Concrete

**Reinforced Concrete:** Concrete in which steel is embedded in such a manner that the two materials act together in resisting forces. The reinforcing steel absorbs the tensile, shear, and sometimes the compressive stresses in a concrete structure. In reinforced concrete, the tensile strength of steel and the compressive strength of concrete work together to allow the member to sustain these stresses over considerable spans. The invention of reinforced concrete in the 19th century revolutionized the construction industry

**Precast concrete:** Concrete cast into structural members under factory conditions and then brought to the building site. A 20th-century development, precasting increases the strength and finish durability of the member and decreases time and construction costs. Concrete cures slowly; the design strength is usually reached 28 days after initial setting. Using precast concrete eliminates the lag between the time on-site concrete is placed and the time it can carry loads. Precast concrete components include slabs, beams, columns, walls, stairways, modular boxes, and even kitchens and bathrooms with precast fixtures  
 * Prestressed concrete:** Concrete reinforced by either pretensioning or posttensioning, allowing it to carry a greater load or span a greater distance than ordinary reinforced concrete. In pretensioning, lengths of steel wire or cables are laid in the empty mold and stretched. The concrete is placed and allowed to set, and the cables are released, placing the concrete into compression as the steel shrinks back to its original length. In posttensioning, the steel in the concrete is stretched after the curing process. Prestressing places a concrete member in compression; these compressive stresses counteract the tensile bending stresses of an applied load. The process was developed by the French engineer Eugène Fressinet in the early 20th century.
 * Concrete-shell** **: ** is a structure composed of a relatively thin shell of concrete, usually with no interior columns or exterior buttresses . The shells are most commonly flat plates and domes , but may also take the form of ellipsoids or cylindrical sections, or some combination thereof. The first concrete shell dates back to the second century