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Concrete is a composite material composed of aggregate bonded together with a fluid cement which hardens over time. Most use of the term "concrete" refers to Portland cement concrete or to concretes made with other hydraulic cements, such as ciment fondu. However, road surfaces are also a type of concrete, "asphaltic concrete", where the cement material is bitumen. In Portland cement concrete (and other hydraulic cement concretes), when the aggregate is mixed together with the dry cement and water, they form a fluid mass that is easily molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix which binds all the materials together into a durable stone-like material that has many uses.〔Zongjin Li; ''Advanced concrete technology''; 2011〕 Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete. Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome. Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete. After the Roman Empire collapsed, use of concrete became rare until the technology was redeveloped in the mid-18th century. Today, concrete is the most widely used man-made material (measured by tonnage). == History == The word concrete comes from the Latin word "''concretus''" (meaning compact or condensed),〔(【引用サイトリンク】url=http://latinlookup.com/word/12124/concretus )〕 the perfect passive participle of "''concrescere''", from "''con''-" (together) and "''crescere''" (to grow). Perhaps the earliest known occurrence of cement was twelve million years ago. A deposit of cement was formed after an occurrence of oil shale located adjacent to a bed of limestone burned due to natural causes. These ancient deposits were investigated in the 1960s and 1970s.〔(【引用サイトリンク】url=http://matse1.matse.illinois.edu/concrete/hist.html )〕 On a human time-scale, small usages of concrete go back for thousands of years. The ancient Nabatea culture was using materials roughly analogous to concrete at least eight thousand years ago, some structures of which survive to this day.〔From The History of Concrete - InterNACHI http://www.nachi.org/history-of-concrete.htm#ixzz31V47Zuuj〕 German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400-1200 BC.〔Heinrich Schliemann with Wilhelm Dörpfeld and Felix Adler, ''Tiryns: The Prehistoric Palace of the Kings of Tiryns, The Results of the Latest Excavations'', (New York, New York: Charles Scribner's Sons, 1885), (pages 203-204 ), 215, and 190.〕〔Amelia Carolina Sparavigna, ("Ancient concrete works" )〕 Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete.〔Jacobsen T and Lloyd S, (1935) ''Sennacherib's Aqueduct at Jerwan,'' Oriental Institute Publications 24, Chicago University Press〕 Concrete was used for construction in many ancient structures. The Romans used concrete extensively from 300 BC to 476 AD, a span of more than seven hundred years.〔 During the Roman Empire, Roman concrete (or ''opus caementicium'') was made from quicklime, pozzolana and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension. Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.〔D.S. Robertson: ''Greek and Roman Architecture'', Cambridge, 1969, p. 233〕 Modern tests show that ''opus caementicium'' had as much compressive strength as modern Portland-cement concrete (ca. ).〔Henry Cowan: The Masterbuilders, New York 1977, p. 56, ISBN 978-0-471-02740-9〕 However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application was also different:〔(History of Concrete ). Ce.memphis.edu. Retrieved on 2013-02-19.〕 Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.〔Robert Mark, Paul Hutchinson: "On the Structure of the Roman Pantheon", ''Art Bulletin'', Vol. 68, No. 1 (1986), p. 26, fn. 5〕 The widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges such as the magnificent Pont du Gard have masonry cladding on a concrete core, as does the dome of the Pantheon. After the Roman Empire, the use of burned lime and pozzolana was greatly reduced until the technique was all but forgotten between 500 and the 14th century. From the 14th century to the mid-18th century, the use of cement gradually returned. The ''Canal du Midi'' was built using concrete in 1670,〔. allacademic.com〕 and there are concrete structures in Finland that date from the 16th century. Perhaps the greatest driver behind the modern usage of concrete was Smeaton's Tower, the third Eddystone Lighthouse in Devon, England. To create this structure, between 1756 and 1759, British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. A method for producing Portland cement was patented by Joseph Aspdin on 1824. Reinforced concrete was invented in 1849 by Joseph Monier.〔(The History of Concrete and Cement ). Inventors.about.com (2012-04-09). Retrieved on 2013-02-19.〕 In 1889 the first concrete reinforced bridge was built, and the first large concrete dams were built in 1936, Hoover Dam and Grand Coulee Dam.〔(Historical Timeline of Concrete ). Auburn.edu. Retrieved on 2013-02-19.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Concrete」の詳細全文を読む スポンサード リンク
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