In the early 1900s, material started oozing out between the outer stones of the first construction period below the 150-foot (46 m) mark, and was referred to by tourists as "geological tuberculosis". This was caused by the weathering of the cement and rubble filler between the outer and inner walls. As the lower section of the monument was exposed to cold and hot and damp and dry weather conditions, the material dissolved and worked its way through the cracks between the stones of the outer wall, solidifying as it dripped down their outer surface.[59]
On August 23, 2011, the Washington Monument sustained damage during the 5.8 magnitude 2011 Virginia earthquake;[67] over 150 cracks were found in the monument.[68] A National Park Service spokesperson reported that inspectors discovered a crack near the top of the structure, and announced that the monument would be closed indefinitely.[69][70] A block in the pyramidion also was partially dislodged, and pieces of stone, stone chips, mortar, and paint chips came free of the monument and "littered" the interior stairs and observation deck.[71] The Park Service said it was bringing in two structural engineering firms (Wiss, Janney, Elstner Associates, Inc. and Tipping Mar Associates) with extensive experience in historic buildings and earthquake-damaged structures to assess the monument.[72]
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Officials said an examination of the monument's exterior revealed a "debris field" of mortar and pieces of stone around the base of the monument, and several "substantial" pieces of stone had fallen inside the memorial.[70] A crack in the central stone of the west face of the pyramidion was 1 inch (2.5 cm) wide and 4 feet (1.2 m) long.[73][74] Park Service inspectors also discovered that the elevator system had been damaged, and was operating only to the 250-foot (76 m) level, but was soon repaired.[75]
On September 27, 2011, Denali National Park ranger Brandon Latham arrived to assist four climbers belonging to a "difficult access" team from Wiss, Janney, Elstner Associates.[70][74] The reason for the inspection was the park agency's suspicion that there were more cracks on the monument's upper section not visible from the inside. The agency said it filled the cracks that occurred on August 23. After Hurricane Irene hit the area on August 27, water was discovered inside the memorial, leading the Park Service to suspect there was more undiscovered damage.[70] The rappellers used radios to report what they found to engineering experts on the ground.[76] Wiss, Janney, Elstner climber Dave Megerle took three hours to set up the rappelling equipment and set up a barrier around the monument's lightning rod system atop the pyramidion;[73] it was the first time the hatch in the pyramidion had been open since 2000.[73]
The external inspection of the monument was completed on October 5, 2011. In addition to the 4-foot (1.2 m) long west crack, the inspection found several corner cracks and surface spalls (pieces of stone broken loose) at or near the top of the monument, and more loss of joint mortar lower down the monument. The full report was issued in December 2011.[18] Bob Vogel, Superintendent of the National Mall and Memorial Parks, emphasized that the monument was not in danger of collapse. "It's structurally sound and not going anywhere", he told the national media at a press conference on September 26, 2011.[74]
More than $200,000 was spent between August 24 and September 26 inspecting the structure.[70] The National Park Service said that it would soon begin sealing the exterior cracks on the monument to protect it from rain and snow.[76][77]
Adobe bricks are rectangular prisms small enough that they can quickly air dry individually without cracking. They can be subsequently assembled, with the application of adobe mud to bond the individual bricks into a structure. There is no standard size, with substantial variations over the years and in different regions. In some areas a popular size measured 8 by 4 by 12 inches (20 cm 10 cm 30 cm) weighing about 25 pounds (11 kg); in other contexts the size is 10 by 4 by 14 inches (25 cm 10 cm 36 cm) weighing about 35 pounds (16 kg). The maximum sizes can reach up to 100 pounds (45 kg); above this weight it becomes difficult to move the pieces, and it is preferred to ram the mud in situ, resulting in a different typology known as rammed earth.
Buildings made of sun-dried earth are common throughout the world (Middle East, Western Asia, North Africa, West Africa, South America, southwestern North America, Southwestern and Eastern Europe.)[5] Adobe had been in use by indigenous peoples of the Americas in the Southwestern United States, Mesoamerica, and the Andes for several thousand years.[6] Puebloan peoples built their adobe structures with handsful or basketsful of adobe, until the Spanish introduced them to making bricks. Adobe bricks were used in Spain from the Late Bronze and Iron Ages (eighth century BCE onwards).[7] Its wide use can be attributed to its simplicity of design and manufacture, and economics.[8]
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 has been widely used, are an adequate composition.[14]
Poured and puddled adobe (puddled clay, piled earth), today called cob, is made by placing soft adobe in layers, rather than by making 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.[21] These are the oldest methods of building with adobe in the Americas until holes in the ground were used as forms, and later wooden forms used to make individual bricks were introduced by the Spanish.[22]
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 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 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 be 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, a lintel 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 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.
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.[28]
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.
The largest structure ever made from adobe is the Arg-é Bam built by the Achaemenid Empire. Other large adobe structures are the Huaca del Sol in Peru, with 100 million signed bricks and the ciudellas of Chan Chan and Tambo Colorado, both in Peru.
In the past, lime based cement binders, such as lime putty, were often used but sometimes with other hydraulic cements, (water resistant) such as a calcium aluminate cement or with Portland cement to form Portland cement concrete (named for its visual resemblance to Portland stone).[8][9] Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder. Concrete is distinct from mortar. Whereas concrete is itself a building material, mortar is a bonding agent that typically holds bricks, tiles and other masonry units together.[10] 2ff7e9595c
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