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KEmerson 01-19-2009 04:31 AM

concrete weight
is there a rule of thumb or real numbers for how much Portland with Vermiculite & Perlite mixed in? I know from reading here that some people use one or the other or both and to a slightly varying amount per. I know much depends on this or that... I just need a ballpark estimate. How heavy my Portland/Verm./Perl hearth is is important information for my project. What's a cu. foot going to weigh?

dmun 01-19-2009 06:14 AM

Re: concrete weight
I think, anyone correct me if I'm wrong, but the weight of the vermiculite concrete is going to be the weight of the dry ingredients. Portland cement weighs 94 pounds per cubic foot. I can't put my finger on the weight of vermiculite: it's going to be about a quarter of the shipping weight of a four cubic foot bag.

And no, I've never heard of anyone mixing vermiculite and perlite for insulating concrete for an oven project.

KEmerson 01-19-2009 06:40 AM

Re: concrete weight
So the water weight doesn't factor in at all? I suppose that makes sense. I hadn't thought of that. Thanks. (Never too old to learn!)

But apparently I'll need to do some more reading as I thought I understood that some people mix Perlite & Vermivculite with the Portland. Is it an either/or and what's the difference? Why one over the other? I'm at the gathering information stage and a lot is being read on the fly. Details like mixing or not mixing all three has so far not been the focus. But how much this thing is going to weigh is a focus for the moment.

Archena 01-19-2009 07:16 AM

Re: concrete weight
Um, how fast does that stuff cure? Water weight should be an issue during curing because the water is still present. Concrete isn't known for fast drying - does the perlite alter it some way?

=Disclaimer= Utter non-expert here.

gjbingham 01-19-2009 07:48 AM

Re: concrete weight
I believe the difference between the perlite and vermiculite are negligable. I think dmun's correct. The four cubic foot bag of vermiculite weighs only around ten - fifteen pounds.

Just curious why weight is such a worry. You've got a lot more weight coming before you're done.

Breven 01-19-2009 08:05 AM

Re: concrete weight
They don't look the same, but I understand the charachteristics to be the same. When I poured my insulation hearth, Home Depot only had a couple of bags of vermiculite left- so mixed in Perlite with it. Vermiculite is a tan color and almost looks like wheat or some sort of grain. Perlite is white and looks like tiny styrofoam balls.

KEmerson 01-19-2009 08:47 AM

Re: concrete weight
Weight is a concern because I want to build into one of those half school buses.

Read here

Jed 01-19-2009 10:25 AM

Re: concrete weight

After a short career as a long haul truck driver, I have developed an eye for safety on the highways, and your concerns about a converted school bus having the capacity to carry the weight of a WFO is well founded.

My first concern would be the capacity of the vehicle, and the weight of not just the oven, but everything else that goes with the project. To get any kind of a kitchen certified by the authorities to provide food to the public, you will have to have a way to wash: hands and utensils at a minimum. Add water weight (for washing) to the wood and the oven, along with any counters, refrigerators, and miscellaneous stuff (like food), and it doesn't take much to overload a vehicle, and that puts everyone at risk... For me, the biggest concern is the ability to stop. Go a slow as you want, but when it is time to stop, it is time to stop, and if the vehicle can't stop, that is a design (or maintenance) issue that can and should be avoided.

With this in mind, I would advise you don't use a perlcrete, or vermicrete in your design. Spend the money and get the good stuff; the lightweight board insulation for under the hearth and the bat insulation for around the dome. This will save a bunch of weight. Consider using a refractory kit oven. I haven't done the math, but these will probably weigh less than a fire brick design, and they may tolerate the rigors of travel better than fire brick.

Consider building the oven on a trailer. There are several examples on this web site, and in Seattle there is a company that has a trailer unit they haul around to cater at events. This takes the weight off the bus, and onto another vehicle. You can equip the trailer with brakes to help with the work of stopping the parade. As you know, working with wood to fire an oven is messy in the sense that wood is always sloughing 'dirt', or wood chips, or bark or whatever. The point is that it is messy. Having the oven on a trailer will help keep that mess out of the kitchen, so to speak...

Good luck with your project. Lots of reading and research at this step of the game will save time and trouble later!


KEmerson 01-19-2009 05:40 PM

Re: concrete weight

Originally Posted by Jed (Post 49298)

With this in mind, I would advise you don't use a perlcrete, or vermicrete in your design. Spend the money and get the good stuff; the lightweight board insulation for under the hearth and the bat insulation for around the dome. This will save a bunch of weight.

Except for structural support, why do I need a concrete hearth? If I can insulate (can I?) well enough and still support the dome, floor etc. must I have a concrete base at all?

dmun 01-19-2009 07:21 PM

Re: concrete weight
I work with metal for a living, and sheets of metal do strange things when they are exposed to heat, particularly on one side. The term of art is "potato chipping". Metal comes out of the rolling mill hard on the outside and softer on the inside, much like bricks do. A steel base will need lots of cross members to keep it rigid, and it wouldn't hurt to send it to the heat treaters for annealing, just for stress relief.

It's also worth mentioning that brick built ovens don't deal with vibration very well. Most of the successful mobile catering ovens are modular ovens: stronger, and fewer elements to vibrate apart.

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