Tuesday, March 4, 2008
Burlapcrete is basically loose-weave burlap, soaked in a modified rapid setting cement. Is there any advantage to using burlapcrete instead of exterior plywood sheathing? The potential advantages are: it doesn’t warp, swell, delaminate, mold, burn, get chewed through by rodents, requires fewer tools and skills to master, and can weather years of direct contact with the elements with no noticed adverse reactions (no warping, splitting, rotting, weakening, molding, no insect or rodent issues). Twenty minutes with a blowtorch does nothing. It’s resilient, not brittle- so a hammer blow bounces off. Later I’ll tell you exactly how to make it- but first a little history.
There are a few folks that have used burlap soaked in cement slurry and draped it over wire form work, therefore being able to build a house with a similar approach as a large piece of sculpture- but I’ve found that burlap prepared this way is weak unless done in several layers. Another way that has been tried is to stretch the burlap over an armature and paint a weak cement solution on it, letting it dry, and adding five to ten layers– which takes about forever unless you have lots of people. I’ve tried a bunch of different mixes while researching how to make ‘burlapcrete’and have come up with one that works so well its just about amazing! My exterior walls on my 40’ diameter round house 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 very strong. 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 it worked fairly well, I mixed 2 parts medium grit sand and 1 part plastic cement. However, it takes a long time to dry so any movement or wind at your work site could keep the burlap moving while it sets up, and then you have a dry, powdery piece of burlap waving in the breeze. In two layers this mix 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 so I was concerned about water vapor condensating in the wall, as it will in any structure when the outside temperature 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 by flexing the sample by hand. 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 with 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 invisible. 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.
Cement-All sets up quickly, and if you use a plastic or metal bucket you won’t have it very long. Get a rubber feed bucket from a livestock supply business, 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. Sometimes the water coming out of my hose was hot, and that changes the setting time as well- too hot and the stuff turns to rock while you’re still mixing it. 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 gallon 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- a drill with a small motor will not have the torque required to mix this stiff solution. I hung my half inch drill 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 in water overnight, cut into pieces that will more-or-less fit in your big rubber feed bucket, about two-feet square works good. Wring 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 that’s soaked into the burlap. If it sets up too quickly, make sure your next piece of burlap isn’t wrung 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 hang 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 t 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 had been thinking of using it for a roof at about three layers thick, but it isn’t cheap at about $18 per 55 lb. sack, and my tests showed that more than 3 coats would be required and I settled on metal lath over stock panels, and non-rapid set construction grout modified with pva fibers as my roofing material. Back to Burlap-Crete in wall applications: concrete binder is less than $10 per gallon, and only one quart is needed per bag of Cement-All if you use the Quickrete brand of concrete binder. 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 with a gloved hand, smoothing on a thinned out layer of Cement-All 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 the wall nice and smooth. You can also add cement coloring powder to a final coat and trowel it or glove it on.
A note on cost effectiveness: the materials cost for a section of wall including the heavy wire formwork aka stock panels, works out to about $1.45 per square foot as of October 2008. This does not include the framework (rebar, 2×4, or etc.) I calculated this based on a 50 square foot section of wall. The components included:
2 sacks Cement All, $18 ea. = $36
2 burlap totes, which are 49 square foot pieces of burlap @ $4 ea. = $8
1/2 container (two quarts) concrete binder: $5
most of a stock panel- they currently cost $23 ea in my area, so I estimated $18
Divided by approx. 50 square feet = $1.44 per square foot.