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Straw Vs. Sawdust  RSS feed

 
James Judd
Posts: 1
Location: Dearborn, United States
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Hello,

My name is James, and I am getting ready to build a cordwood cabin. Instead of traditional mortar, I was thinking of using COB. I live in E. KY, and my question is this: When making COB, can the STRAW be replaced with SAWDUST as the binding agent?

Any help is appreciated,

James
 
Ben de Leiris
Posts: 61
Location: Hinesburg, Vermont
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James,
The cob is definitely a good "mortar" option. As for the sawdust...I'm on the fence. In a traditional, solid cob wall, I'd say no for sure. The whole point of the straw is not a binding agent as much as it is tensile strength. Conventional wisdom says to use "long, strong" straw, because you want those long fibers to "knit" together the otherwise relatively crumbly cob. Since sawdust is just that - dust - it does nothing to tie together the cob. Depending on the quantity used, it may have some effect on the insulative value but would have negligible (or maybe even negative) effect on the strength of the mixture.
That said, I'm not sure that in cordwood construction people use any kind of fiber reinforcement at all. The cob in that case is a mortar after all, not really structural cob. I mean, in a brick wall, there is no fiber in the mortar. I haven't done any cordwood so don't quote me on it. But I'd guess that you'd be fine with strawless cob here.
Put up some pics when you get started!
 
Bryant RedHawk
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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In most cordwood builds there are two rows of mortar (cob in your case and some others) between these two rows, sawdust is usually used for insulation.

Cob that is made with straw will tend to be strong and hold together. Cob made with sawdust will not, as has been mentioned by Ben, have much tensile strength and that means it will tend to crumble.
I've seen traditional cob (made with clay, dirt, sand and straw) used for cordwood building. I've also seen this Trad cob used in cordwood where the builder used "spray foam" for the insulation (it also did an fine job of holding everything together).

I would suggest doing a test wall, that way you will know if using sawdust in your cob will work the way you want it too or not. By doing a test wall, if it should fail, nothing is lost if it should not fail then all is well and new knowledge is gained.
 
Terry Ruth
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I cannot be specific without seeing a soil test and properties but in general adding saw dust to the mix at the wood boundary, a 1-2" layer would help transition the galvanic reaction between the clay binder and wood. I'd start with around 30% of mix then reduce it to 10% at the core, especially if the clay type is a bentonite. It would also provide better quasi-isotropic strength than isotropic straw does not at the boundary for fatigue and flex. I design my concrete foundations with these combined mechanical and physical properties. We (myself included) have been doing the same in composite aircraft structures for decades with unsurpassed proven success.

The best way to reduce cracks and swell/shrinkage is to control the plastic index and flexure property of the mix saw dust will help, bigger fibers less dust better. People throw the "tension" word around that know very little about structures and what properties to look for where. Tension is DEFINITELY not the only mechanical property we use straw for or any other structural reinforcements. For more accurate advice ask a PE.

James, you did use the proper terms the straw is the "binding agent". The silica and magnesium in it binds to the same in clay/soils. Hemp due to it's high silica/mag content binds the best. It also has better mechanical properties. Different species of wood and it's content determines the chemical bond to different clay binders.
 
Ben de Leiris
Posts: 61
Location: Hinesburg, Vermont
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Lots of good points by Terry. In addition to controlling the quasi-isotropic plastic index, you should also calibrate your transitory particle interceptor, or you'll end up with an unwelcome subharmonic oscillation.
Anyway, James, good luck building your composite aircraft structure... I mean cordwood cabin.
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Terry Ruth
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LOL! Funny Ben, I like it thumbs up!! I'm use to people I prove wrong or can't keep up technically with nothing else to post. James follow my advise use the saw dust as I described and like my homes you will have one of the best designs out there. Most of the building industry has been trying to catch up with aircraft for decades with limited success that is why there are so many issues. I get a kick out of the "aircraft quality" sales hype the manufactures use

Then again plastic index and flexure strength properties of composite mixes is nothing new to this industry, just to some that do not understand it.

Screen and separate the material, use the larger particles on the wood and the smaller at the core. THE BEST thing about natural building and composites is we have control over the physical and mechanical properties at any given location.

 
Ben de Leiris
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Well, I'm not on here with the goal of proving other people wrong. I enjoy the forums because people can share their knowledge and experience, and you don't need to be a PE to have a legitimate opinion. In fact, the point I was making is that often your posts go way above the technical level that the average owner/builder is likely after, especially when it comes to natural building. There's nothing wrong with more information if it's relevant to the conversation, but I for one get turned off by all the condescension and self-promotion.
Sorry for the negative detour, James
 
Terry Ruth
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James, Permes, I forgot to mention along with adding the saw dust I described above to the clay in this design do not add straw to the clay. Place the straw with the tubular length along the same direction as the chord wood grain direction so it breathes and handles capillary drainage. This will also provide uni-directional strength along the length, the clay saw dust provides it in multiple directions, the two combined is the best structure known to mankind today.

Engineers are a LARGE part of the manufactured industry, some good and bad building products we use today globally, natural building is no different and actually more complex, many require a license to design for obvious reasons.
 
Rebecca Norman
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Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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Here's a reply from the low-tech practical experience end of things.

We've built lots of buildings here in the Indian Himalayas (Ladakh) with what I guess should be called either "formworked cob" or "packed earth" without any organic material, and also some with wood shavings and sawdust.

The plain cob, a mix of sand and clay (actually technically silt but it worked fine) made very strong walls that hold lots of weight, have survived 20 years just great, have proved to be almost as hard as concrete to break when needed, and do reasonably well when built out in the elements without cover or plaster.

In the past 5 years we've also made some walls for more insulation, where we mixed a lot of wood shavings and sawdust in. Straw is not a waste product here, it's fed to cows, so we didn't have much straw. For a room to hold our water tanks above freezing, we made the new north and west walls out of form-worked packed cob with a hefty mix of the only organic material available as a waste produce here, wood shavings and sawdust from the local lumber yards. It is distinctly less strong than the plain earth mix, but since it's a low roof, a small span, and nobody sleeping under it, we don't mind. It was left unplastered for 4 or 5 years, and was pretty crumbly; if a person stood next to it talking and idly digging with a finger, a bit of damage could be done, though it did eventually reach a tougher layer inside. We finally plastered it last year, so I guess that will help with this kind of erosion.

We also made 2' x '1' x 6" bricks out of a similar material last year and made the three outer walls of a new solar house. Since the wall is 2 feet thick, has a concrete tie beam above, and the house is only one story, we're using it as a residence, with people sleeping underneath.

I don't have a conclusive answer for you, except that in my experience, adding sawdust and wood shavings makes a wall less strong, not more.
 
Ben de Leiris
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Rebecca Norman wrote:... in my experience, adding sawdust and wood shavings makes a wall less strong, not more.

That's been my assumption, without having personally tried it. I can't imagine in what way sawdust would add strength, and it doesn't surprise me that it actually makes it crumbly. Maybe wood shavings, like from a planer, since they have some length along the grain, but I doubt it would work like straw. As for straw, in my experience with cob, a lot of the benefit of the straw comes in keeping the cob from slumping as you build the wall up. I've often thought that once dried, the cob would probably still be rock solid without straw. If you have a solid foundation, cob walls won't be going anywhere. Any if they do want to go somewhere, well I don't know that a little bit of straw is going to stop those heavy suckers. (Bottom line: make a good foundation!) Many forms of masonry have no tensile reinforcement, like adobe, or brick, or rammed earth. It sounds like your buildings are essentially rammed earth. And it sounds like in your experience, they are fine without straw! One time I did demolish a small cob test wall, only 6 inches thick, and I have to say that thing did not want to break up!
Are you familiar with Jhamste Gatsal? It's in Arunachal Pradesh, so not really near Ladakh at all but just wondering. I had some friends over there last fall building a cob house with House Alive.
 
Robert Alcock
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Location: Cantabria, N Spain
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I would say that straw/sawdust is not an either-or issue. They do different jobs in a cob mix.

Straw (or other fibre) is for tensile strength and I would say is necessary in any cob wall.

Sawdust (or other lightweight additive) doesn't add tensile strength but improves insulation value and reduces weight, possibly at a cost in compressive strength. OTOH if you have less weight, you need less compressive strength, as much of the weight the wall needs to hold up is the wall itself.

In the wall of our cob house, we replaced half the sand with wood shavings (basically coarse sawdust) to improve insulation value. So the mix by volume was 40% clay soil, 30% sand and 30% wood shavings, plus straw -- compared with a normal mix of 40% clay soil and 60% sand that we used in the cabin we built first.

I did some basic compressive strength tests (measuring the load required to crush various bricks of each type) which indicated that the compressive strength of the sand-sawdust-clay mix was not significantly different to the normal sand-clay mix. (See photo of compressive strength test. Please note the highly sophisticated and expensive scientific equipment.)

This result was good enough for me, so we built the main wall using this mix. It's held up extremely well, is definitely warmer than standard cob, and has an added minor advantage over the standard mix, that you can drill or nail into it. Only disadvantages are that it dries more slowly and shrinks more when drying than a standard cob mix.

Good luck,

Robert



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Compressive strength test of cob bricks
 
Tom Turner
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Location: high desert and mountains of Idaho and coastal Atlantic Canada (migratory)
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It was my understanding that in building stack-wood that one would mortar the exterior third of the log and the interior third of the log and then leave the center third hollow as a dead air space insulation -or- fill with sawdust insulation.

If you have sawdust available that you want to use, if that is the motivation of the post, do this, but I think I would follow the advice and seek greater structural strength for the mortar, i.e. straw over dust.
 
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