Prestressed concrete

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Prestressed concrete diagram

Prestressed concrete is a concrete construction material which is placed under compression prior to it supporting any applied loads (ie it is "pre" stressed).[1][2]:3–5 A more technical definition is "Structural concrete in which internal stresses have been introduced to reduce potential tensile stresses in the concrete resulting from loads." [3] This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete volume, and is done to improve the performance of the concrete in service.[4] Tendons may consist of single wires, multi-wire strands or threaded bars, and are most commonly made from high-tensile steels, carbon fibre or aramid fibre.[1]: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 compression forces, and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared to conventionally reinforced concrete in many situations.[2]:6[5]

First used in the late-nineteenth century,[1] prestressed concrete has developed to encompass a wide range of technologies. Tensioning (or "stressing") of the tendons may be undertaken either before (pre-tensioning) or after (post-tensioning) the concrete itself is cast. Tendons may be located either within the concrete volume (internal prestressing), or wholy outside of it (external prestressing). Whereas pre-tensioned concrete by definition uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons. Finally, tensioning systems can be classed as either monostrand systems, where each tendon's strand or wire is stressed individually, or multi-strand systems where all strands or wires in a tendon are stressed similtaneously.[5]

Prestressed concrete is used in a wide range of building and civil structures where its improved concrete performance can allow longer spans, reduced structural thicknesses, and material savings to be realised compared to reinforced concrete. Typical applications range through high-rise buildings, foundation systems, bridge and dam structures, silos and tanks, industrial pavements and nuclear containment structures.[6]

Pre-tensioned concrete[edit]

Pre-tensioned bridge girder in precasting bed. Note single-strand tendons exiting through formwork

Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast.[1]:25 The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by static friction.[5]:7

Pre-tensioning is a common prefabrication 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-on-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realised for this method of construction.[5][7]

The amount of bond (or adhesion) 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 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.[5]

Pre-tensioned hollow-core plank being placed

Unlike those of post-tensioned concrete, the tendons of pre-tensioned concrete elements generally form straight lines between end-anchorages. Where "profiled" or "harped" tendons[8] 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.[1]:68–73[5]: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 beams, floor slabs, hollow-core planks, balconies, lintels, driven piles, water tanks and concrete pipes.

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.[1]:25

The tendons are not placed in direct contact with the concrete, but are 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.[1]:25[5]: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), split-wedge anchoring (for strand tendons), and threaded anchoring (for bar tendons).[1]:79–84

Balanced-cantilever bridge under construction. Each added segment is supported by post-tensioned tendons

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 concete by internal grouting of the duct after stressing (bonded post-tensioning); and those where the tendon element is permanently debonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands (unbonded post-tensioning).[1]:26[5]:10

Casting the tendon ducts/sleeves into the concrete before any tensioning occurs allows them to be readily "profiled" to any desired shape including incorporating vertical and/or horizontal curvature. 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.[2]:5–6[5]:48:9–10

Bonded post-tensioning[edit]

Multistrand post-tensioning anchor.

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: 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 behaviours of the final concrete structure.[9]

Bonded post-tensioning characteristically uses tendons each comprising bundles of elements (eg strands or wires) placed inside a single tendon duct, with the exception of bars which are mostly used unbundled. 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 (eg polyethylene) or galvanised steel, and can be either round or rectangular/oval in cross-section.[2]:7 The tendon sizes used are highly-dependent upon the application, ranging from building works typically using between 2-strands and 6-strands per tendon, to specialised dam 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 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.[5]:2

Unbonded post-tensioning[edit]

Unbonded slab post-tensioning. 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 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.[9]:1

Unbonded post-tensioning can take the form of:

  • Individual strand tendons placed directly into the concreted structure (eg 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 (eg restressable anchors, external post-tensioning)
Unbonded slab post-tensioning. 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 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.[9]:1

Individually greased-and-sheathed tendons are mostly 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.

Comparison between bonded and unbonded post-tensioning[edit]

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.[9]:2

The benefits that bonded post-tensioning can offer over unbonded systems are:

  • Increased ultimate strength in flexure
    By bonding the tendon to the surrounding concrete, flexural action at any point in the structure gets directly resisted by tendon strains at that same location. This is compared to unbonded tendons, which will distribute any strains over their full length. By preventing this strain re-distribution, bonding results in significantly higher tensile strains in the tendons as the structure deforms under applied loading. This allows the full Yield Strength of the tendon material to be realised, leading to higher ultimate design load capacity.[2]:16–17[5]:10
  • Improved crack-control
    Cracking occurs in concrete structures once the stress in that part of the concrete exceeds its tensile strength. In this situation, the absence of tendon strain redistribution in bonded post-tensioning results in greater control of the subsequent crack widths. With bonded tendons securely locked into the concrete directly either side of a crack, greater resistance is offered to crack width expansion than do unbonded tendons which disperse any strains over their full length. This characteristic is reflected in the reduced requirements for conventional crack control reinforcement within many design Codes when bonded tendons are used.[9]:4[10]:1
  • Improved fire performance
    The absence of tendon strain redistribution in bonded post-tensioning limits the impact that localised weakening of a tendon's material (due to overheating) will have on the overall structure. As a result, bonded post-tensioned structures may display a higher capacity to resist fire conditions than unbonded structures.[11]
  • Reduced reliance on end-anchorage integrity
    Following tensioning and grouting, bonded tendons are connected to the surrounding concrete along their full length by the high-strength grout. 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.[2]:18[9]:7

The benefits that unbonded post-tensioning can offer over bonded systems are:

  • Ability to be prefabricated
    Unbonded tendons can be readily prefabricated off-site, complete with end-anchorages, thereby facilitating faster installation time during construction. Some additional lead time is usually required to allow for this fabrication process.
  • Improved site productivity
    The elimination of the post-stressing grouting process improves the site-labour productivity of unbonded post-tensioning.[9]:5
  • Improved installation flexibility
    Unbonded tendons of single strands have significantly greater handling flexibility than bonded ducting when being installed, and this gives them a greater ability to be deviated around service penetrations or other in-structure obstructions.[9]:5
  • Reduced concrete cover
    Unbonded tendons may allow some reduction in concrete element thickness, as their smaller size and increased corrosion protection compared to ducted tendons may allow them to be placed closer to the concrete surface (ie reduced concrete cover).[2]:8
  • Simpler replacement and/or adjustment
    Being permanently separated from the concrete structure, unbonded tendons are able to be readily destressed, restressed and/or replaced, should they become damaged or require their tensioning levels to be modified during service.[9]:6
  • Superior overload performance
    Although usually having lower ultimate strength than bonded tendons, unbonded tendons' characteristic of redistributing strain over their full length can give them superior pre-collapse ductility. In extremis, unbonded tendons can resort to a catenary-type action instead of pure flexure, allowing significantly greater deformation before structural failure.[12]

History of problems with bonded post-tensioned bridges[edit]

The popularity of this form of prestressing for bridge construction in Europe increased significantly around the 1950s and 60s. However, a history of problems has been encountered that has cast doubt over the long-term durability of such structures.

Due to poor workmanship or quality control during construction, sometimes the ducts containing the prestressing tendons are not fully filled, leaving voids in the grout where the steel is not protected from corrosion. The situation is exacerbated if water and chloride (from de-icing salts) from the highway are able to penetrate into these voids.

Notable events are listed below:

  • The Ynys-y-Gwas bridge in West Glamorgan, Wales—a segmental post-tensioned structure, particularly vulnerable to defects in the post-tensioning system—collapsed without warning in 1985.[13]
  • The Melle bridge, constructed in Belgium during the 1950s, collapsed in 1992 due to failure of post-tensioned tie down members following tendon corrosion.
  • Following discovery of tendon corrosion in several bridges in England, the Highways Agency issued a moratorium on the construction of new internal grouted post-tensioned bridges and embarked on a 5-year programme of inspections on its existing post-tensioned bridge stock.
  • In 2000, a large number of people were injured when a section of a footbridge at the Charlotte Motor Speedway, USA, gave way and dropped to the ground. In this case, corrosion was exacerbated by calcium chloride that had been used as a concrete admixture, rather than sodium chloride from de-icing salts.
  • In 2011, the Hammersmith Flyover in London, England, was subject to an emergency closure after defects in the post-tensioning system were discovered.[13]

Applications[edit]

Prestressed concrete is the main material for floors in high-rise buildings and the entire containment vessels of nuclear reactors.

Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable.[14] Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a damaged building by holding up a damaged wall or floor until permanent repairs can be made.

The advantages of prestressed concrete include crack control and lower construction costs; thinner slabs—especially important in high rise buildings in which floor thickness savings can translate into additional floors for the same (or lower) cost and fewer joints, since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced constructions with the same thickness. Increasing span lengths increases the usable unencumbered floorspace in buildings; diminishing the number of joints leads to lower maintenance costs over the design life of a building, since joints are the major focus of weakness in concrete buildings.

The first prestressed concrete bridge in North America was the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951.[15] Prestressing can also be accomplished on circular concrete pipes used for water transmission. High tensile strength steel wire is helically-wrapped around the outside of the pipe under controlled tension and spacing which induces a circumferential compressive stress in the core concrete. This enables the pipe to handle high internal pressures and the effects of external earth and traffic loads.

Design agencies and regulations[edit]

In the United States, pre-stressed concrete design and construction is aided by organizations such as Post-Tensioning Institute (PTI) and Precast/Prestressed Concrete Institute (PCI). In Canada the Canadian Precast/Prestressed Concrete Institute (CPCI) assumes this role for both post-tensioned and pre-tensioned concrete structures.

Europe also has its own associations and institutes. 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 pre-stressed design, codes and best practices. In the UK, the Post-Tensioning Association fulfills this role.[16]

Rules for the detailing of reinforcement and prestressing tendons are provided in Section 8 of the European standard EN 1992-2:2005 – Eurocode 2: Design of concrete structures – Concrete bridges: design and detailing rules.

In Australia the code of practice used to design reinforced and prestressed concrete is AS 3600-2009.

See also[edit]

References[edit]

  1. ^ a b c d e f g h i Lin, T.Y.; Burns, Ned H. (1981). Design of Prestressed Concrete Structures (Third ed.). New York, U.S.A.: John Wiley & Sons. ISBN 0 471 01898 8. 
  2. ^ a b c d e f g Federation Internationale du Beton (Feb 2005). fib Bulletin 31: Post-tensioning in Buildings (PDF). FIB. ISBN 978 2 88394 071 0. Retrieved 26 August 2016. 
  3. ^ American Concrete Institute. "CT-13: ACI Concrete Terminology". American Concrete Institute. Farmington Hills, Michigan USA: ACI. Retrieved 25 August 2016. 
  4. ^ Warner, R. F.; Rangan, B. V.; Hall, A. S.; Faulkes, K. A. (1988). Concrete Structures. South Melbourne, Australia: Addison Welsley Longman. pp. 8–19. ISBN 0 582 80247 4. 
  5. ^ a b c d e f g h i j k Warner, R. F.; Faulkes, K. A. (1988). Prestressed Concrete (2nd ed.). Melbourne, Australia: Longman Cheshire. pp. 1–13. ISBN 0 582 71225 4. 
  6. ^ Post-Tensioning Institute (2006). Post-Tensioning Manual (6th ed.). Phoenix, AZ U.S.A.: PTI. pp. 5–54. ISBN 0 9778752 0 2. 
  7. ^ Tokyo Rope Mfg Co Ltd. "CFCC Pre-tensioning Manual" (PDF). MaineDOT. Retrieved 19 August 2016. 
  8. ^ "Tendons having one or more deviations from a straight line, either vertically or horizontally, between the ends of the structure"
  9. ^ a b c d e f g h i Aalami, Bijan O. (5 September 1994). "Unbonded and bonded post-tensioning systems in building construction" (PDF). PTI Technical Notes. Phoenix, Arizona U.S.A.: Post-Tensioning Institute (5). Retrieved 23 August 2016. 
  10. ^ Aalami, Bijan O. (February 2001). "Nonprestresed Bonded Reinforcement in Post-Tensioned Building Design" (PDF). ADAPT Technical Publication (P2-01). Retrieved 25 August 2016. 
  11. ^ Bailey, Colin G.; Ellobody, Ehab (2009). "Comparison of unbonded and bonded posttensioned concrete slabs under fire conditions". The Structural Engineer. 87 (19). Retrieved 22 August 2016. 
  12. ^ Bondy, Kenneth B. (December 2012). "Tow way post-tensioned slabs with bonded tendons" (PDF). PTI Journal. USA: Post-Tensioning Institute. 8 (2): 44. Retrieved 25 August 2016. 
  13. ^ a b Ed Davey and Rebecca Cafe (3 December 2012). "TfL report warned of Hammersmith Flyover collapse risk". BBC News, London. Retrieved 3 December 2012. 
  14. ^ Barrier Cable
  15. ^ Cement & Concrete Basics: Prestressed Concrete | Portland Cement Association (PCA)
  16. ^ PTA Homepage

External links[edit]