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Adobe (/əˈdoʊbi/, UK /əˈdoʊb/,[1] Spanish pronunciation: [aˈðoβe]; Arabic: الطوب) is the Spanish word for mud brick, a natural building material made from sand, clay, water, and some kind of fibrous or organic material (sticks, straw, and/or manure), usually shaped into bricks using molds and dried in the sun. Adobe buildings are similar to cob and rammed earth buildings, but cob and rammed earth are directly made into walls rather than bricks. The Romanian name for this material is chirpici. For a deeper understanding of adobe, one might examine a cob building. Cob, a close cousin to adobe, contains proportioned amounts of soil, clay, water, manure, and straw. This is blended, but not formed like adobe. Cob is spread and piled over the home's frame and allowed to air dry for several months before habitation. Adobe, then, can be described as dried bricks of cob, stacked and mortared together with more adobe mixture to create a thick wall and/or roof. Adobe structures are extremely durable, and account for some of the oldest existing buildings in the world. Compared to wooden buildings, adobe buildings offer significant advantages due to their greater thermal mass, in hot climates, but they are known to be particularly susceptible to earthquake damage.,[2][3] Buildings made of sun-dried earth are common in West Asia, North Africa, West Africa,[4] South America, southwestern North America, Spain (usually in the Mudéjar style), Eastern Europe[5] and East Anglia, particularly Norfolk, known as clay lump.[6] 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, although often substantial amounts of stone are used in the walls of Pueblo buildings.[7] (Also, the Pueblo people built their adobe structures with handfuls or basketfuls of adobe, until the Spanish introduced them to the making of bricks.) Adobe brickmaking was used in Spain starting by the Late Bronze Age and Iron Age, from the eighth century B.C. on.[8] Its wide use can be attributed to its simplicity of design and manufacture, and the economy of creating it.[9] 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. Etymology Church at San Pedro de Atacama, Chile 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, and finally Coptic (c. 600 BC), τωωβε dj-b-t became tobe "[mud] brick." This was borrowed into Arabic as al tob,[10] tuba,[11] or Al-ţŭb.[12] (الطّوب al "the" + ţŭb. "brick") "[mud] brick," which was assimilated into Old Spanish as adobe [aˈdobe], still with the meaning "mud brick." English borrowed the word from Spanish in the early 18th century. Adobe style in Santa Fe, New Mexico 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. (Compare with stucco). Composition An adobe brick is a composite material made of clay mixed with water and an organic material such as straw or dung. The soil composition typically contains clay and sand. Straw is useful in binding the brick together and allowing the brick to dry evenly, thereby preventing cracking due to uneven shrinkage rates through the brick.[13] Dung offers the same advantage and is also added to repel insects.[citation needed] The most desirable soil texture for producing the mud of adobe is 15% clay, 10-30% silt and 55-75% fine sand.[14] Another source quotes 15-25% clay and the remainder sand and coarser particles up to cobbles 2-10 inches with no deleterious effect. Modern adobe is stabilized with either emulsified asphalt or Portland cement up to 10% by weight. The clay content should be a mixture of no more that half expansive clays with the remainder non-expansive illite or kaolinite. Too much expansive clay results in uneven drying through the brick and 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.[15] Adobe bricks Adobe bricks near a construction site in Milyanfan, Kyrgyzstan Bricks are made in an open frame, 25 by 36 cm (10 by 14 in) being a reasonable size, but any convenient size is acceptable. The mixture is molded into a frame, and then the frame is removed after initial setting. After drying a few hours, the bricks are turned on edge to finish drying. Slow drying in shade reduces cracking. The same mixture used to make bricks, but without straw, is used for mortar and often for plaster on interior and exterior walls. Some ancient cultures used lime-based cement for the plaster to protect against rain damage.[citation needed] The brick’s thickness is preferred partially due to its thermal characteristics, and partially due to the stability of a thicker brick versus a more standard-sized brick. 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, rebar or wooden posts. Experience has shown straw, cement, or manure added to a standard adobe mixture can all produce a stronger, more crack-resistant brick.[16] A general testing 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. After it is sealed, the container is shaken vigorously for at least one minute. It is then allowed to sit on a flat surface for a day or so until the soil has settled into layers or remains in suspension. Heavier particles settle out first, so gravel will be on the bottom, 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. Material properties 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[17] call for a minimum compressive strength of 300 lbf/in.2 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/in.2 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 averages out 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 conductivity of adobe is quoted as having an R value of R0 = 0.41 hr*ft2*°F/(Btu*in).[18] To determine the total R value, multiply R0, by the thickness of the adobe wall. From that, 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).[19] The density is 95 lbm/ft3 or 1520 kg/m3. The thermal diffusivity is calculated to 0.0105 ft2/hr or 2.72x10−7 m2/s. Adobe wall construction When building an adobe structure, the ground should be compressed because the weight of adobe bricks is significantly greater than a frame house, and foundation settling may cause cracking in the wall. The footing is dug and compressed once again. Footing depth depends on the region and its ground frost level. The footing and stem wall are commonly 24 and 14 inches, much larger than a frame house because of the weight of the walls. Adobe bricks are laid by course. Each course is laid the whole length of the wall, overlapping at the corners on a layer of adobe mortar. Adobe walls usually never rise above two stories because they are load bearing and have low structural strength. When placing window and door openings, a lintel is placed on top of the opening to support the bricks above. Atop the last courses of brick, bond beams made of reinforced concrete or heavy wood beams 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 wall, finishes can be applied, such as mud plaster, whitewash or stucco. These finishes protect the adobe wall from water damage, but need to be reapplied periodically, or the walls can be finished with other nontraditional plasters providing longer protection. Bricks made with stabilized adobe generally do not need protection of plasters. Adobe roof 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. Roof materials 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. Traditional adobe roof The more traditional flat adobe roofs are functional only in dry climates that are not exposed to snow loads which would call for a steepled roof. Cement may be introduced into the adobe mixture to stabilize and strengthen the composite of mud and organic matter. The heaviest wooden beams are called Vigas which support smaller members called latias [20] upon which brush is then laid atop which the adobe layer applied. Raising a traditional adobe roof To raise a flattened adobe roof, beams of wood or metal should be assembled and span the extent of the building. The ends of the beams should then be attached to the tops of the walls. Taking into account the material from which the beams and walls are made, choosing the attachments may prove difficult. A combination of the bricks and adobe mortar that are laid across the beams creates an even load-bearing pressure that can last for many years depending on attrition. Once the vigas, satias 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 an inch thick to make certain there is ample strength between the brick’s edges and also to provide a relative moisture barrier during rain.[citation needed] Attributes 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 (bricks) was the Bam Citadel, which suffered serious damage (up to 80%) by an earthquake on 26 December 2003. Other large adobe structures are the Huaca del Sol in Peru, with 100 million signed bricks, the ciudellas of Chan Chan and Tambo Colorado, both in Peru (in South America). |
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