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Colloquium:• Introduction
The Context:• Natural Building
• The Buil ding Codes
• Societal Impact Matrix
• Return of The Village
• Habitat For Humanity
• Earthmother Dwelli ng
• Intuitive Design
• Curves of Breath & Clay
• Feng Shui
The Art:• Overview of Techniques
• Nature, Earth & Magi c
• Hybrid House
• Barefoot Architecture
• History of Cob
• Cob Q & A
• Natural Composites
• Compressed Earth B locks
• Adobe Oven
• Earthen Floor
• Earthbags
• Honey House
• German Clay Building
• Straw-bale Dome
• Earthen Plaster & Ali z
• Natural Pai nts
• Bamboo
Technology:• Solar Distiller
• Solar Water Heater
• Composting Toilets
• Watson Wick
• Solar Ovens
Home Page:
networkearth
Compressed Earth BlocksWAYNE NELSON
At the inception in 1976 of Habitat for Humanity International, a
major provider of housing for the world's poor, one of the first
decisions made by the board of directors was to use locally-
available materials as much as possible. This choice is the basis
of a more sustainable building system, which is empowering to the
people of the community, and better for the environment. In my
experience teaching in Africa and elsewhere we have often used
Compressed Earth Block.
Compressed Earth Block (CEB) is one name given to earthenbricks compressed with hand-operated or motorized hydraulic
machines. There are hundreds of types of machines. Some of
these are designed with simple levers which are easily
manufactured in local machine shops, so that in many countries
they have seen widespread use. Other presses contain a variety of
compression mechanisms such as cams, hand-operated hydraulic
assists, toggles and motorized hydraulics. As these presses are
more complex and expensive, they have had less frequent use. In
many areas of the world, proper materials are available for making
CEB, and thus this type of block may be a better choice than any
other building material. The choice to use CEB is dependent on
several factors, including culture, labor force, and most
importantly, the preference of the homeowner.
Hand-operated presses have been used for many decades. Before
that, and still today, some people make the blocks by beating soil
into a wooden mold with a stick. "Rammed earth" is a similar
process, in which case a structure is made as one continuous
mass of compressed earth.
Modern equipment, with hydraulics driven by diesel, gas or electric
motors, may be useful in urban areas or for large multi-house
sites. However, the difficulty of using motor-driven equipment for smaller and more rural projects is hard to grasp until you have lived
in a remote village situation. People in these situations are often
the ones that need the most help with improving their housing, and
compressed earth block can be one of the solutions to their
building needs. However, one must be aware that significant
problems with maintenance, expense of fuel and spare parts, as
well as the tools and expertise needed to repair and maintain the
presses, have been experienced by many local builders who have
tried to use these machines without understanding their proper
place in the local economy.
Advantages of Using Compressed Earth Blocks
Advantages of CEB include: uniform building component sizes,
use of locally-available materials and reduction of transportation.
Uniformly sized building components can result in less waste,
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faster construction and the possibility of using other pre-made
components or modular manufactured building elements. Such
modular elements as sheet metal roofing can be easily integrated
into a CEB structure. The possibility of using such components
can often improve the overall quality of the structure as well.
The use of natural, locally-available materials makes good housing
available to more people, and keeps money in the local economy
rather than spending it to import materials, fuel and replacement
parts. The earth used is generally subsoil, leaving topsoil for agriculture. Building with local materials can employ local people,
and is more sustainable in times of civil unrest or economic
difficulties. People can often continue to build good shelters for
themselves regardless of the political situation of the country.
The reduction of transportation time, cost and attendant pollution
can also make CEB more environmentally friendly than other
materials. Most of the time, soil for CEB can be found on site or
within a short distance. In most of the world's economies, the most
cost-effective transportation is often that provided by people or
animals. In Ghana, for instance, 3' x 5' wagons were built with oldcar wheels and tires. It was less expensive to have a few people
move 20 cement bags, or two 55-gallon drums of sand in one load
with this wagon for two miles up- and down-hill than to hire any
motorized form of transport such as a bus, truck, or tractor with
wagon, because all the latter are dependent on fuel and parts
purchased from economies outside that of the local community.
Innovations in Compressed Earth Brick
Over time, innovation in CEB manufacture has included changing
the original shape from a solid rectangle to one incorporating holes
or grooves in the blocks to allow for the use of steel or bamboo toresist earthquakes. Another innovation was to create interlocking
shapes that don't need to be laid in a bed of mortar, and U-shapes
or tapered bricks for use in reinforced lintels or arches. Floor tiles
and roof tiles can be made on some machines. Sometimes the
press has been used to make bricks which were then fired in a
kiln, although once fired they are no longer referred to as CEBs.
The interlocking shapes of these improved bricks can help to
reduce the skill level needed for homeowners to build their own
homes. In addition, several layers of blocks can be placed in the
wall at a time. A supervisor can then check that the wall is straight
and plumb before giving the OK for mortar to be poured inside the
hollow blocks to lock them together, thus eliminating the small
openings between blocks which would otherwise allow for air
infiltration.
Materials on some of the different machines are available through
the European development agency, GATE, although I wouldn't rely
on their comparisons of speed and ease of operation. Some
presses are shown in their Appropriate Building Materials book by
R. Stulz and K. Mukerji. In addition, the earthbuilding organization
CRATerre in France and the community of Auroville in India have
experience with many types of presses.
My favorite hand-operated press is the "TEK" from University of
Science and Technology in Ghana. The TEK ram is simple and
inexpensive. There is a similar French machine which is great and
simple to operate as well, but it costs ten times as much. Both
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machines create a CEB block using only one operation. The lid is
tripped automatically with the handle motion so there is no
separate operation to close the lid or lock it, nor the need for a
second lever to pull or push. I like the size of the brick from this
press as it is bigger than most. It measures 30 cm long x 23cm x
15cm (12" x 9" x 6"). I have used the 15cm dimension for the width
of the wall, but wall thickness could be made to any of the three
dimensions simply by turning the block. Bigger blocks means
fewer elements that I have to move around as I build, therefore
speeding up the process.
I like to reduce the amount of material I have to use to make a
brick, so "frogs" which dimple the body of the brick as it is pressed
are nice. I have also reduced the thickness of a brick by putting a
one inch board in the bottom of a 6" mold. This simple operation
made my wall 5" thick, still sufficient for a small simple, decent
house (such as Habitat for Humanity builds, which is affordable to
most people in the world).
The Earth Mix
There are several simple publications on select ing earth and using
a press. VITA has a good one, as do some machine
manufacturers. The UN has simple soils testing information, and
many universities can help test soils as well. In addition, soils
engineering has been done in labs for road-base materials which
are basically the same materials that are good to use for walls,
floors and arched roofs. However, there is nothing like experience
to tell how the soil will work, and many indigenous folks know local
soils that are good for different purposes such as building. If you
are on your own, look at cuts at the roadside or river bank to see
how different soil types resist erosion. If the soil resists erosion
well, it could be a good building material.
There are two simple soil tests that I like to use to determine the
proper soil mixture. For the first test, fill a clear jar with straight
sides half full of soil and then three quarters full of water, adding a
pinch of salt. I like to use a plastic peanut-butter jar with a lid. Cap
it tight and shake it up until all is in solution. It is good to let it
soak for a few minutes and then shake the solution again to get it
mixed well. Normally, layers of different particle sizes will form as
it settles. This may take a few minutes with the salt added or a few
hours without salt. The large particles on the bottom of the jar will
be sand, then silt will settle out and finally a clay layer will form on
top. Mark the height of each layer to get a rough calculation of the
percentages of each material. With hand presses 10% clay is a
minimum amount to make strong blocks. To see the clay layer
separately, open the jar and let the sample dry. The clay is a
binder, so it sticks together and will curl up and shrink, pulling
away from the layers below. Think of a mud puddle drying after a
rain with the clay curling up on the top as it dries.
The other test is to build a form 2' long and 1.5" wide x 1.5" tall
that is then filled with moist soil. Wait a week, letting it dry in the
shade. Once the soil sample is dry, you will notice a gap between
the end of the clay body and the end of the box. This gap showshow much the clay shrinks. Acceptable shrinkage is less than two
inches and preferably 1/4" or less. The less shrinkage the better
the soil. If you have a lot of shrinkage, indicating a high clay soil
(over 25% clay), you might add sand to make a better mix. A soil
with a very expansive clay may be rejected after this test, but soils
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in close proximity or lower in the ground may have less expansive
clays. When you find a good soil, it is important to keep testing
the soil even after you start to make bricks to insure quality.
The stability of the clay can also be observed by placing the brick
in a bucket of water after it is compressed, and seeing how quickly
the clay will expand and explode the particles from the brick. If the
brick is stabilized and well cured, placing the brick in a bucket of
water, stream or lake can be a great way to show its water
resistance to those who might doubt that soil can be used to builda good house.
Stabilizers for Compressed Earth Brick
Stabilizers such as cement, gypsum, lime and the liquid types
have been used in the body or on the surface of the bricks.
Sometimes it is less costly to make a soil block with no cement
and then use cement for the mortar joints and a cement plaster
which adheres to the mortar joint. (Cement plaster doesn't stick
well to unstabilized earth bricks, so the mortar joint is used to hold
the plaster.) Natural earth plasters with natural additives can alsobe used on unstabilized walls. These plasters may require
maintenance on a regular basis, which is similar to giving your
house a fresh coat of paint.
When using cement stabilizers, it is very important to cure the
cement by keeping the brick moist. This may mean watering twice
a day for a couple of weeks and covering the wall with plastic or
leaves to keep the sun and wind off and the moisture in. Cement
cures with water, so letting it dry too quickly, especially during the
first week or two of curing, robs it of its strength. Blocks without
stabilizers should also be dried in the shade to avoid excessive
drying around the edges which may cause cracking.
When mixing the stabilizers with the earth, it is important to make
sure they are mixed completely, especially when using smaller
amounts. Using a 3% cement-by-volume ratio takes a lot of mixing
to get the cement distributed thoroughly. I recommend using a
garden rake, although most of the time the stabilized earth is
mixed with shovels in a similar fashion to mixing concrete. I prefer
the rake with its many tines because it mixes more thoroughly,
breaks up particles of dirt, and, if water needs to be added to bring
it to the right consistency, spreads the water through the mix more
easily. I find moving the rake back and forth through the mix easier
and faster than picking it up with a shovel and turning it over. I can
feel when the moisture content is correct more easily with the rake
by how it feels when it is moved through the mix.
Being a person who is always looking for an easier way to do
things, bigger, simpler and faster are things I like. I've heard it said
that if you want to see how to get something done easily, watch
how a lazy man does it. I would caution that you may have to
modify a lazy man's way a bit to get it done right, but I think there
is truth in that statement which can lead to simplified work
processes.
Moisture Content
One of the most unique things about making CEB is the moisture
content. It is common to find people making the mix too wet. The
moisture content of the proper mix is so minimal that it never really
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seems wet. People often have experience with mixing concrete or
adobe, the moisture contents of which are way too high for CEB
mixtures. One simple test is to take a handful of CEB mix and
squeeze it into a ball as tight as you can. If the mix stays in a ball
when you open your hand, the moisture content is good. If it falls
apart in your hand you need to wet it more. When the ball stays
together in your hand, drop it from your waist to the ground. If it
sticks together or breaks in only two or three parts when it hits the
ground, it is too wet. In this case add some more dry ingredients,
keeping the ratio of soil to cement equal to your original mix. If itbreaks into dozens of pieces, add a little water. It is common to err
at this point by adding too much water. The little water you add will
take a lot of mixing to get it thoroughly into the mix, so don't rush
at this point by adding more water than needed, because it is a lot
of work to dry the mix out again.
Why is it good to use as little moisture as possible? Minimal
moisture content results in better strength, water resistance,
durability and thermal mass in the finished block. An
understanding of the structure of soil is helpful in grasping its
interaction with moisture. Imagine that we magnify the size of theparticle and look at it. If you look at a pile of stones you see space
between the stones. This is similar to how sand particles look. If
you try to compress the pile of stones, they really don't compress
because one rests on top of the other. The spaces are similar to
those between the sand particles in a CEB. We want to fill the
spaces with clay because the clay acts to bind the sand particles
together. Sand does not have the characteristic of sticking
together well when it is dry, but the addition of clay binds the sand,
helping the block keep its shape. To get clay to stick together
well, it is first saturated with water. Mixing the wet clay helps the
clay particles to align with each other. Because clay particles are
flat, rather than round or angular like sand, they will stick together or to sand like wet paper.
I like to pull a few bills out of my pocket and toss them in the air to
give an example of how dry, loose clay particles act. They all fall
loosely away from each other. I pick up the bills, laying one neatly
on top of the other, and wet them with water, press them together
to get any excess water out, then throw them in the air again. Now
they fall as one unit and remain stuck together. I can let them dry
and toss them in the air again they again fall as one unit. In a book
that has gotten wet, to open the pages you almost have to take
them apart one by one. This is how wet clay particles bind
together and bind the sand particles together as well.
When a block machine compresses a block, it reduces the volume
by 30%. It does this by mechanically aligning the moist clay
particles, removing the air pockets and sticking the clay to the
sand. If too much water is in the mix, there will be more air space
between the particles when the brick dries. This reduces the
strength and thermal mass, and makes the surface more porous
so it is not as resistant to water and scratches. If there is more
clay than is needed to fill the spaces between the sand particles,
the block becomes weaker because clay compresses more than
sand, especially when wet. However, weaker earth walls can befine too, as we know from the use of cob and adobe. In these
cases the thickness of the wall may need to be increased to carry
the same roof loads.
Production
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How fast can you make blocks and begin building? Efficient
workers using hand presses can make 500 a day, resulting in one
week's work for the walls of one house. I've done 80 blocks in a
day by myself, so you might plan on five to ten people making 500
in a day, or several machines working at the same time in order to
make all the bricks for a house in a day. A hydraulic machine can
make all the blocks for a large house in a day, if you can feed the
soil to it quickly.
After they are pressed, blocks can be set aside to cure. When
using the hand-operated presses, it is not recommended to stack
them on top of each other the first day, and remember to handle
the blocks as little as possible so as not to knock off the fragile
corners. The next day the blocks can be stacked several layers
high to make room for the next batch to be set out. It is possible,
however, to place bricks directly in the wall, either dry-stacked or
mortared, so with enough people and the right equipment the walls
of a simple house can be completed in a day. Some mechanical
soil crushers, sifters and mixers are available to speed the
process. As with all repetitive processes, speed can be increasedby thinking through the steps and organizing the process
efficiently, thus avoiding machine down time.
Most people who build with earth find it quite enjoyable. It can be
just plain fun to make your house from the earth under your feet.
Enjoy the earth God has given us.
Wayne Nelson works with Habitat for Humanity's Department of
the Environment and is an international provider of construction
information and training. Trained as a carpenter and builder, he
has particular experience with creating shelters with compressed
earth blocks in Africa and other countries.
Construction and Environment Resources
Habitat for Humanity International
121 Habitat St., Americus GA 31709
ph 912-924-6935
www.habitat.org
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