<br><br><div class="gmail_quote">On Mon, Feb 9, 2009 at 12:54 PM, RT <span dir="ltr"><<a href="mailto:ArchiLogic@yahoo.ca">ArchiLogic@yahoo.ca</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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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.</blockquote>
<div><br>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.<br>
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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.<br>
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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.<br>
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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.</blockquote>
<div><br>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.<br>
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But back to Laura's question:<br>
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<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
...lime plaster ... begs the question of whether we need to be using<br>
expansion joints on large expanses... lime having different characteristics<br>
than a cement stucco<br>
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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.<br>
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And I think that minimising shrinkage cracks in lime plasters is as much or more about process as it is about specs .<br>
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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.<br>
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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.<br>
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</font></blockquote><div><br>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? <br><br>John<br></div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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-- <br>
=== * ===<br>
Rob Tom<br>
Kanata, Ontario, Canada<br>
< A r c h i L o g i c at ChaffY a h o o dot C a ><br>
(manually winnow the chaff from my edress in your reply)<br></font><div class="WgoR0d">
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