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Did I do my Vermiculite layer wrong? - Forno Bravo Forum: The Wood-Fired Oven Community


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Did I do my Vermiculite layer wrong?

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  • Did I do my Vermiculite layer wrong?

    It's been a week since I did my 4" reinforced concrete layer and 6" vermicrete layer, so I took the forms off today. But the vermiculite layer seems very crumbly, not reassuringly strong at all. I know it only needs compressive strength, but it doesn't even seem that hard -- I can press in with my thumb.

    And the edges just crumble under my fingers. See pic.

    I made my vermicrete layer with a 10:2:3 ratio of vermiculite : portland cement : water (aka 5:1 mix) as advised elsewhere on the board. I mixed the water and cement together first in a 5 gallon bucket before adding to the dry vermiculite in a mixing pan and mixing with a hoe. Seemed to get a nice uniform mix, so I don't think my execution was the problem.

    Did I use the wrong technique? Should I have mixed the vermiculite and dry portland cement first then added water (the other technique I saw mentioned on the board)?

    Could I have kept it too wet after pouring? I covered with a plastic sheet and watered with approx 1/2 gallon from the hose about once a day with a sprinkler nozzle.

    Or am I overreacting? I'm thinking I could "save" the vermicrete by using surface bonding cement around the edges, essentially as prep for the eventual application of stone veneer, and I'm going to have either cast in place concrete countertops or the pizza oven itself holding the top down. Maybe I just need to let it really dry out?

    Advice please!!!!!

    See my build here: http://www.fornobravo.com/forum/f6/3...-tx-19256.html

  • #2
    Re: Did I do my Vermiculite layer wrong?

    Leave the form on to prevent the edge crumbling, other than that it is fine.


    • #3
      Re: Did I do my Vermiculite layer wrong?

      I think that if you look back on several builds you will find others that had the same concerns. A week is not all that long for vcrete to set up. It is probably still wet. I believe that your slab and vcrete were poured on the same day? That could cause a little more drying time also. Misting a concrete slab is recommended. I am not sure that it is necessary for the vcrete layer. It traps enough water in the vermiculite to keep all but the surface and edges damp for weeks. Covering for a few days should be enough. I left my forms in place as long as I could to protect the weak corners. Others, I think have did as you suggested by strengthening the edges with a cement.
      joe watson

      "A year from now, you will wish that you had started today "

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      • #4
        Re: Did I do my Vermiculite layer wrong?

        Thanks experts!

        I had hoped to put the concrete countertops on top of it today, but thinking I should wait another week at least. Instead I'll just pour concrete down the holes in the non-pizza areas of my build.

        Can I agressively help it dry the vermicrete over the next week by things like:
        1) only cover with plastic when it is going to rain
        2) point a box fan at it
        3) ?



        • #5
          Re: Did I do my Vermiculite layer wrong?

          Yes leave the plastic off if it's not raining. This doc may help you.
          As a guide if a 1 mm shower of rain fell on a one metre square slab of vermicrete and all of the rain was absorbed ie no run off then your vermicrete would have taken up one litre of water. 10 mm fall, it could take on 10 L (one bucketful)
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          Last edited by david s; 05-12-2013, 01:53 PM.
          Kindled with zeal and fired with passion.