[GSBN] Control joints (was Re: Lime plaster and expansion joints)
ArchiLogic at yahoo.ca
Tue Feb 10 13:45:07 CST 2009
Date: Mon, 9 Feb 2009, John Swearingen <jswearingen at skillful-means.com>
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
> 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
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
(or perhaps more appropriately named a "drainage plane") at that location
is still essential.
=== * ===
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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