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Crack Control and fiber addmixtures - Forno Bravo Forum: The Wood-Fired Oven Community

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Crack Control and fiber addmixtures

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  • Crack Control and fiber addmixtures

    I noticed the Pompeii instructions say you can add a layer of mortar to the outside of the dome to increase the thermal mass, for those so inclined. Has anyone done this with the addition of fiber reinforcement to that mix to aid in crack control? It certainly helps with concrete shinkage cracking and meets the temperature steel reinforcing requirements in those applications.

    Scott
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  • #2
    Re: Crack Control and fiber addmixtures

    Stainless steel needles. $

    Mark

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    • #3
      Re: Crack Control and fiber addmixtures

      Yeah I use a few different fibers. First is the stainless steel needles for high temp application. They are not actually needles but more of a flat bar with ripples for grip and about 2mm wide by 40 mm long, although I guess there are other sizes. They don't present much problem in application. The only drawback is that you will probably have to buy way more than you need and stainless doesn't come cheap.
      The second kind of fiber I use is a short and extremely fine polyproelene fiber that melts at 160 C This is no good for high temp stuff if you want it for reinforcing, but if used in high temp application the fibers burn away and leave minute pipes where the water can escape. This was developed as an addition to make concrete buildings fire proof. The third fibre i use is a longer plastic one that is used as a replacement to steel reinforcing in structural concrete. Our local council uses it for all the concrete footpaths and I use it for the outer concete shell. It is far simpler and quicker the chicken mesh over a spherical surface.
      Kindled with zeal and fired with passion.

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      • #4
        Re: Crack Control and fiber addmixtures

        Thx1138 used some sisal rope fibers as reinforcing. That should work but don't know how it went. You could easily chop short lengths with a tomahawk, but make sure ou mix it in really well to distribute the fibers evenly.
        Kindled with zeal and fired with passion.

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        • #5
          Re: Crack Control and fiber addmixtures

          I bought my stainless needles by the lb, less than $10.
          Wade Lively

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          • #6
            Re: Crack Control and fiber addmixtures

            I would think fiberglass fibers would stand the heat? Wade, where did you source the stainless needles by the pound?
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            • #7
              Re: Crack Control and fiber addmixtures

              Where did you get your needles? Wlively

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              • #8
                Re: Crack Control and fiber addmixtures

                Glass has a melting point around 900 C which is way out of our range, but ordinary glass fibers react with the material. You need to get alkaline resistant fiberglass.
                Kindled with zeal and fired with passion.

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                • #9
                  Re: Crack Control and fiber addmixtures

                  Thanks David, I've found a supplier of Alkali resistant fibers and I can get relitively cheep ($32) delivered for 4 - 1# bags (way more than I need). I think this would be the preferable way to go from a cost and workability standpoint.

                  What I really want to know if this is a good idea, or does it cause more problems?

                  Ordinarily on any concrete, stucco, or motar work its alway a benifit to add fiber reinforcement. But with this high temp application I wonder if everything will move together and still be contained?

                  Is the cooefficient of thermal expansion of the glass fibers the same as the firebrick and mortar? Does this really matter?

                  Just trying to work it arround in my head....cost isn't much and I only want to do this once....not that it hasn't been fun so far. (I'm actually on the last row now and its not fun at this point!)

                  For those who have done this, did it work out ok?
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                  • #10
                    Re: Crack Control and fiber addmixtures

                    I've had trouble with the outside cracking, but since going to fibres, problems gone. There is considerable expansion with the heat and steam pressure build up. A render mixture that has lime in it is apparently more elastic. Try a 4:1:1 sand ,cement, lime mix for the outer shell, only needs to be thin, after all it's only a weather coating.
                    Kindled with zeal and fired with passion.

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                    • #11
                      Re: Crack Control and fiber addmixtures

                      Originally posted by Cypress View Post
                      Where did you get your needles? Wlively
                      I bought them from this supplier. Look up the contact info and call him, he can sell by the pound. I also bought my Insblock 19 here for $1.00 sq ft.

                      http://stores.ebay.com/HIGH-TEMP-REFRACTORY-STORE
                      Wade Lively

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                      • #12
                        Re: Crack Control and fiber addmixtures

                        thanks Wade, did you use this to parge the outside of your dome? How much did you use and did it work well?
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                        • #13
                          Re: Crack Control and fiber addmixtures

                          It is hard to remember. I bought around 2 lbs I think. I told him what I was doing and he recommended the amount, there is a recommended ratio depending on product used.

                          I mainly used mine for my dome plug and vent transition which I cast. What needles I had left over went into the cladding coat on the dome. My castings are holding up well so far. Can't say about the cladding because I added a layer of perlcrete over the top.
                          Wade Lively

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                          • #14
                            Re: Crack Control and fiber addmixtures

                            I have some questions I need your help:
                            - I just have finished the dome and have added a layer of mortar to the outside of the firebricks (so no insulation on the outside yet). Then I came across with this subject. What is the best next step ? a) Add another layer of mortar mixed with stainless needles. b) add another layer of mortar on a chicken mesh. c) or just leave it this way (no extra layer of mortar with reinforcement).

                            - And should I start curing the oven before applying insulation of after insulation ?

                            Btw, just curious, I don't understand why stainless is used here. Does this have anything to do with rust ?

                            thanks.
                            Phi

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                            • #15
                              Re: Crack Control and fiber addmixtures

                              The thickness of your refractory mortar over your bricks depends on how thick you want your dome to be. It's purpose is to add thermal mass and added strength. I think chicken wire that is galvanized could be a problem because zinc's melting point is around 420 C and the oven dome could well exceed this at times. Heat will accelerate any reaction. I think that is why stainless needles are recommended. The curing before or after insulating has been addressed on another thread somewhere, the consensus seemed to be that either way was ok but there are advantages and disadvantages for both.
                              Kindled with zeal and fired with passion.

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