Volume 10 Issue 3, September , pp. Matthews x S. Search for articles by this author. Author Affiliations. Key: Open access content Subscribed content Free content Trial content. Keywords: concrete. Full Text References. Related content. Towell , V.
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Chandra , P. Mainville , E. Main Recommendations of Part 2. Asset management. Content tools. Add to Favorites Cite this Track Citations. 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 , lintels , driven piles , water tanks and concrete pipes. 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.
Tendon encapsulation systems are constructed from plastic or galvanised steel materials, and are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal grouting 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. When the tendons are tensioned, this profiling results in reaction forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure.
In bonded post-tensioning, prestressing tendons are permanently bonded to the surrounding concrete by the in situ grouting of their encapsulating ducting after tendon tensioning. This grouting is undertaken for three main purposes: to protect the tendons against corrosion ; 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. 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.
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Ducting is fabricated from a durable and corrosion-resistant material such as plastic e. Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end-anchorages to formwork , 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 corrosion.
Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of longitudinal movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a corrosion -inhibiting grease , usually lithium 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. 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 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. 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. 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 s,  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. 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.
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:. ICC tower , Hong Kong m Sydney Opera House Kai Tak Terminal Hong Kong Ocean Heights 2 , Dubai m Eureka Tower , Melbourne m Torre Espacio , Madrid m Concrete is the most popular structural material for bridges, and prestressed concrete is frequently adopted.
Concrete dams have used prestressing to counter uplift and increase their overall stability since the mids. 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. 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. 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 prestressing 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 utilise layers of horizontally-curved hoop tendons for containment in combination with vertically looped tendons for axial wall prestressing.
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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.
Gateway Bridge Brisbane, Aust. Incheon Bridge South Korea. Autobahn A73 Itz Valley, Germany. Ringhals nuclear plant Videbergshamn, Sweden.
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Worldwide, many professional organizations exist to promote best practices in the design and construction of prestressed concrete structures. It is important to note that 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:. From Wikipedia, the free encyclopedia.
Form of concrete used in construction. Unbonded slab post-tensioning. Above Installed strands and edge-anchors are visible, along with prefabricated coiled strands for the next pour. Below End-view of slab after stripping forms, showing individual strands and stressing-anchor recesses. World Tower , Sydney m Ostankino Tower Moscow, Russia. CN Tower Toronto, Canada. Norcem silos Brevik, Norway. Roseires Dam Ad Damazin, Sudan.
Wanapum Dam Washington, US. Design of Prestressed Concrete Structures Third ed. Retrieved 26 August American Concrete Institute. Retrieved 25 August Post-tensioned concreted is "structural concrete in which internal stresses have been introduced to reduce potential tensile stresses in the concrete resulting from loads.