Prestressed concrete

Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast. The amount of bond (or adhesion) 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 for bonding than bundled-strand 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 concrete elements, such as shallow beams and hollow-core 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 slabs, balconies, lintels, driven piles, water tanks and concrete pipes. ==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. 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) or galvanised steel, and can be either round or rectangular/oval in cross-section. • 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. 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. The elimination of the post-stressing grouting process required in bonded structures improves the site-labour productivity of unbonded post-tensioning. ==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 encapsulationPrestressing tendons requiring permanent monitoring 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 polyethylene 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 protectionIn 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, 1985A single-span, precast-segmental structure constructed in 1953 with longitudinal and transverse post-tensioning. Corrosion attacked the under-protected tendons where they crossed the in-situ joints between the segments, leading to sudden collapse. • UK Highways Agency, 1992Following 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 five-year programme of inspections on its existing post-tensioned bridge stock. The moratorium was lifted in 1996. • Pedestrian bridge, Charlotte Motor Speedway, North Carolina, US, 2000A multi-span steel and concrete structure constructed in 1995. An unauthorised chemical 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, 2011Sixteen-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 ("Viadotto Petrulla"), Sicily, Italy, 2014One span of a twelve-span viaduct collapsed on 7 July 2014, causing four injuries, 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, EnglandThe 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. and World Tower, Sydney; St George Wharf Tower, London; CN Tower, Toronto; Kai Tak Cruise Terminal and International Commerce Centre, Hong Kong; Ocean Heights 2, Dubai; Eureka Tower, Melbourne; Torre Espacio, Madrid; Guoco Tower (Tanjong Pagar Centre), Singapore; Zagreb International Airport, Croatia; and Capital Gate, Abu Dhabi UAE. File:International Commerce Centre.jpg|ICC tower, Hong Kong484m 2010 File:Guoco Tower, Singapore, under construction - 20141006.jpg|Guoco Tower, Singapore290m 2016 File:Aerial view of the Sydney Opera House.jpg|Sydney Opera House1973 File:Kai Tak Cruise Terminal in June 2014.jpg|Kai Tak TerminalHong Kong 2013 File:World Tower 2014-08-22.jpg|World Tower, Sydney230m 2004 File:Ocean Heights Dubai Marina.jpg|Ocean Heights 2, Dubai335m 2016 File:Eureka Tower LC.JPG|Eureka Tower, Melbourne297m 2006 File:Torre Espacio (Madrid) 06.jpg|Torre Espacio, Madrid230m 2008 File:Capital Gate.jpg|Capital Gate, Abu Dhabi18° 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, 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 girders 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, the underlying rock strata, or both. Such anchors typically comprise tendons of high-tensile bundled steel strands or individual threaded bars. A tendon is grouted to the concrete or rock at its far (internal) end and has a significant de-bonded free length at its 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 (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, Brisbane Australia; Incheon Bridge, South Korea; Roseires Dam, Sudan; Wanapum Dam, Washington, US; LNG tanks, South Hook, Wales; Cement silos, Brevik Norway; Autobahn A73 bridge, Itz Valley, Germany; Ostankino Tower, Moscow, Russia; CN Tower, Toronto, Canada; and Ringhals nuclear reactor, Videbergshamn Sweden. File:Gateway Bridge aerial4.JPG|Gateway BridgeBrisbane, Aust. File:Incheon Grand Bridge.jpg|Incheon BridgeSouth Korea File:Itztalbruecke 2005-07-24.jpg|Autobahn A73Itz Valley, Germany File:Ostankino Tower, 2015.JPG|Ostankino TowerMoscow, Russia File:Toronto - ON - Toronto Harbourfront7.jpg|CN TowerToronto, Canada File:Norcem Brevik fra sjøen.JPG|Norcem silosBrevik, Norway File:Roseires Dam.jpg|Roseires DamAd Damazin, Sudan File:Wanapum Dam from West Shore - downstream 10360031.jpg|Wanapum DamWashington, US File:Gas storage tanks at LNG terminal - geograph.org.uk - 1025438.jpg|LNG tanksSouth Hook, Wales File:Ringhals.JPG|Ringhals nuclear plantVidebergshamn, 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 (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. These organizations are not the authorities of building codes 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 1992-2:2005 – Eurocode 2: Design of Concrete Structures; • US Standard ACI318: Building Code Requirements for Reinforced Concrete; and • Australian Standard AS 3600-2009: Concrete Structures. ==See also==