Concrete is a compound made from sand, gravel, and cement, while cement is a mixture of minerals that become hard when water is added, binding the sand and gravel into a solid mass. Although concrete is traditionally considered an Ancient Roman invention, earlier forms have been found, such as in eastern European floors that date to around 5000 BC and in some ancient Indian stupas. The first civilization to use concrete extensively, however, was the Ancient Egyptian, where concrete has been found and dated as early as 2500 BC. However, it was the Romans who around 100 BC found that a far stronger material resulted from mixing a volcanic ash obtained from Pozzuoli with their normal lime-based concretes. This type of concrete came to be called pozzolana, which is any siliceous (or siliceous and aluminous) material containing little or no cement in itself but if finely divided and mixed with water will react with calcium hydroxide to form a cement-like material. In fact, it is from the Latin caementum, which refers to the materials, and concretus, which refers to the process of binding the materials together, that the modern-day names for these two materials are derived.
   The Romans also experimented with lightweight aggregates and used them in concrete in the cast dome of the Pantheon around AD 120. The Pantheon dome features coffering, or square sections carved out of the concrete to reduce the dome's weight but not its strength. Coffering became a widely used classical feature that later on assumed a more decorative function in such ceilings as flat timber roofs and small barrel vaults. Concrete was mainly used for Roman foundations, however, in which workers framed the concrete wall with a diagonal brick or stone setting called an opus reticulatum. Onto this pattern, the Romans then put a stone or stucco veneer to protect the concrete from moisture. The opus reticulatum was beneath the veneer and allowed it to adhere better, but when this design was rediscovered in the Renaissance revival of classicism, it was reused merely as a decorative pattern on the outside of Renaissance palace walls.
   Searching for even more inventive architectural forms, the Romans began to experiment with the use of bronze bars embedded in concrete, thereby creating the first reinforced concrete. But since the differing thermal expansion of the two materials often caused a problem in concrete breakage, its use never became widespread. The Romans had already begun to add horsehair to concrete to make it less likely to shrink when dried. They also added animal blood to make the concrete more frost-resistant.
   The rediscovery of concrete had to wait until 1756, however, when the British engineer John Smeaton created the first Portland cement. Then, in 1892, François Hennebique patented a ferro-cement, or reinforced concrete, which was threaded with steel to create not only a stronger material, but also to solve the problem of how to join the separate building materials together. Thus, with the integration of the binding material into the building material, the problem of the monolithic joint was solved. This innovation paved the way for larger unencumbered interior spaces and more highly technical structures. Reinforced concrete was first used in industrial buildings, but in 1903 Auguste Perret first introduced its use in domestic architecture in Paris with his eight-story apartment building constructed at 25 bis Rue Franklin.
   In the 20th century, Pier Luigi Nervi is best known for his aesthetic concrete designs that reflect his ideas on the integration of math and nature in order to create stronger yet more aesthetically pleasing feats of engineering. This Italian architect, after concluding his studies at the University of Bologna, began to experiment with wide-spanning construction in the 1920s on a series of airplane hangar commissions. He then went on to construct in Florence in 1931 a soccer stadium made entirely of reinforced concrete, called the Stadio Artemia Franchi. Using simple geometry and prefabricated concrete pieces, Nervi's structural ideas were very accessible and economical and therefore were widely adapted in the reconstruction of Europe after World War II. In 1959, Nervi built his famous Palazzetto dello Sport in Rome for the 1960 Olympics. Here he created a round concrete building with a concrete roof that resembles from the outside a tent-like canopy with its corners "staked" into the ground by flying buttresses. Made of precast concrete, this compressive form-active dome is one of the best-known unencumbered interior spaces.
   Marcel Breuer, Eero Saarinen, and Felix Candela began in the 1950s to experiment with a "softer," more expressive aesthetic for their concrete designs. Candela, a Spanish-born Mexican architect, is best known for his creation of a thin-shelled dome, which is a more efficient use of concrete and has minimal tensile forces. Clearly influenced by the hyperboloid structures of Antoni Gaudí in Barcelona, Candela, called the "shell builder," maintained that basic geometry, not complex math, was the key to successful lightweight construction in concrete, and that thicker concrete did not necessarily make the material any stronger. The Xochimilco Restaurant (Los Manantiales), built in Mexico City in 1958, demonstrates Candela's interest in modeling concrete into organic forms. With its corners appearing to balloon upward, propelled by wind, this thin-shelled concrete hyperbolic building resembles a giant ocean shell. Candela's aesthetic was further developed by J0rn Utzon in his Sydney Opera House, completed in 1973. These thin-shelled, organic-styled concrete roof coverings were more recently the source of inspiration for the Millennium Dome, a massive, mast-supported dome built in southeast London for the year-long millennium celebrations held there in 2000. Constructed by Richard Rogers and the structural engineer Buro Happold, this massive building is currently an indoor sports arena.
   Innovative modern uses for concrete also include the earlier daring types of cantilevering. In the 1940s, architects working in the United States, such as Walter Gropius and Marcel Breuer, continued the domestic use of cantilevered concrete, first seen in the work of Le Corbusier in France. This use of concrete could provide strong, horizontal lines and a crisp geometric framework consistent with the prevalent International style. It was Frank Lloyd Wright, however, who became best known for his use of cantilevered concrete in the American home, where porch terraces and widely overhanging rooflines formed the prairie-style aesthetic that Wright is so famous for. Wright's Fallingwater, built as a vacation home in rural Mill Run, Pennsylvania, in the 1930s, reveals the most daring use of cantilevered concrete. The Kaufmann family initially wanted a home that would overlook a waterfall, but instead Wright designed the house directly on top of the waterfall, with a strongly horizontal tiered appearance that mimicked the rocky levels of the waterfall.
   The house is therefore constructed with two 15-foot-wide rectangular cantilevers that form a stepped terrace directly over the water and a 6-foot-wide concrete slab cantilevered out from a bedroom to create a porch. Although reinforced with steel and supported by parapets, the walls began to crack over the years and were not fully repaired and reinforced until 2002. Since these 20th-century innovations, architects continue to fuse the structural and aesthetic application of reinforced concrete in new ways.

Historical Dictionaries of Literature and the Arts. . 2008.

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