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  • #16
    Re: The Picton Pizzeria

    Originally posted by Hendo View Post
    Ian,

    From the photo of your formwork at http://www.fornobravo.com/forum/phot...arth-floor.gif, it appears that you?re only using the plastic over the plywood base ? so appearance on the sides from plastic creases shouldn?t be a problem. And no-one will see creases in the concrete under the hearth slab except the wood gatherer!

    ...
    Um, how the heck do you get the plywood base out? if it's still in there (which it will be on mine - I don't want it badly enough to try getting it out!) all you're gonna see from underneath is plywood.


    Or did I miss something?
    "He is no fool who gives what he cannot keep to gain what he cannot lose." - Jim Elliot

    "Success isn't permanent and failure isn't fatal." -Mike Ditka
    [/CENTER]

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    • #17
      Re: The Picton Pizzeria

      Originally posted by Hendo View Post
      As far as covering the concrete slab after the pour, if it were me, I wouldn?t hesitate covering it as soon as it has set with a plastic membrane. I had an impervious membrane under my slab ? steel flooring ? and covered the freshly poured slab with plastic after 12 hours or so. I gave it a good hose down first, and during the next 7 days I ensured that it remained moist under the plastic. It actually stayed like that for 28 days as there wasn?t any action going on ? at least on the hearth slab. This was late February and quite warm. The idea is to keep the concrete moist to ensure those (exothermic) chemical reactions continue so as to achieve a good strong slab.

      See my post at http://www.fornobravo.com/forum/16/o...+cure#post7244 for more info on moist curing, including advice from the Cement and Concrete Association of Australia. I?ve always favoured plastic film over the concrete as it retains moisture more reliably than sheets or hessian etc. which you have to keep hosing down. You can just cover it and forget it basically. I?ve taken my cue from the many building sites which adopt this practice.

      Good luck for the pour next weekend!

      Cheers, Paul.
      Yes covering concrete with plastic during the cure stage or part of it is an acceptable practice but this is generally in regards to a slab on grade not when poured on another impermeable surface such as plastic.

      As far as water being used to "ensure those chemical reactions continue" this is just not correct (if what you meant is different that my explanation) sometimes what we mean doesn't always come across in written words. This process happens on its own from start to finish without the addition of external water applications. The point of adding moisture via misting or putting chemical cure on the surface is to retain moisture within the slab and to retard or slow this process by retaining a high moisture content.

      In the first stages of concrete cure there is a tremendous amount of heat generated and this heat needs to dissipate somewhere. The heat generated between the 2 sheets of plastic has no where to go but internal, increasing the internal temperature which accelerates the process and drastically weakens the structure.
      In this application having just the center wood covered is certainly better than the whole slab sitting on a plastic sheet and then being covered by one, but as I tell our customers, my philosophy is "concrete is hard enough to put in, we do it right because taking it out is even worse" so why take the chance?

      As long as were are citing sources this is where I got my info from.
      Home
      http://www.palmisanoconcrete.com

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      • #18
        Re: The Picton Pizzeria

        Originally posted by Unofornaio View Post

        In the first stages of concrete cure there is a tremendous amount of heat generated and this heat needs to dissipate somewhere. The heat generated between the 2 sheets of plastic has no where to go but internal, increasing the internal temperature which accelerates the process and drastically weakens the structure.
        Does this mean that putting the perlite concrete layer on top of the slab at the same time is bad as heat will be retained in the bottom concrete slab.
        I saw it mentioned on another link that it is OK to do the concrete slab and insulation layer at the same time.

        I only ask because I've convinced a friend to help me do the concrete work (ie I supply home brew and he does the heavt work).

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        • #19
          Re: The Picton Pizzeria

          Ihughes,
          Not at all. In this case the difference would be like wrapping something in saran wrap as apposed to just putting a towel over it. Let me re-iterate, putting plastic over a slab on grade or an elevated slab as "cure" is OK the difference is when you have BOTH sides against an impermeable surface. The issue here is the concrete getting too hot.
          You should be fine..
          Just as a side note: If we we were to really split hairs on the subject your slab covered with plastic after it has set up and most of the initial heat has dissipated (later that evening) would probably be fine, the point is it is not an industry accepted method and has been proven to drastically reduce the psi.

          Then again, about a year a go I did a slab on grade for a buddy of mines addition to their house. It was in the dead of summer here and this week we were having temps in the 100-106 range (horrible) but it was only 3 yards. I usually will NOT pour concrete after 7am winter or summer but sometimes there is nothing you can do. the only time we could get it on that day was @ 10:00am so I had them add a full dose of retardant. So the truck gets there I read the ticket to make sure everything is right check the slump shoot it out and about 3/4 of the way done the first part was so hard I could not bull float it the truck hasn't even left yet well I knew something was up because although it was 100 this shouldn't happen. To make a long story short the guy that took the order (not the batch-master) put accelerator instead of RETARDANT on the order. The concrete company sent a crew out the next week to remove the slab refund the money and reimburse my friend for my "bill". the concrete had gotten so HOT it literally turned to dust, after a week you could still scratch it with your fingernail. It was like when you mix cement, wash the wheel burro out and dump it on the ground, the silt that collects after the water evaporates very, very strange.. I believe I almost died that day trying to at least get it "flat" so we could put a topping on it if necessary, then after it didnt even set up..I was
          http://www.palmisanoconcrete.com

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