RMH Cracking after laying too much cob...

So caveat: please don't be too hard on me. I know.

I was building a RMH by myself over the last 5 months and it was becoming tedious mixing and laying cob alone.

I enlisted a buddy to help me and we attacked the RMH... we laid far more cob in 8 hours than I would have imagined. Probably a literal ton of cob +-200 pounds.

So as it's been drying it has also been cracking.... I'm thinking I will fill the cracks w/ a clay slip that has a little sand mixed in but hoping to find a good way to do that or other options at this point.

My wife would be very unhappy if I had to tear down the cob to start fresh so I'm going to avoid that. Full stop.

Thanks y'all.

Hi Chris;
What mix did you use to make your cob?   Three-part sand to one-part clay?
Also, it appears that you used all cob with no solid rock buried inside.
All cob will work but it does not have the heat-holding capability of solid rock, and it cracks.
Fill your cracks with wet sandy clay (sand helps with cracking).

What "kind" of sand did you use?   Local free sand?
The first RMH I built, I used free sand from the RR, very clean (no rocks) but very fine.
My cob was constantly cracking, and I was constantly filling the cracks with new sandy cob.
Now I use only a commercial graded medium builder's sand, apx $15 at Home Depot.
This creates a superb cob mortar that resists cracking and also looks great.

I  suggest using large rocks and graded sand to finish your build you will be very happy with the results.

Thank you so much Thomas!

I used not quite a 3:1 mix-- closer to a 2.6:1 mix

AKA a 13:4 I guess...

The sand I used is from a local cement and masonry place that the sand grains come in all sizes from small to large, it feels especially grainy/rough in the hand. I got 2 tons for $60.

Sounds like for the rest of the project I may want to just go get that medium grain... I think I will.

Are you saying there is a decent chance the cracks did not come from doing too much cob at one time?

& I did line bricks and rocks all through the middle of the cob. I just didn't take pictures of it....

Thank you again so much and Cheers!!!

That is a LOT of shrinkage. Your clay must be really fine and pure... what is the source like? My native clay is glacial till, around 30% clay/silt, 30% sand, and 40% gravel to stone. For mass cob, I just pick out stones bigger than a golf ball and go. I do use some straw away from high-heat areas, which also helps minimize cracks. I get hairline cracks every 6" to a foot or so, in general, less where the cob was applied stiff. How wet was the cob when you built it?

Not that it helps now on the existing cob, but I would mix quite a bit of gravel (rough or crushed would be best) into the sandy cob, and save the finer mix for the last half-inch or inch on the surface. Apply that before the mass cob dries out, and wet the surface to get a good bond. This in addition to using as many rocks in the internals as possible.

Assuming you have not yet reached the absolute finished dimensions of your cob, once the mass is thoroughly dry it will not shrink more and you can hide the patched cracks with a smooth finish coat. If you have cob attached tightly to the hot inner surface or combustion core, you will get some heat expansion and contraction which may cause surface cracks to appear when it is hot. For this reason as well as keeping the core hot in use, I would never apply cob directly to the core brick, but put a layer of somewhat flexible insulation around the core first.

Ahh, excellent that you have stone in the mass!
I highly recommend using the graded sand, you will like it.

When I built the mass on my studio rmh, I was lucky enough to have access to a large amount of slate.
I created a "cob lasagna" by layering the stone and filling any air pockets with cob.  I would place apx 4-6" or 2 layers a day.
Working alone, that was more than enough work and it allowed some drying time between layers.
Plenty of builds have had large amounts of cob applied quickly.

In my case, I choose to enclose the mass with clay brick.  
More mass, and I think it looks better, also cracking is a non-issue.
 

In case Thomas' advice on the cracks wasn't clear enough, the filler would work best if it is very sandy with just enough clay to hold  it together firmly. That will minimize shrinkage and cracking of the filler material.

The sand you describe with a range of sizes sounds like a fine material; you just need to use enough of it in the mix to reduce shrinking, along with plenty of larger stone and some straw so that cracks don't open wide.