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

[RT]

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 ?

>> 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.


>  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.


-- 
=== * ===
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)
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