[GSBN] Control joints (was Re: Lime plaster and expansion joints)

[John Swearingen]

Comments bellowed:


On Tue, Feb 10, 2009 at 11:45 AM, RT <ArchiLogic at yahoo.ca> wrote:

> Date: Mon, 9 Feb 2009, John Swearingen <jswearingen at skillful-means.com>
> wrote:
> Re: Lime plaster and expansion joints
>
>
>  We know that the steel reinforcing in reinforced concrete works because
>>> the coefficients of thermal expansion are similar for the two materials so I
>>> don't think that the cracking that Jeff is talking about is due entirely
>>> to juxtapostion of the two different materials, plaster and steel.
>>>
>>
>
>  Well.....I dunno about coefficients, but I have seen cracks along steel
>> members.  I think what happens is that the steel superheats while the skin
>> doesn't.
>>
>
> That's more about temperature differentials rather than different
> coefficients
> isn't it ?


Yes, I guess that's more the point.

>
>
>  In an infill bale wall, this might mean pre-stressing the mesh by pulling
>>> down to the foundation and/or sill plate only and providing a connection at
>>> the top of the panel that only restricts lateral movement of the panel.
>>>
>>
>  Now who's gonna do that?
>>
>
> I do it with any construction (ie whether it be something small like a
> piece of furniture or large, as in a building ( ie With a sliding dovetail
> stiffenning rib to restrain a table top you would fix only one end of the
> rail. With wide plank flooring you fix only the tongue edge. With a large
> pane of glass you allow clearance on three edges ) where there are going to
> be different rates of movement between  different components whether the
> movement be due to humidity, temperature or structural loading so I don't
> see why one wouldn't do it with a plastered wall panel.


It's just that here in earthquack country, we are required to make our mesh
very, very tight so that slippage does NOT occur when the ground moves the
building.

>
>
>   I would venture that a panel size should probably
>> not exceed 144 sq ft., with control joint spacing in any direction not
>> exceeding 12 ft whereas with Portland cement mixes, [snip]
>>
>
>
>  All very well, but what about the fact that you're making a crack in a
>> wall that doesn't have a paper backing to deal with the water than comes
>> rushing in?
>>
>
> Which begs the question: Why would you *not* install proper flashing at a
> control joint knowing full well that there will be a crack at that location
> ?
>
> True, that flashing would not be a strip of paper simply because it
> wouldn't do much good against a bale substrate but the presence of a
> flashing (or perhaps more appropriately named a "drainage plane") at that
> location is still essential.


Right.  You would have to install flashing paper over all or most of the
bale wall in order to keep water out of the crack made by an expansion
joint.  Where would it all end, I ask you?  We want the plaster to bond with
the bales for a variety of reasons, among which I would include laziness on
our part, and bale/plaster adhesion that is important structurally.

With care we can limit cracking and/or seal/patch cracks that occur and so
limit any leakage and subsequent damage to an acceptable level.  There is a
pitfall here, though, because the owner and future owners have to assume a
degree of vigilance and responsibility to survey the building for new
cracks.

John

>
>
>
> --
> === * ===
> Rob Tom
> Kanata, Ontario, Canada
> < A r c h i L o g i c  at  ChaffY a h o o  dot  C a >
> (manually winnow the chaff from my edress in your reply)
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com_______________________________________________
> GSBN mailing list
> GSBN at greenbuilder.com
> http://greenbuilder.com/mailman/listinfo/GSBN
>



-- 
John Swearingen

Skillful Means
www.skillful-means.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.sustainablesources.com/pipermail/gsbn/attachments/20090210/4ba474db/attachment.htm>