POST-TENSIONING Principles and Applications to Concrete Elements Prepared by: Danny M. Francisco. C.E., G.E. Tel. 6770359 (Office) Mobile: 0557124793 Email: danilo.francisco@aramc
[email protected] o.com
Objectives: 1. To know the brief history of post tensioning. 2. To understand the basic principles of post-tensioning. 3. To identify the components of post tensioning. 4. To know the applications of post tensioning
Objectives: 1. To know the brief history of post tensioning. 2. To understand the basic principles of post-tensioning. 3. To identify the components of post tensioning. 4. To know the applications of post tensioning
History •
The first patent for prestressed concrete was issued to P.H. Jackson of San Francisco in 1886. He obtained a US patent for tightening steel tie rods in artificial stones (concrete blocks) and concrete arches used for slabs and roofs.
Shortly thereafter, in 1888, C.E.W. Doehring from Germany also obtained a patent for prestressing concrete slabs with metal wires. However, modern development of prestressed concrete is usually attributed to Eugene Freyssinet of France. In 1928, Freyssinet begun to use high-strength steel wire for prestressing concrete.
In 1940, Professor Gustav Magnel of Belgium developed a system of curve, multi -wire tendons in flexible rectangular ducts.
The first use of post-tensioning in the US was on the Walnut Lane Bridge in Philadelphia in 1949. This landmark bridge had precast girders post -tensioned with the Magnel system.
Ulrich Finsterwalder- German Civil Engineer
Tie-Back Anchorage
Tie-Down Anchorage
Over the years, there have been a number of significant technological developments that have helped advance the state-of -the-art of post tensioning and have contributed to its continued growth. These developments include: 1. Introduction of strand system 2. Development of ductile iron castings for single strand tendons 3. Introduction of the “load-balancing” design method 4. Introduction of “ “banded” tendon layout for 2-way slab system 5. Segmental bridge construction 6. Use of computers for analysis and design 7. Formation of the Post-Tensioning Institute 8. Improvements in corrosion resistance
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In the United States, early bonded post -tensioning system used high-strength stress-relieved steel wires, bars, or strand. The button-headed tendon system used ¼ inch wire bundled together, greased, and wrapped with Kraft paper as sheathing. The wires ran through a stressing head and were “button headed” to anchor them. Machinery was used to cold -upset the ends of the wires to create the button -head anchors. The post-tensioning elongation was held with shims or a threaded nut.
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The button-headed tendon system had two major problems. The first problem was that the tendons had to be an exact length. Any deviation between the tendon length and the length between forms required either a new tendon or moving the edge forms before pouring the concrete.
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Second, because shims and stressing washer ended up on the outside edges of the constructed slab, they have to be covered with second concrete pour.
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The first strand tendon system – developed by Edward K. Rice and others at Atlas Prestressing Corporation- used ½ inch sever-wire prestressing strand and an anchorage assembly manufactured of coiled wire and a plate and anchored with two half wedge chucks. The strand was also greased and wrapped with Kraft paper.
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Since 1985, post-tensioning usage has continued to grow at a rapid pace, averaging 8.5% annual growth as shown.
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In the last 50 years, prestressed concrete has grown to be a multibillion-dollar industry in North America and is used in many different construction applications. Figure shows the relative usage of posttensioning by market value.
Principles: •
Prestressing General: Pre-stressing is a method of reinforcing concrete. Externally applied loads induce internal stresses (forces) in concrete duri ng the construction and services phases of a member. The concrete i s pre-stressed to counteract these anticipated stresses during the service life of the member.
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There are two commonly used methods of pre-stressing concrete. One is called pre-tensioning. The prefix “pre” means that the prestressing steel is stressed before the concrete is cast. This me thod consists of first stressing high- strength steel strands or wires between buttresses, and then casting the concrete around the steel. Once the concrete has reached a certain specified strengt h, the steel is cut between the ends of the member and the buttresses to transfer the pre-stressing force to the concrete. This process typically takes place at a precast plant and requires th e complete pre-tensioned concrete member to be trucked out to the job site and then assembled.
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The other method of pre-stressing concrete is called post-tensioning. The prefix “post” means that the prestressing steel is stressed after the concrete is cast. Instead of stressing the high-strength steel between buttresses at a precast plant. The steel is simply installed on the job site after the contractor forms up the member. The high-strength steel is housed in a sheathing or duct that prevents it from bonding to the concrete. The steel is attached to the concrete at the ends of the member by especially design anchorage devices. Once the concrete has cured (hardened), the steel is stressed to induce forces in the concrete. Posttensioning has all of the advantages of pre -stressed concrete while allowing the builder the freedom to construct the member in any location, including its final position in the structure (cast -in-place).
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Types of post-tensioning system: In most post-tensioned construction, the pre-stressing tendons are embedded in the concrete before the concrete is cast. These internally post-tensioned systems can be either bonded or unbonded. In some bridge and retrofit applications, the post-tensioning tendons are mounted outside the structural members. These are referred to as external post-tensioning systems.
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In un-bonded systems, the strand is kept unbonded to the surrounding concrete throughout its service life. In bonded systems, grout is injected in the ducts to bond the pre-stressing strand to the surrounding concrete after it has been stressed. Once the grout has cured (hardened), the system behaves as an integral system without any relative movement between steel and concrete. Most of the internally grouted post-tensioned systems are considered to be bonded. Unbonded systems allow relative movement between the strand and surrounding concrete throughout it service life. Most single -strand systems and all external post-tensioning systems fall under this category.
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Unbonded Post-Tensioning Systems: The tendons in an unbonded system typically consist of single-strand that coated with corrosion-inhibiting coating and protected by extruded plastic sheathing. This allows the strand to move inside the plastic sheathing and prevent the ingress of water. The strands are anchored to the concrete using ductile iron anchors and hardened steel wedges. The tendon is supported by chairs and bolsters along its length to maintain the desired profile. Figure shows the typical components and construction sequence for an unbonded system.
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Depending on the exposure of the single-strand unbonded system, it can be classified as a standard or encapsulated system. Encapsulated systems are required for aggressive environment where there is a possibility of tendon exposure to chlorides or other deleterious substances. Encapsulated tendons are designed to prevent any ingress of water during and after construction. Figure shows an example of a standard and encapsulated tendon.
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Bonded Post-Tensioning System: Bonded post-tensioning systems consist of tendons with multi strands or bars. The strands or bars are placed in corrugated galvanized steel, high density polyethylene (HDPE) or polypropylene (PP) ducts. Depending on the site conditions and system used, the strands may be installed before the concrete is placed or the ducts may be installed without the strands. The strands are then pulled or pushed through the ducts. Once the concrete has hardened, the tendons are stressed and ducts filled with grout. Inlets and outlets are provided at high/low points to ensure that the grout fills the ducts completely. Figure shows t he components of a typical multi strand grouted system. The grout provides an alkaline environment and protect the pre-stressing strands from corrosion. It also bonds the strands to the surrounding concrete.
Illustrations: •
Concrete has a low tensile strength but is strong in compression. The tensile strength of concrete is about 10% of its compressive strength. As a result, plain concrete members are likely to crack when loaded.
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Consider a beam of plain concrete
As the load increases, the beam deflects slight and then falls abruptly. Under load, the stresses in the beam will be compressive in the top, but tensile in the bottom. We can expect the beam to crack at the bottom and break, even with a relatively small load, because of concrete’ s low tensile strength. In order to resist tensile stresses or counter the low tensile strength which plain concrete cannot resist, it can be reinforced in two ways by using steel reinforcing bars or by pre-stressing.
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In the usual reinforced concrete beam, the concrete cannot be used efficiently, certain forces may be applied to the beams that result in a member in which all the concrete can resist the bending stresses.
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Introducing a means of pre-compressing the tensile zones of the concrete members to offset anticipated tensile stresses will reduce or eliminate cracking and will produce a more durable concrete member. By pre -compressing a concrete element, so that when flexing under applied load, it will still remain in compression and achieving a more efficient design of the structure.
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The function of Prestressing (pre-tensioning or POST-TENSIONING) is to place the structure under compression in those region where load causes tensile stresses. Compressive stresses introduced into areas where tensile stresses under load will resist or annul these tensile stresses.
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So the concrete now behaves as if it had a high tensile strength of its own and, provided that the tensile stresses do not exceed the pre compression stresses, cracking cannot occur in the bottom of the beam.
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Tension caused by the load will first have to cancel the compression induced by the posttensioning before it can crack the concrete. The pre-compression stresses can also be designed to overcome the diagonal stresses. The normal procedure is to design to eliminate cracking at working loads.
However, bending is only one of the conditions involved. There is also shear. Vertical and horizontal shear forces are set up within a beam and these will cause diagonal tension and diagonal compression stresses of equal intensity. • As concrete is weak in tension, cracks in a reinforced concrete beam will occur where the diagonal tension stresses are high, usually near the support. In pre-stressed concrete, the compression stresses can also be design to overcome these tension stresses. •
Simple illustration #1: • Row of books •
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Prof. Gustav Magnel, one of the pioneers of prestressed concrete used this illustration and simply explained to his students using stack of books.
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Illustration #2:
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Figure 1.4 shows a plainly reinforced concrete simple-span beam and a fixed cantilever beam cracked under applied load.
Reinforced concrete cracked under load (Fig. 1.4)
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Figure 1.5 shows the same unloaded beams with prestressing forces applied by stressing high strength tendons. By placing the prestressing low in the simple-span beam and high in the cantilever beam, compression is induced in the tension zones; creating upward camber.
Post-tensioned concrete before loading (Fig. 1.5)
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While figure1.6 shows the two prestressed beams after loads have been applied. The loads cause both the simple-span beam and cantilever beam to deflect down, creating tensile stresses in the bottom of the simple-span beam and top of the cantilever beam.
Post-tensioned concrete after loading (Fig. 1.6)
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The designer shall be able to balance the effects of load and prestressing in such a way that tension from the loading is compensated by compression induced by the posttensioning. Figure 1.6 shows that tension is eliminated under combination of Figures 1.4 and 1.5. Also construction materials (concrete and steel) are used more efficiently; optimizing materials, construction effort and cost.
Fig. 1.4
Fig. 1.5
Fig. 1.6
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One of the things that happen to a concrete, or a masonry wall, is that they are subjected to forces that cause them to flex and bend. Examples of this includes slabs on ground where the edges of the slabs are forced upward by swelling soil, elevated concrete slabs where gravity and other applied loads pull down on the slab in between supports, and walls that might be subjected to lateral forces from wind or seismic activity. This bending creates high tensile forces that can cause the concrete and masonry to crack. This is where the use of reinforcing is applied.
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Illustration #3:
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Since steel has a high capacity to resist tensile forces, it can be embedded in the concrete at the tension zones (the area that tensile failures could occur) allowing the tensile forces to be handled by the reinforcing steel. Adding post-tensioned reinforcing combines the action of reinforcing the tension zones with the advantages of compressing the concrete or masonry structure.
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Additional benefits are obtained when the posttensioned reinforcing is installed in a draped profile instead of running in a straight line. A typical draped profile in an elevated concrete slab would route the post-tensioned reinforcing through a high point over the slab’ s supports, and through a low point in between those supports. Now, optimum efficiency is obtained in the tension zones, the concrete is being compressed, and the post-tensioned reinforcing is creating an uplift force in the middle of the spans where it is needed the most.
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Consider a beam with a force P applied at each end along the beam’ s center line.
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This force applies a uniform compressive stress across the section equals to P/A.
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Consider next a vertical load W applied to the section and the corresponding bending moment diagram applied to this alone.
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The stress distribution from the flexure of the beam is calculated from M/Z , where M is the bending moment and Z is the section modulus. By considering the deflected shape of the beam, it can be seen that the bottom surface will be in tension. The corresponding stress diagram can be drawn.
As previously discussed that concrete is strong in compression but not in tension. Only small tensile stresses can be applied before cracks that limit the effectiveness of the section will occur. • By combining the stress distribution from the applied pre-compression and the applied loading, it can be seen that there is no longer any tension. •
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In addition, the technique known as “load balancing” offers the designer a powerful tool. In this, forces exerted by the prestressing tendons are modeled as equivalent upward forces on the beam or slab. These forces are then proportioned to balance the applied downward forces. By balancing a chosen percentage of applied loading, it is possible to control deflections and also to make the most efficient use of the beam or slab depth.
Load Balancing by Prestressing
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In order to use the load balancing technique, the pre-stressing tendons must be set to follow profiles that reflect the bending moment envelop from applied loadings. Generally, parabolic profiles are used.
Comparison between Post-tensioning and Pretensioning: •
Post-tensioning can be done on the job site. From reinforcing bars placement, installation of duct, steel form erection, concrete pouring, curing and post-tensioning of strands until erection or placement.
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Bridge abutments and piers can be done simultaneously while fabricating the post tensioned I-girders (beams).
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Pretensioning normally requires large open areas usually in a factory and being done between heavy end anchorage, bulkheads or abutments.
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Camber is the upward deflection in flexural members due to an eccentrically applied prestressing force. Camber is divided into two categories i.e., initial camber and long -term camber. Initial camber is induced at transfer of the prestressing force at the time of release. It is the net upward deflection calculated by algebraically summing the smaller downward deflection caused by the beam self -weight ( ∆beam) and the larger upward deflection ( ∆ps) caused by the prestressing force applied at an eccentricity ‘e’ below the center of gravity of the section.
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The components of camber due to self weight and prestressing force.
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For straight tendon profile: ∆beam = 5wLs4 384EcI ∆ps = PeLs2 8EcI The magnitude of initial camber is the difference (∆ps - ∆beam) between the above two values.
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The three figures below illustrate the elastic components of beam camber and deflection immediately after released.
Load applied to the beam at released
Deflection due to beam weight
Camber due to prestressing force
Definition of commonly used terms: •
Aggressive Environment – An environment in which structures are exposed to direct or indirect application of deicing chemicals, seawater, brackish water, or spray from these water sources; and salt-laden air as occurs in the vicinity of seacosts. Aggressive environment also include structures where stressing pockets are wetted or are directly in contact with soils which contain chloride levels considered y the geotechnical engineer to be harmful to metals.
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Anchor Cavity – The opening in the anchor or anchor block designed to accommodate the strand passing through and the proper seating of the wedges.
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Anchor Nut – Threaded device that screws onto a threaded bar and transfers the force from the bar to the bearing plate.
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Anchorage – A mechanical device comprising all components required to anchor the prestressing steel and permanently transfer the post-tensioning force from the prestressing steel to the concrete.
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Anchorage Zone – The portion of the member through which the concentrated prestressing force is transferred to the concrete and distributed more uniformly across the section. Its extent is equal the largest dimension of the cross section. For anchorage devices located away from the end of the member, the anchorage zone includes the disturbed regions ahead of and behind the anchorage. The general expression for combined general and local zones.
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Anticipated set – The expected movement of the wedges into the anchorage during the transfer of the prestressing force to the anchorage device. This is that set which was assumed to occur in the design calculation of post-tensioning forces immediately after load transfer
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AUTS – Actual Ultimate Tensile Strength: The actual breaking strength obtained in tests on a single representative strand or bar, breaking outside the anchorage. For multi-strand or bar tendons, AUTS equals the AUTS of a single tendon elements (strand, bar) times the number of such elements in the tendon. Representative samples must be from same coil of strands or the same bar from which strands or bars are cut and used in connection efficiency tests.
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Back -Up Bars – Reinforcing bars placed in concrete in the anchorage zone to position the anchor and help in distributing the loads.
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Banded Tendons – Group(s) of closely speced tendons in slabs placed together in a narrow strip, usually along the column line.
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Bar – Bars used in post-tensioning tendons conforming to ASTM A722, Standard Specification for Uncoated HighStrength Bar for Prestressing Concrete. Bars have a minimum ultimate tensile strength of 150,000 psi (1035 Mpa). Type I Bar has a plain surface and Type II Bar has surface deformations. Post-tensioning bars are high-strength steel bars, normally available from 16mm to 44mm (5/8 to 1 3/4in) diameter and usually threaded with coarse thread.
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Barrel Anchor – A cylindrical metal device housing the wedges and normally used with a bearing plate to transfer the prestressing force to the concrete.
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Barrier Cable – High-strength steel strand erected around the perimeter of a structure and at open edges of ramps to prevent automobiles and pedestrians from falling over the open sides.
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Basic Bearing Plate – Flat plate bearing directly against concrete meeting the analytical design requirements. Covered by this definition are square, rectangular, or round plates, sheared or torch cut from readily available steel plates, normally ASTM A36.
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Bearing Plate – A plate which bears directly against the concrete and is part of an overall anchorage system. A steel hardware that transfers the tendon force into a structure.
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Blowout – A localized concrete failure which occurs during or after stressing.
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Bonded Tendons – Tendon in which prestressing steel is bonded to concrete either directly or through grouting.
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Bursting Steel – Reinforcing steel used to control the tensile bursting forces developed at the bearing side of the anchor as the concentrated anchor force from prestressed tendon spreads out in all directions.
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Cable – A term used by some to denote a prestressing strand or a single-strand tendon.
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Camber – An upward deflection that is caused by the application of prestressing force. Camber is intentionally built in a structural element or form to improve appearance or to nullify the deflection of the element under the effect of loads, shrinkage, and creep.
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Chair – Hardware used to support or hold posttensioning tendons or reinforcing. Bars in their proper position to prevent displacement before and during concrete placement.
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Coating – Material used to protect the prestressing steel from corrosion and reduce the friction.
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Confinement Reinforcement - Non-prestressed reinforcement in the local zone. Confinement reinforcement in the concrete ahead of tendon anchorage is limited to the local zone. Confinement reinforcement consists of spirals, orthogonal reinforcing bars, or a combination of both.
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(Cont’ d) d) Confinement reinforcement: For basic bearing plates, confinement reinforcement is required in that volume of concrete in which compressive stresses exceed acceptable limits for unreinforced concrete determine by rational analysis. For special bearing plate, confinement reinforcement is system dependent as determined by tests on individual anchorages. Test block reinforcement, in the portion surrounding the special bearing plate and immediately ahead of it, must represent the confinement required in the local zone for that particular system.
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Coupler – A device designed to connect ends of two strands together, thereby transferring the prestressing force from end to end of the tendons. The means by which the prestressing force maybe transferred from one partial-length prestressing tendon to another tendon.
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Creep – Time dependant deformation (shortening) of concrete under sustained stress (load).
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Curvature Friction – Friction resulting from bends or curves in the specified prestressing tendon profile.
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Distributed Tendons – Single or group of tendons in a slab that are uniformly distributed, usually perpendicular to the bonded tendons and spaced at a maximum of eight times the slab thickness or 5 feet (1.5 M).
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Duct – A conduit (plain or corrugated) to accommodate prestressing steel for post tensioning installation.
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Eccentricity – Distance between the center of gravity of the concrete cross- section and center of gravity of the prestressing steel at any point along the length of the member.
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Effective Prestress - Stress remaining in prestressing steel after all losses have occurred.
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Elastic Shortening – The shortening of a member that occurs immediately after the application of the prestressing force.
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Elongation – Increase in the length of the prestressing steel under the applied prestressing force.
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Encapsulated System – A post-tensioning system that prevents the ingress of water into the tendon tendon during during all all stages of construction, and isolates the strand and anchorage from contact with concrete.
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Fixed End Anchorage – An anchorage at the end of a tendon where prestressing jack is not attached during stressing operations. Fixed-end anchorages are typically attached from contact with concrete.
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Friction Loss – The loss of force in a prestressing tendon resulting from friction created between the strands and sheathing due to curvature and wobble during stressing.
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General zone – The region in which the concentrated prestressing force spreads out to a more linear stress distribution over the cross section of the structural member (Saint Venant Region). It includes the local zone. The general zone extends from the anchorage along the axis of the member for a distance equal to the overall depth of the member. The height of the general zone equal to the overall depth of the member.
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Grout – A mixture of cementitious materials and water, with or without mineral additives, admixtures or fine aggregate, proportion to produce a pumpable consistency without segregation.
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GUTS – Guaranteed Ultimate Tensile Strength: This is the tensile strength of the material that can be assured by the Manufacturer. GUTS should not be confused with “fpu” the specified ultimate tensile strength (AASHTO LRFD). (The term “GUTS” has been replaced by two definitions, “MUTS” and “ AUTS” by the Post-Tensioning Institute.)
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HDPE – Acronym for High Density Polyethylene plastic. HDPE has a minimum density of 0.941 gram per cubic centimeter in post-tensioning.
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Initial Concrete Strength – The strength of the concrete necessary for the post-tensioning operation.
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Initial Prestress – The force in the tendon immediately after transferring the prestressing force to the concrete. This occurs after the wedges have been seated.
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Inlet – The opening used to inject grout into the duct.
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Intermediate Anchorage – An anchorage located at any point along the tendon length, which can be used to stress a given length of tendon without the need to cut the tendon. Normally used at concrete pour breaks.
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Jack Calibration – A chart showing the related gauge pressure to actual force applied to a tendon.
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Jack Gripper Plates – Steel plates designed to hold the jack grippers in place in the jack.
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Jack Grippers – Wedges used in the jack to hold the strand during the stressing operation.
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Jack – – A mechanical device (normally hydraulic) used to apply force to a prestressing tendon.
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Jacking Force – The maximum temporary force exerted by the jack on the tendon.
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Live End – Stressing End.
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Local Zone – The local anchorage zone is the volume of concrete surrounding and immediately ahead of the anchorage device where the concrete compressive stresses exceed acceptable values for unconfined concrete (concrete without confinement reinforcement). The local zone is defined as a rectangular prism of concrete surrounding the bearing plate and any integral confinement reinforcement. The transverse dimension of the prism are equal to those of the bearing plate, including any integral confinement, plus the supplier’ s specified minimum edge covers. The length of the local zone extends over the distance from loaded concrete surface to the bottom of each bearing surface of the anchorage device plus maximum dimension of the bearing surface.
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Material Certification – Documentation from manufacturer that confirms that the quality of material supplied meets all project requirements.
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Modulus of Elasticity – Ratio of stress to corresponding strain for tensile or compressive stresses below proportional limit of material.
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Monostrand – One single-strand.
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Multistrand – More than one single-strand in a tendon.
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MUTS – Minimum Ultimate Tensile Strength: When measured as a force, for a single strand or bar breaking outside of the anchorage or the multiple of those single strand or bar forces for multi-strand or bar tendons; MUTS is the force equal to the nominal cross-sectional area of strand , or bar, times their nominal ultimate tensile stress.
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Non- Aggressive Environment – All environments not specifically defined as aggressive, including enclosed buildings.
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Outlet – Opening to allow the escape of air, water, grout and bleed water from the duct during grouting operation.
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P/T Coating – material used to protect against corrosion and reduce friction between prestressing steel and sheathing. For unbonded application P/T coating should meet or exceed the performance criteria outlined in the PTI Specifications for Unbonded Strand Tendons.
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Pocket Former – A temporary device used at the stressing end to provide a cavity that can be grouted after the prestressing operation is complete.
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Post-Tensioning System – This is the proprietary system where the necessary hardware (anchorage, wedges, strands, bars, couplers, etc.) is supplied by a particular manufacturer or manufacturers of post-tensioning components and may also include ducts and local zone reinforcement.
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Post-tensioning – Method of prestressing in which steel is tensioned after concrete has hardened.
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Potable Water – Water defined by EPA (Environmental Protection Agency) to meet drinking water standards.
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Prestressed Concrete – Structural concrete in which internal stresses are introduced to reduce potential tensile stresses in concrete resulting from applied loads.
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Prestressing Steel – High-Strength steel, most commonly 7-wire strand, used to impart prestress forces to concrete.
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The steel element of a post-tensioning tendon, which is elongated and anchored to provide the necessary permanent prestressing force.
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Pretensioning – A method of prestressing in which the tendons are tensioned before the concrete has been placed.
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Profile – The path of a tendon in concrete from end to end.
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Pump – A hydraulic pump used to provide hydraulic pressure to the stressing jack.
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Reference Point – The painted mark placed on a tendon tail used to measure the elongation of a tendon after stressing.
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Seating Loss – The relative movement of the wedges into the anchor cavity during the transfer of the prestressing force to the anchorage resulting in some loss of prestressing force.
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Set – The total movement of point on the strand just behind the anchoring wedges during load transfer from the jack to the permanent anchorage.
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Set movement is the sum of slippage of wedges with respect to the anchorage head and the elastic deformation of the anchor components.
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Sheathing – A material encasing prestressing steel to prevent bonding of the prestressing steel with the surrounding concrete, provide corrosion protection, and contain post-tensiong coating.
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Slab Bolster – Continuous hardware used to support or hold post-tensioning tendons in place prior to and during concrete placement.
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Special Bearing Plate – Any hardware that transfers tendon anchor forces into the concrete but does not meet analytical design requirements. Covered by this definition are devices having single or multiple plane bearing surfaces, and devices combining bearing and wedge plate in one piece. They normally require confinement reinforcement.
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Split Pocket Former – A temporary temporary two two-piece device used at the intermediate end during casting of the concrete to provide an opening in the concrete, allowing the stressing equipment access to the anchor cavity.
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Stage Stressing – Sequential stressing of tendons in separate steps or stages in lieu of stressing all the tendons during the same stressing operation.
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Strand Slippage – Slippage or relative movement of strand with respect to wedges during force transfer. See seating loss.
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Strand – High-strength steel wires helically placed around a center wire. For bonded tendons typically a 7wire strand.
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Stressing End Anchorage – The anchorage at the end of a tendon where the prestressing jack is attached to the tendon during stressing operations.
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Stressing Equipment – Consists normally of a jack, pump, hoses, and a pressure gauge.
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Stressing Force – See jacking force.
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Stressing Pocket – The void created by the pocket former between the stressing anchor and the edge of the concrete to allow access for stressing equipment. After stressing, this void is filled in with an approved grout to provide protection for the tendon end.
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Stressing Record – A permanent record of the actual tendon elongations after stressing produced by the Inspector.
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Stressing End – The end of the tendon at which the prestressing force is applied.
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Temperature Tendons – Tendons used to resist shrinkage and temperature stresses.
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Tendon Group – More than one strand of Prestressed steel tied together to form a tendon.
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Tendon Support System – The required support bars, chairs, bolsters, and other accessories required to maintain the tendon profile. Tendon Tail – The excess strand beyong the stressing-end anchor.
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Tendon – In post-tensioned applications, the tendon is a complete assembly consisting of anchorages, prestressing steel, and sheathing with post-tensioning coating for unbonded applications or ducts with grout for bonded applications. A single or group of prestressing elements and their anchorage assemblies, which impart a compressive force to a structural member. Also included are ducts, grouting attachment and grout. The main stressing element is usually a high strength steel member made up of a number of strands, wires or bars.
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Thixotropic – The property of a material that enables it to stiffen in a short time while at rest, but to acquire a lower viscosity when mechanically agitated. The process is reversible.
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Threshold Inspector – This is a term employed by certain states to define a qualified professional engineer who inspects structures of certain defined parameters, and who also inspects the post-tensioning tendons.
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Ultimate Strength – The tension force or stress that is required to fail a steel element in tension.
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Unbonded tendon – Tendon in which prestressing steel is prevented from bonding to concrete and is free to move relative to concrete. The prestressing force is permanently transferred to concrete at the tendon ends by the anchorages only.
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Water reducing Admixture – An admixture that either increases the slump of freshly mixed grout without increasing the water content or maintains the slump with reduced amount of water due to factors other than air entrainment.
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Wedge Plate – The hardware which holds the wedges of a multistrand tendon and transfers the tendon force to the bearing plate.
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Wedges – Pieces of tapered metal with serrations, which bite into the prestressing steel (strand) during transfer of the prestressing force.
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Wedge Set – See seating loss.
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Wobble friction – The friction caused by the unintended deviation of the tendon.
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Wire – A single, small diameter, high strength steel wire, typically the basic component of strand.
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Yield Strength – The stress at which a material exhibits a specific limiting deviation from proportionality of stress to strain.
Anchor assembly
Barrel Anchor
Banded Tendons
Bearing Plate
Barrier Cable
General zone
Local Zone
Stressing End Anchorage
Temperature Tendons Wedge Plate
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Materials used in Post-tensioning:
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Prestressing Steel:
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Strand – The most commonly used prestressing material in not only North America is a 7-wire carbon steel strand. Seven-wire has a center wire enclosed tightly by six helically wound outer wires. Strand conforming to ASTM A416 Grade 270 has a minimum ultimate strength of 279 ksi (1860MPa). Grade 250 is also available (with an ultimate strength of 250 ksi) for use, for barrier cable applications. ASTM A416 also sets forth other requirements for strands, such as strand size, tolerances, workability, bending, fatigue, stress corrosion and hydrogen embrittlement, and bond. For each grade, there are two types of steel: low relaxation and stress relieved (normal relaxation).
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Almost all of the prestressing strand supplied today is low relaxation steel. Up until 1970’ s, s, stress-relieved strands were common; however they are rarely used today. Relaxation is defined as the reduction in force over time in a highly stressed tendon at a given elongation. Low relaxation strand must conform to the Supplement I requirements of ASTM A416, which limit relaxation loss after 1000 hours of testing to 2.5% at 70% of minimum ultimate tensile strength (MUTS) or 3.5% at 80% of MUTS.
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Stress relieved strand, in contrast, is not subject to any relaxation loss limit under ASTM A416. Relaxation losses for such tendons typically run at 4.5%, 8%, and 12% of the initial stress in the free tendon (i.e., strand is not associated with a concrete element) for an initial stress equal to 0.6, 0.7, and 0.8 of MUTS, MUTS, respective respectively.
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Minimizing relaxation loss reduces overall prestress losses, and as a result enables the designer to take advantage of a higher final prestressing force after all other losses have occurred.
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If a structure is exposed to an aggressive environment, the designer may elect to specify a corrosion protective coating for the strand such as epoxy coating or hot dip galvanizing. Epoxy coated strands shall conform to ASTM A882, Standards Specification for Epoxy-Coated SevenWire Strand. Galvanized strand shall conform to ASTM A475-98, Standard Specification for ZincCoated Steel Wire strand. Neither epoxy -coated or galvanized strand are widely used in general post-tensioning applications in the United States. The designer should evaluate the local availability of these materials before specifying.
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Bars – Prestressing bars are high-strength steel bars that are cold-stressed to not more than 80% MUTS and then stress relieved to produce the desired mechanical properties. They have a minimum ultimate tensile strength of 150 ksi (1036 MPa). Prestressing bars are rolled from properly heat ingot – or strand-cast steel. They can be manufactured as a smooth round (Type I) or with deformations similar to a common reinforcing bar (Type II). Deformed prestressing bars have deformations that are arranged in a thread pattern permitting the use of screwon couplers and nuts. Plain round bars must be threaded before they can be used with nut/bearing plate anchoring system. Bars used in post-tensioned structures Bars for Prestressed Concrete, including must meet the requirements of ASTM A722, Specifications for Unbonded High-Strength Supplementary Requirements S1 and S2. These requirements include chemical composition, dimension, and tensile properties.
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Wires – Wires used for prestressing generally conform to ASTM A421, Uncoated Stress-Relieved Wire for Prestressed Concrete. Rods are used to manufacture wires by open hearth or elastic furnace process. Heat treatment is then used to stressed relieve the wires so that the desired mechanical properties are achieved. Wires are manufactures with various cross-sectional shapes and surface conditions: round versus oval, smooth versus indented, ribbed or crimped. ASTM A421 also has a supplement for low -relaxation wires. Wires are rarely used for post-tensioning applications in the United States; however, they are still used to a greater degree in other parts of the world.
Types of Prestressing tendons: Typical shapes and commonly available diameters.
Properties of Prestressing Steel •
Mechanical Properties: Certain mechanical properties of prestressing steel must be known to properly design a post-tensioned structure. ASTM specifications identify requirements for: MUTS fpu; yield limit fpy, modulus of elasticity Ep; and the total elongation under load. In most cases, the design strength of unbonded tendon fps will substantially less than the yield limit fpy. For bonded construction, the designstrength will be greater than or equal to fpy. Typically mechanical properties for low-relaxation strands, wires, and bars are shown in the table below.
Prestressing Steel
f pu
f py
Ep
Relaxation
ksi (MPa)
% Elongation [Gauge Length]
ksi (MPa)
ksi (MPa)
0.90 fpu
28,500 (196,50 0)
3.5 [24 in (610 mm)]
[2.5% @ 70% MUTS] or [3.5% @ 80% MUTS]
0.85 fpu
29,000 (200,00 0)
4 [10 in (250 mm)] 4 [10 in (250 mm)]
4 [20 bar dia.] 7 [10 bar dia.]
Low-Relaxation 7-Wire strand Grade 270 per ASTM A416/416M
270 (1860)
Stress-Relieved Wire per ASTM A421/421M
235 - 250 (1620 1725)
Low-Relaxation Wire per ASTM A421/421M
235 - 250 (1620 1725)
0.90 fpu
29,000 (200,00 0)
Prestressed Bars Grade 150 per ASTM A722
150 (1035)
Type I: 0.85 fpu Type II: 0.80 fpu
29,700 (205,00 0)
[2.5% @ 70% MUTS] or [3.5% @ 80% MUTS]
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The typical values shown in the table are often used for design purposes; however, the actual material properties for the prestressing steel supplied to the project may vary and may exceed specification minimums.
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Knowing the actual properties can be important during inspection and future rehabilitation. For example, when evaluating out-of -tolerance elongations during stressing, the design engineer should compare the actual values (e.g. the modulus of elasticity) as given on the supplierprovided mill certificates and the value assumed in design. In many instances, the difference may explain the observed elongation.
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Ductility – Ductility is an essential property of a prestressing material. Standard specifications prescribe ductility requirements, which are usually expressed as a minimum percent elongation in the gauge length under total load. For ASTM A416 prestressing strand, the minimum elongation is specified as 3.5% using gauge length of not less than 24 inches (610mm). For ASTM A722 prestressing bars, the minimum percent elongation after rapture is 4% and 7% for type I and type II prestressing bars, respectively.
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Static and Fatigue Testing – Tendons in prestressed concrete structures and ground anchors normally do not experience stress cycling significant enough to cause fatigue problems. For those applications where fatigue is a concern, such as post-tensioned bridge and cable-stayed bridges, fatigue resistance can be increase by proper material selection and anchorage design. Tendon fatigue will depend on the type of structure and whether the tendon is bonded or unbonded. The strand-wedge connection is the most sensitive part of a tendon in regards to fatigue resistance.
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Where fatigue is a possible concern, the design engineer should confirm that the intended post tensioning system has been dramatically tested and qualified in accordance with the PTI Acceptance Standards for Post -Tensioning Systems for bonded tendons and in accordance with the post-tensioning project specifications for unbonded tendons. For unbonded systems on bridges, AASHTO requires that a representative anchorage and coupler specimen, as well as tendon, be dynamically tested without failure, 500,000 cycles from 60 to 66 percent of MUTS, and 50 cycles from 40 to 80 percent MUTS.
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Application of Post-tensioning Today, post-tensioning is used for a wide range of applications including:
