Prestressed concrete

[6]: 7 Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured off-site from the final structure location and transported to site once cured.

[1]: 26 [6]: 10 Casting the tendon ducts or sleeves into the concrete before any tensioning occurs allows them to be readily profiled to any desired shape including incorporating vertical or horizontal curvature or both.

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.

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.

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.

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.

Research on the durability performance of in-service prestressed structures has been undertaken since the 1960s,[14] and anti-corrosion technologies for tendon protection have been continually improved since the earliest systems were developed.

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.

Modern prestressing systems deliver long-term durability by addressing the following areas: Several durability-related events are listed below: 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.

[1]: 12  Its wide range of application is reflected in its incorporation into the major design codes covering most areas of structural and civil engineering, including buildings, bridges, dams, foundations, pavements, piles, stadiums, silos, and tanks.

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.

[41] 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 girders or planks.

[45][46] Prestressing is also frequently retro-fitted as part of dam remediation works, such as for structural strengthening, or when raising crest or spillway heights.

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.

[49] 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.

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.

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.

[55]: 594–598 [56] 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.

[57] Some notable civil structures constructed using prestressed concrete include: Gateway Bridge, Brisbane Australia;[58] Incheon Bridge, South Korea;[59] Roseires Dam, Sudan;[60] Wanapum Dam, Washington, US;[61] 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.