Prestressed concrete is a form of

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

used in construction. It is substantially "prestressed" ( compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. Post-tensioned concreted is "structural concrete in which internal stresses have been introduced to reduce potential tensile stresses in the concrete resulting from loads." This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete and is done to improve the performance of the concrete in service. Tendons may consist of single

wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...

s, multi-wire strands or threaded bars that are most commonly made from

high-tensile steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...

carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

aramid fiber Aramid fibers, short for aromatic polyamide, are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic-rated body armor fabric and ballistic composites, in marine cordage, mar ...

. The essence of prestressed concrete is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent

compression forces In mechanics, compression is the application of balanced inward ("pushing") forces to different points on a material or structure, that is, forces with no net sum or torque directed so as to reduce its size in one or more directions.Ferdinand ...

and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared with conventionally

reinforced concrete Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...

in many situations. In a prestressed concrete member, the internal stresses are introduced in a planned manner so that the stresses resulting from the imposed loads are counteracted to the desired degree. Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans, reduced structural thicknesses, and material savings compared with simple

. Typical applications include

high-rise buildings A tower block, high-rise, apartment tower, residential tower, apartment block, block of flats, or office tower is a tall building, as opposed to a low-rise building and is defined differently in terms of height depending on the jurisdictio ...

, residential slabs, foundation systems,

bridge A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or rail) without blocking the way underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somethi ...

and

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...

structures,

silo A silo (from the Greek σιρός – ''siros'', "pit for holding grain") is a structure for storing bulk materials. Silos are used in agriculture to store fermented feed known as silage, not to be confused with a grain bin, which is used t ...

s and

tanks A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engine ...

, industrial pavements and nuclear containment structures. First used in the late-nineteenth century, prestressed concrete has developed beyond pre-tensioning to include post-tensioning, which occurs after the concrete is cast. Tensioning systems may be classed as either ''monostrand'', where each tendon's strand or wire is stressed individually, or ''multi-strand'', where all strands or wires in a tendon are stressed simultaneously. Tendons may be located either ''within'' the concrete volume (internal prestressing) or wholly ''outside'' of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons.

Pre-tensioned concrete

Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned ''prior'' to the concrete being cast. The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by

static friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

Pre-tensioning is a common

prefabrication Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located. The term is u ...

technique, where the resulting concrete element is manufactured remotely from the final structure location and transported to site once cured. It requires strong, stable end-anchorage points between which the tendons are stretched. These anchorages form the ends of a "casting bed" which may be many times the length of the concrete element being fabricated. This allows multiple elements to be constructed end-to-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized. The amount of bond (or

adhesion Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be ...

) achievable between the freshly set concrete and the surface of the tendons is critical to the pre-tensioning process, as it determines when the tendon anchorages can be safely released. Higher bond strength in early-age concrete will speed production and allow more economical fabrication. To promote this, pre-tensioned tendons are usually composed of isolated single wires or strands, which provides a greater

surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...

for bonding than bundled-strand tendons. SpannbetonFertigdecke Montage

Unlike those of post-tensioned concrete (see below), the tendons of pre-tensioned concrete elements generally form straight lines between end-anchorages. Where "profiled" or "harped" tendons are required, one or more intermediate ''deviators'' are located between the ends of the tendon to hold the tendon to the desired non-linear alignment during tensioning. Such deviators usually act against substantial forces, and hence require a robust casting-bed foundation system. Straight tendons are typically used in "linear" precast elements, such as shallow beams, hollow-core planks and slabs; whereas profiled tendons are more commonly found in deeper precast bridge beams and girders. Pre-tensioned concrete is most commonly used for the fabrication of structural beams, floor slabs, hollow-core planks,

balconies A balcony (from it, balcone, "scaffold") is a platform projecting from the wall of a building, supported by columns or Corbel, console brackets, and enclosed with a balustrade, usually above the ground floor. Types The traditional Malta, Malte ...

lintel A lintel or lintol is a type of beam (a horizontal structural element) that spans openings such as portals, doors, windows and fireplaces. It can be a decorative architectural element, or a combined ornamented structural item. In the case of w ...

s, driven piles,

water tanks A water tank is a container for storing water. Water tanks are used to provide storage of water for use in many applications, drinking water, irrigation agriculture, fire suppression, agricultural farming, both for plants and livestock, chemic ...

and concrete

pipes Pipe(s), PIPE(S) or piping may refer to: Objects * Pipe (fluid conveyance), a hollow cylinder following certain dimension rules ** Piping, the use of pipes in industry * Smoking pipe ** Tobacco pipe * Half-pipe and quarter pipe, semi-circula ...

Post-tensioned concrete

Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned ''after'' the surrounding concrete structure has been cast. The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete. Once the concrete has been cast and set, the tendons are tensioned ("stressed") by pulling the tendon ends through the anchorages while pressing against the concrete. The large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is "locked-off" at the anchorage. The method of locking the tendon-ends to the anchorage is dependent upon the tendon composition, with the most common systems being "button-head" anchoring (for wire tendons), split-wedge anchoring (for strand tendons), and threaded anchoring (for bar tendons). Go Between Bridge construction (2)

Tendon encapsulation systems are constructed from plastic or

galvanised Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...

steel materials, and are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal

grout Grout is a dense fluid which hardens to fill gaps or used as reinforcement in existing structures. Grout is generally a mixture of water, cement and sand, and is employed in pressure grouting, embedding rebar in masonry walls, connecting secti ...

ing of the duct after stressing (''bonded'' post-tensioning); and those where the tendon element is permanently ''de''bonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands (''unbonded'' post-tensioning). Casting the tendon ducts/sleeves into the concrete before any tensioning occurs allows them to be readily "profiled" to any desired shape including incorporating vertical and/or horizontal

curvature In mathematics, curvature is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane. For curves, the canonic ...

. When the tendons are tensioned, this profiling results in

reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law *Chain reaction (disambiguation). Biology and me ...

forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure.

Bonded post-tensioning

In bonded post-tensioning, tendons are permanently bonded to the surrounding concrete by the ''

in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...

ing of their encapsulating ducting (after tendon tensioning). This grouting is undertaken for three main purposes: to protect the tendons against

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

; to permanently "lock-in" the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain structural behaviors of the final concrete structure. Bonded post-tensioning characteristically uses tendons each comprising ''bundles'' of elements (e.g., strands or wires) placed inside a single tendon duct, with the exception of bars which are mostly used unbundled. This bundling makes for more efficient tendon installation and grouting processes, since each complete tendon requires only one set of end-anchorages and one grouting operation. Ducting is fabricated from a durable and corrosion-resistant material such as plastic (e.g.,

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...

) or

steel, and can be either round or rectangular/oval in cross-section. The tendon sizes used are highly dependent upon the application, ranging from

building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fun ...

works typically using between 2 and 6 strands per tendon, to specialized

works using up to 91 strands per tendon. Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end-anchorages to

formwork Formwork is Molding (process), molds into which concrete or similar materials are either precast concrete, precast or cast-in-place concrete, cast-in-place. In the context of concrete construction, the falsework supports the shuttering mold ...

, placing the tendon ducting to the required curvature profiles, and reeving (or threading) the strands or wires through the ducting. Following concreting and tensioning, the ducts are pressure-grouted and the tendon stressing-ends sealed against

Unbonded post-tensioning

Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of

longitudinal Longitudinal is a geometric term of location which may refer to: * Longitude ** Line of longitude, also called a meridian * Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicl ...

movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a

-inhibiting grease, usually

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

based. Anchorages at each end of the tendon transfer the tensioning force to the concrete, and are required to reliably perform this role for the life of the structure. Unbonded post-tensioning can take the form of: * Individual strand tendons placed directly into the concreted structure (e.g., buildings, ground slabs) * Bundled strands, individually greased-and-sheathed, forming a single tendon within an encapsulating duct that is placed either within or adjacent to the concrete (e.g., restressable anchors, external post-tensioning) For individual strand tendons, no additional tendon ducting is used and no post-stressing grouting operation is required, unlike for bonded post-tensioning. Permanent corrosion protection of the strands is provided by the combined layers of grease, plastic sheathing, and surrounding concrete. Where strands are bundled to form a single unbonded tendon, an enveloping duct of plastic or galvanised steel is used and its interior free-spaces grouted after stressing. In this way, additional corrosion protection is provided via the grease, plastic sheathing, grout, external sheathing, and surrounding concrete layers. Individually greased-and-sheathed tendons are usually fabricated off-site by an

extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

process. The bare steel strand is fed into a greasing chamber and then passed to an extrusion unit where molten plastic forms a continuous outer coating. Finished strands can be cut-to-length and fitted with "dead-end" anchor assemblies as required for the project.

Comparison between bonded and unbonded post-tensioning

Both bonded and unbonded post-tensioning technologies are widely used around the world, and the choice of system is often dictated by regional preferences, contractor experience, or the availability of alternative systems. Either one is capable of delivering code-compliant, durable structures meeting the structural strength and serviceability requirements of the designer. The benefits that bonded post-tensioning can offer over unbonded systems are: * Reduced reliance on end-anchorage integrity
Following tensioning and grouting, bonded tendons are connected to the surrounding concrete along their full length by high-strength

. Once cured, this grout can transfer the full tendon tension force to the concrete within a very short distance (approximately 1 metre). As a result, any inadvertent severing of the tendon or failure of an end anchorage has only a very localised impact on tendon performance, and almost never results in tendon ejection from the anchorage. * Increased

ultimate strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

flexure A flexure is a flexible element (or combination of elements) engineered to be compliant in specific degrees of freedom. Flexures are a design feature used by design engineers (usually mechanical engineers) for providing adjustment or compliance ...

With bonded post-tensioning, any

of the structure is directly resisted by tendon strains at that same location (i.e. no strain re-distribution occurs). This results in significantly higher

tensile In physics, tension is described as the pulling force transmitted axially by the means of a string, a rope, chain, or similar object, or by each end of a rod, truss member, or similar three-dimensional object; tension might also be described as t ...

strains in the tendons than if they were unbonded, allowing their full

yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...

to be realised, and producing a higher ultimate load capacity. * Improved crack-control
In the presence of concrete cracking, bonded tendons respond similarly to conventional reinforcement (rebar). With the tendons fixed to the concrete at each side of the crack, greater resistance to crack expansion is offered than with unbonded tendons, allowing many design codes to specify reduced reinforcement requirements for bonded post-tensioning. * Improved fire performance
The absence of strain redistribution in bonded tendons may limit the impact that any localised overheating has on the overall structure. As a result, bonded structures may display a higher capacity to resist fire conditions than unbonded ones. The benefits that unbonded post-tensioning can offer over bonded systems are: * Ability to be

prefabricated Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located. The term is u ...

Unbonded tendons can be readily prefabricated off-site complete with end-anchorages, facilitating faster installation during construction. Additional lead time may need to be allowed for this fabrication process. * Improved site

productivity Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...

The elimination of the post-stressing grouting process required in bonded structures improves the site-labour productivity of unbonded post-tensioning. * Improved installation

flexibility Stiffness is the extent to which an object resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is. Calculations The stiffness, k, of a bo ...

Unbonded single-strand tendons have greater handling flexibility than bonded ducting during installation, allowing them a greater ability to be deviated around service penetrations or obstructions. * Reduced

concrete cover Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete ( ACI 130). The concrete cover depth can be measured with a cover meter. Purpose of provision of conc ...

Unbonded tendons may allow some reduction in concrete element thickness, as their smaller size and increased corrosion protection may allow them to be placed closer to the concrete surface. * Simpler replacement and/or adjustment
Being permanently isolated from the concrete, unbonded tendons are able to be readily de-stressed, re-stressed and/or replaced should they become damaged or need their force levels to be modified in-service. * Superior overload performance
Although having a lower ultimate strength than bonded tendons, unbonded tendons' ability to redistribute strains over their full length can give them superior pre-collapse

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

. In extremes, unbonded tendons can resort to a ''

catenary In physics and geometry, a catenary (, ) is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, superficia ...

''-type action instead of pure flexure, allowing significantly greater

deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...

before structural failure.

Tendon durability and corrosion protection

Long-term durability is an essential requirement for prestressed concrete given its widespread use. Research on the durability performance of in-service prestressed structures has been undertaken since the 1960s, and anti-corrosion technologies for tendon protection have been continually improved since the earliest systems were developed. The durability of prestressed concrete is principally determined by the level of corrosion protection provided to any high-strength steel elements within the prestressing tendons. Also critical is the protection afforded to the end-anchorage assemblies of unbonded tendons or cable-stay systems, as the anchorages of both of these are required to retain the prestressing forces. Failure of any of these components can result in the release of prestressing forces, or the physical rupture of stressing tendons. Modern prestressing systems deliver long-term durability by addressing the following areas: * Tendon grouting (bonded tendons)
Bonded tendons consist of bundled strands placed inside ducts located within the surrounding concrete. To ensure full protection to the bundled strands, the ducts must be pressure-filled with a corrosion-inhibiting grout, without leaving any voids, following strand-tensioning. * Tendon coating (unbonded tendons)
Unbonded tendons comprise individual strands coated in an anti-corrosion grease or wax, and fitted with a durable plastic-based full-length sleeve or sheath. The sleeving is required to be undamaged over the tendon length, and it must extend fully into the anchorage fittings at each end of the tendon. * Double-layer encapsulation
Prestressing tendons requiring permanent

monitoring Monitoring may refer to: Science and technology Biology and healthcare * Monitoring (medicine), the observation of a disease, condition or one or several medical parameters over time * Baby monitoring * Biomonitoring, of toxic chemical compounds, ...

and/or force adjustment, such as stay-cables and re-stressable dam anchors, will typically employ double-layer corrosion protection. Such tendons are composed of individual strands, grease-coated and sleeved, collected into a strand-bundle and placed inside encapsulating

outer ducting. The remaining void space within the duct is pressure-grouted, providing a multi-layer polythene-grout-plastic-grease protection barrier system for each strand. * Anchorage protection
In all post-tensioned installations, protection of the end-anchorages against corrosion is essential, and critically so for unbonded systems. Several durability-related events are listed below: * Ynys-y-Gwas bridge, West Glamorgan, Wales, 1985
A single-span, precast-segmental structure constructed in 1953 with longitudinal and transverse post-tensioning.

Corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

attacked the under-protected tendons where they crossed the ''in-situ'' joints between the segments, leading to sudden collapse. * Scheldt River bridge, Melle, Belgium, 1991
A three-span prestressed cantilever structure constructed in the 1950s. Inadequate

in the side

abutment An abutment is the substructure at the ends of a bridge span or dam supporting its superstructure. Single-span bridges have abutments at each end which provide vertical and lateral support for the span, as well as acting as retaining walls ...

s resulted in tie-down cable

, leading to a progressive failure of the main bridge span and the death of one person. * UK Highways Agency, 1992
Following discovery of tendon corrosion in several bridges in England, the Highways Agency issued a moratorium on the construction of new internally grouted post-tensioned bridges and embarked on a 5-year programme of inspections on its existing post-tensioned bridge stock. The moratorium was lifted in 1996. * Pedestrian bridge,

Charlotte Motor Speedway Charlotte Motor Speedway (previously known as Lowe's Motor Speedway from 1999 to 2009) is a motorsport complex located in Concord, North Carolina, outside Charlotte. The complex features a quad oval track that hosts NASCAR racing including th ...

, North Carolina, US, 2000
A multi-span steel and concrete structure constructed in 1995. An unauthorised

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...

was added to the tendon grout to speed construction, leading to corrosion of the prestressing strands and the sudden collapse of one span, injuring many spectators. * Hammersmith Flyover London, England, 2011
Sixteen-span prestressed structure constructed in 1961. Corrosion from road de-icing salts was detected in some of the prestressing tendons, necessitating initial closure of the road while additional investigations were done. Subsequent repairs and strengthening using external post-tensioning was carried out and completed in 2015. * Petrulla Viaduct, Sicily, Italy, 2014
One span of the viaduct collapsed on 7 July due to corrosion of the post-tensioning tendons. * Genoa bridge collapse, 2018. The Ponte Morandi was a cable-stayed bridge characterised by a prestressed concrete structure for the piers, pylons and deck, very few stays, as few as two per span, and a hybrid system for the stays constructed from steel cables with prestressed concrete shells poured on. The concrete was only prestressed to 10 MPa, resulting in it being prone to cracks and water intrusion, which caused corrosion of the embedded steel. * Churchill Way flyovers,

Liverpool Liverpool is a city and metropolitan borough in Merseyside, England. With a population of in 2019, it is the 10th largest English district by population and its metropolitan area is the fifth largest in the United Kingdom, with a popul ...

, England
The flyovers were closed in September 2018 after inspections revealed poor quality concrete, tendon corrosion and signs of structural distress. They were demolished in 2019.

Applications

Prestressed concrete is a highly versatile construction material as a result of it being an almost ideal combination of its two main constituents: high-strength steel, pre-stretched to allow its full strength to be easily realised; and modern concrete, pre-compressed to minimise cracking under tensile forces. Its wide range of application is reflected in its incorporation into the major design codes covering most areas of structural and civil engineering, including buildings, bridges, dams, foundations, pavements, piles, stadiums, silos, and tanks.

Building structures

Building structures are typically required to satisfy a broad range of structural, aesthetic and economic requirements. Significant among these include: a minimum number of (intrusive) supporting walls or columns; low structural thickness (depth), allowing space for services, or for additional floors in high-rise construction; fast construction cycles, especially for multi-storey buildings; and a low cost-per-unit-area, to maximise the building owner's return on investment. The prestressing of concrete allows "load-balancing" forces to be introduced into the structure to counter in-service loadings. This provides many benefits to building structures: * Longer spans for the same structural depth
Load balancing results in lower in-service deflections, which allows spans to be increased (and the number of supports reduced) without adding to structural depth. * Reduced structural thickness
For a given span, lower in-service deflections allows thinner structural sections to be used, in turn resulting in lower floor-to-floor heights, or more room for building services. * Faster stripping time
Typically, prestressed concrete building elements are fully stressed and self-supporting within five days. At this point they can have their formwork stripped and re-deployed to the next section of the building, accelerating construction "cycle-times". * Reduced material costs
The combination of reduced structural thickness, reduced conventional reinforcement quantities, and fast construction often results in prestressed concrete showing significant cost benefits in building structures compared to alternative structural materials. Some notable building structures constructed from prestressed concrete include:

Sydney Opera House The Sydney Opera House is a multi-venue performing arts centre in Sydney. Located on the foreshore of Sydney Harbour, it is widely regarded as one of the world's most famous and distinctive buildings and a masterpiece of 20th-century architec ...

and

World Tower The World Tower is a residential skyscraper in Sydney, Australia. Designed by Fender Katsalidis, it stands at a height of , making it the second tallest residential building in the city, surpassed by Greenland Centre. Construction began in ...

, Sydney;

St George Wharf Tower St George Wharf Tower, also known as the Vauxhall Tower, is a residential skyscraper in Vauxhall, London, and part of the St George Wharf development. At tall with 50 storeys, it is the List of tallest buildings and structures in London, eighte ...

, London;

CN Tower The CN Tower (french: Tour CN) is a concrete communications and observation tower in downtown Toronto, Ontario, Canada. Built on the former Railway Lands, it was completed in 1976. Its name "CN" referred to Canadian National, the railway c ...

, Toronto;

Kai Tak Cruise Terminal Kai Tak Cruise Terminal is a cruise ship terminal on the former Kai Tak Airport runway in Hong Kong. Its completion date was delayed into 2013 due to re-tendering. Following an international competition, Foster + Partners was chosen to ...

and

International Commerce Centre The International Commerce Centre (abbreviated ICC) is a 118- storey, commercial skyscraper completed in 2010 in West Kowloon, Hong Kong. It is a part of the Union Square project on top of Kowloon station. It was the 4th tallest building in ...

, Hong Kong; Ocean Heights 2, Dubai;

Eureka Tower Eureka Tower is a skyscraper located in the Southbank precinct of Melbourne, Victoria, Australia. Construction began in August 2002 and the exterior was completed on 1 June 2006. The plaza was finished in June 2006 and the building was offici ...

, Melbourne; Torre Espacio, Madrid; Guoco Tower (Tanjong Pagar Centre), Singapore;

Zagreb International Airport Zagreb Franjo Tuđman Airport ( hr, Zračna luka Franjo Tuđman Zagreb) or Zagreb Airport ( hr, Zračna luka Zagreb) () is an international airport serving Zagreb, Croatia. It is the largest and busiest airport in Croatia. In 2019 it handled 3. ...

, Croatia; and

Capital Gate Capital Gate, also known as the Leaning Tower of Abu Dhabi, is a skyscraper in Abu Dhabi that is over tall, 35 stories high, with over of usable office space. Capital Gate is one of the tallest buildings in the city and was designed to incline 1 ...

, Abu Dhabi UAE. File:International Commerce Centre.jpg, ICC tower, Hong Kong
''484m 2010'' File:Guoco Tower, Singapore, under construction - 20141006.jpg, Guoco Tower, Singapore
''290m 2016'' File:Aerial view of the Sydney Opera House.jpg,

''1973'' File:Kai Tak Cruise Terminal in June 2014.jpg, Kai Tak Terminal
Hong Kong ''2013'' File:World Tower 2014-08-22.jpg,

, Sydney
''230m 2004'' File:Ocean Heights Dubai Marina.jpg, Ocean Heights 2, Dubai
''335m 2016'' File:Eureka Tower LC.JPG,

, Melbourne
''297m 2006'' File:Torre Espacio (Madrid) 06.jpg, Torre Espacio, Madrid
''230m 2008'' File:Capital Gate.jpg,

, Abu Dhabi
''18° lean 2010''

Civil structures

Bridges

Concrete is the most popular structural material for bridges, and prestressed concrete is frequently adopted. When investigated in the 1940s for use on heavy-duty bridges, the advantages of this type of bridge over more traditional designs was that it is quicker to install, more economical and longer-lasting with the bridge being less lively. One of the first bridges built in this way is the

Adam Viaduct The Adam Viaduct is a grade II listed concrete underbridge in Wallgate, Wigan. The bridge, constructed in 1946, is the earliest prestressed concrete railway bridge in the United Kingdom, with only some examples in Switzerland being earlier. It ...

, a railway bridge constructed 1946 in the UK. By the 1960s, prestressed concrete largely superseded reinforced concrete bridges in the UK, with box girders being the dominant form. In short-span bridges of around , prestressing is commonly employed in the form of precast pre-tensioned

girder A girder () is a support beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a stabilizing ' ...

s or planks. Medium-length structures of around , typically use precast-segmental, ''in-situ'' balanced-cantilever and incrementally-launched designs. For the longest bridges, prestressed concrete deck structures often form an integral part of cable-stayed designs.

Dams

Concrete dams have used prestressing to counter uplift and increase their overall stability since the mid-1930s. Prestressing is also frequently retro-fitted as part of dam remediation works, such as for structural strengthening, or when raising crest or spillway heights. Most commonly, dam prestressing takes the form of post-tensioned anchors drilled into the dam's concrete structure and/or the underlying rock strata. Such anchors typically comprise tendons of high-tensile bundled steel strands or individual threaded bars. Tendons are grouted to the concrete or rock at their far (internal) end, and have a significant "de-bonded" free-length at their external end which allows the tendon to stretch during tensioning. Tendons may be full-length bonded to the surrounding concrete or rock once tensioned, or (more commonly) have strands permanently encapsulated in corrosion-inhibiting grease over the free-length to permit long-term load monitoring and re-stressability.

Silos and tanks

Circular storage structures such as silos and tanks can use prestressing forces to directly resist the outward pressures generated by stored liquids or bulk-solids. Horizontally curved tendons are installed within the concrete wall to form a series of hoops, spaced vertically up the structure. When tensioned, these tendons exert both axial (compressive) and radial (inward) forces onto the structure, which can directly oppose the subsequent storage loadings. If the magnitude of the prestress is designed to always exceed the tensile stresses produced by the loadings, a permanent residual compression will exist in the wall concrete, assisting in maintaining a watertight crack-free structure.

Nuclear and blast

Prestressed concrete has been established as a reliable construction material for high-pressure containment structures such as nuclear reactor vessels and containment buildings, and petrochemical tank blast-containment walls. Using pre-stressing to place such structures into an initial state of bi-axial or tri-axial compression increases their resistance to concrete cracking and leakage, while providing a proof-loaded, redundant and monitorable pressure-containment system. Nuclear reactor and containment vessels will commonly employ separate sets of post-tensioned tendons curved horizontally or vertically to completely envelop the reactor core. Blast containment walls, such as for

liquid natural gas Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6) that has been cooled down to liquid form for ease and safety of non-pressurized storage or transport. It takes up about 1/600th the vol ...

(LNG) tanks, will normally utilize layers of horizontally-curved hoop tendons for containment in combination with vertically looped tendons for axial wall pre-stressing.

Hardstands and pavements

Heavily loaded concrete ground-slabs and pavements can be sensitive to cracking and subsequent traffic-driven deterioration. As a result, prestressed concrete is regularly used in such structures as its pre-compression provides the concrete with the ability to resist the crack-inducing tensile stresses generated by in-service loading. This crack-resistance also allows individual slab sections to be constructed in larger pours than for conventionally reinforced concrete, resulting in wider joint spacings, reduced jointing costs and less long-term joint maintenance issues. Initial works have also been successfully conducted on the use of precast prestressed concrete for road pavements, where the speed and quality of the construction has been noted as being beneficial for this technique. Some notable civil structures constructed using prestressed concrete include:

Gateway Bridge The Sir Leo Hielscher Bridges are a side-by-side pair of road bridges on the Gateway Motorway (M1), which skirts the eastern suburbs of Brisbane, Queensland, Australia. The western bridge carries traffic to the north and the eastern bridge car ...

, Brisbane Australia;

Incheon Bridge The Incheon Bridge () is a reinforced concrete cable-stayed bridge in South Korea. At its opening in October 2009, it became the second bridge connection between Yeongjong Island and the mainland of Incheon. The Incheon Bridge is South Korea's ...

, South Korea;

Roseires Dam The Roseires Dam ( ar, خزان الروصيرص) is a dam on the Blue Nile at Ad Damazin, just upstream of the town of Er Roseires, in Sudan. It consists of a concrete buttress dam 1 km wide with a maximum height of 68 m, and an earth dam ...

, Sudan; Wanapum Dam, Washington, US; LNG tanks, South Hook, Wales;

Cement silos A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

, Brevik Norway; Autobahn A73 bridge, Itz Valley, Germany; Ostankino Tower, Moscow, Russia;

, Toronto, Canada; and Ringhals nuclear reactor, Videbergshamn Sweden. File:Gateway Bridge aerial4.JPG,

Brisbane, Aust. File:Incheon Grand Bridge.jpg,

South Korea File:Itztalbruecke 2005-07-24.jpg, Autobahn A73
Itz Valley, Germany File:Ostankino Tower, 2015.JPG, Ostankino Tower
Moscow, Russia File:Toronto - ON - Toronto Harbourfront7.jpg,

Toronto, Canada File:Norcem Brevik fra sjøen.JPG, Norcem silos
Brevik, Norway File:Roseires Dam.jpg,

Ad Damazin, Sudan File:Wanapum Dam from West Shore - downstream 10360031.jpg, Wanapum Dam
Washington, US File:Gas storage tanks at LNG terminal - geograph.org.uk - 1025438.jpg, LNG tanks
South Hook, Wales File:Ringhals.JPG, Ringhals nuclear plant
Videbergshamn, Sweden

Design agencies and regulations

Worldwide, many professional organizations exist to promote best practices in the design and construction of prestressed concrete structures. In the United States, such organizations include the

Post-Tensioning Institute The Post-Tensioning Institute (PTI) is a nonprofit organization committed to advancing the quality, safety, efficiency, profitability, and use of post-tensioning systems. PTI's manuals and technical guides provide guidance on the design, construct ...

(PTI) and the Precast/Prestressed Concrete Institute (PCI). Similar bodies include the Canadian Precast/Prestressed Concrete Institute (CPCI), the UK's Post-Tensioning Association, the Post Tensioning Institute of Australia and the South African Post Tensioning Association. Europe has similar country-based associations and institutions. It is important to note that these organizations are not the authorities of

building code A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...

s or standards, but rather exist to promote the understanding and development of prestressed concrete design, codes and best practices. Rules and requirements for the detailing of reinforcement and prestressing tendons are specified by individual national codes and standards such as: *European Standard

EN 1992 In the Eurocode series of European standards (EN) related to construction, '' Eurocode 2: Design of concrete structures'' (abbreviated EN 1992 or, informally, EC 2) specifies technical rules for the design of concrete, reinforced concrete and pre ...

-2:2005 – Eurocode 2: Design of Concrete Structures; * US Standard ACI318: Building Code Requirements for Reinforced Concrete; and *

Australian Standard Standards Australia is a standards organisation established in 1922 and is recognised through a Memorandum of Understanding (MoU) with the Australian government as the primary non-government standards development body in Australia. It is a c ...

AS 3600-2009: Concrete Structures.

References

External links

The story of prestressed concrete from 1930 to 1945: A step towards the European UnionGuidelines for Sampling, Assessing, and Restoring Defective Grout in Prestressed Concrete Bridge Post-Tensioning Ducts

Federal Highway Administration The Federal Highway Administration (FHWA) is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two programs, the Federal-aid Highway Program a ...

Historical Patents and the Evolution of Twentieth Century Architectural Construction with Reinforced and Pre-stressed Concrete
{{Use dmy dates, date=April 2017 Building materials Concrete buildings and structures Reinforced concrete Structural engineering fr:Béton#Béton précontraint sv:Armerad betong#Spännarmerad betong