Concrete countertop for RMH?

Hey All,
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.

Here's a youtube detailing the process of making a concrete countertop
https://www.youtube.com/watch?v=TUAPO8otVrY

I'm posting here to see if anyone out there has done this.  And is there any reason why I would not want to do this?

Any and all feedback is appreciated.
Thanks
Rich

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.

All that work in the video making a mold and he pours in bagged concrete.

What you want to do is GFRC. It will be stronger, lighter, denser and no holes to fill in. Spray on the face coat then hand pack in the backer.

GFRC does not use the coarse aggregate you find in common concrete. It is a 50/50 mix of cement/sand with other ingredients...alkoli resist fiber, pozolin, polymer, plasticizer, etc....

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 agree with travis. They can be completely different in color/blotching than you thought and planned. Same with the finished surface. The more you sand/polish the more air bubbles that are exposed.

Thats not a bad thing depending on expectations. If you expect a super smooth top like you would get with marble you will be disappointed.

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.

William Bronson wrote: 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.

A couple questions for you if you happen to see this. I just fired up my newly built rocket mass heater. It has performed admirably, although, the concrete countertop that holds the cooktop just failed this morning. It was not unexpected but did happen sooner than I anticipated. It was the second small burn since completion. I used RapidSet Mortar mix for the countertops. It states on the bag "not intended for extended exposure over 212*F," just as the CementAll states. I couldn't find the document you mentioned with the higher temp info.

Do you know if this mix, RapidSet Mortar Mix, would work the same as the CementAll if amended in the way you recommend? What are the ratios of CementAll, blasting sand and Rockwool? Say one 55lb bag, to ?lbs sand, to ? handfuls of shredded Rockwool? Also, does the Rockwool appear in the finished surfaces; I'm guessing it does, but wonder how badly?

I didn't add any extra aggregate to the Mortar mix and they turned out nicer than I expected. I happen to have a bag of Mortar Mix left and a batt of Rockwool and would love to pour a new top so I can continue to cure/dry out the stove. Thank you for any thoughts you might share!

Not only did I not finish that project, I've not been able to find that initial document.
I would not pursue using cement all with expectations of success.
I will continue to mess around with it because I don't need success, I need dopamine...

But your results might actually dissuade me.
For what it's worth Cement All is the Rapid Set product that has the largest portion of calcium aluminate cement, which is the heat resistant portion.

I've moved on to stainless/steel in places where it can radiate heat on one side.

You might want to try glass cooktop from electric stoves.

It would be fun to find out of Rich, the op, made the RMH and the concrete countertops.

I love these older threads.

I don't have concrete countertops though I feel they are neat looking.

Thank you for the thoughts William. I protected the concrete with a layer of super wool and it seems to have improved the situation. There are three slabs in total on top of the stove, but my only concern is with the one directly over the top of the firebox core. Since I have the materials necessary, I plan to pour a new slab for the area, let it cure much longer and create space for a solid and sealed layer of cfb to encase the countertop edges. She's fired up and roaring right now, so it looks like I have a bit of time to see if there are any more issues. I'll report back with any useful info.

P.S. This is a Continental Walker inspired cook stove. Thanks!