Tuesday, March 4, 2008

[papercreters] Burlap-Crete

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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