[GSBN] Lime plaster and expansion joints

[RT]

On Sun, 08 Feb 2009 13:00:14 -0500, <gsbn-request at greenbuilder.com> Jeff  
Ruppert <jeff at odiseanet.com> wrote:
>
> In looking at the cracks it seems that the expansion of the frame in a
> vertical direction places the panel in tension and creates the cracks.
> The vertical expansion joints don't do anything for this action.

> The interior bale walls on the same project which are not exposed to the
> extremes of the weather have not cracked.
>
> In doing it over I would have found a way to not attach the plaster to
> the frame.  I am not sure how this could have been done easily, but
> steel frames expand and contract with temperature so much that attaching
> a rigid material like plaster to it can be problematic.


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.

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.


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.

-- 
=== * ===
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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