Cracking cob

Hey everyone,
I'm new to building with cob and I love it so far but I'm running into a few problems. I'm currently build my chicken coop with cob as a test. I started just using the clay soil I have, it pretty much just red clay. Instead however that if you don't add sand it will crack. Well I decided to start adding masonry sand that I had to the kix. Well now it looks like that mixture is cracking quite a bit. I'm not too sure if the mix with no sand will crack as it hasn't fully dried, but so far it shows no signs of cracking. What am I doing wrong, not enough straw?

Hello,

For one thing your cob + masonry sand could be causing your mix to dry to fast. Consider watering your creation to slow down the drying speed.

The other thing I wonder is have you added straw, this gives cob more structure and helps with the consistency and over all performance even when drying. You didn't say if you added any carbon material - cow/horse manure, straw, wood chips, etc.

Maybe post some pictures....

All the best,

Yes I have been adding straw, right now I'm going to go back to not adding sand, I weigh 220lbs and I put all my weight on it and didn't even act like it wanted to give. I'm also posting a couple of pictures. Maybe the cracks are no big deal just don't have much experience.

Jami is correct you are cracking because your soil is too expansive, mix has a low plastic index and high moisture content which is good for hygrothermal mass and bad for structures, high expansion/shrinkage, and you are too thin with weight issues. I did not look at your climate but if marine or cold(freeze/thaw) it will only get worse. Cut back on the soil content, add more aggregates. Try adding soil stabilizers such as Type S lime, OPC and/or or fly ash or other pozzolans..start with around 10%. Sounds like you got some decent compression strength but do not be fooled by it, it is still very low @ 225 PSF for a wall. Some framing members in a home design is a good idea unless you are a COB pro backed by a soil geotech test lab and PE.

Ah well that sounds about right for me. So use less clay and more sand. Basically I have doubly useless soil. Can't grow anything on my clay and can't build anything with my clay. Thought I'd found the perfect building material. I do live in Louisiana so we don't really have winters per se.

Roman, we have that red dirt close by me in KS in OK, it holds massive amounts or water/vapor sticks to everything, can't get it off your car easy. Don't get me wrong the "glue" "binder" property you want but not too much. Your too thin too the weight of the COB will crack it in shear. I see alot of verticals. Start at around 12", 300MM.

So let me see if I understand better. Use about 10% of my clay to 90% of the masonry sand and straw and make my walls a minimum of 12" thick? Even for a chicken coop. Or are you saying I can continue how I'm building as long as my walls are 12" thick. Good thing is i started my coop as a pole barn style structure so i dont have framing for it to be formed around. Sorry I'm not exactly an engineer but I appreciate the help. Also what's a good way to make sure I'm only using 10% of my soil? Maybe I should try my other clay soil i have. That layer of clay isn't very thick after that is a light tan Sandy clay.

Yes you did the correct thing start here. Most use a 1' square 2x4 test frame to get the mix right. Cut back on the red dirt, at least do a jar test to see how many fines are in the soil and how much clay....that should give you an idea or %s. Add some lime or fly ash, xperiment. Then build. The norm is around 10% clay to 70 aggregate BUT that is just a guide. If you want less expansion/shrinking use a different binder like lime or fly ash, that is IF your aggregates do not hold together well with less clay. Play with the thickness as another variable. Make sense?

Yes that makes sense. I did the jar test on it and didn't really see any lines. All I saw was some sediments in the bottom and the rest of it looked like clay water.

There you go too much red clay. You know now what to do

Yea just sucks cause I didn't want to spend any money but guess I don't have much choice. Where do you get lime or fly ash whatever that is.

Type S lime is cheap, less than $10/90 lb bag. Fly ash I get free at my local utility company. It is kiln waste you recycle into your build. Check at building supply houses, very cheap. If you cant find them try 10% white portland cement very cheap $10/90 lb bag. Fly ash makes PC stronger try both if you can.

Type S lime contains more magnesium and calcium it is best. OPC (portland cement) pollutes our atmosphere from kilns, FA is a by product, so does lime if you get down to it, but the contents of either are excellent binders.

Cost a little more but if you can find OPC with chopped fiberglass fibers (usually less expensive s-glass vs e) that will add alot of compression strength and take care of the expansive clay binders. Pros can build COB structures with FRPs, FG or bassalt chopped/rebar composites reduce the soil issues that require no framing and last the test of time.

I have so much stuff in my head I can't remember it all at once. Another thing you can do and probably should if you cannot get rid of the cracks is add a lime plaster or wash to the exterior and interior. That will do alot, and/or add a siloxane sealer. You can use earth or iron oxides for color and/or natural striations like rammed earth very cheap. It will look and perform very well. As Jami said wet the lime so it adheres well and cures over a 30 day period. Lime self heals cracks naturally itself as it absorbs CO2. If it is very humid out when you do this use a type SA or N lime. 24-36" overhangs and 1/8 - 1/4" per linear foot inch grade.

Hope that helps and good luck keep up the good work, trials and errors will get you there

OK let me understand this. It's better to use my masonry sand than type s lime or fly ash? So I'll use that until I run out then I'll either get another load of sand or buy type s lime or fly ash. Im understanding correctly that type s lime is the type you use for lime plastering right? And what do you mean 24-36" overhangs? Like my roof over hang? And go up by 1/4" per foot when applying cob?

Watched some videos and read a little more and everything is starting to click I think. Cob is very hard when dry, so in my mind cob was more clay than anything. Actually cob needs to be more sand than anything else. Cob gets its strength from clay and straw. The clay does nothing more than sticks everything together. Have I understood this correctly?

Right you just want enough binder to hold the aggregates together. Your clay type is super glue you don't need much. I'd keep going by reducing the clay see if the cracks go away. If that does not work go get a bag of type s lime yes for plastering and white portland cement with FG if you can afford it and add it more to the surface layers. To cut cost you can also combine the two with a total percent starting at around 10-20% of the mix on the surface around 2" in depth. If you cannot afford the FG just find a white one. They make gray if you want a deeper color. You can pigment the white with iron oxides sold at lowes or home depot too. This is where you can get creative with the surface. You can also use different sand colors towards the surface. If you can get the FA free try it, it can be used to reduce or eliminate OPC some don't like. Lime and/or OPC on the surface should do it as a plaster. Another option is add some lime to water and spray it as a wash that can also be colored. Read up on that.

While you are experimenting take the roof overhang out 24" past the wall to start and grade the ground away from the wall 1/4" per foot so it drains. Take some water to the roof see how the wall performs. Water should bead upon the wall if not go to a concrete supply house and get some Siloxane with 40% solids spray it on the wall with a pump sprayer see if it sticks and beads up with no erosion.


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I called my local feed store and they only have hydrated and non hydrated lime. Would either of those work as a plaster?

You are better off going to a home depot or lowes,etc, and getting a plaster type s lime. Those high calcium limes are more for agriculture and are not as strong w/o the magnesium.

You might try them in your garden to raise the PH or gypsum for high clay contents like yours check the utility company for it free too.

OK well I'll try the hydrated lime first, I went ahead and got some and the guy said that's what people use it for so I'll give it a shot. Now applying it, can I apply it to my newly built cob wall or is there a waiting period of like a month? And every video I find they say you have to slake the time for at least 24 hours or more. What is slaking? Just soaking it in water? And I got my fingers crossed but I dug a little deeper and got passed the red dirt, and hit some light greyish Sandy clay. After I saw that I realized I had a big pile of that stuff and went to go look at it dry. It had very small almost of cracking and it feels to be comprised of mostly sand. I did the jar test and it had about 1/8" of clay and the rest looked like sand. If it had that much sand in it I would've that you couldn't mold it very well, but it retains its shape quite well.

Why not give it a try, who knows perhaps your walls might grow some hemp or a close by relative and you can use it instead of straw . We use to have a lime expert out here named Bill Bradberry goggle some of his thread, he lived in a COB home too. Getting a little out of my league I'm more of designer than tradesman. From my conversations with him you want to install it on a wet surface and keep wetting it since it gets harder as it hydrates or absorbs CO2. He said 2-4 weeks wetting twice daily that is why I don't use it often. You are mixing in soil and using it as a earth_lime plaster the test will be how well it adheres to the COB you have. So I'd say keep it all wet a while using a hose.

I didn't know you used top soil, you want to get your soil from down past the top soil.