I'm getting closer to finally building my RMH. My mass will be "framed in" using bricks, I'll fill the void of the mass with 8 inch pipes and pea gravel. Cob is not an option for this build. For the top of the mass I'm looking and making a 2 inch thick concrete countertop type thingy.
Hi Rich; As long as that concrete is far enough away from the core it should be ok. Might crack on you.
Why is cob not an option ?
If it is because of the availability of clay. Then you should know , that if your mass is contained you can fill with any old dirt. Just don't use new dirt !
No need for clay and dirt will hold heat better than pea gravel.
Originally, I stayed away from this thread because I know nothing about rocket mass heaters, but I have poured a lot of concrete countertops. Assuming, like others have said, you stay away from the intense heat, I see no reason why it should not work.
My issue with concrete countertops have always been with coloring the concrete. Coloring dyes just do not work, paint mixed in with the concrete can work, but a person has to go really dark, because the concrete will end up far lighter (hunter green will end up pale green for instance). Painting after the concrete has dried is okay, but expensive, and often peels.
As for the actual method of construction, I have done pre-cast, which was when I measured carefully, made a form and then poured and transported the concrete slab into place, but I will never do that again. It was way too much work. Since then I have formed and poured in place (but with the understanding this was traditional countertops) and it was much, much easier. Reinforcing wire was used in place, rebar, etc then poured 1-1/2 inches thick, formed to have an edge.
Finishing can be a bear, it is concrete after all, but there is a trick to make mistakes go away...Bondo! If you read the can, yep right there on it, it says it can be used on concrete. That can make rough areas smooth again.
To get "high strength concrete" just use more cement in your mix. I used sand for an aggrgate in my mix, then added more cement to make sure it got around all the particles well, and super-mixed it.
Again, all my projects have been for counterops, but here is a picture on one. (I tried to find a photo without my wife in it, but they were all yellow-looking photos for some reason.)
I'm gonna throw this out there.
Use RapidSet CementAll as the cement portion and an aluminum oxide or garnet blasting sand as your aggregate.
Add handfuls of rockwool into the mix to resist cracking.
Now use it wherever you like, heat be damned, and you should be fine.
I've used this mix, plus perlite to form the feed, burn tunnel and riser on a j tube rocket stove.
It's very durable, resistant to heat an physically strong.
The addition of perlite makes it physically weaker,not strong enough to support weight over a large span, but insulative and easily worked with saws and drills.
My under construction batch box is made of slabs of this mix.
I left out the aluminum oxide sand, they seemed superfluous.
RapidSet CementAll isn't officially a refractory cement.
It is an Calcium aluminate cement, rated to 2300 degrees in at least one official msds document,and about $20.00 a bag Vs. 50 bucks for "real" refractory cement.
It sets up crazy fast which is either a bug or a feature...
With all that said, I have also had great luck with cob and/or soil cement over steel mesh, wires shelves, or sheet metal.
I recommend screwing some hardware cloth to the sheet metal to create a bond.
I plan on topping some of my own bells with aluminum sheet pans from the scrap yard, filled with a CementAll mix, topped with soil.
I want to be able to get back into the bell for inspection, and each of these will form a large access panel.