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: 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.
Bonded post-tensioning has prestressing tendons permanently bonded to the surrounding concrete by the in situ grouting of their encapsulating ducting following tendon tensioning. This grouting is undertaken for three main purposes: Bonded post-tensioning characteristically uses tendons each comprising bundles of elements e. This bundling make 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. 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.
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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 mostly fabricated off-site by an extrusion process.
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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 to use 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 significance as a ubiquitous, modern construction material. 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 permanently. 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: Building structures are typically required to satisfy a broad range of structural, aesthetic and economic requirements. Significant among these include: The prestressing of concrete allows "load-balancing" forces to be introduced into the structure to counter the loadings which will apply in-service.
This provides many benefits to building structures:. Some notable building structures constructed from prestressed concrete include: ICC tower , Hong Kong m Sydney Opera House Kai Tak Terminal Hong Kong St George Wharf , London m Ocean Heights 2 , Dubai m Eureka Tower , Melbourne m Torre Espacio , Madrid m Of the wide range of alternative methods and materials that are available for the construction of bridges, concrete remains the most popular structural material, and prestressed concrete, in particular, is frequently adopted.
Concrete dams have used prestressing to counter uplift and increase their overall stability since the mid s. 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 used to 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 under all storage conditions. Prestressed concrete is long-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, while blast containment walls for LNG tanks and similar will normally utilise layers of horizontally-curved hoop tendons for containment in combination with vertically looped tendons for axial wall prestressing.
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.
Prestressed concrete - Designing Buildings Wiki
Some notable civil structures constructed using prestressed concrete include: Gateway Bridge Brisbane, Aust. Incheon Bridge South Korea. Autobahn A73 Itz Valley, Germany. Ringhals nuclear plant Videbergshamn, Sweden. Worldwide, many professional organizations exist to promote best practice in the design and construction of prestressed concrete structures.
Europe has similar country-based associations and institutions. 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 the European Standard EN From Wikipedia, the free encyclopedia.
This makes it more resistant to shock and vibration than ordinary concrete , and able to form long, thin structures with much smaller sectional areas to support equivalent loads.
Prestressed concrete was patented by San Franciscan engineer P. H Jackson in , although it did not emerge as an accepted building material until 50 years later when a shortage of steel , coupled with technological advancements, made prestressed concrete the building material of choice during European post -war reconstruction.
It is now commonly used for floor beams , piles and railways sleepers, as well as structures such as bridges , water tanks, roofs and runways. Generally, prestressed concrete is not necessary for columns and walls , however, it can be used economically for tall columns and high retaining walls with high bending stresses. As a general rule, traditional reinforced concrete is the most economic method for a span of up to 6 m.
Prestressed concrete is more economical when spans are over 9 m. Between 6 and 9 m, the two options must be considered according to the particular requirements as to which is the most suitable option. Steel used for prestressing may be in the form of wire or tendons that can be grouped to form cables. Solid bars may also be used.
Wire is made by cold- drawing a high carbon steel rod through a series of reducing dies. The wire diameter typically ranges from mm and may be round, crimped or indented to give it better bond strength. Another form of tendon is strand which consists of a straight core wire around which is wound in helixes around further wires to give formats such as 7 wire 6 over 1 and 19 wire 9 over 9 over 1.
Similar to wire tendons, strand can be used individually or in groups to form cables. The process of prestressed concrete can be either through pre-tensioning or post-tensioning.
This process involves the stressing of wires or cables by anchoring them at the end of a metal form, which may be up to m in length. Side moulds are then fixed and the concrete placed around the tensioned wires. The concrete hardens and shrinks, gripping the steel along its length, transferring the tension from the jacks to exert a compressive force in the concrete.
Once the concrete has reached the desired strength, the tensioned wires are released from the jacks. To create shorter members, dividing plates can be placed at any point along the member which, when removed, permit the cutting of the wires. This follows the reverse method to pre-tensioning , whereby the concrete member is cast and the prestressing occurs after the concrete is hardened.
This method is often used where stressing is to be carried out on site after casting an insitu component or where a series of precast concrete units are to be joined together to form the required member. The wires, cables or bars may be positioned in the unit before concreting, but bonding to the concrete is prevented by using a flexible duct or rubber sheath which is deflated and removed when the concrete has hardened. Stressing is carried out after the concrete has been cured by means of hydraulic jacks operating from one or both ends of the member.
Due to the high local stresses at the anchorage positions it is common for a helical spiral reinforcement to be included in the design. When the required stress has been reached, the wire or cables are anchored to maintain the prestress. The ends of the unit are sealed with cement mortar to prevent corrosion due to any entrapped moisture and to assist in stress distribution. Anchorages used in post-tensioning depend on whether the tendons are to be stressed individually or as a group.
Most systems use a form of split cone wedges or jaws which act against a form of bearing or pressure plate. There are many different post-tensioning systems.