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Rocko Bonaparte 02-11-2011 06:44 PM

Setting a perlcrete pad despite the cold
My plan has been to install a slab of perlcrete as the pad over which to build a pizza oven. The general method was to pour the perlcrete into a wooden form set on top of the firebricks off the ground. The bricks would have an old shower curtain over them to keep the cement from getting on to them. My assumption was after some cure time, I could remove the form, pull out a few bricks, get some hands under the thing, lift it up, and rest it down on top of my cinder block base with some mortar ready to go. I have not had much luck with this.

In the first attempt I had used concrete instead of cement. After two weeks, I had something that crumbled apart right as I tried to pick it up. It would break off right at the corners. That's when I realized I should have been using cement. So I have tried again with a 1:6 ratio of Type I Portland cement to perlite. The slab is poured about 4.5" thick; that's about how much I had yielded when all was done. The pad is a square four feet to a side. There is also two meshes of rebar inside.

The past few days, the weather got down to freezing overnight and there was some rain in the forecast. I had covered it over in a tarp for that whole time. I think it had gone roughly 24 hours w/o a tarp when the weather was still okay, then it got covered the night things were supposed to get bad. I think this will be the last freezing night. I checked the tarp and noticed a lot of condensation on it that I removed.

What has me a little afraid is I found a small chunk not level with the risk that I managed to break off. I found I could bust that little piece in half just about the same way I could with the perlite and concrete, so I'm afraid I screwed this up again.

I am wondering if this method is even possible, or if I should be doing something else to prepare the base for my oven. I can get more materials tomorrow before noon when the place here closes, and I'd want to use the weekend to get this right. Or if you guys think it needs to just cure some more in warmer weather, then I'll leave it alone.

Given I already blew my first slab, I want to get this working right ASAP. I've had my firebricks and clay for awhile now just waiting to be used. :(

tusr18a 02-11-2011 07:25 PM

Re: Setting a perlcrete pad despite the cold
The Pompeii Oven construction guide available free through Forno Bravo can provide you with some solid direction on how to pour a perlcrete slab. Have not seen someone try to pour the slab and then mortar it onto the base. Most slabs are poured in place using rebar to provide strength.

dmun 02-12-2011 06:32 AM

Re: Setting a perlcrete pad despite the cold

I have not had much luck with this.
I dare say.

Insulating concrete has a lot of compressive strength, meaning it's hard to crush, but almost no tensile strength, meaning if you try to pick it up in one or two places it's going to fall apart, exactly as you have demonstrated. It's also going to do the same thing if you are trying to bridge a gap with the stuff. It has to sit on top of a rigid structure, like a reinforced concrete pad.

Tscarborough 02-12-2011 07:17 AM

Re: Setting a perlcrete pad despite the cold
It has to be poured in place, it is not movable, and you do not want or need any rebar/mesh in it.

Neil2 02-13-2011 10:57 AM

Re: Setting a perlcrete pad despite the cold
"I am wondering if this method is even possible,"

No. Its not. You want, from the bottom up:

1 Foundation or slab on grade.
2 Walls
3 Structural suspended slab of reinforced concrete.
4 Perlcrete or vermicrete layer
5 Hearth bricks.

Do not allow any Portland based concrete or perlcrete to freeze while curing.

Tscarborough 02-13-2011 11:41 AM

Re: Setting a perlcrete pad despite the cold
A gentleman came into the store yesterday, and I could tell just by talking to him that it was Rocko. I think we got him lined out now on the pad and insulation, and how to form it.

Rocko Bonaparte 02-14-2011 02:51 PM

Re: Setting a perlcrete pad despite the cold
Yeah it's a small world after all. When I explained on the other side of the counter that I'm a moron and I'm trying to make a pizza oven, I think he figured it out. ;)

I spent the weekend preparing the slab on top. I had a bunch of fast-drying concrete I mixed with some Maximizer, and I'm kind of wondering if it's already cured. It was real hard just from curing overnight. I'm thinking I'll keep the support underneath until Friday but I'll probably start screwing with it Wednesday.

Now I need to figure out what to do with that perlcrete pad I made. The first time I made a pad with 1:4 concrete:perlite, the edges broke right off when I lifted them up, but the core of it was pretty strong. I had to break the center up by lifting it up and dropping it down onto my patio slab. This new pad is 1:6 cement:perlite and might hold up a smidgeon better. I'm thinking of picking it up with a helper suspending it with two scraps of cement board for support and see if it'll survive the brief walk over to the concrete slab. With a little fresh perlcrete spread on the new pad, I am thinking I'll get the center part that matters to transfer and I can use a little fresh perlcrete to keep it all together.

I assume if that fails I can bust it up, crush it back up, mix it with a little fresh perlcrete, and just pour it on top like I should have in the first place. I just don't want to mix up a huge bunch of perlcrete again. And I certainly don't want to just dispose the perlcrete I've already made.

Tscarborough 02-14-2011 06:38 PM

Re: Setting a perlcrete pad despite the cold
You can use the chunks for the new pad. It only has to have compressive strength, nothing else. Put the big ones in place and place the new around them, plus mix the smaller chunks in with the new mix.

Rocko Bonaparte 02-17-2011 08:46 AM

Re: Setting a perlcrete pad despite the cold
Oddly enough, the perlcrete slab held up when I picked it up and moved it. Some of the edges crumbled off, so I can see the problems with using it to span a space. But I was impressed with how we could walk it over to the concrete pad and slide it up on there without the whole thing just cracking in half. I had spread some fresh perlcrete under it to help bind it, and mortared the sides by mixing up some cement with the sand I had used to help level that original perlcrete slab.

lwood 02-18-2011 05:00 PM

Re: Setting a perlcrete pad despite the cold
Now that's a first!!!

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