Comparison of non-prestressed (top) and prestressed concrete beam (bottom) under load:1. Non-prestressed beam without load2. Non-prestressed beam with load3. Before concrete solidifies, tendons embedded in concrete are tensioned4. After concrete solidifies, tendons apply compressive stress to concrete5. Prestressed beam without load6.
Precast and prestressed concrete In the field of structural concrete construction, two basic concepts are generally applied in practice: conventionally reinforced and prestressed concrete. In conventionally reinforced concrete, ordinary un-prestressed.
Prestressed beam with loadPrestressed concrete is a form of used in construction. It is substantially 'prestressed' during its fabrication, in a manner that strengthens it against tensile forces which will exist when in service.: 3–5This compression is produced by the 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, multi-wire or threaded bars that are most commonly made from, or.: 52–59 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 and of ductile high-strength steel when subject to.
This can result in improved structural capacity and/or compared with conventionally in many situations.: 6 In a prestressed concrete member, the internal stresses are introduced in a planned manner so that the stresses resulting from the superimposed 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, reduced structural thicknesses, and material savings compared with simple. Typical applications include, residential slabs, and structures, and, and.First used in the late-nineteenth century, prestressed concrete has developed beyond to include, 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 bridge girder in precasting bed, with single-strand tendons exiting through thePre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast.: 25 The concrete bonds to the tendons as it, following which the end-anchoring of the tendons is released, and the tendon are transferred to the concrete as compression by.: 7Pre-tensioning is a common 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 ) 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 for bonding than bundled-strand tendons.
Pre-tensioned hollow-core plank being placedUnlike 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.: 68–73: 11 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, driven, and concrete. Post-tensioned tendon anchorage; four-piece 'lock-off' wedges are visible holding each strandPost-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast.: 25The 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.: 25: 7 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), anchoring (for strand tendons), and anchoring (for bar tendons).: 79–84.
Balanced-cantilever bridge under construction. Multi-strand post-tensioning anchorIn bonded post-tensioning, prestressing tendons are permanently bonded to the surrounding concrete by the of their encapsulating ducting (after tendon tensioning). This grouting is undertaken for three main purposes: to protect the tendons against; to permanently 'lock-in' the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain 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. ) or steel, and can be either round or rectangular/oval in cross-section.: 7 The tendon sizes used are highly dependent upon the application, ranging from 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, 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 and the tendon stressing-ends sealed against.: 2 Unbonded post-tensioning. 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.Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a -inhibiting, usually based. Anchorages at each end of the tendon transfer the force to the concrete, and are required to reliably perform this role for the life of the structure.: 1Unbonded post-tensioning can take the form of:.
Individual strand tendons placed directly into the concreted structure (e.g. Buildings, ground slabs), or. 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.: 1Individually greased-and-sheathed tendons are usually fabricated off-site by an 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.: 2The benefits that bonded post-tensioning can offer over unbonded systems are:. Reduced reliance on end-anchorage integrityFollowing 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.: 18: 7. Increased inWith bonded post-tensioning, any of the structure is directly resisted by tendon at that same location (i.e.
No strain re-distribution occurs). This results in significantly higher strains in the tendons than if they were unbonded, allowing their full to be realised, and producing a higher ultimate load capacity.: 16–17: 10. Improved crack-controlIn the presence of concrete, 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.: 4: 1. Improved fire performanceThe 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 beUnbonded tendons can be readily prefabricated off-site complete with end-anchorages, facilitating faster installation during construction. Abu Dhabi18° lean 2010Civil 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, a railway bridge constructed 1946 in the. 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 10 to 40 metres (30 to 130 ft), prestressing is commonly employed in the form of precast pre-tensioned or planks. Medium-length structures of around 40 to 200 metres (150 to 650 ft), typically use precast-segmental, in-situ. For the longest bridges, prestressed concrete deck structures often form an integral part of. 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.: 61 Nuclear and blast-containment structures 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.: 585–594Nuclear 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 (LNG) tanks, will normally utilise layers of horizontally-curved hoop tendons for containment in combination with vertically looped tendons for axial wall prestressing.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. Videbergshamn, SwedenDesign 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 (PTI) and the Precast/Prestressed Concrete Institute (PCI).
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