oven, untill i noticed .... (see pictures)

    Has anyone had this problem and successfully found a way to fix it?

    Also the cement i am using is called cement fondue ... it was $$$$

    I was thinking

    [1] Make/Cut the whole bigger in an effort to fit in more cement fondue? With its current size and mimensions i think it will be difficult to squeeze it in

    [2] ?? i dunno what else to do .. please help

  • #2
    Re: Help please: A huge hole right thru my cladding

    What does that cement fondue consist of. It doesn't look like expanded perlite or vermiculite. It looks like aggregate/cement against the fire-brick. You might want to check what's in that fondue. That layer looks to be less than 2 inches thick, 4 inches is recommended. The only thing you can do is patch it with heat-stop which is a mortar, no aggregate. So you can just trowel it in.
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    • #3
      Re: Help please: A huge hole right thru my cladding

      When you make up your next batch to go over the top take a small amount of your mix and remove the big stone, mix it up a bit on the stiff side then trowel it in. No big problem there. I see your following the Allen Scott directions to the T. What are your plans for the oven, bread, pizza, and everything else?


      • #4
        Re: Help please: A huge hole right thru my cladding

        Hi Iwood,

        Cement fondue is a product from france. Its a type of CAC and its typically used for high temp applications . .eg fireplaces and woodfire ovens. Basically the strength of it all falls down to what type of aggregate you use. I used basalt(normal blue stone rock).

        But now with my situation i was thinking of maybe making the fondue blend but instead use crushed firebrick to get deeper into the gaps.

        Hi FiV

        Yeap pretty close too alan scots design. I plan to use it for all sorts of things, pizzas, baking, ribs and roasts.

        Do you think the cement fondue + crushed firebrick might work ?


        • #5
          Re: Help please: A huge hole right thru my cladding

          gummz, what you have is commonly called a "rock pocket" here in the USA. In the construction trades there are proprietary products which expand upon curing and that is how these are commonly dealt with. The holes are packed as well as possible and the cured material expands and grips the undercut exposed rock. However, you are using calcium aluminate cement (same as what I used in my refractory) and I think mixing cements would be problematic. I would suggest that you use the same aggregate that you used for your mix but either smaller pieces (either crushed or sorted for much smaller size) to fill the space as best as possible. That way there will be less possible problems with different expansions of the material. Undoubtedly there will be some small voids but unless you really vibrated your pour and were unusually lucky there are bound to be others unseen.

          When I made my mix I used 1/4 minus crushed basalt and that provided both the fines and the larger aggregate for my "concrete". I did that both for ease of mixing as well as uniformity of expansion of the final product. What did you use for sand in your mix?

          Hope this helps,


          • #6
            Re: Help please: A huge hole right thru my cladding

            Hi Wiley,

            Many thanks for the info.

            I used "sydney sand" .. its just normal washed sand. I did get worried i may have other "bubbles" in other areas ... i did run around it with a rubber mallet.

            Things i learned (and also for others to observe)
            [1] This problem came because i was trying to pour the concrete in a small gap that also had reo in the way
            [2] This entire project has been done by myself and this CAC does harden up fast. I would really look at getting and extra one if not two pairs of hands to help quickly mix and distrubute the cement.
            [3] Appears thickness > 2~3 inces is best. At the back of my oven where its about 3~4 inches thick, its a nice deep black marbel colour and feels really solid

            How fine do you think i need to get the basalt down too? I used 10mm bluestone , should i bash it till i get it down to a quarter of this size??

            Also what are you thoughts on cutting out a bit of the cement to fix in the "patch" a bit better?




            • #7
              Re: Help please: A huge hole right thru my cladding

              gummz, Personally I would put on good eye protection and take a handful of your 10 mm pebbles and mash them on a suitable anvil (piece of railroad iron or whatever you have that has some mass) using a good sized hammer. Do it in a box so you can collect the bits and pieces either to use as they are broken or to re-smack until you have a suitable quanity in sizes from fine sand size to 1/8th the size of the pebbles. My inclination would be for a higher percentage of fines and I would mix it on a mortar board or small tub and push the mix ino the hole with something like a putty knife. I think I would dampen the hole first with a wet sponge so that it doesn't steal the moisture from the mix as you press it in.

              As for consistancy of the mix: would keep the mix somewhat plastic (on the order of something you can squeeze tight in your fist and not squeeze out any water and when released it holds it shape. A drier mix having less shrinkage than a wet one. When I mixed my 1/4 minus basalt it was pretty straightforward as the fines to larger pieces ratio was close on optimal and I simply used five of the basalt to one of the cement by volume. You might want to go a bit richer than that. The mixture I used was very nice to work with, troweling easily to a nice surface.

              As you mentioned the calcium aluminate cement sets up really fast and so even if you have to keep some pressure holding the stuff tight against the dome until it gets its initial set you won't be holding for long, a few minutes at best.

              Anyway, that is how I would approach the problem. As for making the hole larger: I would think along the lines of "less is more" and keep enlargement of the hole to a minimum.

              Again that is just how I would approach the problem. I would think there would be others with answers/suggestions worth considering.



              • #8
                Re: Help please: A huge hole right thru my cladding

                Lots of unnecessary work there.

                Take the mix that you are using and pick out the big stones, you have no need to crush them. Don't make the hole bigger that is just a waste of time and will hurt you more then help you. The bumpy uneven crevices will help to hold your patch in. I suggest you use your current mix and pick out the big stones so that you don't have to buy a big bag of something and use a little bit of it. Crushing the big stones will do nothing for structure for a small patch such as that. Mix it up on the dry side and pack it in. The less water you use in concrete the stronger it sets. This is why on big bridge projects the concrete must pass the slump test that shows the proper amount of water was used when mixing.

                You may get a bit of slumping when you fill this hole. It will look like the concrete is sagging at the bottom and pulling at the top. So fill your hole most of the way and leave a bit of room for a second coat. The second coat will require you to remove all the stones if you want a smooth surface.

                It's an easy fix and there is no need to get all crazy about patching a small hole.

                You can trust me on this I work with concrete for a living.

                Best of luck,


                • #9
                  Re: Help please: A huge hole right thru my cladding

                  Hi Guys,

                  Really appreciate the support and advice.

                  The cement fondue's thermal resistance properties rely pretty much on the type of aggregate used. By removing the "larger" stones i'm worried i'll be left with something thats just like mortar .. and i thin mortar shrinks right?

                  Can i also ask for a bit more advice when i put the fondue over/ontop of the vault of my oven ... how can i ensure all the bubbles come out?? Do i get a plank of wood and lay it over sections of the cement and tap it????

                  Greatly appreciated,




                  • #10
                    Re: Help please: A huge hole right thru my cladding

                    Hello Gummz,

                    I had a problem like yours and made patch by sifting the same dry cement material through a 6mm mesh screen to remove the big rocks. Get a good layer on the inside surface against the fire brick then when it sets up mix a little more and put it over the first layer while it is still damp. No need to stress over fixing a void like this and no problem with removing the rocks by sifting your patch mix.

                    "Pouring" a dry mix to finish this application (oven) is problematic. We're not pros with professional equipment. Here's what I would do. Work in small sections and pack the cement with a tamper or a long piece of your iron bar, then work your way across doing the other areas until you are done. If you are using forms then it is possible to make an effective vibrator from a hand-held electric sander or an impact drill with a blunt shaped bit (maybe a bolt). Hold the tool against the form near the top to get an idea of how it works then move the tool across the form in the area of fresh cement. It works pretty good.....just don't over do it since all you want to do is remove the big air bubbles and fill the potential voids. Over vibrating could settle heavy rocks.

                    Best of luck,

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                    Here is the link to my oven number 2 construction photos!


                    • #11
                      Re: Help please: A huge hole right thru my cladding

                      gummz, I agree with you concerning the properties of the final product being determined by the qualities of the aggregate. That is why I suggested crushing/smashing the basalt pebbles to create the fines and medium sized aggregate of the repair material.

                      As for getting the bubbles out, there are two options as I see it:

                      Option one, pour the mix wet and vibrate it to bring the bubbles to the surface. This would reduce /remove the voids caused by entrained air, to remove trapped air (between the form and the mix) one needs to move a tool along the surface of the molds/forms. However, this has the downside in that wet mixes have problems with shrinkage and cracks and usually have less final strength.

                      And option two: have somebody assist you to mix and hand to you a fairly
                      stiff mix in a pail and you place the material in handfulls/trowel fulls and tamp the material in small "lifts", tamping the material as you go with the butt of the trowel. They would do the mixing and you would do the placing. This would be slower than a simple "mix and pour in the forms/mold" but you would be compacting each lift so that the chance of voids etc would be small. The downside is the need for a second person who is clever enough to pick up the flow of the work so that they can supply you material as you need it so that you can work continously.

                      The second is what I did with my WFO except I was working alone.
                      I mixed and placed each bucket of mix with the idea of creating polygonals designed to separate along the joins. You might want to check out my thread "Steel Dome Oven" under alternative types to see some pictures. If someone were assisting and handing you mixed ready to place buckets so you only had to place the material and not mix you could perhaps create a continuous pour. It should have less voids and fewer problems with shrinkage.

                      Hope this helps,


                      • #12
                        Re: Help please: A huge hole right thru my cladding

                        what strength properties are you concerned about? This is not a bridge that your going to drive 40 ton trucks over. You have more steel in there then a house foundation. don't mix in too much water and you won't have a shrinkage issue. Think about it if you put a stone facade and grout in the stones how much shrinkage do you see...none. If your worried about a small hole like that affecting structure then how do you think a window opening in a poured foundation will affect it's strength. If it were a problem then everyone's basements would look more like a bomb shelter no doors or windows.

                        As for the top it looks like you will need at least some side forms along that rebar. once you get your concrete in take a thin stick and stab it up and down in the concrete like your making butter in a butter churn and that will get the bubbles out. over the top just take your trowel and work it in well and you will be fine