[GSBN] Lime plaster and expansion joints

Tue Feb 10 00:11:12 UTC 2009

On Mon, Feb 9, 2009 at 12:54 PM, RT <ArchiLogic at yahoo.ca> wrote:

>
>
> 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.  My theory goes something like thisL  The sun heats the plaster and
the plaster heats the steel and the steel gets REALLY hot because it's
backed by all this great straw insulation and because it's really hot it
gets really big, but the skin isn't hot (except maybe right around the
steel) and so the steel swells and the skin doesn't, ergo crack.

>
>
> Here in Canada where exterior temperatures can be at either extreme of the
> thermometer, it's common practise to keep the structural frame completely
> inside of the thermal envelope as much as is possible in order to minimise
> the amount of thermally-induced movement of the frame.
>
> And brittle materials like glass and masonry (including plaster) should be
> mechanically isolated so as to avoid the destructive effects of movement of
> the frame, whether those stresses are thermally induced or as a result of
> structural loads, even if the frame is entirely inside of the thermal
> envelope. ie One would never use fixed connections with masonry (and
> plaster) at both ends of a structural steel member. One end can be a fixed
> connection but the other should be a "roller" type connection ie free to
> move.
>
> 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 would mention that often we might have a steel
brace frame inserted into a wood wood frame structure, with the mesh
attached to the wood.  So then you have to think of the steel as kind of an
alien material behaving differently than the rest.

>
>
>
> But back to Laura's question:
>
>  ...lime plaster ... begs the question of whether we need to be using
>> expansion joints on large expanses... lime having different
>> characteristics
>> than a cement stucco
>>
>
> Control joists are intended to deal with cracking due to stresses from
> structural loads as well as those induced by temperature and shrinkage (T&S)
>  so I think that it would be imprudent to omit them or significantly change
> the spacing when used with lime plasters.
>
> And I think that minimising shrinkage cracks in lime plasters is as much or
> more about process as it is about specs .
>
> For instance, lime mortars should be well-aged before application in order
> to work properly, the mix for the scratch & brown coats cannot be too rich
> (as is the case with PC mortar mixes), the brown coat should be floated the
> day following application to further compact it (unlike with PC mortar
> mixes) and the plaster needs to be protected from dryouts due to sun and
> wind exposure during the lengthier (than Portland cement plasters) setting
> period -- all to minimise the amount of shrinkage that *will* occur during
> the curing period.
>
> I don't know what the "official" recommendations for control joint spacing
> with lime plasters might be but given the incremental degree of attention
> and amount of time that is needed for lime plasters to be done properly and
> given that that incremental attention and time is likely to be circumvented
> (probably the biggest reason why the use of Portland cement plasters
> displaced lime plasters) 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, I wouldn't want to
> exceed a panel size of 100 sf with max spacing between control joints at 10
> ft although the "official" specs say that larger panel sizes and greater
> spacings are acceptable.
>

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?

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)
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-- 
John Swearingen

Skillful Means
www.skillful-means.com
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