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Inside Arch Question - Forno Bravo Forum: The Wood-Fired Oven Community


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Inside Arch Question

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  • Inside Arch Question

    Long time lurker with a question regarding my inside arch. I am building a 42" inside diameter oven with a 16" interior height. I have a laid a soldier course and buttressed the soldiers with reinforced insulating castable. I went to my local refractory supplier with the form for my arch and I asked for some help selecting tapered bricks for the arch. As we began testing various taper configurations, I was leaving a gap between the bricks for mortar. The supply yard guy said "Why are you leaving gaps"? I said "mortar". He said no need for mortar. Sure enough the arch is good and strong with no mortar. My question is: as I set the bricks in my dome will outward pressure cause the arch to be pushed out? I wouldn't expect the arch to collapse mind you, but I have the feeling with nothing adhering the arch bricks to the side walls that the outward pressure from the dome will cause the arch to be pushed over. Any opinions??
    Attached Files

  • #2
    If your buttresses move the arch will fail regardless in my experience no mortar means that it cannot erode due to heat and thus the bricks are less likely to move


    • #3
      If you have something buttressing the insulating layer you will be fine, if not, there is not enough mass to support the arch.


      • #4
        The flatter the arch the greater the outward thrust. From the pic provided it all looks solid to me. The problem of using insulating castable is that the denser it is the stronger and less insulating it will be. Conversely the lighter it is the more insulating it will be, but also the weaker it will be. Hopefully you have struck a reasonably happy medium so that it will be strong enough to support the sideways thrust of the arch whilst not being too conductive and acting like a heat sink. What is the density of the brew as there are
        many insulating castables available?

        This design problem with your configuration is one of the many reasons that the popularity of the hemisphere has become established over centuries.
        Last edited by david s; 01-13-2016, 07:27 PM.
        Kindled with zeal and fired with passion.


        • #5
          I'd be a fool if I said I wasn't worried about the dome collapsing when I remove the form because of this design. But...the buttress has 1/2" rebar sunk into the concrete floor every 10" that is tied together by three horizontal lengths of rebar bent to the shape of the verticals. That having been said, anyone know who is the patron saint of WFOs? I'll be praying before I take away the form.

          I still have my question regarding the arch. Again, I'm less concerned about the arch collapsing than I am about it tipping over from the outward pressure of the dome. Perhaps I should build the out arch and its abutment before I set any of the dome bricks dome behind the inner arch?

          david s: here's the product I used. http://www.pryorgiggey.com/TDS/Econo...e%2025G-td.pdf

          Thanks to all for your assistance.


          • #6
            That stuff has less than a quarter the density of the bricks it is to support and has pretty low strength. Even with heavy rebar in the middle its ability to act as a buttress would be minimal IMO, but only service will determine if it'll work ok. I would have gone for double that density and used random fibre reinforcing instead of heavy rebar. There will be no problem about it standing up when forms ar removed, it is the continual expansion and contraction in service that is the concern. As Tscar said you need something solid on the outside of the insulation at the base.
            Last edited by david s; 01-14-2016, 11:13 AM.
            Kindled with zeal and fired with passion.


            • #7
              I have room to pour some regular concrete outside the insulating layer. How high up the base do you think I'd need to pour?


              • #8
                Up to the top of your arches for the entry and as high as the soldier course is for the rest of the dome.
                Last edited by david s; 01-14-2016, 02:17 PM.
                Kindled with zeal and fired with passion.


                • #9
                  4" thick, 16" to the top of the soldiers equal about 6 cu ft of concrete. Small price for a measure of reassurance. Thanks again.


                  • #10
                    Make sure you pin it to the supporting slab. You could also taper it so it's thinner, say 3" at its top.
                    Kindled with zeal and fired with passion.


                    • #11
                      In my opinion, your Dome is very flat. A perfect circle would be 21" tall in the center and you are at 16". The main thrust is at the top of the soldier course. You could lock the soldier course in with a flat metal band that encircles the dome right at the top of the soldier course. You have to get creative in how you tie the ends together at the vent opening, but there are several options. Since you have already cast your insulating layer, we will be anxious to hear how it turned out, or if you added additional buttressing. I suspect that the insulating layer will be robust enough to keep the soldiers in place, but only by removing the interior support can that be confirmed. I am curious to see if you used mortar for the rest of the dome, or dry stacked it like the arch? When the arch is tight, as in your picture, then Refractory cement instead of Refractory mortar would be appropriate for those joints to add stability and ease of construction. I agree with the salesman that those tight joints do not require a mortar/cement, but I would fill them anyway because they will open up when firing and customers tend to notice things like that. Since you are the customer, you get to decide how much an open joint will bother you. When your focus is on pizza, an open joint is the last thing you will be looking at.

                      There is no reason to remove the support for the arch until the dome is complete, and you can simply stack some brick or even just use a stick as a brace in the center of the arch to support it. (again, the use of refractory cement in this arch would have helped resist the thrust of the dome.) Once you get the outer arch and vent done, the mass will be sufficient to support the inner arch, so you are just talking about temporary support.

                      You have a nice oven going on there, keep posting pictures of your progress so we can see it go up.
                      The cost of living continues to skyrocket, and yet it remains a popular choice.


                      • #12
                        Thanks dakzaag for your comments. I believe I will put the form back under the arch as your comments make a lot of sense. I took the form out because family, friends and neighbors all want to see inside the oven - even though there's nothing to see but forms, supports and sand.

                        Yes my dome is flat. When I started getting interested in this project several years ago, the idea of a (modified) Neapolitan oven got stuck in my head and I'm pretty stubborn so the idea wasn't going away; even though I knew it would be more costly and take more time.

                        Only the arches will be dry stacked, although the idea of refractory cement does sound interesting. The rest of my oven is 9x4.5x2.5 medium duty bricks mortared with homebrew. Right now, I'm thinking I will pour another buttrress outside the insulating layer. I have room on my stand and it seems the simplest solution to gain a good measure of reassurance for the long haul. I'll try and remember to post some photos as I make progress.
                        Thanks again for all the comments.