One of the oldest building methods, adobe is simply clay and sand mixed with water. Often, chopped straw or other fibers are added for strength. The mixture is then allowed to dry in the desired shape. Usually adobe is shaped into bricks that can be stacked to form walls.
Various claims are made about the optimal proportions of clay and sand (or larger aggregate).
Some say that the best adobe soil contains 15% – 30% clay to bind the material together. Others say equal proportions of clay and sand are best to prevent cracking or fragmenting of the bricks.
The blocks can either be poured into molds and dried, or pressed into blocks. Adobe colored with clay and polished with natural oil makes an attractive and resilient floor.
To protect the walls and reduce maintenance, adobe buildings usually have large overhanging eaves and sizeable foundations.
Adobe can be plastered over with cob or lime-based mixes for both appearance and protection.
Adobe has good thermal mass, meaning that it is slow to transmit heat or cold. It is not a good insulator, however, so insulation can be added (preferably on the outside), or a double wall built with airspace or insulation in between.
The traditional thick, un-insulated adobe has proven to perform best in regions without harsh winters or where daily sun is predictably available during those cold periods.
Adobe (US i/əˈdoʊbi/, UK /əˈdoʊb/; Spanish: [aˈðoβe], from Spanish: mud brick, from Arabic) is a building material made from earth and often organic material.
Most adobe buildings are similar to cob and rammed earth buildings. Adobe is among the earliest building materials, and is used throughout the world.
Adobe bricks are most often made into units weighing less than 100 pounds and small enough that they can quickly air dry individually without cracking and subsequently assembled, with the application of adobe mud, to bond the individual bricks into a structure.
Modern methods of construction allow the pouring of whole adobe walls that are reinforced with steel.
In dry climates, adobe structures are extremely durable, and account for some of the oldest existing buildings in the world. Adobe buildings offer significant advantages due to their greater thermal mass, but they are known to be particularly susceptible to earthquake damage if they are not somehow reinforced.
Cases where adobe structures were widely damaged during earthquakes include the 1976 Guatemala earthquake, the 2003 Bam earthquake and the 2010 Chile earthquake.
Buildings made of sun-dried earth are common throughout the world (Middle East, West Asia, North Africa, West Africa,South America, southwestern North America, Spain, and Eastern Europe.)
Adobe had been in use by indigenous peoples of the Americas in the Southwestern United States, Mesoamerica, and the Andean region of South America for several thousand years.
The Pueblo people built their adobe structures with handfuls or basketfuls of adobe, until the Spanish introduced them to making bricks. Adobe bricks were used in Spain from the Late Bronze Age and Iron Age, (8th century B.C. onwards).
Its wide use can be attributed to its simplicity of design and manufacture, and economics.
A distinction is sometimes made between the smaller adobes, which are about the size of ordinary baked bricks, and the larger adobines, some of which may be one to two yards (1–2 m) long.
The word adobe /əˈdoʊbiː/ has existed for around 4,000 years, with relatively little change in either pronunciation or meaning.
The word can be traced from the Middle Egyptian (c. 2000 BC) word dbt “mud brick.”
As Middle Egyptian evolved into Late Egyptian, Demotic (Egyptian), and finally Coptic (c. 600 BC), τωωβε dj-b-t becametobe “[mud] brick.”
This was borrowed into Arabic as al tob, tuba, or Al-ţŭb.(الطّوب al “the” + ţŭb. “brick“) “[mud] brick,” which was assimilated into Old Spanish asadobe [aˈdobe], still with the meaning “mud brick.”
English borrowed the word from Spanish in the early 18th century.
In more modern English usage, the term “adobe” has come to include a style of architecture popular in the desert climates of North America, especially in New Mexico.
An adobe brick is a composite material made of earth mixed with water and an organic material such as straw or dung.
The soil composition typically containssand, silt and clay. Straw or rice husk are useful in binding the brick together and allowing the brick to dry evenly, thereby preventing cracking due to uneven shrinkage rates through the brick.
Dung offers the same advantage. The most desirable soil texture for producing the mud of adobe is 15% clay, 10-30% silt and 55-75% fine sand.
Another source quotes 15-25% clay and the remainder sand and coarser particles up to cobbles 50 to 250 mm (2 to 10 in) with no deleterious effect.
Modern adobe is stabilized with either emulsified asphalt or Portland cement up to 10% by weight. This is known as Super Adobe.
No more than half the clay content should be expansive clays with the remainder non-expansive illite or kaolinite.
Too much expansive clay results in uneven drying through the brick resulting in cracking, while too much kaolinite will make a weak brick.
Typically the soils of the Southwest United States, where such construction is in use, are an adequate composition.
Adobe walls are load bearing, i.e. they carry their own weight into the foundation rather than by another structure, hence the adobe must have sufficient compressive strength.
In the United States, most building codes call for a minimum compressive strength of 300 lbf/in2 (2.07 newton/mm2) for the adobe block.
Adobe construction should be designed so as to avoid lateral structural loads that would cause bending loads.
The building codes require the building sustain a 1 g lateral acceleration earthquake load.
Such an acceleration will cause lateral loads on the walls, resulting in shear and bending and inducing tensile stresses.
To withstand such loads, the codes typically call for a tensile modulus of rupture strength of at least 50 lbf/in2 (0.345 newton/mm2) for the finished block.
In addition to being an inexpensive material with a small resource cost, adobe can serve as a significant heat reservoir due to the thermal properties inherent in the massive walls typical in adobe construction.
In climates typified by hot days and cool nights, the high thermal mass of adobe mediates the high and low temperatures of the day, moderating the living space temperature.
The massive walls require a large and relatively long input of heat from the sun (radiation) and from the surrounding air (convection) before they warm through to the interior.
After the sun sets and the temperature drops, the warm wall will then continue to transfer heat to the interior for several hours due to the time-lag effect.
Thus, a well-planned adobe wall of the appropriate thickness is very effective at controlling inside temperature through the wide daily fluctuations typical of desert climates, a factor which has contributed to its longevity as a building material.
Thermodynamic material properties are sparsely quoted. The thermal resistance of adobe is quoted as having an R-valueof R0 = 0.41 hr-ft2-°F/(Btu-in) and a conductivity of 0.57 W/(m-K) quoted from another source.
A third source provides the following properties: conductivity=0.30 Btu/(hr-ft-°F); heat capacity=0.24 Btu/(lbm-°F); density=106 lbm/ft3(1700 kg/m3).
To determine the total R-value of a wall for example, multiply R0 by the thickness of the wall.
From knowledge of the adobe density, heat capacity and a diffusivity value, the conductivity is found to be k = 0.20 Btu/(hr-ft-°F) or 0.35 W/(m-K).
The heat capacity is commonly quoted as cp = 0.20 Btu/(lbm*F) or 840 joules/(kg-K).
The density is 95 lbm/ft3 or 1520 kg/m3.
The thermal diffusivity is calculated to be 0.0105 ft2/hr or 2.72×10−7 m2/s.
Poured and Puddled Adobe Walls
Poured and puddled adobe (puddled clay, piled earth) today called cob, is made by placing soft adobe in layers, rather than making by individual dried bricks or using a form.
Puddle is a general term for a clay or clay and sand based material worked into a dense, plastic state.
These are the oldest methods of building with adobe in the Americas until holes in the ground were used as forms and then later wooden forms used to make individual bricks were introduced by the Spanish.
Bricks made from adobe are usually made by pressing the mud mixture into an open timber frame.
In North America, the brick is typically about 25 by 36 cm (10 by 14 in) in size.
The mixture is molded into the frame, which is then is removed after initial setting.
After drying for a few hours, the bricks are turned on edge to finish drying.
Slow drying in shade reduces cracking.
The same mixture, without straw, is used to make mortar and often plaster on interior and exterior walls.
Some ancient cultures used lime-based cement for the plaster to protect against rain damage.
Depending on the form into which the mixture is pressed, adobe can encompass nearly any shape or size, provided drying is even and the mixture includes reinforcement for larger bricks.
Reinforcement can include manure, straw, cement, rebaror wooden posts.
Experience has shown straw, cement, or manure added to a standard adobe mixture can all produce a stronger, more crack-resistant brick.
A test is done on the soil content first.
To do so, a sample of the soil is mixed into a clear container with some water, creating an almost completely saturated liquid.
The container is shaken vigorously for one minute.
It is then allowed to settle for a day until the soil has settled into layers.
Heavier particles settle out first, sand above, silt above that and very fine clay and organic matter will stay in suspension for days.
After the water has cleared, percentages of the various particles can be determined.
Fifty to 60 percent sand and 35 to 40 percent clay will yield strong bricks.
The New Mexico State University Extension Service recommends a mix of not more than 1/3 clay, not less than 1/2 sand, and never more than 1/3 silt.
Adobe Wall Construction
The earthen plaster removed exposing the adobe bricks at Fort St. Sebastian in France
The ground supporting an adobe structure should be compressed, as the weight of adobe wall is significant and foundation settling may cause cracking of the wall.
Footing depth is to below the ground frost level. The footing and stem wall are commonly 24 and 14 inches thick, respectively.
Modern construction codes call for the use of reinforcing steel in the footing and stem wall.
Adobe bricks are laid by course. Adobe walls usually never rise above two stories as they are load bearing and adobe has low structural strength.
When creating window and door openings, alintel is placed on top of the opening to support the bricks above.
Atop the last courses of brick, bond beams made of heavy wood beams or modern reinforced concrete are laid to provide a horizontal bearing plate for the roof beams and to redistribute lateral earthquake loads to shear walls more able to carry the forces.
To protect the interior and exterior adobe walls, finishes such as mud plaster, whitewash or stucco can be applied.
These protect the adobe wall from water damage, but need to be reapplied periodically.
Alternatively, the walls can be finished with other nontraditional plasters that provide longer protection.
Bricks made with stabilized adobe (cement added) generally do not need protection of plasters.
The traditional adobe roof has been constructed using a mixture of soil/clay, water, sand and organic materials.
The mixture was then formed and pressed into wood forms, producing rows of dried earth bricks that would then be laid across a support structure of wood and plastered into place with more adobe.
Depending on the materials available, a roof may be assembled using wood or metal beams to create a framework to begin layering adobe bricks.
Depending on the thickness of the adobe bricks, the framework has been preformed using a steel framing and a layering of a metal fencing or wiring over the framework to allow an even load as masses of adobe are spread across the metal fencing like cob and allowed to air dry accordingly.
This method was demonstrated with an adobe blend heavily impregnated with cement to allow even drying and prevent cracking.
The more traditional flat adobe roofs are functional only in dry climates that are not exposed to snow loads.
The heaviest wooden beams, called vigas, lie atop the wall. Across the vigas lie smaller members called latillas and upon those brush is then laid.
Finally, the adobe layer is applied.
To construct a flat adobe roof, beams of wood were laid to span the building, the ends of which were attached to the tops of the walls.
Once the vigas, latillas and brush are laid, adobe bricks are placed. An adobe roof is often laid with bricks slightly larger in width to ensure a greater expanse is covered when placing the bricks onto the roof.
Following each individual brick should be a layer of adobe mortar, recommended to be at least 25 mm (1 in) thick to make certain there is ample strength between the brick’s edges and also to provide a relative moisture barrier during rain.
Depending on the materials, adobe roofs can be inherently fire-proof.
The construction of a chimney can greatly influence the construction of the roof supports, creating an extra need for care in choosing the materials.
The builders can make an adobe chimney by stacking simple adobe bricks in a similar fashion as the surrounding walls.
Adobe Around the World
The largest structure ever made from adobe is the Bam Citadel.
Other large adobe structures are the Huaca del Sol inPeru, with 100 million signed bricks, the ciudellas of Chan Chan and Tambo Colorado, both in Peru.