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40" pompeii, first time builder

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  • #46
    If ithis were my oven I’d be using the burnout pp fibres, AR glass fibres and melt extract fibres. But that is because I have had a lot of experience in handling all of these fibre as well as having them all on hand. I wouldn’t use the basalt fibres because they do pretty much the same as the AR fibres and they tend to clump way more.

    However, for you, who may find access to both basalt fibres and the melt extract fibres both difficult expensive and maybe unavailable in smaller quantities, as well as inexperience in handling, mixing and placing a mix containing them I’d suggest you just add the burnout fibres and the AR glass fibres.
    Kindled with zeal and fired with passion.


    • #47
      I'll let you know how it goes. I have pretty good concrete and masonry experience and access to a pro mason who might help. I think the tricky part is going to be finding these fibers. Off to Google I go.

      But any help from USA folks would be appreciated.

      Also, input on the overall weight of the dome would be nice. I don't want to buy a bunch of materials I don't need and will have to get rid of at the end.

      Thanks all!


      • #48
        Once you work out the total volume (in litres), using the formula D=M/V and density of both concrete and castable of approx 1.8 kg/litre,( isn’t metric wonderful) you can calculate the weight in kgs, then finally converting to imperial.
        Kindled with zeal and fired with passion.


        • #49
          There should be a table somewhere that simply has the answer. I did a quick Google search against the FB Forum and this question has been asked many times since at least 2010 and it's never been answered with a simple number. Which is what the typical builder would want to know.

          Dome volume

          38" sphere. V≈34​3.1416(19)3 = 16.66 cu ft
          36" sphere. V≈34​3.1416(18)3 = 14.14 cu ft

          The different is 2.52 cu ft, or 1.26 cu ft for a 2" thick hemisphere.

          Vent volume

          22" round, 6" high barrel = 5.28 cu ft
          20" round, 6" high barrel = 4.36 cu ft

          The difference is about 1 cu ft, or .5 cu ft for a 2" thick barrel vault vent.

          That makes 1.76 for the dome and vent, plus some for the flat area to attach the anchor plate.

          Let's round up to 2 cu ft to be safe.

          Assuming 3:1:1:1 is about 135 pcf (this is a guess based on commercially available castable refractory), then the weight of a 2" sphere (36" ID, 38" OD), with a 2" vent is about 270 lbs.

          2*135=270 lbs
          2*130=260 lbs

          How is my math? Let me know if this is wrong. This is also less than I had expected, and it puts buying KS-4+ back into the mix as a possibility. Five bags of 55 lb KS-4+ isn't terrible in terms of cost. And it's real refractory.

          Any thoughts?

          Of course, the point of this exercise is not to do math, but for me to work out how many raw materials to buy. How do I convert final dome weight estimates to purchased material volumes -- assuming I go with homebrew?

          Thanks all!


          • #50
            You can find AR fiber at most concrete supply companies, even the big AMZ site carries AR fibers, relatively cheap.
            Google Photo Album []


            • #51
              Thanks Utah. Yes, I found AR fiberglass on Amazon. I see that they are used for poured concrete kitchen counters, so they're popular enough for supply on Amazon.

              But I only found this for burnout fibers. Is this what I'm looking for?
              Last edited by UtahBeehiver; 02-11-2024, 11:58 AM. Reason: Removed hyperlink


              • #52
                I had to remove the commercial hyperlink, not allowed on blog but you can post non hyperlink info, ie PDF (spec sheets, etc) as long as it is not commercial. You have to ask David S on the burnout fibers, he is our expert.
                Google Photo Album []


                • #53
                  Got it. Is this acceptable?
                  Attached Files


                  • #54
                    Or this:

                    Polypropylene Fiber for Cement Concrete Fiber Additive, Impact Resistant Anti Seepage Fiber Reinforcement Concrete Mix for Building Wall Garden Anti Cracking, White (19 mm, 3.6 LB)

                    About this item
                    • Solved Crack Trouble: Mixing Polypropylene fiber and mortar to make the mortar stronger, increase the vibration resistance of the cement, so to reduce the cracking problem, which can suitable for strengthening cement projects
                    • Material: Polypropylene Fiber is different from ordinary fiberglass fiber, polypropylene fiber has the same length and standard, smooth overall, ordinary fiber is rough and varying standards.
                    • Widely Application: Anti seepage polypropylene fiber for cement for concrete reinforcement are good at increasing cement density, so they can reduce infiltration to some extent, can be widely applied to countertop, fireplace surrounds, basement, pond, exterior wall and other wet places.
                    • Easy to Use: you will get 1 packs of polypropylene fiber , just blend sand, cement and fibers for concrete, then add the right amount of water, and stir evenly, Add more for high impact or severe freeze or thaw areas.
                    • Friendly Customer Service: If you have any issue during using Polypropylene Fiber, pls contact us at any time, we will reply u in first time while we have received your message.


                    • #55
                      Thanks for understanding the hyper links. David is the best resource on whether the AR fibers shown are correct or not.
                      Google Photo Album []


                      • #56
                        Thanks. Being clear. Amazon has obvious AR fiberglass fibers for strength. No problem there. Those are the ones used by people making concrete counters and things like that.

                        I am trying to see if these polypropylene fibers are the right material for burnout.

                        Thanks all.


                        • #57
                          I’m currently away until Sunday with mobile battery/charging issues and limited reception, so can’t give considered answers for a while.
                          Kindled with zeal and fired with passion.


                          • #58
                            Definitely no rush! We're pouring the foundation this week.... hopefully.


                            • #59
                              Those fine fibres are the ones to use as burnout fibres. Because they’re so fine, they don’t really help much in reinforcing. But they do work well to hold the wet mix together and assist in controlling wet shrinkage crack problems. Thicker polypropylene fibres are good for reinforcing once concretes are dry and cured, but unsuitable for our purposes because they will melt. That’s why you need the AR glass fibres which are good to around 900C. Be careful where you source them. The AR means alkaline resistant by coating them with zirconium which is expensive. Some cheaper ones have the bare minimum of zirconium coating so they can call them AR. Source your AR fibres from a reputable dealer rather than the cheapest ones on the net.
                              Last edited by david s; 02-11-2024, 06:19 PM.
                              Kindled with zeal and fired with passion.


                              • #60
                                Thanks David.

                                Does anyone have a guess as to whether my math is right on the weight of a 36" cast dome. I am seriously considering using KS-4+, but would need to know how much to buy. The quote is for 6 55# bags for 330 lbs of material for $310, plus $150 to ship. $460.

                                What does everyone think? Worth it? I'm estimating home-brew at around $150.

                                One other things. Can anyone point me to a good thread(s) for building outdoor kitchen cabinets out of concrete blocks? I want to have a 36" workspace to the right of the oven, a 36" built-in grill and a landing space on the other side of the grill. One question is how form and pour the hearth that holds the grill and forms the opening for the stainless doors under the grill.