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Difference between Cob and Adobe?

Chelle Lewis


Joined: Dec 10, 2009
Posts: 417
Location: Hartbeespoort, South Africa
    
    1
I went looking and found this:
Adobe is a natural building material made from sand, clay, and water, with some kind of fibrous or organic material (sticks, straw, dung), which is shaped into bricks using frames and dried in the sun.
Maybe it is cob when applied freehand and adobe when baked as bricks in the sun first..... Or cob is sand clay and only straw...  not just any fibrous material...  ?

Just interested. Some people recommend one and not the other... and doesn't look like a big difference to me.

Chelle
Glenn Kangiser
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Joined: Dec 31, 2009
Posts: 236
Location: Central California
If using just the natural resources on your land and you had clay without enough sand content ie: 30% clay 70% sand is ideal for most types of earth building, then it may be better to make your soil into adobe bricks.  Without enough sand and aggregates, shrinkage will be a lot greater, therefore the bricks may work better.

Straw is your rebar and holds things together and if you were short on sand and aggregate, more straw could help in making the cob work without as much shrinkage as well as better reinforcement.

Cob is a monolithic structure usually with rounded walls and reinforced with straw,  so it is immensely stronger than adobe brick walls and done properly will resist even large earthquakes.

In India they have developed methods of making mud bricks with interlocking sides and laid diagonally to resist earthquakes quite well also.  They have also developed a method of putting a waterproof coating on the sides of the brick to prevent water damage... concrete and or rock chips etc on the exposed edge.


- Glenn -
Chelle Lewis


Joined: Dec 10, 2009
Posts: 417
Location: Hartbeespoort, South Africa
    
    1
Thanks. It does sound appealing enough to give it a try.

When you say straw ... can that be bushveld grasses?

And sand.... can this be river sand? It is not fine like beach sand. A bit more rough.

I have a very sticky clay-type substance under river sand down by the river... have to dig down past the river sand. So maybe 30% of this ... and 70% river sand... and grasses that grow naturally here. Would that count as a cob mixture? I have a sense it would harden pretty well if compacted. Is it possible to make a dome roof with this or will it not hold up? Sort of mound up some wet river sand into a size enough for rabbits to rest in.... as a mould.... and then pack this mixture over and compact down and leave to dry with an igloo entrance.... then dig out the river sand when it is dry and hard. Do you think this could work.... or collapse? Not too high... less than a meter including the cob.... half a meter high inside.

Or do you think this is better done as adobe bricks?
Glenn Kangiser
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Joined: Dec 31, 2009
Posts: 236
Location: Central California
Yes, Chelle - I think it would work fine.  It is common for the sand and clay to stratify separately and they should make cob just fine I think.  At the very least your material bears experimenting with.  Bushveld grasses, it sounds like you are not from around here.   Africa?

The river sand and gravel will work fine.  The clay will have to be experimented with.  The organic clays near the surface are not as good.  Learn all you can about their properties through experimentation.  I call it "becoming intimate with your natural resources."

Grass after it drops it's seed generally always makes straw that will reinforce cob whether it be fine grass straw or coarse as /in Rice straw.  Take one piece of grass - pull on it try to break it.  5 lbs 10 lbs 50 lbs of pull?  Now multiply that times hundreds of pieces of straw in your cob and you will see where it gets it's strength.  Some people have used vines and other things.  Native Americans used sticks and twigs embedded in mud to support second story floors.

Key to making this work reliably is keeping water off of the top of it.

I would say you could make a safe small animal dome from cob alone if your clay sand mix works out.  It doesn't melt in the rain but it needs a roof over it for long term safety.  It will likely be safer than an adobe dome.  I am studying a book by Hassan Fathy that tells a safe way to make mud brick domes.  It is called Architecture for the poor.

Seems you had a web link .. guess I will have to check it out to see what you are up to.

Here is my clay oven - a cob dome similar to what you are talking about built by an amateur - (me) in the early days of the underground cabin.

[/img]
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
Chelle, the coarser sand would be even better than beach sand and even larger rocks in cob is not a problem unless you are into getting a fine finish.  Poke holes -roughen for a bond - in the surface for a grip if you are going to add on or plaster it later.  A roof can be tin or wood or slate. 

This bear hot tub heater has sat out for about 7 years with no roof.  It is now missing ears and a hand I think but still there



It would not have lost anything with a roof over it. 

Several coats of boiled linseed oil can help preserve exposed cob.  The larger amount of sand in it also prevents a lot of damage.
Chelle Lewis


Joined: Dec 10, 2009
Posts: 417
Location: Hartbeespoort, South Africa
    
    1
Thanks so much.  I had to look up what a Troglodyte is… Someone who lives in a cave! I guess tht really fits with your underground architecture. Pretty neat.  

Yes, Chelle - I think it would work fine.  It is common for the sand and clay to stratify separately and they should make cob just fine I think.  At the very least your material bears experimenting with.  Bushveld grasses, it sounds like you are not from around here.    Africa?
That is obviously what has happened….. stratified. I was rather thrilled to discover it. It really stinks though. Probably even be good to add in tiny bits to my garden beds. I am keen to see what this new treasure will produce for me. I just have to wait for the river to subside now…. Rainy season and it is in flood…. Real fun finding ut what has survived each flooding. It can rise 10 meters overnight. Took and enormous tree once… well toppled it and it took me SUCH a long time to get it hacked up to move it. I am in South Africa. Live along the Crocodile River that runs into Hartbeespoort Dam.

The river sand and gravel will work fine.  The clay will have to be experimented with.  The organic clays near the surface are not as good.  Learn all you can about their properties through experimentation.  I call it "becoming intimate with your natural resources."
I could use the top stuff in the garden then. I will make some bricks and see what mix works best.

Grass after it drops it's seed generally always makes straw that will reinforce cob whether it be fine grass straw or coarse as /in Rice straw.  Take one piece of grass - pull on it try to break it.  5 lbs 10 lbs 50 lbs of pull?  Now multiply that times hundreds of pieces of straw in your cob and you will see where it gets it's strength.  Some people have used vines and other things.  Native Americans used sticks and twigs embedded in mud to support second story floors.
No wonder all this interest. The uses are huge! A second story.. wow. So it must be dried grass and not green grass. OK. I will harvest in autumn… the river will go quiet then. So busy making Food Forest at the moment anyway.

Key to making this work reliably is keeping water off of the top of it.
Just cover with plastic? Or plastic under a final coat of the stuff?

I would say you could make a safe small animal dome from cob alone if your clay sand mix works out.  It doesn't melt in the rain but it needs a roof over it for long term safety.  It will likely be safer than an adobe dome.  I am studying a book by Hassan Fathy that tells a safe way to make mud brick domes.  It is called Architecture for the poor.
For the rabbits it would only be an outdoor shelter from the sun… I think it would be very cool inside….. they will be locked up at night behind rock and cement. Not safe to leave them out without this I think.

Seems you had a web link .. guess I will have to check it out to see what you are up to.
Not very good at keeping it updated… just too much to do. New Year’s resolution…. Ho hum.

Here is my clay oven - a cob dome similar to what you are talking about built by an amateur - (me) in the early days of the underground cabin.
Such a neat idea! Artistic. Really like it.    And the bear is too much! LOL. A hot tub heater.... you are really into things. So super. 

Chelle
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
Chelle, sounds like an extremely interesting place to live.

If the clay stinks pretty bad it is likely organic - again - I would at least experiment with it.

Some clays such as my mineral clay are not recognizable as clay until broken up and mixed with water.  They look like soft rock and are very hard to dig.  They are claystone when dry and are even considered bedrock around here in the gold country.  When broken up and mixed with water they become excellent building clay that is not overly expansive and goes back to being very hard when dry. 

The clay, sand and straw needs to be well mixed - I use machines - Bobcat tires mostly squishing it.  Most use their feet and do it on a tarp to facilitate rolling it over while squishing it..



Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
For more ideas, I found an old pix of our cob conversation pit with the grand kids in it.  All benches floor etc in this area are cob.



The cat oven is to the left just above the back of this bench.
Chelle Lewis


Joined: Dec 10, 2009
Posts: 417
Location: Hartbeespoort, South Africa
    
    1
The Troglodyte wrote:
Chelle, sounds like an extremely interesting place to live.

If the clay stinks pretty bad it is likely organic - again - I would at least experiment with it.
I'll let you know. It will probably be a couple of weeks before I get to it though with the water so high.

Some clays such as my mineral clay are not recognizable as clay until broken up and mixed with water.  They look like soft rock and are very hard to dig.  They are claystone when dry and are even considered bedrock around here in the gold country.  When broken up and mixed with water they become excellent building clay that is not overly expansive and goes back to being very hard when dry. 
That is interesting. Never heard of that before. Sounds perfect. I have some soft rock too... in between very hard rock..... but is limestone I think. 

The clay, sand and straw needs to be well mixed - I use machines - Bobcat tires mostly squishing it.  Most use their feet and do it on a tarp to facilitate rolling it over while squishing it..
I want to get a cement mixer... you think that would be good to use? .... or need to be pushing down on it? Maybe start with the mixer and finish with feet...

Chelle
Chelle Lewis


Joined: Dec 10, 2009
Posts: 417
Location: Hartbeespoort, South Africa
    
    1
The Troglodyte wrote:
For more ideas, I found an old pix of our cob conversation pit with the grand kids in it.  All benches floor etc in this area are cob.

The cat oven is to the left just above the back of this bench.
Glenn, this is so SUPERB! It makes me really hope I can get some use out of mine too... Nice looking grandkids.

Did you seal the seats or something? Is it like sitting on bare earth?

Chelle
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
Chelle, we sealed the cob to keep the clothes clean.

We used an acrylic sealer.  Concrete cure and seal.  There are other acrylics that would work.  Elmers glue mixed with water would work and it is milk based.  Clear water base polyurethane would work and be more durable but less natural and expensive. 

For a more natural solution, a drying oil such as boiled linseed works but takes a while to dry and darkens the cob.  Possibly a milk based sealer would work also.  I need to look more into the natural sealers.

I suggest digging out a bit of the soft rock - smashing it up a bit and mixing it with water.  Watch for places that are slick to walk on when it rains but harden too hard to dig in the summer when they dry around rocky areas - it is a good chance that they are mineral clay.  Rocks decomposing by acids from tree leaves etc. make mineral clay.  There may still be solid rocks between them.  The clay is generally a foot or more down and may be around areas that have hardened into or by limestone.

Cement mixers are not too great for cob - feet or vehicle tires work better as it is made pretty dry - a cob is a lump that you can pick up and build with.  The mixers work better for soil cement which makes solid but rather soft floors and is wet more like mixing concrete, soil with around 5 to 7% Portland cement added as a stabilizer.  I have experiment with lots of alternative building so will try to include relevant information.

I am not sure what natural and unnatural resources (store stuff- ) you have available or want to use but I will try to tell you what I know of.

Glenn
Ernie Wisner
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Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
I have issues with putting portland cement in earthen materials. the cement uses a totally diffrent way of moisture management then the cob and in my experiance weakens it over time. but it might work better in places that are dryer then oregon. i can comment on earth cement floors and say if you want cement look up lime cement and roman cement neither use the portland mix.
both are burnt rocks but they handle water diffrently over time. 


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


                                          


Joined: Jan 16, 2010
Posts: 46
I love that converstion pit, Glenn.  It amazes me something that has straw mixed in can end up with such a nice finish.  Or do you put a smooth coat over the rough cob?
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
I finished it as I built it with a rubber tile float to follow the contours and leave a smooth finish.

Larger flat areas and some contours can be worked well with a swimming pool trowel.  A bit of water splashed on as finishing it will help to smooth it but you have to work it more as the water dries to get the smooth finish-- usually within an hour or less things will change if the clay is not all too wet.

After it is well dried the sealer can be put on it - usually a few weeks.
                                          


Joined: Jan 16, 2010
Posts: 46
And the straw doesn't stick out at all?  Amazing.
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
Ernie wrote:
I have issues with putting portland cement in earthen materials. the cement uses a totally diffrent way of moisture management then the cob and in my experiance weakens it over time. but it might work better in places that are dryer then oregon. i can comment on earth cement floors and say if you want cement look up lime cement and roman cement neither use the portland mix.
both are burnt rocks but they handle water diffrently over time. 


I agree that it does not work that well as far as a floor material, Ernie.  It stops the clay shrinkage - it goes off like concrete - takes a set in around an hour and is pretty water resistant, but for floors it is softer and less durable than plain cob.  I agree with you that the surface is not as durable on the soil cement floors also and it is subject to chipping, denting and other damage.

Our bedroom floor is pretty lightly used and it is pretty good in there.  Not so good in the heavier used areas.

We have since gone to the CBRI light duty concrete floor with surface color troweled on as the floor is finished.  It seems to be the most durable yet and only uses 1/2 to 3/4 inch of concrete, so is still not too bad.  It would be a different topic though.

Portland cement is commonly used as a stabilizer in rammed earth walls in some areas but they are still built using a concrete bond beam and columns between the rammed earth in engineered designs.  In some areas they say the portland cement is not even necessary in the rammed earth walls.  It is a California builder and writer that was doing that.  I did one building with some walls and foundation his way.
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
fardarrigger wrote:
And the straw doesn't stick out at all?  Amazing.


The straw seems to just stay down under the surface and you slick it down with the rubber tile grout float.  If i was bothered with a persistent piece of straw I would pull it out but generally I don't recall any problem.  It softens when wet then gets tough when all is dry again.
                                          


Joined: Jan 16, 2010
Posts: 46
Very cool, thanks Glenn.  It makes a really organic surface that looks very comfortable.  Cob, it's not just for walls.  And ovens,  And bears...lol
                    


Joined: Oct 23, 2011
Posts: 0
This is just general information and opinionation about clay.  I copied myself from the "when clay isn't clay" thread, because I think it's really important information for natural builders.

As a ceramicist I'd like to point out that clay particles are flat, and silt particles, while the same size, are round.  This is important!  You can press your thumb into clay, and the detail of your fingerprint will be visible because the flat particles have slid up against one another in a new arrangement, with an interlocking structure.

Silt can act very much like clay when it's wet, but it doesn't have the same "sticky" or "slick" feeling, and that's because it's little balls rolling against each other.  Silt does NOT have the same structural strength as clay.  I'd say it's very important for a person to know what their dirt is made of before building a house out of it.

The "jar test" can be extremely misleading for silty soils.  The fine silt particles will settle on top for sure, but that doesn't turn them into clay!

The best test for questionable clay/silt is to make a snake, or whatever you want, and let it dry completely.  Clay will be hard, difficult to break, but silt will crumble easily.

Here in northern cali we have volcanic silt as well.  I used very silty dirt in a light straw clay mix as the wall infill of my cabin, and it worked well enough for this purpose.  The straw is the structural component of this wall system, though.  I used the same mud for my rough coat of plaster (not because I didn't know it was silt, but out of desperation to get my house liveable before it snowed), and that's when the silt really became apparent and problematic.  The mud layer crumbles so easily we had to cover the walls entirely with cloth or wooden boards.  (if I wanted to, I could probably get a higher clay mix and continue mud plastering the walls, but we decided this cabin isn't worth the effort as we hope to live in here for less than five years)

I think using silty soil is no substitute for clay, especially for high-clay content mixes like cob.  Telling yourself "it might work" could lead to a nightmare of a building, and possibly dangerous structural compromises.

I'd take Erica's advice and mine some clay studios for leftovers.

Or, (actually maybe after you've gone to clay studios and really get a feel for what clay is) you can hunt around for natural deposits.  They exist, usually in a big layer that gets exposed when things get water-logged and that part of the hillside literally slips downhill.  Clays on the west coast seem to be white or grey, thankfully making them easy to spot.  The red dirt is the iron rich volcanic ash and crushed up lava, not at all like the "red clay" back on the east coast.  We have way newer geology on this side of the continent. 
Ernie Wisner
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Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
Trog-
ya ive seen some of those rammed earth houses with the cement stabilizer.

Ive not been impressed with the moisture content or the Pcement wicking.
thats the problem its the wicking. cob/earth will breath out the water and cement retains it thats not good in my experiance. cement floors are great for durability not so good for me these days in the walking/standing on them department.

I am an advocat for using the best thing for the job in a building. however the cement has never seemed to be the right thing for walls and those things you want to last for many years. theres also the issue of what happens when you dont want the house. cement in the mix means you have to take it to the dump while just earth and linseed oil can and does simply melt into the landscape.

its a matter of choice IMO and every person will have to make the choice that suits them best. for me its just earthen work and good mixes. but i have more experiance than many and its easy for me to amend a cob mix. here in oregon we are also graced with a huge amount of good clay close to anyplace you might want to build.

i think the cement stabilizer might be a way around lack of good clay cause it would work OK with silt/top soils. Thanks for the info by the way; i never thought of getting a rubber float. 
Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
Thanks for the reply, Ernie.

With the lack of experienced people in this area I have studied quite a bit then did a lot of hands on experimentation.  I take all I can learn from others and try to share what I find also.  I appreciate it when people like you are also willing to share.

I have found the mineral clays of the Calaveras formation - if I recall correctly, to be very dry claystone (known as porphyry clay to the gold miners here) and not like I would have ever thought of as being a good mineral clay.  I found that once it is broken up and water gets into it it becomes what I am sure is some of the best cob or earthbuildng clay in California.

We have a buildings around from the gold rush days that are either adobe brick from it, or even slate or schist rocks mortared with it then plastered over the outside.

Chew Kee store in Fiddletown is a Chinese rammed earth store from gold rush days and it was found to be a rammed earth foundation with no rocks under the rammed earth.  It survived for better than 150 years before the state I think it was had it restored  to a bit better than original.  I highly doubt that it was stabilized with anything except sand and clay.  David Easton told about restoring it in his book. 

My point here is that clay will not always be found as the wet pliable mass we think of as clay.  Some of the best mineral clay for building with may be high and dry and hard as a rock.  As I recall the Calaveras porphyry clay is mostly decomposed Greenstone along the mother lode and it is considered a bedrock.  Between the embedded decomposing rocks there are tons of great building clay.  As it is dug out and broken by the backhoe or Bobcat it becomes a resource that is ready to be molded into whatever we want to make of it.  Miners also picked there way through it with picks and shovels so it does not require a machine, but that does make it a lot easier.
                    


Joined: Oct 23, 2011
Posts: 0
Some of the best mineral clay for building with may be high and dry and hard as a rock.


This is great information.  Sounds like this clay was buried deep for a long time, hence the hard packed density.  I don't know if you'd locate that kind of thing anywhere other than mining areas.  Good clay is a great score, but if you can't FIND it under the dirt.....not so useful.    Still I'll add old (gold specifically do you think?) mining sites to the mental inventory I've started as places to scout for natural clay deposits. 

I'm interested in measuring the elevation of the clay we found, as I suspect there's a layer going through the whole mountain and it'd be fun to see if you could locate it again on the other side too!

Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
They say that our mountains were at one time about two miles taller.  The granite pushed up in the Yosemite area and hence heat and pressure on the volcanic formation turned some of it into greenstone.  Smoke and heat added carbon tracks through the porphyry clay - kind of a broken, angled chunk clay with carbon tracks through it.

You can see it where I excavated part of my great room in the lower lever of the underground cabin.  The left side wall.  It is so hard that a pretty tough backhoe will only scrape about 2 inches at a time, but the resulting powder and claystone chunks mix great with water, sand and straw making a very strong cob. 

The cob in the above pictures was made from it.

















Glenn Kangiser
volunteer

Joined: Dec 31, 2009
Posts: 236
Location: Central California
A good study in earthquake performance of adobe structures including vaulted roofs from Iran.  Small PDF File



www.archidev.org/IMG/pdf/3.pdf
sherry chang


Joined: May 13, 2013
Posts: 1
Glenn Kangiser wrote:If using just the natural resources on your land and you had clay without enough sand content ie: 30% clay 70% sand is ideal for most types of earth building, then it may be better to make your soil into adobe bricks.  Without enough sand and aggregates, shrinkage will be a lot greater, therefore the bricks may work better.

Straw is your rebar and holds things together and if you were short on sand and aggregate, more straw could help in making the cob work without as much shrinkage as well as better reinforcement.

Cob is a monolithic structure usually with rounded walls and reinforced with straw,  so it is immensely stronger than adobe brick walls and done properly will resist even large earthquakes.

In India they have developed methods of making mud bricks with interlocking sides and laid diagonally to resist earthquakes quite well also.  They have also developed a method of putting a waterproof coating on the sides of the brick to prevent water damage... concrete and or rock chips etc on the exposed edge.


Can you recommend where to look up this techniques in India? I'm going to start a project in Taiwan where is really humid and earthquake zone. I'm interested in using cob than adobe.
Thanks for your time.
 
 
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