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Compare Concrete & Clay
Questions on Earth Blocks

Question: Compare Concrete & Clay

Clay bricks have been in use for more than a thousand years and so they are well known compared to concrete, that has been around for only 200 years.

There still are those that prefer clay to concrete claiming that concrete exhibits poor resistance to damp, and cracks more than clay brick walls do.

Clay bricks are made in a process that starts with a suitable blend of clays that have to be mined, aged, then milled/mixed to even consistency. The clay is then extruded through a special press and sliced to size. These unburnt bricks are dried out before being placed in a kiln that is heated to between 700c and 1100c. Thereafter, when the firing is complete, the bricks need to be cooled and classified as to colour and strength.
Technically clay and concrete products have different properties that affect the way they should be built into a wall. Clay bricks tend to expand after manufacture in the first few years of their life about 3mm to 5mm over 10 metres of wall length. So expansion joints need to be provided. Concrete bricks on the other hand tend to shrink about the same amount (partly curing and partly drying out) usually in the first 6 months after construction. Concrete masonry walls, just like clay, need contraction joints.
Contrary to common belief, concrete bricks have relatively low moisture absorption of about 5% to 7%, whereas clay bricks are very variable, depending on the burning and type of clay, can range from 5% to 20% moisture intake. Clay bricks need to be wet before laying otherwise they suck the moisture out of the mortar, whereas concrete bricks need to be laid dry. If they are wet they ‘float’ and the mortar does not acquire sufficient early stiffness to keep the wall from sagging out of shape.

Stength & Durabilty

Building regulations stipulate the same strength in clay and concrete bricks for a specific design of a wall. Durability is generally a function of strength. The clay brick by nature of its manufacturing process hardens from the outside inwards. If the outer face is damaged it is possible that the soft inner could deteriorate. Concrete on the other hand tends to have the same strength throughout.

To obtain resistance to weathering, clay face bricks need to be greater than 30 Mpa (megapascal) and concrete about 20 Mpa. Concrete tends to deteriorate at temperatures above 450C.

Concrete brick are formed in a mold and then allowed to cure (preferably for about 28 days). Clay brick are cut from a block of clay and kiln-fired after the addition of various powders to achieve the desired color.

Clay brick have lower water absorption rates, higher strength (up to 5,000psi vs 1,200-1,800psi), have higher heat resistance, weather better, hold their color better (vs colored concrete). 

*psi (pound per square inch)

Clay Paver Bricks versus Concrete for Walkways & Patios

Paving clay bricks are especially designed for ground cover and traffic and can withstand severe temperatures and weather conditions. Traditional clay paver bricks can be purchased with beveled edges, chamfered edges (to help with water run off) or standard edges and come in many different colours and finishes. Prices for clay paver bricks and concrete pavers vary, but in general a homeowner can expect to pay more for concrete pavers than traditional clay bricks.



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Question: Questions on Earth Blocks

Q: What are Earth Blocks?

A: Earth Blocks, or compressed earth blocks (CEBs), are machine-made adobes, a modern variation on an ancient building material. The blocks are typically composed of 60% soil with high clay content (30-35% is ideal), 35% sand, and 5% lime as a stabilizer to create a water-resistant block. The percentage of each ingredient will vary from region to region depending on soil composition. Each of our earth press machines uses a high pressure hydraulic ram to form approximately 7.5 blocks per minute or 450 blocks per hour.

Q: How are Earth Blocks different than adobes?

A: Sun-dried adobes are made by pouring a mixture of mud and straw into forms on the ground, with the frequent addition of asphalt emulsion, a petroleum by-product, as the most widely used stabilizer. The adobes cure in the sun for as little as two weeks before being used and considerably longer depending upon weather conditions, but do not reach full strength for at least 30 days. Among the reasons we prefer earth blocks is their consistent size and quality, the speed of production, and the more natural and non-toxic stabilizer. In addition, their uniform size makes for a more rapid building process, with associated savings in labour costs, as the courses of blocks are even and only a thin layer of slurry between courses is required as opposed to a thick mortar joint. Finally, earth blocks have a much higher PSI, a measure of compressive strength, than sun-dried adobes which typically average between 300 and 350 PSI. While sun-dried adobes are sufficiently strong enough to meet the state of New Mexico's code requirement of 300 PSI, Earth Blocks typically test out at over 750 PSI. Strengths as high as 1800 PSI have been recorded for stabilized Earth Blocks.

Q: What are the dimensions of Earth Blocks?

A: The different machines on the market today produce blocks of varying size. Our machines have the capacity to produce blocks with the following dimensions and weights:

25.4cm × 35.56cm × 10.16cm - 18kg.
20.32cm × 35.56cm × 10.16cm - 14.5kg.
17.78cm × 35.56cm × 10.16cm -12.7kg.

Note: thickness may be adjusted between 7.62cm and 12.7cm

Q: How much does an Earth Block building cost?

A: Building costs vary depending on site location, soil composition, and architectural design. If the site dirt is not suitable for making blocks, either dirt is trucked on site to make blocks or the blocks are made elsewhere and trucked to the site. Trucking costs for dirt or blocks can depends on your country, the location of the site and amount of dirt or number of blocks needed. The design of the building ultimately determines the number of blocks and man hours involved in construction. Curved walls, multiple stories, and eccentric details are more difficult to construct and may require highly skilled labourers, thus creating more labour costs.

Q: How do you figure the number of earth blocks needed?

A: Typically, 5,000 blocks are needed for the exterior of approximately 140 square metres, single-story structure. There are approximately 28 blocks per square metre of wall space in a 25.4cm thick wall, and approximately 39 blocks per square metre in a 35.56cm wall. Deduct for doors and windows and add 5% for breakage.

Q: How much dirt do I needed?

A: For 25.4cm blocks, each block weighs approximately 18.4kg; 50 blocks = 1 ton/~ 65 blocks = 1 yard of dirt (±1.3 tons/yard depending on dirt composition and moisture content)

Q: How do I know if the site dirt is suitable for making Earth Blocks?

A: Soils need to be tested by a qualified testing laboratory to make an accurate analysis of whether or not it is useable for construction with Earth Block. The following four factors are important to consider:

" Plasticity Index - the percent of clay in the soil, a very important factor in determining the binding quality of the block, and the percentage of aggregate to be added
" Sieve Analysis - the granular distribution of the sample in terms of clay, sand, and gravel
" PSI - a measure of the compressive strength of the block which must meet code requirements
" Modulus of Rupture - a measurement of the resistance to fracture

A common general test is the cigar test. Take a handful of dirt, form it into the shape of a cigar, lay it on a flat surface and push it slowly over the edge. If the cigar stays intact for the first inch you may have soil worthy of making Earth Blocks.

Q: How does Earth Block construction compare to conventional building techniques?

A: An Earth Block building is not only healthy for the individual, but also for the planet. Dirt, an Earth Block's primary ingredient, is a renewable, non-toxic natural resource. Earth Block walls are virtually breathable, soundproof, bug-proof, fireproof, and even bulletproof! Requiring less transportation of materials than conventional construction methods, Earth Blocks have a lower embodied energy than conventional building materials. Energy savings are immediate with thermal mass equating to lowered heating and cooling requirements. The durability of an Earth Block building means it will last for centuries! Ancient earthen structures still stand today in certain parts of the world.

Q: What does it mean to have a 'breathable' wall?

A: A breathable wall is porous and allows indoor vapors, toxins, odours, and moisture or humidity to pass slowly through the wall to the outside, as well as fresh air from the outdoors to enter the building, thereby maintaining a healthier indoor environment. In addition to promoting the health of the occupants and facilitating temperature control, the longevity of the wall is enhanced by its porosity and flexibility. Insulation and other synthetic materials such as stucco applied to Earth Block walls will hinder its breathability, whereas lime or clay plasters do not.

Q: What is thermal mass?

A: Thermal mass is the ability of a material to retain heat. The thick walls of an Earth Block building create thermal mass, protecting the occupants from extreme temperatures with a building that stays cool in the summer and warm in the winter. Heating and cooling requirements in an Earth Block building are substantially reduced in an Earth Block home.

Q: What is embodied energy?

A: Embodied energy is the total amount of energy required to manufacture a product; including the initial removal of the raw material, transportation, processing, distribution, and its final usage. The more embodied energy in a product the greater the carbon dioxide (CO2) emissions, and thus, contribution to the greenhouse effect and global warming. Although the block machine requires diesel fuel, only ten gallons are needed to produce 3,000 blocks. Moreover, substituting bio-diesel reduces pollution and makes use of a waste product. Compared to a 10'× 10' wood-framed wall that has an embodied energy equivalent to 6 gallons of gasoline, an adobe wall of the same size only requires about 0.2 gallons of gasoline.

Q: Are Earth Blocks safe in earthquake-prone areas?

A: Special techniques may be used to resist ground movement in seismic zones, including horizontal wire ladder reinforcement, vertical steel-reinforced concrete columns, and through-tied galvanized stucco wire on both sides of the walls. Shake table tests in Berkeley, California and Peru have clearly demonstrated the effectiveness of these procedures.

Q: Why stabilize Earth Blocks?

A: Using 5-8% lime or Portland cement stabilizes the blocks and eliminates concerns about potential water damage during construction. Concrete footers keep the blocks from coming in contact with the ground and a cement bond beam protects the top. Stabilized Earth Blocks are water-resistant, but need protective plastering to be considered completely weather-proof. The compressive strength (PSI) of the blocks is also greatly enhanced by stabilization.

Q: Which stabilizer is best?

A: We believe that lime is a superior product. However, in many instances Portland cement is used as a stabilizer and is very effective. Unlike cement, lime is a more natural product, more breathable, has self-healing properties which aid in the repair of any cracks which may appear, has a lower embodied energy, and even absorbs CO2.

Q: What is the R-value and U-value of Earth Blocks?

A: The R-value, or resistance to heat flow, is the most common used measurement for determining building material's insulating properties. The greater the R-value of a material, the better it is at resisting heat loss (or heat gain). An earth block wall has very little insulative or R-value. Although proponents of other wall systems point to a high R-value as advantageous due to the ability to resist heat loss or gain, an earthen structure with breathable walls has the advantage of U-value (BTU/hr/ft2/°F). The U-value is based on the materials thermal transmittance abilities given a difference between the interior and outside temperatures. The lesser the U-factor of a material, the better it is at transmitting heat. Consequently, the interior temperature of the building changes slowly. A 16" adobe wall has a U-factor of .163 compared to .490 for concrete blocks.

Q: Does an Earth Block building require insulation?

A: In extreme climates Earth Block buildings can be double walled with an air space in between that is either left empty or filled with dirt or foam. Walls can be covered with foam sheets or sprayed foam for added temperature maintenance on those sides of the building that are exposed to extreme temperatures.

Q: Where can I see photos of the Earth Block process?

A: Go to: http://www.flickr.com/photos/tierraycal/sets. If the link isn't working, please cut and paste the URL into your browser.



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