Appalachia--Science in the Public Interest

PRESSED EARTH BLOCKS

by Thom Price

1. Introduction

Anyone who has been in a lumber yard recently knows that
prices for solid wood building products are high and quality is
low. This is because we are running out of trees. Each year the
timber industry cuts smaller and smaller trees to fulfill the
high demand for lumber. These days, it is not unusual to find a
2 x 4 with the pith on one side and traces of bark on the other.
Indiscriminate harvesting of trees to meet demands for lumber
cannot continue forever. We must reduce the amount of wood used
in building houses if our forests are to have a fighting chance
for survival.

To approach construction more responsibly we must adopt
building materials that,

1) require little energy to manufacture;

2) are native to the area in which they are used, requiring
little transportation;

3) do not degrade the environment or rob
the planet of its vital organs (trees are the earth's lungs);

4) are safe to handle and live in;

5) are affordable to the poor; and

6) will last for centuries. Reconstituted wood products --
made from wood clearcut from forests, broken down, mixed with
glue and reformed into beams and boards -- do not meet these
criteria. Neither do steel, concrete, burnt bricks nor synthetic
materials which require vast amounts of energy to produce and
transport and compound pollution problems.

What material known to humans could possibly meet such
stringent criteria? Mud! Using earth to build homes pre-dates
written history and, in many poor regions of the globe, is still
quite common. Mud is found almost all over the world and --
pardon the pun -- is "dirt" cheap. Earth can be made into
building materials that provide better insulation than
conventional materials, are stronger than burnt bricks, are
durable in harsh weather, and will last for generations. The
trick for turning common dirt into a suitable building material
is to add a small amount of stabilizer (e.g. cement or lime) for
strength and durability, and then compact it so it will hold
together. Adobe, burnt brick, rammed earth, cob construction and
pressed earth block are a few varieties of earth building
techniques. Of these, pressed earth block (PEB) construction is
superior because the stabilizers used and the pressure applied to
the blocks make PEB a more rugged and durable building material.
This supplement will give a basic procedure for using PEB
construction to build safe durable buildings without contributing
to the destruction of the environment.

Advantages of PEB

PEB buildings stay cooler in the summer and warmer in the
winter than conventional buildings.
The material costs for PEB walls are very low.
Soil suitable for earth block can be found in most areas.
In areas where the soil is not naturally suited to PEB, it can be
easily modified.
PEBs are fireproof.
PEBs provide excellent thermal storage for houses
employing passive solar designs.
PEBs are twice as strong as burnt brick.
PEB construction requires very little wood and has minimal
impact on the environment. One program in Africa has built PEB
homes with fiber-concrete roofs that require only 1/2 tree per
house. This contrasts favorably with similar houses built of
burnt brick which require twenty trees worth of energy.
Because PEB construction requires a lot of human labor, it
can help build community and family relations.
PEB walls are very attractive. If desired, colorants can
be added to the mixture for various hues.
Tremendous satisfaction can be derived from digging your
own house from the earth.

Disadvantages

Manufacturing PEBs is time consuming and labor intensive.
In earthquake zones, PEB homes are not safe unless the
blocks are used as fill for timber frame or poured concrete post
and beam construction.
Because of the long cure periods, time must be well
managed to ensure labor is used efficiently.
* The outside of a PEB house must be protected from the
direct impact of rain and splash from the roof.

2. Technique

This supplement focuses on techniques for making and using
pressed earth block, particularly in the construction of homes.
However, it is far from a comprehensive home building guide.
Those who do not have extensive experience in building houses may
wish to consult other sources, especially for information on
foundations, roofs, interior surfaces and block laying. See
reference list.

Preparing the Foundation

The foundation for a PEB house can be either cement or stone
with mortar. Often, rocks are a by-product of digging and
sifting soil to make blocks. It may be wise to incorporate these
rocks into the foundation.

Care should be taken to ensure that the foundation is level.
It needs to be at least one foot higher than the grade and should
be as deep as the local frost line. To avoid creating a ledge
that might retain water, the foundation should be no wider than
the wall it supports. Surrounding ground should slope away from
the house and proper drainage beds should be created around the
perimeter of the house to minimize the chance of standing water
coming in contact with the blocks. Additionally, the eaves of
the house should extend at least two feet beyond the walls to
provide protection against rain and splash. PEBs are very
vulnerable to erosion and deterioration from standing water and
heavy rains. The roof and foundation should be like a wide
brimmed hat and a good pair of boots that protect the earth
blocks from excessive moisture.

Digging the Soil

As a first step, all organic matter, including the topsoil,
should be removed from the subsoil by digging down about a foot.
This top layer should be saved and later replaced on the
excavated site and planted with vegetation to prevent erosion.
To prepare the subsoil for block making it should be processed
through a sifter. A sifter can easily be constructed by making a
4' x 4' frame of 2 x 4s and covering the bottom with 1/4" wire
mesh. This sifter can then be suspended like a hammock between
two trees or posts and the soil worked through it. Rocks that do
not sift through should be removed and saved for other uses.
When there is sufficient sifted soil to make blocks, a load
should be mixed while the process of digging and sifting
continues. It is important to protect both the sifted and
unsifted soil from rain and excessive moisture.

Testing the Soil

Subsoil is typically made up of a combination of clay, sand
and silt. To prepare for making PEBs, a test should be conducted
to determine the proportion of each of these components present
in the soil in preparation for block making. It may be necessary
to supplement certain components to obtain the optimum ratio for
block making. The ratio does not have to be precise, but
proportions should be about 75% sand, 15% clay and 10% silt.
Having less than 10% silt does not pose a serious problem, but if
there is too much silt, sand and clay should be added. Having
more than 25% clay can result in excessive shrinkage when the
blocks dry and can yield a weaker block. Too much clay should be
off-set by adding sand. Sand can be purchased or, better yet,
can often be easily and cheaply acquired from a local creek bed.
In the rare instance that there is insufficient clay in the soil,
Bentonite clay can usually be purchased from builder supply
stores that carry masonry products to use as a supplement.

To determine the composition of a soil sample, simply fill a
wide mouth glass jar half way with sifted soil. Add a few
teaspoons of salt and fill the jar with water. Place a lid on
the jar and shake vigorously for three minutes. Allow the
contents of the jar to settle overnight. The soil will settle
into three layers which can be measured with a ruler to determine
the respective proportions.

After establishing the composition of the soil and adding
clay or sand as necessary, a shrinkage test should be performed.
The shrinkage test will indicate how much stabilizer needs to be
added to the soil mix. To conduct a shrinkage test, first make a
wooden trough with interior dimensions 1.5" x 1.5" x 24".
Moisten the soil to be tested enough so that it will stick
together, but not so much that it will stick to your hand when
pressed in your palm. Another way to check for proper moisture
content is to make a ball of soil and drop it on a hard surface.
If properly moistened, the ball should break into small chunks
but it should not pulverize. Once the soil is properly
moistened, it should be packed firmly in the trough (completely
filling the trough) and allowed to dry in a sunny window for
three days. After three days, the dried soil should be pushed to
one end of the trough and the gap measured. The size of the gap
will indicate how much stabilizer needs to be added to the soil
to give the blocks optimum strength and durability:

1/2" or less 1 part stabilizer to 18 parts soil
1/2" to 1" 1 part stabilizer to 16 parts soil
1" to 1-1/2" 1 part stabilizer to 14 parts soil

Mixing Soil, Stabilizer and Water

Mixing soil, stabilizer and water can be done either quickly
or inexpensively. The quick method is to use a rented or
borrowed electric concrete mixer. The inexpensive method is to
use a good wheelbarrow or build a shallow box (4' x 4' x 6") and
mix the soil with a concrete mixing hoe. Regardless of which
method is chosen, the dry ingredients should be mixed first. If
the ratio is to be 1:16, add one shovel-full of stabilizer, 16
shovels-full of soil, mix and repeat. Once the dry ingredients
are mixed, water can be added. Water should be added sparingly.
Adequately moistened soil may appear too dry, but the urge to add
more water should be resisted. The proper moisture content is
determined using the same method employed before the shrinkage
test.

Making Blocks

The most important tool for making blocks is a stabilized
block press. New block presses are difficult to find in the
U.S., but they can sometimes be purchased used. International
relief organizations, such as Habitat for Humanity in Americus,
GA, can be helpful in locating a press. The U.S. distributor of
the most common press, the "CINVA-Ram," stopped importing it in
the 1980s (It sold for $175).

The manufacturer of the CINVA-Ram is:

Metalibec
Carrera 08B, No. 18-30
Bogota, Columbia
Phone: 011-57-12613277

In using the press, it is very important to fill it with the
proper amount of soil mixture. A scoop can be made from wood or
sheet metal to measure the proper amount for each block. If the
press is not completely filled, the block will not be subjected
to adequate pressure and will be weak. If the press is over-
filled, the block will be too big or the press may break.

The CINVA-Ram requires 130 pounds of force be applied to the
lever for each block. If water drips from the press when the
block is compressed, this indicates that the soil in the press is
too wet and should be discarded. To make removing the blocks
from the press easier, a kerosene/oil mixture can be lightly
sprayed on the inside of the press before filling.

Curing

As the blocks are removed from the press, they should be
inspected for defects and carefully stacked on a clean, flat,
level surface in the shade. One way to conserve space and
provide shelter for the blocks is to construct the roof for the
house first, mount it on removable posts over the foundation (but
not where it will interfere with wall construction) and use the
floor of the house as a curing area. The blocks should be
stacked in groups of three, side by side,with a one inch space
between blocks. The blocks should be layered, alternating
direction for each layer, with no more than five layers per
stack. The stacks should be sprinkled with water daily
throughout the duration of the wet cure and covered to hold in
moisture. The duration of the wet cure period depends on the
type of stabilizer used. After the wet cure is complete, the
blocks should be uncovered and allowed to stand for the duration
of the dry cure. Never underestimate the importance of careful
curing.

Building Walls

Building walls is the most challenging part of PEB
construction. Those who are not familiar with masonry should
seek assistance and information from other sources, such as those
listed at the end of this supplement. Construction with pressed
earth blocks is essentially the same as construction with burnt
brick. The blocks made by the CINVA-Ram are 3-1/2" x 5-1/2" x
11-1/2". Allowing 1/2" for mortar between blocks, building units
(foundation size, placement of doors and windows, etc.) should be
in one foot increments. There are a number of methods available
for wall construction. The timber frame or poured concrete post
and beam methods use a single layer of PEB as in-fill and rigid
foam covered with stucco on the outside. Another option for
construction is the plain block method. The plain block method
employs a double layer of PEB connected by brackets, leaving an
air space or using foam between the layers. Blocks can be
connected using mortar or can be dry stacked and then cemented
with an eighth inch of fiberglass bonding cement (Surewall brand)
on both sides.

To build walls using the plain block method, first nail
together the door frames and brace them in place. Next, without
using mortar, lay the first course of blocks around the perimeter
of the house to establish the proper placement. Then, using 1/2"
of mortar under the first course and between blocks, lay up
several layers at the corners and door jams, staggering the
joints. Be sure to moisten the blocks before they are laid
unless fiberglass cement is being used instead of mortar. Once
laid, the corners will act as guides for constructing the rest of
the walls. Stretch mason's twine across the length of the wall
to keep the courses straight. Move the twine up with each course
to keep level. Use a 4' level to check for plumb. Blocks can be
cut using a brick layer's hammer or a masonry chisel.

Once up to the window level, concrete window sills should be
poured. These sills should be constructed to shed water away
from the window. Next, the window frames should be nailed
together and braced in place. Continue laying block until the
tops of the window and door frames are reached. Use large
timbers to construct lintels over the windows and doors. The
lintels should be as wide as the wall, as thick as two or three
courses, and should extend at least one foot beyond the sides of
the opening. Fill in between the lintels with bricks. Make
forms and pour a binder beam of reinforced concrete around the
top perimeter of the wall. If the roof was made and suspended
above the structure before building the walls, the joists can be
notched and integrated into the binder beam and the temporary
support posts removed once the concrete has cured.

Completing Construction

If the roof was not erected before the walls, it can be done
once the binder beam has cured. In the interest of protecting
the environment, roofing options other than plywood and asphalt
shingles should be considered. Finishing windows, doors, and the
interior is generally the same as with other types of
construction, though a single layer of PEBs can be used for
inside walls. For a durable finish on the outside of the home,
mortar can be left to ooze from the joints and the rest of the
exterior walls covered with stucco. Instead of stucco, -- if you
want people to see all of your hard work -- a coat of masonry
sealer can be used to protect the exterior. This coat should be
reapplied about every two years. Plaster, sheetrock or furring
strips can be used for the interior walls or they can be left
bare. In addition to blocks, the CINVA-Ram can also make thin
tiles for a very durable floor surface.

3. Stabilizers and Mortar

Though it is commonly used in pressed earth block
construction, Portland Cement is expensive, from both an
environmental and monetary standpoint. Cement requires a huge
amount of energy to produce. It is usually made by centrally
located large corporations and must be transported across great
distances to the consumer. Considering that even a small house
can require several tons of stabilizer, cement is an expensive
choice.

Fortunately, there are alternatives that can be used. The
first consideration in choosing a stabilizer should be the
environmental impact of its manufacture and the distance over
which it must be transported. If you live near one of the more
efficient cement plants (cement production can consume energy
across a broad range, varying anywhere from 2 to 10 megajoules
per kg) and cement can be purchased for a fairly low price, it
may be the best option. However, for most bagged cement, this
will not be the case. Using lime instead of cement or adding it
as a supplement to cement is one way to reduce the environmental
and economic costs of the stabilizer.

Stabilizers

Slaked Lime (calcium hydroxide or agricultural lime) is
pulverized limestone that, when wetted expands and gets hot,
breaking down into a fine powder without the use of additional
heat. Lime is the primary ingredient in Portland Cement;
however, in its unprocessed form, it requires twice as long to
cure as processed cement (2 weeks wet cure, 2 weeks dry cure, 4
more weeks max strength). Slaked lime mixes better with high
clay soils than cement and makes a stronger brick if this type of
soil is used. To use slaked lime, simply add lime to the soil in
the proportions determined by the shrinkage test. Once the
lime and soil are mixed and properly moistened, the mixture
should be covered and allowed to stand for one or two days before
blocks are made, moistening the mixture periodically. After this
curing period, any remaining lumps should be broken down before
pressing blocks.

Portland Cement is made from lime and clay that is mixed,
fired at 1450 degrees Celsius, and ground along with a trace of
gypsum into a fine gray powder. Portland Cement works best with
sandy soils. To use cement, mix with the soil in proportions
prescribed by the shrinkage test and add water. This mixture
should be pressed into block within about a half hour of being
wetted. If the mixture begins to harden, it should be discarded.
Blocks stabilized with cement should wet cure for 1 week and dry
cure for 1 week; they reach maximum strength after 2 more weeks.

A Lime/Cement Combination may be a good compromise, reducing
the amount of cement used while requiring less time for curing
than a pure lime stabilizer. To use a combination stabilizer,
equal amounts of lime and cement will be needed. First add the
lime to the soil, mix, moisten and keep covered for 1 to 2 days.
After this period, break down any remaining lumps, add cement and
moisten as necessary before making blocks. The blocks should be
allowed to wet cure for 10 days and dry cure for 10 days; they
reach maximum strength after 20 more days.

The cure times given are approximate and may vary depending
on temperature, humidity and air currents. Construction of walls
may begin any time after the dry cure is complete but blocks
should not be placed under extremely heavy stress until the
maximum strength period has passed.

Mortars

Mortars, like stabilizers, can be made using various
proportions of cement and lime. A good environmentally sound
mortar for PEBs can be made using 1 part Hydraulic Lime, 4 parts
sand and enough water to make the mortar workable.

Pre-packaged mortars, like Brixment brand type-N Coplary
Cement, can be used to make a very strong but expensive and
environmentally costly mortar. A homemade version of this mortar
can be made by mixing 1 part Portland Cement with 6 parts sand
and enough water to make workable.

Another strong mortar can be made by combining 1 part
Portland Cement, 4 parts lime, 32 parts sand and water.

When mixing mortar, mix all dry ingredients thoroughly
before adding water. After laying up bricks in mortar, keep the
mortar moist for a day or so.

4. Typical Cost for a PEB Home.

Here is a rough estimate of what you can expect to spend for a
20' x 20' cabin using pure lime in the stabilizer and mortar,
masonry in the foundation, pressed earth tile floor, and a tin
roof. This estimate assumes that the lintels and rafters are
made from timber cut from the owners property and milled using a
borrowed portable bandsaw mill.

Concrete and Rebar (footer, window sills and binder beam $300
Slaked Lime 165
Hydraulic Lime 30
Sand (10 tons) 160
Gravel (11 tons) 100
Windows, salvage (6) 60
Doors, salvage (2) 25
Roof 400
Rafters 200
Window and door frames 100
Lintels 75
Insulation 65
Masonry sealer, caulking 200
Misc. hardware 100
$1980

REFERENCES

Folsom, C. D. "You Can Build with Soil-Cement Blocks" The
Mother Earth News. 39 (June, 1978), 28-31.

Houben, H. and H. Guillaud. Earth Construction Primer.
Brussels: United Nations, 1984.

Kern, Ken. Owner Built Home. New York: Scribner's Sons, 1972.

Stultz, Roland. Appropriate Building Materials. Croton-on-
Hudson, NY: Intermediate Technology Publications, 1981.

Volunteers in Technical Assistance. Making Building Blocks with
the CINVA-Ram Block Press. Arlington, VA: VITA, 1966.

Wolfskill, Lyle A., Wayne A. Dunlap and Bob McGalaway. Handbook
for Building Houses of Earth. Washington, DC: U.S. Agency for
International Development (available from: National
Technical Information Service, Springfield, VA 22161).