Cob House Construction 2

[msquared48]

OK all you Limeys. Here's your chance to shine!

Can you reinforce a Cob House with rebar, considering the construction and materials? Would it even be worth it?

I have a friend that is considering building one in the mountains of Colorado and I am very hesitant to become involved with any advice here to her on the subject, especially considering Haiti.

Even if it could be done, would the rebar do any good structurally considering the materials used?

I understand that Cob construction and buildings have been used in the UK for over 1000 years, some in existence today and 1000 years old. Concerned about earthquakes though as I do not know of the seismicity in the UK as opposed to Colorado.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Corn cobs?

 

[msquared48]

Just wake up Hokie?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Cob

Cob is a very old method of building with earth and straw or other fibers. It is quite similar to adobe in that the basic mix of clay and sand is the same, but it usually has a higher percentage of long straw fibers mixed in. Instead of creating uniform blocks to build with, cob is normally applied by hand in large gobs (or cobs) which can be tossed from one person to another during the building process. The traditional way of mixing the clay/sand/straw is with the bare feet; for this reason, it is fairly labor intensive. Some of the process can be mechanized by using a backhoe to do the mixing, but that diminishes the organic nature of it. Because of all the straw, cob can be slightly more insulating than adobe, but it still would not make a very comfortable house in a climate of extreme temperatures. The wonderful thing about cob construction is that it can be a wildly freeform, sculptural affair. I've seen some very charming homes made this way. Cob was a common building material in England in the nineteenth century, and many of those buildings are still standing.

A variant of cob is what is commonly called "light straw/clay". This is made with the same long fibers of straw which is tossed like spagetti with a sauce of clay slip. The idea is to coat the straw fibers with enough of the clay to get them to stick together, but not so much that it makes a gummy clump. This material is then tamped into a form and left to set up enough to remove the form. Light straw walls could be useful for interior partitions and even exterior walls if it is thick enough. Such walls would be quite a bit more insulating than cob, but they require a timber frame of some sort because the straw itself would not be load bearing.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[fannanxx]

http://cobhouse.net/

 

[msquared48]

Very interesting - some special reinforcing mentioned (helical rods - corkscrew fashion) by British Engineers in retrofit, plus the UK is NOT in a seismic zone. Bad for Colorado application.

Taken from the above link:

http://www.youtube.com/watch?=ljoBV_UJRi4&feature=relateds

Does anyone have any examples of this retrofitting reinforcing mentioned?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ussuri]

Cob houses were never engineered. I suspect they were built by the locals to rules of thumb. Those that were over the top will be the ones that have stood for hundreds of years. Those that were not, fell down and were forgotten about.

Even if you could reinforce it, how would you determine what material properties to use in your design? As it is essential dried mud, how would you ensure a bond between the reinforcement? If using steel, how would you prevent corrosion? Would you be better using a plastic mesh material? I would have lots of questions.

You might try speaking with someone like the Building Research Establishment (BRE):

http://www.bre.co.uk/

Or maybe CIRIA:

http://www.ciria.org/service/Home/A...502_20080929T115140HomePage.aspx?Section=Home

A few google hits:

http://www.sustainablebuild.co.uk/CobBuilding.html

http://www.earthedworld.co.uk/modules.php?op=modload&name=News&file=article&sid=4

http://www.amazon.co.uk/Cob-Builders-Handbook-Hand-sculpt-Your/dp/0965908208

http://www.amazon.co.uk/Building-Cob-Step-step-Guide/dp/1903998727

 

[Ussuri]

A couple of papers submitted to The Structural Engineer. The second one contains some test data.

Prefabricated earth constructions in the UK and Europe - 16 September 2008

The performance of cob as a building material
R.H. Saxton, BSc(Eng), CEng, MICE
School of Civil & Structural Engineering, and the Centre for Earthen Architecture, University of Plymouth
- 4 April 1995