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Vermicrete Layer Advice - Forno Bravo Forum: The Wood-Fired Oven Community

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Vermicrete Layer Advice

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  • Vermicrete Layer Advice

    I need some quick advice...

    I was planning to buy the vermiculite today for my vermicrete layer so that tomorrow I could pour the 10cm of concrete slab and then the 10cm of vermicrete before the concrete set.

    That said, the supplier I found (£11 /100l bag) wont let me collect after 15:30 - and I got there at 15:31 (pain in the @ss I know...)

    So - can I pour the concrete and vermicrete layers separately? What is the risk of doing this? Or should I delay pouring the concrete until I can pour the vermicrete as well?

    M.

  • #2
    Re: Vermicrete Layer Advice

    You do not normally do them both at the same time anyway.

    Comment


    • #3
      Re: Vermicrete Layer Advice

      Great - thanks! I was worried that if the two layers weren't bonded together, bad things would happen

      M.

      Comment


      • #4
        Re: Vermicrete Layer Advice

        Because the vermicrete layer contains so much water it is advisable to leave it as long as possible (a few weeks if you can bear to wait that long) to allow it to dry before building over it and trapping the moisture it contains.
        Kindled with zeal and fired with passion.

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        • #5
          Re: Vermicrete Layer Advice

          I haven't used vermiculite, but using perlite at the recommended water ratio, it is pretty dry. For a 4-1 mix using 4cuft perlite, 1 cuft portland, and 10 gallons of water, most of the water will be used in hydration of the cement and unless you want to there is no reason to wait more than a day or 2 to build over it.

          http://www.perlite.org/perlite_info/...e_concrete.pdf

          Comment


          • #6
            Re: Vermicrete Layer Advice

            "You do not normally do them both at the same time anyway."

            Particularity since the insulation layer has a different "footprint" than the slab. The insulation layer should only be under the hearth bricks and dome bricks. You have to set up new forms on the recently poured slab surface.

            If the insulation layer extends to the edge of the suspended slab, you will have created a very broad entryway for moisture.

            On that note, make sure the surface of your structural slab has positive drainage and that there are no low spots or "bathtubs" to hold water. If you haven't sloped your slab you may have to do some grinding at this point.

            Keep both the slab and the vermicrete layer well supplied with water = permanently damp, for at least 7 days. (You can start build on either layer within a few days). I do not share David's concern with trapping water. I think a properly cured concrete/vermicrete layer is more important. Add lots of water.
            Last edited by Neil2; 08-03-2012, 03:06 PM.

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            • #7
              Re: Vermicrete Layer Advice

              Originally posted by Tscarborough View Post
              I haven't used vermiculite, but using perlite at the recommended water ratio, it is pretty dry. For a 4-1 mix using 4cuft perlite, 1 cuft portland, and 10 gallons of water, most of the water will be used in hydration of the cement and unless you want to there is no reason to wait more than a day or 2 to build over it.

              http://www.perlite.org/perlite_info/...e_concrete.pdf
              Approximately one third of the volume of vermiculite or perlite of water is required to make a vermicrete or perlcrete slab. The water does not disappear when hydration takes place. It does not magically get lighter(this can be tested by weighing a sample as it dries to calculate water loss) sure you can build over it the next day, but this does make the water removal more difficult later.
              I often weigh castings to calculate water loss, it is quite an accurate measure if you know the weight ofbthe dry casting
              Kindled with zeal and fired with passion.

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              • #8
                Re: Vermicrete Layer Advice

                It doesn't disappear, it is incorporated into the crystalline structure of the paste.

                Comment


                • #9
                  Re: Vermicrete Layer Advice

                  When making vermicrete or perlcrete you require about double the content of water that you would normally use in a regular concrete mix. Given that a regular concrete slab still contains a fair proportion of mechanical water that eventually escapes to the atmosphere a vermicrete slab has proportionally a huge amount to be removed. This can easily be demonstrated by making a test slab and weighing the dry ingredients including the water, then weighing the slab daily to calculate the water loss over time. If I have time I'll do it and graph the results for you.
                  Kindled with zeal and fired with passion.

                  Comment


                  • #10
                    Re: Vermicrete Layer Advice

                    Interesting. Slab laid (it's still wet, so I could theoretically give it a positive slope) - but my query is why does it need a slope?

                    It'll all be encased in a large brick house - I agree that there will be some steam - but why would the water condense under the vermicrete layer?

                    Just curious...

                    M.

                    Comment


                    • #11
                      Re: Vermicrete Layer Advice

                      Sloping the hearth down to the outside is a method of avoiding water getting under the oven if it is in the weather. If you are doing an enclosure it really doesn't apply.
                      Kindled with zeal and fired with passion.

                      Comment


                      • #12
                        Re: Vermicrete Layer Advice

                        Cool - thanks! Was worried for a moment there!

                        Slab's now poured, pictures uploaded in my other thread:
                        http://www.fornobravo.com/forum/21/n...k-17939-2.html

                        M.

                        Comment


                        • #13
                          Re: Vermicrete Layer Advice

                          "but my query is why does it need a slope?"

                          You don't need much of a slope. A 1/4 inch from the center out is enough.

                          I always proceed on the principal that water will find a way in. (Do I need to tell a Londoner this ?)

                          Comment


                          • #14
                            Re: Vermicrete Layer Advice

                            Originally posted by Neil2 View Post
                            I always proceed on the principal that water will find a way in. (Do I need to tell a Londoner this ?)
                            I only live here I was born in a country with much less rain (Italy!). I've ended with what I think is a level slope... may pour some water tomorrow to test it and see what to do... I could always angle the vermicrete layer a tad so that if water gets into the oven, it'll come out...

                            Trying to visualise the over opening now... undecided between a full arch or the straight-sided one....

                            Comment


                            • #15
                              Re: Vermicrete Layer Advice

                              Spinal I don't quite understand. This oven is not the same as the one on your other thread. Are you building two at once? Hope you have insulation under that floor it looks rather like normal concrete from the photo.
                              Kindled with zeal and fired with passion.

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