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Homebrew cast over yoga ball

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  • david s
    replied
    I like to do the drying fires after insulation before proceeding to the outside render. This reduces the possibility of steam pressure build up cracking the outer shell. Your vent plan seems sound. A removable hat should work well.if after around an hour of firing, your outer shell feels hot to the touch, then your insulation is wet. Under normal operation when the insulation is dry the outside will only feel cosy warm.
    Use the fine polypropylene fibres in theouter shell mix if you still have them, but they don’t impart a lot of flexural strength as they’re too fine. The give quite good compressive strength though. Remember to mix them about double the time that you’d think as they take a long time to disperse properly. The AR (alkaline resistant) glass fibres give good flexural strength. The guys who do GFRC countertops use them a lot for strength so go to them for sourcing. Very easy to work with.

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  • strangelove
    replied
    Thx for all the help david s I will try the 1:4:1 mix and try it. Thought about leaving a 100mm circle on the top, let everything dry, then cover the hole with cling wrap and make a hat out of the cement:sand:lime so I can lift it if I need to eliminate water after a longer period of wet weather, you think that would work? Paint it with some waterproof color later.

    The AR glass fibres aren't the same as the polypropylene fibres used in the homebrew?

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  • david s
    replied
    Rockwool is difficult to vermicrete against. You’ve done well particularly with such a thin layer. I always do the drying fires before applying the stucco layer to avoid trapping moisture. If you want the outer stucco layer to be white then you will find that the cement wil make it grey unless you use white cement. A good mix is 4 parts sand, one part cement, one part hydrated lime. This mix is not waterproof but you can always paint it white with an acrylic paint to make it waterproof. Personally I think white is a mistake as there will be some soot falling from the flue cap to dirty it up. I usually apply the stucco in one single layer about 15 mm thick and mix in AR glass reinforcing fibres due to time constraints , but most do two layers over chicken wire, 24 hrs apart. A wet sponge over the surface will even out any trowel marks, Holding moisture in that outer stucco layer for a week will enhance its strength. I do this by wrapping the whole oven in cling wrap.
    Last edited by david s; 07-20-2018, 01:13 PM.

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  • strangelove
    replied
    Think the second layer 8f homebrew was good, think it stuck really well to the existing layer.

    Added 90mm of rockwool and covered it with a thin layer of vermicrete. The vermicrete was a bit difficult to get to stick so I had to use a form for the lower part 8f the dome, and the thickness ended up around 2cm,a bit thin, but ran out of vermiculite.

    I want to get a nice finnish on the outside, and make it waterproof. Would also like it to be white.
    Thinking about mixing lime, sand and concrete and use that as a Finnish, or just sand and concrete. Opinions on this?
    Ratios to use?
    Would the lime solution be rain proof?
    Will add pictured later

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  • strangelove
    replied
    david s
    Thx for a quick answer!
    i will try to add another layer tonight then.
    Do you think its safe to remove the sand today? its been 48 hours.

    After this layer i will wait atleast a week before i add my insulation, hope its workable enough with the rockwool, bought 45mm thick, maybe should go for 2 layers of that and make sure the joints arent at the same place

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  • david s
    replied
    Apologies, I missed your last post. 150 mm diam is the correct size for your oven. Usually 0.55 mm thick 304 stainless. The chimney location should be placed in front of the oven mouth (crossdraft), so that the heat rises, is then drawn down before exiting out the flue. Look at the many builds on this site. If the flue is placed in the actual chamber (updraft) a lot of heat is lost. Judging by your pic, you got it right.

    If you plan on adding another layer over your casting, do it ASAP, while there is still moisture in the casting, to achieve a decent bond between the layers. Homebrew requires a minimum of a week of damp curing to achieveI anything like full strength. I think it is better to do the insulation before lighting any fires.The reasoning behind this is that for example, a clay chimney will almost certainly crack unless it is insulated, because there is a huge temperature difference between the inner and outer surfaces and therefore also thermal expansion. The same principle applies with an uninsulated oven. I tried rockwool for two ovens partly because it is a little cheaper than ceramic fibre blanket (for me) and very little difference in insulation value. However I wouldn’t use it again as it’s harder to cut, more compressible, much more difficult to add the vermicrete layer against and more water absorbent. I concluded the small saving not worth it. But as all these products vary they may not necessarily have the same characteristics, this was only my experience of the materials I used.
    Last edited by david s; 06-26-2018, 03:51 AM.

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  • strangelove
    replied
    So i made my sand castle and casted the homebrew over it, it was ok, maybe the first batch was a bit dry, see if i have cracks in it when i remove the sand.
    It may be a bit thinner then i planned, so i may have to cast another layer. Should i do that as soon as possible or should i wait for it to get dry?
    i casted sunday, and think ill remove the sand wednesday, and maybe cast another layer and patch the inside of the dome if its to much cracks.

    i sprayed it with water twice a day.

    Should i start some small curing fire before i insulate it?
    im gonna try rockwool and cover it with 10:1 vermicrete

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  • strangelove
    replied
    david s
    Finally ive found the time to start my build!
    ive casted the base of concrete ontop of my wooden stand.
    And got my first cast of bottom insulation in place.
    one thing i noticed was that i used alot more vermiculite then expected, used abit more then 1 bag (100 litres) and i had to use a bit more water then i thougt, 5verm:1portland:2water, but i saw no excess of water in the bottom of the wheelbarrow.

    1. Should i keep the vermicrete under a tarp during the nights and when rain is expected and let the sun and wind help with the elimination of water uncovered?
    2. Should i remove my form ASAP to help it get rid of water? (I think i will use it again when i put my firebricks inplace and cast to get an even surface)
    3. got a neighbour that can help me get a chimned, works with sheet metal, so i can get it in any diameter and thickness? is 150mm ID ideal for a 700mm oven?
    3.a what thickness of the stainless steel?
    3.b is there a best spot for the chimney? how far from the center of the oven?

    Added pics in link to imgur.com due to file size

    https://imgur.com/a/554pXKR
    Last edited by strangelove; 06-05-2018, 02:28 PM.

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  • david s
    replied
    Sorry, no they're not. They provide quite good compressive strength but negligible flexural strength. There are thicker and longer plastic fibres that would work, but the normal reinforcement for concrete is steel bar.

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  • strangelove
    replied
    Thanks for quick answer, ill go to the store today to shop for material.
    The polypropylene fibres isnt enought to strenghten the slab?

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  • david s
    replied
    Not a bad idea to cast around a few wooden dowels near the centre of the concrete slab. They can be knocked out easily to reveal drain holes to help the elimination of water from your vermicrete insulation. Most builders would recommend a 100 mm concrete slab containing steel reinforcing bar. At only 50mm yours looks a bit thin.

    1. I don't because the vermicrete mix requires about double the amount of water to that of a standard concrete mix, because each grain is made up of tiny absorbent holes. This means that after the hydration process has used up the water from the mix there is still lots of free water present. The problem is getting the water out. see my attached experiment
    2. Yes
    3. usually the insulating slab is cast in place, but you could do it separately. Just let its weight sit it in place, no need to stick it down.
    4. Yes at least a week, see attached experiment again.
    Vermicrete insulating slab copy.doc.zip
    Attached Files
    Last edited by david s; 04-18-2018, 11:53 PM.

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  • strangelove
    replied
    I will start my casting this weekend i hope.
    I will do the bottomplate from concrete and the bottom insulation like jumbalaya in his 6th post https://community.fornobravo.com/for...ent-oven-build.
    i will keep the concrete under plast and spray it for 3 days or so. some questions...
    1. do i need to treat the vermicrete in that way too?
    2. For the vermicrete, ive read that a 5:1 ratio is used for the bottom insulation, is it okay to use that for the whole bottom insulation part?
    3. when i move the insulation to the concrete, should i "glue" it with a ring of vermicrete or how do i make it stick to the concrete?
    4. do i need to wait a week for the vermicrete to cure before i place the fire bricks and do the last casting to make a nice even surface of fire bricks and vermicrete?

    i will cast both parts in the garage, and move outside for the next step, building sand mold and cast the dome.

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  • david s
    replied
    The diameter of the flue pipe depends on the internal diameter of your oven. A 750 mm dial oven requires a 150 mm diam pipe.thin galvanised steel will rust out better to use thin (0.55mm) stainless steel. A clay pipe is an option but needs to be insulated on the outside or it will crack.Using an anchor plate is problematic because of the high heat the fixings must be stainless or they'll rust away. Drilling into the refractory is asking for cracks to develop as it weakens it and any solid metal fixing will expand before the surrounding refractory causing stress. The same would apply if you embed some steel bars in the refractory for support. My solution, attached, is to cut three tags in the thin pipe, bend them out 90 degrees and insert the pipe as a slightly loose fit to allow for expansion. The joint can then be packed around and up about 500 mm with a 5:1 vermicrete mix which is strong enough and will allow some expansion against it.There are of course other ways to do it, you're the builder.

    Click image for larger version  Name:	IMG_1676.jpg Views:	1 Size:	1.06 MB ID:	404047
    Last edited by david s; 04-17-2018, 05:14 AM.

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  • strangelove
    replied
    Originally posted by david s View Post
    Here is a build that would be worth you reading.

    #1
    Thx for that "guide" read it and hope i am wiser now.

    Have ordered vermiculite, the polypropylene fibres and clay (things i had to order on internet), and made up a list of things that i need.
    Think ill need about:
    10kg of lime
    80kg of portland cement
    100kg of sand, should i buy any special sand, right now ive put sandbox-sand in my shoppinglist (0-2mm) what fraction is prefered for the sand?
    22 fire bricks. 230x114x50 mm.

    A chimney, but not sure what to buy and how i should place it. i dont want it to crack up my dome so it will be a loose fit, and i think i wanna use a connector, but i guess its the same cracking problem with that.
    Should i place some metal bars and let the chimney connector rest on that or should i leave a rim for the connectior to rest on?
    do i want the chimney close to the door or close to the dome?

    i must use a steel pipe and not something enamelled?
    im thinking of 150mm diameter and the length of it is just to keep the smoke out of my face?

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  • david s
    replied
    Here is a build that would be worth you reading.

    #1

    Leave a comment:

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