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Help! Straw Substitute?  RSS feed

 
Conner Choi
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Hi there,

I'm building a cob house and I have plans and everything but I'm having alot of trouble finding the right kind of straw this time of year. I've read that rice straw and many other straws are unsuitable for cob construction because they lack the tensile strength that is present in barley, oat, and wheat straws. Can anyone validate this? I have alot of long dry grasses on the property I'm building on, would this be suitable?

Any help is appreciated.

 
Bill Bradbury
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Location: Richmond, Utah
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Hi Conner,
I can't say for sure, but you could make a test wall and find out real easy. That's what I like to do even if you have everything worked out, just to be sure.
 
Conner Choi
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Thanks! What are some solid testing methods I could use?
 
Terry Ruth
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Conner Choi wrote:Hi there,

I'm building a cob house and I have plans and everything but I'm having alot of trouble finding the right kind of straw this time of year. I've read that rice straw and many other straws are unsuitable for cob construction because they lack the tensile strength that is present in barley, oat, and wheat straws. Can anyone validate this? I have alot of long dry grasses on the property I'm building on, would this be suitable?

Any help is appreciated.


I do not see in strawbale building code that rice is excluded, meaning code provides a "prescriptive path" that has been tested and passed the test of time: http://thelaststraw.org/wp-content/uploads/IRC_StrawbaleConstructionAppendix_Approved_10.4.13.pdf

AR103.7 Types of straw. Bales shall be composed of straw from wheat, rice, rye, barley, or oat.

AR103.8 Other baled material. The dry stems of other cereal grains shall be acceptable when approved.
by the building official.

It is the highest silica content you want, hemp is very high 80%+. Silica composes 71 percent of the ash of Equisetum telemantia, 51 percent of the
straw of barley, 67 percent of the straw of wheat, and 46 percent of the straw of oats . The native grasses and weedy grasses as well as the cereal crops have varying amounts of silica.

If you wanted to spend the money you could have atterburg soil test, or if do you know how to do a jar test to get an idea of how much binder is your soil that will determine how well it bonds to straw or grass? If it is low you can add cements like fly ash, lime, portland cement to stabilize it and made it stronger. You could take a sample you make in PVC pipe about 5-6 inch in diameter to take to a lab for compression testing if a code official wants it to satisfy a building permit. Or, if no building codes just make up a wall and stomp on it a few times you'll get a feel fast for how strong it is and whether it cracks. Test some sealers on the test sample too or any other renders you are planning on.
 
Katy Whitby-last
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Location: North East Scotland
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I wondered wether we might be able to make use of all the soft rush that grows on our land for when we do some cob construction. How can I find out it's silica content easily?
 
Terry Ruth
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Katy, I'm not aware of how to get silica content outside of going to a chem lab. I didn't mean to say silica content is everything but, it is responsible for tissue strength or tensile we want alot of. Hemp also grows tall, in sand little soil, little water, no or little pesticides or irrigation, it is just a super hardy and strong plant all around.

The easiest way is to try and tear or pull it apart and note it's resistance compared to other straws or grasses. When surrounded in soil it got it's silica content from it will form a chemical bond. The soil or clay in COB is the glue or binder, the fiber or what holds it together and resist cracking is the fiber (chopped straw, grasses). If it is a weaker tensile straw/grass keep it long, add more clay content or other cements until a sample test looks good....Go with that or take that to a lab for compression, tensile, shear test if you really want to know. Labs can give you all the "mechanical properties" and "thermal properties" of a mix. When I run test I document my mix ratio's. I even take pictures of the mix in the dry state. If it don't work back to the drawing board ..Lots of variables that are difficult to quantify without the proper tools.
 
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