Prestressed concrete is a structural material that allows for predetermined, engineering stresses to be placed in members to counteract the. The unique characteristics of pre-stressed concrete allow predetermined, and build lighter and shallower concrete structures without sacrificing strength. Compressive Strength Added. Compressive stresses are induced in prestressed concrete either by pretensioning or post-tensioning the steel reinforcement. As the steel reacts to regain its original length, the tensile stresses are translated into a compressive stress in the concrete.
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Pre-tensioning process Pre-tensioned bridge girder in precasting bed.
Note single-strand tendons exiting through the formwork Pre-tensioned concrete is a variant of prestressed prestressed concrete structures where the tendons are tensioned prior to the concrete being cast. 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 prestressed concrete structures fabricated.
This allows multiple elements to be constructed end-on-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale prestressed concrete structures be realised for this method of construction.
Higher bond strength in early-age concrete allows more economical fabrication as it speeds production. To promote this, pre-tensioned tendons are usually composed of isolated single wires or strands, as this provides a greater surface area for bond action than bundled strand tendons.
Where "profiled" or "harped" tendons  are required, prestressed concrete structures or more intermediate deviators are located between the ends of the tendon to hold the tendon to the desired non-linear alignment during tensioning.
Straight tendons prestressed concrete structures 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.
Post-tensioned concrete[ edit ] Post-tensioned tendon anchorage. Four-piece "lock-off" wedges are visible holding each strand Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast.
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 prestressed concrete structures compression being applied to the concrete once the tendon is "locked-off" at the anchorage.
Prestressed concrete - Wikipedia
Each added segment is supported by post-tensioned tendons Tendon encapsulation systems are constructed from plastic or galvanised steel prestressed concrete structures, 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: This bundling make for more efficient tendon installation and grouting prestressed concrete structures, 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 formworkplacing 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. Installed strands and edge-anchors are visible, along with prefabricated coiled strands for the next pour 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 greaseusually 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. Individual strand tendons placed directly into the concreted structure e. End-view of slab after stripping, showing individual strands and stressing-anchor recesses 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 prestressed concrete structures 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 prestressed concrete structures. 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 prestressed concrete structures for the project.
Comparison prestressed concrete structures bonded and unbonded post-tensioning[ edit ] Both bonded and unbonded post-tensioning technologies prestressed concrete structures 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.