Compressed Earth Blocks, River Raisin Institute

Intro to Compressed Earth Block, with illustrations

Compressed Earth Blocks

WAYNE 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 earthen bricks 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, 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 old car 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 to resist 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 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 selecting 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 shows how 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 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 build a 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 also be 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 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 it breaks 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 the particle 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 be fine 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

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 increased by 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

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