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Kiko Denzer on Art
[Cob] rebarHenry Raduazo raduazo at cox.net
Mon Jun 25 17:46:51 PDT 2012
Alpha testers are the pioneers and scientists of the world they try things that have never been done before in order to advance building technology, Often they suffer the consequences of failure. Anyone putting steel in cob is an alpha tester. An alpha test of this type should run at least 10 years at a minimum and possibly 30 years to be meaningful. Why so long? Buildings are designed to last for many years and the problems might not show up in short term testing. A first problem that might occur is "popping". Have you ever attached something like a board to a cob wall with long screws. If you have a nice long screw and slowly tighten it on a board the head of the screw will start taking up tensile forces even imbedding its self into the board until you reach the limit of the cob and then the screw will pop loose of the cob and begin turning freely in the hole. No amount of additional turning will get the screw to engage the cob wall. Something similar to this might happen when you install a 10 or 15 foot piece of rebar in a cob wall. As the weather gets hotter the steel will expand faster than the cob putting the cob around the rebar under tension. As the weather gets cooler the steel will shrink more then the cob causing the cob around the steel bar to be under compression. The question is: Will the steel bar pop free of the cob wall and just slide free relative to the cob as it expands and contracts? If it does then the steel bar will not reinforce cob. A second problem is in-situ disintegration of the steel. Will steel rebar rust out over time? At what rate will it rust? How much holding power will the rusted steel have? Even if the steel is rust proofed will chemicals in the earth attack the rust proof coating? The knurling on rebar is plenty adequate for concrete is it adequate for cob or should we use a more extreme knurling to give the bar more grabbing power? The only way to answer these questions is for some adventurous Alpha tester to try them out. I do alpha testing on buildings that I do not care about. My wood shed for example has three alpha teats running at the same time. 1) I have a steel dead man anchor, a wood dead man with a galvanized steel wire and an all wood deadman held together with galvanized screws. These anchors attach a 7 foot by 32 foot shed roof to three short cob wall segments 2) I also have sections of the shed wall built with wood fiber cob instead of straw fiber cob. 3) The roof its self is made of a bamboo mat covered with paper/clay/sand plaster and waterproofed with boiled linseed oil on one section and driveway sealer on another. I may come out some day after a big storm or hurricane and find the roof of my shed laying in my back yard, and that will give me a chance to analyze how and why the roof failed. People who use steel rebar in cob walls should be aware that they are Alpha testers and they should also be ready to accept the fact that their experiment may fail. Ed On Jun 25, 2012, at 5:17 PM, Anthony Novelli wrote: > Damon, > > You offer excellent points, though I would caution against over-generalizations e.g., "we don't need rebar too"; as use of cob purely as a mass wall system *may* be achieved without such reinforcement. But when cob is promoted as something you can build damned near anything with - without the necessary caveats - you end up with those unnecessary collapses and messed up contractual arrangements or botched public projects. History is a great teacher, and it is important to note - as you say - that when used in ways obvious to a careful look at what has stood the test of time, you have a good chance at success. Our egos and other factors can get us into trouble as much in building as anywhere... and that can apply as much to what we don't know to what we do - or think we do. Entropy rules. > > Tony > > P.S. Just one example... a friend who took what he learned in a cob workshop or two and set out to build a large rectilinear cob home... which had two very tall, very thin (I think they were maybe 12-14" at base tapering to 8-10" at top), finely crafted cob walls running parallel with a glass wall at the end, and large timbers spanning for the roof. Without buttressing, shear or other reinforcement and an appropriately light roof structure this building was dangerous to an extreme, and would have been improved by the benefit (and cost) of code. > > P.P.S. One more... a permacutlure teacher of mine that had permitted several straw bale structures showed an example of his work in Latin America, where the straw bale walls were left unpinned, had no bond beam, a serious bamboo truss system, and a TILE ROOF. Each of these systems on their own may be fine, but not understanding the shortcomings as well as the strengths of each creates seriously out of balance structural equations when combining them. Cob would have been a superior choice for this kind of roof system, though long unbuttressed or unreinforced cob walls can be problematic as well. > > > > On Jun 25, 2012, at 1:13 PM, dhowell at pickensprogressonline.com wrote: > >> Tony: ...the services of tensile strength that rebar offers in earthen and concrete structures is justified to ridiculous extreme. >> >> Damon: Possibly. But also, cob has been proven to hold up just fine without rebar. So which side is doing it the right way? Whichever side has the most money, I presume. Generally cobbers are poor people and we all should be able to build our homes with what materials we can afford and not be denied housing because of code costs. There are cheaper ways than concrete, steel and sheet metal. Believe me, I think about what will make my building stand up as I'm sure anyone who looks to build their own home would. Tensile strength is an important issue. I bet after a few collapses and possibly deaths, traditional cob builders noticed the more straw in the wall, the more weather resistant. Straw provides our tensile strength, we don't need rebar too. Cob does have its place and California is hard on it. But they are adapting it to their environment. What is required in one place may not be required in another. Let's not pretend like this is a brand new material that the world has just discovered. > > > _______________________________________________ > Coblist mailing list > Coblist at deatech.com > http://www.deatech.com/mailman/listinfo/coblist
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