[GSBN] Steel mesh in clay plaster (?)

Wed Aug 28 00:20:30 UTC 2013

Hi Martin

Ok I will make it 3c worth

What are we really talking about here? 
The conversation is muddled.

Are we taking about  1) internal reinforcing of cob walls? or 2) reinforcing of clay plasters? or 3) providing only external reinforcement to hold up cob walls? 
because all three conditions are not equal.

1)The inside of a cob wall is most unlikely to be anything other than dry in service unless the design has serious flaws, so I have no great issue with steel reinforcing in there. It is very predictable and works.  But as I say, in NZ we have adopted a lets-be-quite-sure attitude and protected the steel, although bamboo or fibreglass or even  PVC pipes might do just as good a job if well engineered and built.  We are considering introducing polyprop geogrids for horizontal reo for cob in the next Standards revision currently underway. 
I can't see straw in the mix doing the job of a few well placed vertical steel rods either, esp if faced with high vertical accelerations as well as horizontal forces, but straw does prevent micro cracking and no doubt also prevent the chess cutter effect some seem concerned about.

2) If I wanted to put reinforcing in clay plaster to toughen it up I would chopped straw, or other fibre,  or go to Cintoflex D polypropylene mesh. 
I would not use nylon unless I only wanted very minor gains in strength as it can stretch under load. 
If you were to reinforce plaster for whatever reason I would remain adamant that steel is most unlikely to be a good option in earth plasters for reasons of corrosion, even if galvanised. Plaster layers are going to be exposed to the atmosphere and unless in an extremely arid zone, there will be some moisture present.  It takes at least 25mm of dense alkaline concrete to protect steel from corrosion when exposed to air - I can't see 25mm or even less - or more- of clay giving anything like that protection.  And I have seen clay strip galvanising off steel adobe moulds quite rapidly where the clay was a touch on the acidic side, but of course the clay is always wet in this instance.  

3)  I would have doubts about a reinforced clay skin being able to adequately reinforce massive cob walls on their own in an earthquake no matter how well done for several reasons, one being the adhesion or lack of it to the substrate when severe out of place forces are incurred. I measured a 100mm horiz out-of-plane displacement between layers of unreinforced (apart from straw) cob after the Christchurch earthquake in walls at least 600mm thick - and that was where the walls were still standing. The only possible exception I would consider would be if the reinforced layers were very well tied together through the wall.  I have seen research in Sydney being done on external bamboo rods being applied to adobe buildings in a mesh pattern to provide retrofitted earthquake reinforcing. The layers were tied through the thickness of the wall with polyprop rope, and then covered in clay plaster to make then look good. They stood up in shake table tests very well, and I have no doubt this would work for cob too, but there is still the issue of tying bond beams to the footings to work out.  

And that makes it 4c worth

Graeme   

On 27/08/2013, at 6:31 PM, martin hammer <mfhammer at pacbell.net> wrote:

> Thanks for your 2c worth Graeme.
> 
> However your advice to “don’t do it” seems unduly emphatic for the evidence you present.  I agree steel (especially thin gauge mesh) should be galvanized or otherwise protected if possible.  Not only in clay plaster, but lime and cement as well.  But one would expect the trowel rusting you and Andre observed to occur.  It has the moisture and oxygen it needs to rust.  However it seems that all the evidence and thoughts presented in this GSBN discussion suggest that once the soil dries, the rusting arrests.
> 
> At this point I’m more convinced that steel (maybe even unprotected) in clay plaster (or cob, as Art Ludwig has described) is not in danger of significant corrosion.  I can imagine extreme conditions potentially presenting a problem (e.g., at the base of a wall where a plaster is routinely wet from frequent rain), but in general is seems to be safe enough.  I’ve used Cintoflex also, and like it in many ways (though the thought of plastic mesh in clay plaster sometimes makes me squirm) and I’ve read about the use of geomesh yielding great test results.  But these materials may not be available in developing regions where the Strawbale Tutorial might be used.  I’m looking for an array of materials to choose from, taking into account availability, durability, cost, and performance.
> 
> Martin
> 
> 
> On 8/26/13 6:14 PM, "Graeme North" <graeme at ecodesign.co.nz> wrote:
> 
>> My 2c worth
>> 
>> Unprotected steel in clay plasters - don't do it.
>> 
>> In the NZ Earth Building Standards steel reinforcing is to be galvanised or painted with zinc rich paint or asphalt as a "belt and braces" approach even though we have little evidence of reinforcing steel rusting or otherwise in clay, but we were influenced by observations already made about how quickly steel tools rust if clay is left on them.  
>> 
>> For plasters we are now using light polypropylene meshes such as Cintoflex D - or C if we want a heavier mesh. Its  much easier to use than any steel meshes I have seen, is compatible with clay, very cheap, and corrosion free.
>> For any horizontal reinforcing inside earth walls we use heavier polypropylene geogrid square or triaxial  meshes, and shake table tests seem to give great results.
>> 
>> cheers
>> 
>> Graeme  
>>    
>> 
>> Graeme North FNZIA
>> Graeme North Architects
>> 49 Matthew Road
>> RD1
>> Warkworth 0981
>> 
>> www.ecodesign.co.nz <http://www.ecodesign.co.nz> 
>> 
>> 
>> 
>> 
>> On 26/08/2013, at 10:49 AM, Kyle Holzhueter <nihondaigaku.kairu at gmail.com> wrote:
>> 
>>> I think it depends on the soil and climate.  Rule of thumb in Japan is to avoid contact of earth plaster and metal.  In cases where it can't be avoided, cover the metal lath with a alkaline plaster first.  As Andre suggested, you'll notice how trowels not cleaned will rust differently when working with earth and lime plasters.  I've also noticed that my trowels will rust faster in Japan than North America.  I suspect that soil pH may be one factor.
>>> Regarding natural reinforcement, in traditional Japanese earthen storehouse construction, there is a practice called tarumaki (barrel wrapping) that involves embedding straw rope in wet plaster:  Vertical rope on roughly 20cm centers, more plaster, than horizontal rope on roughly 20cm centers secured with bamboo nails. Repeat depending on thickness of wall.  There's an interesting instructional video series on this practice that used to be for sale.  If anyone is interested, I could see if the videos are still available.
>>> I've attached two photos of an improvised non-traditional version on a straw bale wall using palm rope.
>>> 
>>> Kyle
>>> 
>>> *************************************************
>>> Kyle Holzhueter  PhD (Bioresource Sciences)
>>> Nihon University, Graduate School of Bioresource Sciences
>>> Architectural and Regional Ecological Design Studio
>>> 1866 Kameino Fujisawa Kanagawa 252-0880 Japan
>>> 
>>> カイル・ホルツヒューター　博士（生物資源科学）
>>> 日本大学大学院　生物資源科学研究科                       
>>> 建築・地域共生デザイン研究室                    
>>> 〒252-0880 神奈川県藤沢市亀井野1866
>>> 
>>> TEL/FAX: 0466-84-3364
>>> Mobile: 080-3477-9841
>>> http://hp.brs.nihon-u.ac.jp/~cnes/index0.html
>>> http://holzhueter.blogspot.com/
>>> http://japaneseplastering.blogspot.jp/
>>> *************************************************
>>> 
>>> 
>>> <P1090046.JPG><P1090073.JPG>_______________________________________________
>>> GSBN mailing list
>>> GSBN at sustainablesources.com
>>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
>> 
>> 
>> _______________________________________________
>> GSBN mailing list
>> GSBN at sustainablesources.com
>> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
> _______________________________________________
> GSBN mailing list
> GSBN at sustainablesources.com
> http://sustainablesources.com/mailman/listinfo.cgi/GSBN

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.sustainablesources.com/pipermail/gsbn/attachments/20130828/77c0f06c/attachment.htm>