Concrete

=__Concrete__= ==Concrete is a manufactured mixyure of cement and water, with aggregates of sand and stones, which hardens rapidly by chemical combination to a stonelike, water and also fire resisting solid of great compressive but low tensile strenght. There are different types of concrete. Reinforced concrete was developed to add the tensile strenght of steel to the compressive strenght of mass concrete.== ==There are also three concrete developments of the 20th century, the first one is the concrete-shell construction, it permits the erection of vast vaults and domes with a concrete and steel content so reduced that the thickness is comparativly less than that of and eggshell. the second one is precast concrete construction, it employs bricks, slabs, and supports made under optimal factory conditions to increase waterproofing and solidity, to decrease time and cost in erection, and to reduce expansion and contractions. The last one is prestressed concrete, that provides bearing members into which reinforcement is set under tension to produce a live force to resist a particular liad. Since the member acts like a spring, it can carry a greater load than an unstressed member of the same size== Concrete Reinforced Concrete Prestress Concrete