[GSBN] Steel mesh in clay plaster (?)
martin hammer
mfhammer at pacbell.net
Thu Aug 29 17:19:51 UTC 2013
Hi Graeme,
Thanks very much for your additional cents (and sense). All you stated
sounds right to me. I started this subject with steel mesh reinforced clay
plaster on straw bale walls (your item #2), but it expanded (nicely) to cob
with Art Ludwig’s testing. I’ve used and believe in Cintoflex polypropylene
mesh also (within its limits, like anything else), I was just looking to
expand the array of mesh choices for the purposes of the Tutorial I
mentioned.
Re: nylon, you’re right that it is known to stretch. But I’ll offer Darcey
Donovan’s successful monotonic and shake table testing of the Pakistan
system that uses nylon fishing net, as a counter balance to your concern.
There could have been stretching of the nylon (especially at the base of the
straw bale wall where crushing of plaster began to free the mesh), but the
bottom line is that it performed under a peak ground acceleration twice the
1994 Northridge CA earthquake (and approximately equivalent to the 2005
Kashmir earthquake).
I’m a believer in through-tying reinforcement (bamboo, various types of
mesh, etc) on opposite sides of the core (straw bale, adobe, straw-clay,
cob, etc) of a wall, as an effective way of stiffening a wall and resisting
in-plane and out-of-plane loads. There has been much success retrofitting
unreinforced adobe walls with bamboo in this manner. There is an excellent
paper on testing at U of Technology, Sydney Australia by Dominic Dowling and
others:
http://services.eng.uts.edu.au/~ddowling/Attachments/SismoAdobe05%20Dowling%
20(B)%20UTS%20(final).pdf
Marcial Blondet and Daniel Torrealva have also been doing work in Peru with
reinforced adobe (bamboo, cane, geomesh).
Martin
On 8/27/13 7:20 PM, "Graeme North" <graeme at ecodesign.co.nz> wrote:
> 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:
>
>> Re: [GSBN] Steel mesh in clay plaster (?)
>> 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
>> <x-msg://48/graeme@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>
>>> <http://www.ecodesign.co.nz <http://www.ecodesign.co.nz/> >
>>>
>>>
>>>
>>>
>>> On 26/08/2013, at 10:49 AM, Kyle Holzhueter <nihondaigaku.kairu at gmail.com
>>> <x-msg://48/nihondaigaku.kairu@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/
>>>> *************************************************
>>>>
>>>>
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