Here's a photo of the house before burlap: a ferrocement ring at the base, encasing steel studs, covered in stock panels. http://www.annesleyphoto.com/Building/ferroring7511.jpg
Here's the structure from another angle, partly covered in burlap-crete. http://www.annesleyphoto.com/Building/burlap7516.jpg
There's a few folks that have used burlap soaked in cement slurry and draped it over wire formwork, therefore being able to build a house with a similar approach as a large piece of sculpture. Another way is to stretch the burlap over an armature and paint a weak cement solution on it, building up layers-- which takes about forever unless you have lots of people. I've tried a bunch of different mixes and have come up with one that works so well its just about amazing: my exterior walls are made this way, 1,200 square feet!
I found that portland and sand and water, in any combination generally don't do well for burlap-crete, as the result is fairly easy to crack and then powders and seperates from the burlap. I tried adding latex paint which was suggested on numerous web pages as a way to add elasticity, but it wasn't strong at all. I suspect in several layers either of these approaches could work, but I was looking for a thin shell that would be strong enough to use as a wall without building up layer after layer... I tried plastic cement, which is a bagged cement product with clay and other plasticizers and that worked fairly well, mixed 2 parts medium grit sand and 1 part plastic cement. However, it takes a long time to dry and in two layers is only so-so for strength and it does crack easily when moderate pressure is applied. Then I tried traditional sculpture recipes using 20 minute casting plaster, which in two layers was generally stronger than the cement-and-burlap products, and was rock hard in 20 minutes. The problem with that stuff is it isn't weatherproof at all, and dissolves in damp conditions and I was concerned about water vapor condensating in the wall, as it will in any structure when the outside temp is lower than the inside temp unless you have a vapor barrier paint on the inside of your walls or something similar. So then I tried 5 minute post hole cement, which failed miserably but did set up rapidly. It crumbled and powdered into oblivion very readily when even mild pressure was applied. I tried a bunch of other things, and finally tried rapid set non-shrinking grout, which in two layers (one layer half overlapping the next like shingles) is very strong, about as rigid as 1/2" plywood coated in stucco. You can crack it with a lot of force or a hammer blow, so I added latex conctrete binder to the mix and that added a fair bit of elasticity to it so that when it dried, it was tougher and when enough force was applied to break it, it would bend slightly before breaking, and only a fine hairline crack would appear as opposed to a large shattering pattern. In 30 minutes it achieves 3,000 psi compressive strength, 6,000 overnight, and 9,000 after 28 days. (That's what the bag says, Cement-All fast set non-shrinking grout.) It sets up underwater, is highly durable in wet locations, and has a sandy beige color when dry. The seams between pieces of burlap feather into each other and disappear, and when dry all it takes is dampening to be able to blend another layer of burlap-crete onto it without any cracking and the splice becomes seamless. If you add too much concrete binder (a latex glue), it will shrink when drying and cracks will appear, so I misted the walls for half an hour while drying, and came up with appropriate amounts of concrete binder to add. This stuff sets up quickly, and if you use a plastic or metal bucket you won't have it very long. Get a rubber feed bucket, a big one about three feet around and a little over a foot high. That way when the stuff turns into rock which it will until you find out what mix to use for your climate, you can flex the bucket and the stuff will drop out. I tried it on cold days and warm days and windy sunny days: each time I needed a different amount of water. The basic mix I found worked well was to use a plastic 5 gallon bucket to mix it in and then pour it into the rubber feed bucket for use with the burlap. The sacks of Cement-All are 55 lbs. and you can mix a sack in a 5 gal. bucket, but as soon as you're done mixing, pour it into the rubber bucket and clean your plastic one immediately. Using a 1/2" drill with a mixing paddle is probably mandatory, I hung mine from a rope at a comfortable working height so all I had to do was pull the trigger to mix the stuff. It depends on the brand of concrete binder as to how much of the stuff to use. Sika brand required 3 quarts of binder and 2 quarts of water per 55 lb. bag of Cement-All grout. Supercrete brand concrete binder is a lot thicker, and slightly less expensive, so that I can use 1 quart of it and 5 quarts of water. Put the binder and water in your plastic bucket first, then pour half the sack of Cement-All in and stir with your paddle on a drill for about 30 seconds, add half of what's left and stir again, and add the last of it and stir until creamy smooth, about a minute or two. Be sure you've got your burlap soaking water overnight, cut into pieces that will more-or-less fit in your big rubber feed bucket, about two-feet square works good. Ring it out a little and dip it in, working the Cement-All into it from both sides by turning the burlap over a few times. Work your hands around the bottom of the rubber bucket as the Cement-All will start to harden there but you can get it workable again by pressing it around with your GLOVED hands and it will mix with the water from the burlap. If it sets up too quickly, make sure your next piece of burlap isn't rung out so well, a little water goes a long way. Soupy is good, but don't go beyond pea soup consistency in terms of making it more runny. You'll need a ton of large wooden clothes pins to hold the burlap onto your wire wall, I used stock panels, but you could get really sculptural with this stuff. Because it really does set up FAST, I put the clothes pins onto the wire about a foot above where I'll be putting the burlap, so that it will be all ready to go the moment I need them. Oil the clothes pins, otherwise they'll become part of your project on a permanent basis. To hold the middle of the burlap against he wire, press the clothespins around the wire trapping the burlap against them, this leaves an interesting divot in your wall, which can be filled later with more Cement-All if you don't like the look of it. Seriously, if you overlap each layer half way, it ends up 2 layers thick and is as strong as a 'real' house covered in stucco. Then you can do whatever you want inside, out of the rain and sun and wind while you finish your project. It's non-flammable, too, and you can cut out the doors and windows as needed later on, using a cutting wheel or sawzall. I'm thinking of using it for a roof at about three layers thick, but it isn't cheap at about $18 per 55 lb. sack. The concrete binder is less than $10 per gallon, and only one quart is needed if you use the Quickrete brand. In an overlapping pattern, a sack will yeild about 25 square feet of soaked burlap. One person can work quickly enough to deal with the entire process alone if pre-planning is employed. Wear a dust mask while mixing the stuff, its dusty. When hard, its fairly rough in texture, but by dampening it and then smoothing on a thinned out layer of Cement-All with binder with your gloved hand, from the remnants of your rubber bucket full of stuff when there isn't enough left to coat an entire piece of burlap, you can even out the contours and make it nice and smooth. You can also add cement coloring powder to a final coat and trowel it or glove it on. I'm not at 'my' computer now, I'll add a follow up with pictures soon. John A
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|current||12:04, 8 November 2008||720 × 455 (67 KB)||Crasch||Here's a photo of the house before burlap: a ferrocement ring at the base, encasing steel studs, covered in stock panels. http://www.annesleyphoto.com/Building/ferroring7511.jpg Here's the structure from another angle, partly covered in burlap-crete. htt|
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