- Soils that qualify for both Compressed Earth Block and Rammed Earth
are common in most areas. Consider that most of the continents are
granitic and decomposed granite is normally perfect having the
ratios of feldspars to quartz that are appropriate for compaction.
The basaltic soils are a little more difficult and many times
require additional clay added. The basic formula is 30% clay and the
balance loam and small aggregate. Caliche which is usually a
misnomer for decomposed limestone soils is the common subsoil of the
alluvial plain which dominates the south
Texas
landscape, much of the Midwest and most of the deep south as well as
most of the
Caribbean
. In The Dominican Republic it is named for the coral reefs that
underly the island and is somewhat compactable depending on the
area. The use of decomposed limestone can be problematic unless
modified with either the addition of clay,
Portland
or lime if necessary.
- Soils that are bentonitic or highly expansive are normally
unsuitable for earth construction without modification. The shrink
and swell capacity of these soils, related to their clay content can
cause the block to be highly susceptible to moisture, even high
humidity, however the acid test is how the clays actually perform
under compaction and even poor performance can be offset by
stabilization. Soil cracking after rainfall may indicate expansive
soil. Soil must be tested to determine its suitability. The ideal is
a block or wall that looks pretty and has a lot of strength but even
ugly block and marginal soils can be used to build a structure that
will last for centuries.
- Desirable qualities for
soil construction materials include:
- Strength
- Low
Moisture Absorption
- Limited
Shrink/Swell Reaction
- High
Resistance to Erosion and Chemical Attack
- availability
·
3.1 Soil Testing
- Soil
testing techniques vary, and include laboratory as well as field
testing. Testing is done in three phases: laboratory testing,
construction mix testing, and quality control testing. Laboratory
testing should always be done early in the design process, using
representative samples of soil intended for use. (See Resources
section for laboratories.) Engineering properties for which soils
are tested include permeability, stability, plasticity and
cohesion, compactibility, durability, and abrasiveness. Shrinkage,
swelling and compressive strength are important aspects of soil
suitability.
- Again, it is possible
to alter soils to make them suitable for construction by
stabilizing them. Stabilizing soil helps to inhibit the shrink and
swell potential, and aids in the binding of soil components. Soil
can be stabilized through chemical or mechanical means or both.
For information on mechanical methods, see Section 5.0 on rammed
earth.
·
3.2 Chemical Soil Stabilization
- Lime,
portland
, and pozzolan (high silica volcanic ash) can be used as chemical
additives. Lime is most effective on clay soils, and can be used
in combination with
portland
and pozzolan. Hydrated lime, as opposed to quick lime, should be
used. Lime is inexpensive, but care must be taken to protect
workers from breathing in lime dust. Cement is relatively
inexpensive, but requires large energy inputs in its production
process. However, cement produces the strongest block and will
substitute for clay poor soils where lime will not and the normal
usage of between 5 and 10% minimizes the embodied energy
especially when compared to concrete and lumber products (source).
Pozzolan exists in plentiful supply in
Texas
, but is not readily available commercially.
- (The Center
for Maximum Potential Building Systems (CMPBS) in
Austin
is experimenting with the use of pozzolan as an additive and
offers considerable expertise in earth materials use. See the Resources
section.)
·
3.3 Strength of tested earth and caliche block
- Unfired Compressed
Earth Block with addition of 5-10% cement can easily pass the
Uniform Building Code standards for compression with an average of
960 psi.
- Rammed earth walls have
been tested with a compressive strength of 30 to 90 psi
immediately after forming. Ultimate compressive strength should
reach 450-800 psi. If cement is added, compressive strength will
increase.
- The Uniform Building
Code for single and two story buildings requires block bearing
capacity of 300 psi bearing strength. Blocks manufactured with a
hydraulic press have been tested with a bearing capacity
immediately after production of 700 psi. Such soil block continues
to cure, until blocks reach a typical bearing capacity of 1000 psi.,
far exceeding requirements of the Uniform Building Code and HUD
standards. Cement can be added to the soil block mixture to reach
a bearing capacity of 2500-3900 psi.
·
3.4 Soil Handling
- The use of earth as
building materials is inexpensive for materials costs, but
emphasizes labor in construction methods. The right equipment and
coordinated labor are important in the soil material construction
process. Even a small structure may require at least 15 tons of
earth. This material must be moved and handled at least twice. A
front end loader, skidsteer or tractor equipped with a shovel or
back hoe will be necessary for on-site extraction of soil
materials as well as processing the soil and loading the
machinery. A large flat area with good drainage is necessary for
handling and processing the materials as well as making the
blocks. The building footprint should also be accessible by truck
for rammed earth construction.
·
4.0 Block Production Methods
- A backhoe and or a
front end loader will be needed to dig the soil on-site or handle
soils imported. Soils are then tested to prove their
compactability and to determine any needed additions such as sand
or clay. The next step of hydrating and mixing has traditionally
been the largest labor and time investment being done either by
hand or with a front end loader. The use of concrete and stucco
mixers have proven ineffectual for large projects such as a home,
however there are earth mixing or blending machinery available
that are especially cost effective for adding Portland or lime and
for adding water in dry areas. Source
- Sun Dried Adobe
- Molding techniques may
be in the form of monolithic walls (See Rammed
Earth) or molded into blocks or bricks. For the latter, the
mix is poured into molds, or pressure molded using special
machinery. These methods provide for a variety of standard and
custom size and shapes of block. With the hand mold technique, the
prepared mix is poured into damp or oiled molds, spread evenly,
and the molds are shaken slightly to ensure even filling of the
forms. The blocks are then removed and allowed to cure before
stacking.
- Air curing must occur
for 10-14 days before the block can be used in construction.
Protection from direct sunlight for 5 days and protection from
rain throughout the curing process are important. Drying bricks
may be temporarily covered with tarps or plastic sheeting, but
these must be removed for curing to continue. Once bricks are
sufficiently cured, they can be set on end to continue drying.
- With a wheelbarrow and
gang forms, a crew of two can produce 300 to 400 bricks per day.
With the addition of a plaster mixer and gang forms for 500
bricks, this production can be doubled. The addition of a front
end loader with a driver will additionally increase production.
- Compressed Earth Block
- Compressed earth or
soil block can be manufactured on site with a variety of
block-making machines, including hydraulic presses, mechanical
presses, and various combinations. Some mechanical presses are
small enough to be operated by hand (Cinva-Ram, for instance).
With a mobile industrial block machine, powered by a diesel
engine, as many as 800 blocks can be produced per hour. Compressed
soil blocks can be used immediately. They continue to cure and
gain strength after they are installed. When green (before they
are cured), they can be readily shaped or nailed into with hand
tools.
Compressed Earth Block come in two basic types, The
vertical press where the block are normally 10” x 14” (there are
many variations) that are fixed with the height of the block nominally 3
½” which is variable due to the variability of the soil. These block
are treated like Adobe in that they need to be mortared and cut to fit. The
Horizontal Press are of a fixed dimension normally 4” x
14”(again there are variations) with a length of the block variable
from 2” to 12” depending on the machine. These blocks do not require
mortar and can be dry stacked with ease by basic skilled workers, the
block can also be custom sized to minimize cutting for electrical,
plumbing and wall changes.
·
4.3 Mortaring
- Mortar for blocks must
be applied to the entire surface of the block, as opposed to
ribbon mortar beds often used with conventional brick. Full
surface mortaring allows for maximum compressive strength. The
same soil used in block making, mixed with water to form a slurry,
is usually used as a mortar for binding blocks together into
floors and walls. Cement can be added to the mortar mix, but this
increases the cost. The main advantage of cement mortar is
stabilization.
·
4.4 Design Methods
- Block size can be
varied easily to accommodate a variety of designs. Walls can be
sculptured, rounded, or formed into keystone arches to create
custom effects. Relatively unskilled labor can be utilized in
construction with compressed earth block.
- Design of structural
walls using any soil material block must take into account wall
height and thickness, size of block, mass value (source),
and the desired style and finish. Wall height-to-thickness ratio
must be adequate for stability (source).
- Because thermal mass
equates to insulation in soil block a minimum of 12 inches is
needed for a comfortable abode.
- Earth block structures
need not have the "pueblo" style if this is not desired.
In fact a gable or hip roof can protect the home better while
offering solar protection from western exposures. A bond or collar
beam is necessary if the roof is supported by the walls. This will
serve to spread the loads over the entire wall, and stabilize the
tops of the walls from horizontal movement. (See code)
Plasters
- Soil blocks are
typically stuccoed or plastered to
prevent them from getting wet, however, any veneer or siding can
be used on Pressed Earth Block as they can hold a nail or staple.
Interior finishes are normally plaster (structolite) or earth
plasters that are simple to apply and maintain. Petroleum based
finished do not work well with earth unstabilized earth block and
cement plasters do not stick to asphalt stabilized adobe. A common
mix for a stabilized interior mud plaster is 5%
Portland
to 30% minimum clay fine screened with window screen. Exterior mud
plaster will need 6 to 10%
Portland
with 30% minimum clay and 1/8” screen.
- Fully stabilized
structures do not require any exterior finish unless desired for
aesthetics.
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