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UNIT 7
           Concrete and reinforced concrete are used as building materials in every country. In many, including the United States and Canada, reinforced concrete is a dominant£¨Ö÷ÒªµÄ£© structural material in engineered construction£¨½¨ÔìµÄ½¨ÖþÎ. The universal£¨Í¨Óõģ©nature of reinforced concrete construction stems from£¨¹éÒòÓÚ£©the wide availability of reinforcing bars£¨¸Ö½î£©and the constituents£¨×é³É²¿·Ö£©of concrete, gravel,sand, and cement, the relatively simple skills required in concrete construction£¨Ê©¹¤£©, and the economy£¨¾­¼ÃÐÔ£©of reinforced concrete compared to other form of construction. Concrete and reinforced concrete are used in bridges, buildings of all sorts£¨¸÷ÖÖ¸÷Ñù£©, underground structures, water tanks, television towers, offshore oil exploration and production structures£¨½üº£Ê¯ÓÍ¿ª²ÉºÍÉú²ú½á¹¹£©, dams, and even in ships.
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Mechanics of Reinforced Concrete
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 Concrete is strong in compression but weak in tension. As a result, cracks develop£¨Ðγɣ©whenever£¨Ã¿µ±£©loads, or restrained shrinkage£¨ÊÕËõÏÞÖÆ£©or temperature changes, give rise to£¨µ¼Ö£©tensile stresses in excess of£¨³¬¹ý£©the tensile strength of the concrete. In the plain concrete£¨ËØ»ìÄýÍÁ£©beam, the moments due to applied loads are resisted by an internal tension-compression couple£¨À­Ñ¹Á¦Å¼£©involving tension in the concrete. Such a beam fails very suddenly and completely when the first crack forms. In a reinforced concrete beam, steel bars£¨¸Ö½î£©are embedded in the concrete in such a way that the tension forces needed for moment equilibrium after the concrete cracks can be developed in the bars.
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           The construction£¨Ê©¹¤£©of a reinforced concrete member involves building a form or mold£¨Ä£¾ß£©in the shape of the member being built. The form must be strong enough to support the weight and hydrostatic pressure£¨¾²Ë®Ñ¹Á¦£©of the wet concrete, and any forces applied to it by workers, concrete buggies£¨Áϳµ£©, wind, and so on. The reinforcement£¨¸Ö½î£©is placed in this form and held in place£¨¹Ì¶¨¾Í룩during the concreting£¨ÓûìÄýÍÁ½½Öþ£©operation. After the concrete has hardened, the forms are removed£¨²ð³ý£©.

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Factors Affecting Choice of Concrete For a Structure
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            The choice of whether a structure should be built of concrete, steel, masonry, or timber£¨Ä¾²Ä£©depends on the availability£¨¿ÉµÃÐÔ£©of materials and on a number of£¨Ðí¶à£©value decisions£¨¼ÛÖµÅжϣ©.

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    Economy    Frequently, the foremost£¨×îÖØÒªµÄ£©consideration is the overall cost£¨×Ü·ÑÓã©of the structure. This is, of course, a function of the costs£¨·ÑÓú¯Êý£©of the materials and the labor necessary to erect them. Frequently, however, the overall cost is affected as much or more by the overall construction time£¨×ܵĽ¨Ôìʱ¼ä£©since the contractor and owner must allocate£¨·ÖÅ䣩money£¨×ʽð£©to carry out the construction and will not receive a return on this investment £¨ÊÕ»ØͶ×Ê£©until the building is ready for occupancy£¨¾Óס£©. As a result, financial savings£¨²ÆÎñµÄ½ÚÔ¼£©due to rapid construction may more than offset£¨×ãÒÔµÖÏû£©increased material costs. Any measures designer can take to standardize the design and forming£¨¼Ó¹¤£©will generally pay off£¨Ê¹È˵ÃÒ棩in reduced overall costs.