Prestressed concrete is a method for overcoming concrete's natural weakness in tension. Concrete is a construction material composed of Cement (commonly Portland cement) as well as other cementitious materials such as Fly ash and Slag In Physics String Tension is the magnitude of the pulling force exerted by a string cable chain or similar object on another object It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. A floor is the walking surface of a room or vehicle Floors vary from simple dirt in a cave to many-layered surfaces using modern technology A bridge is a Structure built to span a Gorge, Valley, Road, railroad track, River, Body of water Span is a section between two intermediate supports eg of a beam or a Bridge. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. In Physics String Tension is the magnitude of the pulling force exerted by a string cable chain or similar object on another object Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 A cable is one or more Wires or Optical fibers bound together typically in a common protective jacket or sheath Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume Stress is a measure of the average amount of Force exerted per unit Area. A compression member is a general class of structural elements of which a Column is the most common specific example Traditional reinforced concrete is based on the use of steel reinforcement bars, rebar, inside poured concrete. Reinforced concrete is Concrete in which reinforcement bars (" Rebars quot or fibers have been incorporated to strengthen a material that would otherwise be Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 A rebar, or reinforcing bar, is a common Steel bar and is commonly used in Reinforced concrete and reinforced Masonry structures Concrete is a construction material composed of Cement (commonly Portland cement) as well as other cementitious materials such as Fly ash and Slag

Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Pre-tensioned concrete

Stressed ribbon bridge

Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Prefabrication is the practice of assembling components of a Structure in a Factory or other Manufacturing site and Transporting complete Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. Balcony (from Italian balcone, scaffold cf High German balcho, beam balk probably cognate with Persian term بالكانه For lintel as a decorative element see Lintel (architecture For beam as load-bearing member see beam An innovative bridge construction method using pre-stressing is described in stressed ribbon bridge. A bridge is a Structure built to span a Gorge, Valley, Road, railroad track, River, Body of water This article is concerned with a particular type of suspension bridge the stressed ribbon type

Bonded post-tensioned concrete

Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). Physical compression is the result of the subjection of a material to Compressive stress, resulting in reduction of Volume. In situ (ɪn siːˈtuː is a Latin phrase meaning in the place. The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 WikipediaNaming A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. For the mechanical technology see Hydraulic machinery and Hydraulic cylinder Hydraulics is a topic of science and Engineering For other uses see Jack or Carjacking. A jack is mechanical device used to lift heavy loads or apply great forces When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. In Mechanics, and Physics, Hooke's law of elasticity is an approximation that states that the amount by which a material body is deformed (the A wedge is a triangular shaped tool a compound and portable Inclined plane, and one of the six classical Simple machines It can be used to separate The duct is then grouted to protect the tendons from corrosion. Grout is a Construction material used to embed Rebars in Masonry walls connect sections of pre-cast Concrete, fill voids and seal joints (like Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. Adobe bricks are a Natural building material made from Sand, Clay, water and some kind of fibrous or Organic material ( Sticks, Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-stressing is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge. Falsework consists of temporary structures used in Construction to support spanning or arched structures in order to hold the component in place until its construction is sufficiently As its name implies a segmental bridge is a Bridge built in short sections (called segments i The advantages of this system over unbonded post-tensioning are:

1. Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
2. Tendons can be easily 'weaved' allowing a more efficient design approach.
3. Higher ultimate strength due to bond generated between the strand and concrete.
4. No long term issues with maintaining the integrity of the anchor/dead end.

Unbonded post-tensioned concrete

Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 Extrusion is a process used to create objects of a fixed cross-sectional profile The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). Maintenance repair and operations or maintenance repair and overhaul (MRO is fixing any sort of mechanical or Electrical device should it The advantages of this system over bonded post-tensioning are:

1. The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 to either side).
2. The procedure of post-stress grouting is eliminated.
3. The ability to de-stress the tendons before attempting repair work.

Picture number one shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack, shown in picture number nine, until it is stretched to achieve the specified tension.

1. Rolls of post-tensioning cables	2. Pulling anchors for post-tensioning cables	3. Pulling anchors for post-tensioning cables	4. Pulling anchors for post-tensioning cables
5. Post-tensioning cables stripped for placement in pulling anchors	6. Positioned post-tensioning cables	7. Post-tensioning cable ends extending from freshly poured concrete	8. Post-tensioning cable ends extending from concrete slab
9. Hydraulic jack for tensioning cables	9. Cable conduits in formwork

Applications

Prestressed concrete is the predominating material for floors in high-rise buildings, foundations for residential buildings in soft soil areas, bridges and in the construction of water towers and water tanks. A skyscraper is a tall continuously habitable Building. There is no official definition or a precise cutoff height above which a building may clearly be classified as a skyscraper A foundation is a Structure that transfers loads to the earth Soil, often typeset as SOiL, is a four piece rock band from Chicago Illinois United States founded by Shaun Glass Tom Schofield Tim King and Adam Zadel A water tower, watershed, or elevated water tower is a large elevated water storage container Water tanks are liquid storage containers these tanks are usually storing water for human consumption Post-tensioning is also used to reinforce the large concrete chambers in nuclear reactors. This article is a subarticle of Nuclear power. A nuclear reactor is a device in which Nuclear chain reactions are initiated controlled Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable. Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made. The advantages of prestressed concrete include lower construction costs; thinner slabs - especially important in high rise buildings in which floor thickness savings can translate into additional floors for the same (or lower) cost and fewer joints, since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced constructions with the same thickness. Increasing span lengths increases the usable unencumbered floorspace in buildings; diminishing the number of joints leads to lower maintenance costs over the design life of a building, since joints are the major locus of weakness in concrete buildings. The first prestressed concrete bridge in North America was the Walnut Street Bridge in Philadelphia, Pennsylvania. The Walnut Street Bridge in Philadelphia Pennsylvania crosses the Schuylkill River between Center City and West Philadelphia. Philadelphia (ˌfɪləˈdɛlfiə It was completed and opened to traffic in 1951 ^[1].

References

External links

• Precast/Prestressed Concrete Institute – Resources and information on Precast and Prestressed Concrete
• Canadian Precast/Prestressed Concrete Institute – Resources and information on Precast and Prestressed Concrete, mostly technical information about precast prestressed concrete in Canada.