Rethink Your Life!
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The Work of Art and The Art of Work
Kiko Denzer on Art
[Cob] update on trench issuesLance Collins collinsl at bigpond.net.au
Tue Aug 19 23:49:39 CDT 2008
I'm not an expert on concrete but I have used it on lots of small projects and I read a lot. On lists like this we make general comments about clay, straw, concrete etc. These things have a wide range of properties and it's easy say things which are not correct in all circumstances. e.g. concrete wicks moisture. Well yes and no. Structural concrete usually does but with a higher proportion of cement it can be made waterproof (ever see a concrete water tank?) >Can a cob wall be built on a concrete footing / stem wall or slab? Yes. >Earlier assertions (paraphrasing) said "no" cuz the moisture wicks up >through the concrete and can cause a failure at the cob/concrete >junction. Hope I got that right. > >Am wondering if concrete's tendency to wick could be moderated. It can. > >Thoughts and questions ... > > * How about a good vapor barrier between the concrete and the > ground? Should moderate the wicking effect. I'd expect to see a sheet of plastic before a concrete slab is poured. Not only to stop wicking but to stop water draining out of the concrete before it's fully cured. > * How about a good drainage system under the concrete, much like > that prepared prior to a rubble trench? That should also reduce > the wicking. A good idea. Can you be sure the plastic sheet won't get the odd hole in it before the concrete pour? > * How about extending the concrete vertically 12", or 18" above > grade. That should contribute to the concrete's evaporative drying. > >Would a combination of these or other techniques make concrete an >acceptable material as a cob wall footing? Structurally acceptable: definitely. Politically acceptable: maybe not. >The last bullet point seems most relevant to Tim's thinking. If a >mortared urbanite stem wall can be used, I should think a concrete stem >wall of similar dimensions could also be used, since urbanite is >concrete chunks. Is my thinking valid? What am I missing? > >I understand the shortcomings of concrete on the environment and would >prefer not to use it. But ... in some situations it could be quite >useful. And if an urbanite wall has similar behaviors perhaps it is not >an optimum solution? > >This has been a great conversation. Caused me to think of several >different topics. Some good info has been provided. I'd urge all >participants to practice patience during the dialog. There is sooooooo >much information lost in printed communications. We don't have all the >visual and verbal clues and can easily misinterpret humor, sarcasm, >irony, etc. This list is a wonderful resource. Thank you all. I have a shed with a concrete floor. Plastic sheet first, poured earth (5% cement) to level off then 40 mm of concrete laid in metre wide strips. It's been down seven years and no moisture problems (rainfall about a metre a year with most in winter). The shed is divided into two parts (workshop/living). The dividing wall is 250mm of poured earth (no additives of straw, cement etc.). Warning: It may be just a fluke that my soil is excellent for cob without adding sand or clay. YMMV Lance (in Aus)