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Sand Particle Size for Cob?

Dustin Rhodes

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posted 7 years ago

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Hello Everyone,

I've searched through several forum topics recently, and have seen a lot of discussion about the Clay component of Cob, but not as much on the sand side of things:

What average particle size should the sand be? I know that ultra fine beach sand is not good, but how small is too small, or too big?

I have much Decomposed Granite (DG) at my disposal, with particle size ranging from 1/16" to 1/4" - should I sift out the larger stuff, or would it be okay to use as is?

Any answers or speculation y'all have would be most helpful.

Thanks, fellow Permies!

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Bryant RedHawk

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I shoot for around a 50 market mesh size which is .3mm or 0.0117 in. in size (play ground sand has most particles in an acceptable range of sizing for cob making).

63, 31.5, 16, 8, 4, 2, 1 mm, 500μm., 250, 125, 63μm. screen sizes are commonly used to grade aggregates in the construction industry in Europe.

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Daniel Ray

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Really any sharp sand will do the job. I've even done a few good shovels of gravel just to take up space in my cob for bench building. I think if you can still stomp it, it will work.

http://www.spiritwoodnaturalbuilding.com

Dustin Rhodes

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Thank you both!

Daniel: that is wonderful news!! Have you noticed any structural or cosmetic issues with the gravel (crumbling/excessive cracking)? What was the approximate ratio of sand and gravel? Were there any surfacing issues with the large gravel sticking out after trimming/shaping the surface, or did they push in easily? (or did you mix this separately as infill for the middle of the bench only?)

This idea is looking promising!!

Daniel Ray

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Dustin,

I did 4 buckets of clayey soil and 1 bucket of gravel. The clayey soil is 20% clay 80% sandy soil. I didn't think it would be the greatest for finishing anything so I only used it for the "inside" of the bench where it was surrounded by normal cob. It is completely dry and doesn't show any issues. I only did it because I had a small pile of gravel getting in my way, but I always bury garbage material in my cob. My bathroom wall has a broken waffle iron somewhere inside.

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Rebecca Norman

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You could try out a few cobs using different proportions of your available materials, let them dry, and see how you like them. Kick them, rub them, pour water on them, see if they crack while drying (though a drier mix may help), etc. That's generally a good way to prepare for earth building on a new site.

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Glenn Herbert

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posted 7 years ago

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Concrete is made with gravel for a reason - aside from economizing on the processed cement component, the aggregate makes the structure stronger. My native clay (glacier-deposited) is well over half sand and gravel and rocks; I just pick out the stones that are too big, depending on the intended use, and the rest is perfect cob. Getting a finished surface is the issue I have, and I do find that it is easy to push the stones that are too large into the mass while the cob is still pliable.If you are adding a harder finishing surface like lime plaster, having stones sticking out a bit (less than about half the desired finish layer thickness) gives good tooth to connect the finish layer to the base.

John C Daley

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Frpm my experience, if you have a grading of course 1/4 inch to really fine clay particles I would use that.
In Australia we often use a 1/4 inch screen to sieve soil for earth bricks.

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Dustin Rhodes

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Thanks Glenn, John, for your input; we seem to be building consensus here, and your experiences are very encouraging - when I'm ready to start attempting construction in a few years(small shed that doesn't need a permit) I'll make some test blocks with the DG just to confirm, but it looks like it should work well!

John C Daley

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What is DG?

Dustin Rhodes

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Decomposed Granite, sorry; certain types of granite in my area, when weathered, crumble into a VERY coarse sand.

john Harper

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posted 6 years ago

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I currently live among the loess deposits on either side of the Missouri River basin. Has anyone had experience making cob with loess subsoils? I understand that along the Yangtze River basin houses and other structures are traditionally excavated out of loess soils.

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