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How does the pre-stressing force transmit in post-tensioning beams/slabs?

  • 1.  How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-17-2018 02:26 PM
    Edited by Tirza Austin 08-17-2018 04:34 PM
    It's known that for the pre-tensioned members, the tendons are stressed first and soon the concrete is placed which leads to a (what I call) perfect bonding between the tendon and the surrounding concrete. This achieved bonding is the main key to transfer the stresses. In case of post-tensioning, the tendon is placed through ducts and stressed later after the concrete is poured, which means there's no bond (at least not a perfect one) between the tendon and concrete. So how would the pre-stress transmission work here? I understand that for bonded tendons the grout will help in building that friction between the two surfaces and hence transferring the stresses, but what about the un-bonded ones? Why would we use the un-bonded system?

    ------------------------------
    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    +86 183 5193 3411
    ------------------------------


  • 2.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-19-2018 12:50 AM
    Edited by Tirza Austin 08-19-2018 12:50 AM
    Think of the post-tensioning tendon and its end anchorages as a large "clamp" that compresses the concrete element.  Remember that the reason for pre-stressing concrete is to impart compression to offset tension forces (and therefore stresses) imparted by service loads on the concrete member.  Pre-tensioning and post-tensioning both are pre-stressing.  Pre-tensioning is impractical for very large cast-in-place elements.

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    Richard Lenz, Life M. ASCE
    Retired
    West Chester OH

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    Original Message


  • 3.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?
    Best Answer

    Posted 08-19-2018 02:45 PM
    Edited by Muhannad Husain 09-05-2018 11:03 AM
    ​I feel that some clarification is in order:


    Pre-tensioning (commonly referred to as "Prestressing") needs "bulkheads" to support the prestressing jacks, since the concrete is not there yet. Since it's unlikely that such bulkheads can be installed on the jobsite, they are restricted to fixed locations at precast concrete plants. Strands are usually 1/2" or 5/8" diameter, of 7-twisted wires, 270 ksi ultimate strength. "Low Relaxation" strands are most common for minimizing priestess losses. Prestress is usually released when concrete strength reaches 70% of its 28-day strength. To accelerate that in a plant, steam-curing is used. The prestressing force in a strand is developed by friction. This together with the "bursting effect" at the strand end, are usually quite modest given the small diameter of strands. I can't give the Precast/ Prestressed Concrete industry justice in such few lines but I can refer you to the website of PCI, the "Precast/ Prestressed Concrete Institute" of USA (www.pci.org). If interested in metric version, you can refer to its Canadian sister CPCI (www.cpci.ca). They both have great literature.

    Post-Tensioning is mostly done on job sites, even though it is used occasionally in precast concrete plants. Its scale varies widely from a 1/2" dia. Monostrand with, to a tendon of dozens of strands. PT relies only on the end jack being kept in place after being "seeded" (its wedges being pushed in the end cone after jacking is completed). PT strands as well as PT tendons come in lubricated sheaths to minimize losses of prestress due to friction. Sheaths allow for flexible profiles that fit moment diagrams best. Monostrands are very common in light PT of cast-in-place concrete slabs. (Also, I used it in Architectural precast concrete). No grouting can be injected in such tight-fitting sheaths. Large-scale heavy PT, using tendons can accommodate sheath grouting after PT completion, but the PT force is still maintained by the end jack; we can't rely on grouting for development of tendons. Due to the total reliance on jacks at the ends, these jacks should be protected in patched recesses. Bursting end zones are particularly critical in PT. While the PT hardware, "Trumpets" and their surrounding reinforcement are designed to mitigate this issue, heavy PT needs special attention to the design of those bursting zones where every possible mode of failure needs to be identified and accounted for. Applications of PT include, in addition to CIP structures, segmental counter-cast precast concrete held together by heavy PT, usually in bridges. Also used at a stadium roof: Zayed Sports City in UAE. More info on PTI "Post-Tensioning Institute" website: Post-Tensioning Institute - post-tensioning research, code, development and marketing

    Post-tensioning remove preview
    Post-Tensioning Institute - post-tensioning research, code, development and marketing
    PTI is a professional association dedicated to research, specifications, design recommendations and providing information on post-tensioning and construction technology through publications and education.
    View this on Post-tensioning >





    ------------------------------
    Neil Kazen,
    M.Eng., M.Sc., P.Eng..
    F.ASCE, F.CPCI, Award of Merit- PCI
    Retired Structural Engineering Manager
    Toronto ON
    ------------------------------

    Original Message


  • 4.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-19-2018 12:50 AM
    Edited by Tirza Austin 08-19-2018 12:49 AM
    Post-tensioning forces are transferred to the concrete by anchorages at the ends of the tendons.  The end that the tendons are stressed from typically uses a steel chuck bearing on a cast-in steel anchorage.  The opposite end may be anchored in by a similar setup or may have a flared tendon end cast into the concrete.

    Here are a couple of material suppliers that have some good info on the systems.

    VSL | Post-tensioning strand systems | Post tension
    Vsl remove preview
    VSL | Post-tensioning strand systems | Post tension
    VSL designs, manufactures and installs durable, state-of-the-art post-tensioning systems that comply with international standards and approval guidelines for both new and existing structures.
    View this on Vsl >



    Post-Tensioning
    Dywidag-systems remove preview
    Post-Tensioning
    DYWIDAG Post-Tensioning Systems - Reliability and a long Service Life for your Project. For decades, we have been your competent contact for bridge, industrial and tank structures in Construction Engineering.
    View this on Dywidag-systems >



    Another good resource is the Post Tensioning Institute.  Post-Tensioning Institute - post-tensioning research, code, development and marketing
    Post-tensioning remove preview
    Post-Tensioning Institute - post-tensioning research, code, development and marketing
    PTI is a professional association dedicated to research, specifications, design recommendations and providing information on post-tensioning and construction technology through publications and education.
    View this on Post-tensioning >



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    Robert Holland P.E., M.ASCE
    Engineer
    Oldcastle Precast
    Spokane Valley WA
    (509) 536-3349
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    Original Message


  • 5.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-25-2018 01:34 PM

    Thank you so much for the valuable comments. I may conclude that:

     

    -post-tensioned members basically do transfer stresses by means of wedge actions in end zones, regardless the adoption of bonded or unbonded tendons.

    -grouting is introduced in bonded tendons as a way of protection only.

     

    Am I right?

     

    Since the difference between bonded and unbonded is the grout injection, does that mean for two identical beams (same material properties, same prestress applied, etc, but one with bonded and the other with unbonded tendons) the stress distribution and prestress transfer would be nearly the same and follow the same trend?



    ------------------------------
    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    86183 51933411
    ------------------------------

    Original Message


  • 6.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-26-2018 10:35 PM
    The "bonded" is used only in prestressing strands.

    Grouting of PT tendon sheaths does not constitute "bonding" for design purposes, even though it has some helpful bonding effect.

    A similar phenomena, where a helpful material is not counted in design, is caulking in the joints of architectural precast concrete panels: when it hardens it can acquire great strength, yet we don't count it in design; simply because we can't quantize its effect. I remember a situation where we were removing such a panel: after hooking it to crane, it's hardware connections were cut, the crane operator tried lifting it assuming that caulking was nothing to worry about. The result was that the panel didn't move but the crane fell. A surprising demonstration of the strength of aged caulking that we couldn't quantize. Think of sheath grouting as similar.

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    Neil Kazen F.ASCE
    FCPCI, Award of Merit- PCI
    Retired tructural Design Manager
    Toronto ON
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    Original Message


  • 7.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-27-2018 02:35 PM
    Edited by Tirza Austin 08-27-2018 02:35 PM
    Post Tension Institute (PTI) specifies both bonded and unbonded as does ACI 423 in their current guides. A bonded tendon still transfers force from the grout to the duct to the concrete, whereas an unbonded tendon that is damaged is completely lost.

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    Martin Fradua P.E., F.ASCE
    Vice President
    Huntington NY
    (631)470-2019
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    Original Message


  • 8.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-27-2018 11:28 AM
    Edited by Tirza Austin 08-27-2018 11:30 AM
    You are correct that unbonded post-tensioning has forces transferred through wedge action.  This is also true for bonded post tensioning.  The bonding does several things, provides protection to the tendons and if the tendons break the effect of the breakage is localized to the area of the break.  The grouting in the tubes allow the force from the tendons to transfer through the grout and then the duct into the concrete.  The transfer of stressing forces from a broken tendon through the bonded duct is similar to the a pre-stress strand into concrete.

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    David Thompson P.E., M.ASCE
    Principal
    KTA Structural Engineers Ltd.
    Calgary AB
    (403) 246-8827
    ------------------------------

    Original Message


  • 9.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-28-2018 10:26 PM
    Edited by Tirza Austin 08-28-2018 10:25 PM

    I’m concerned with the statement, "and if the tendons break the effect of the breakage is localized to the area of the break.  The grouting in the tubes allow the force from the tendons to transfer through the grout and then the duct into the concrete. "

    What's the development length for a tendon with 200 tonnes (440 kips) PT? How about 900 tonnes (2000 kips), my heaviest PT to date?


    Further, in a tendon of several strands, not all strands are in full touch with the grout.


    ------------------------------
    Neil Kazen F.ASCE
    Structural Design Manager Transportation Division
    Toronto ON
    ------------------------------

    Original Message


  • 10.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-29-2018 11:44 AM
    Edited by Tirza Austin 08-29-2018 11:43 AM
    I would agree with your concern about transferring tensioning force through friction with large tendons. You just have to look at slippage of strands in pre-stressing after the strands are cut.

    There are grouted ducts used in slabs where strands or small tendons are used that my statement would apply. 

    Our firm has not used unbonded post-tensioning since the 1990's.  Our concern was the sensitivity of the tensioning system to construction methods and quality leading to corrosion, but that is a different topic

    ------------------------------
    David Thompson P.E., M.ASCE
    Principal
    KTA Structural Engineers Ltd.
    Calgary AB
    (403) 246-8827
    ------------------------------

    Original Message


  • 11.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-29-2018 05:17 PM
    Edited by Tirza Austin 08-29-2018 05:17 PM
    Of course, we grout the sheaths whenever possible; the only exception being monosttands where the sheath fits so tiredly on the lubricated strand, leaving no room for grouting. As mentioned earlier, we know that there is some bonding in grouted PT, but we can't quantize it. Our calculations have to show total reliance on end anchors. Extra measures are taken to protect those anchors; properly embedded in covered recesses. 

    I gave an example of another material of great strength that we can't officially rely on as we can't quantize it: caulking in the joints between architutal precast panels. Strength that was enough to tip the crane that tried lifting a panel with its hardware connections cut.

    ------------------------------
    Neil Kazen, M.Eng., M.Sc., P.Eng.
    FASCE, FCPCI, FEC,
    Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
    Toronto, Ontario, Canada
    ------------------------------

    Original Message


  • 12.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-30-2018 10:03 AM
    Edited by Tirza Austin 08-30-2018 10:02 AM
    Actually this is a general comment regarding Post-Tensioning [PT].

    In Nuclear Containment PT construction there is a requirement to check the existing tension load at given periods.

    Also, from time to time the PT is released and a section of the wall removed for Steam-Generator replacement and then Re-tensioned.

    In a number of cases, when the large wall section has been removed, significant delamination has been discovered. Crystal River was shut down as a result of not being able to resolve the further delamination when re-tensioned.

    Any comments on the delamination occurrence and a solution? 

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    Allen Hulshizer P.E., F.ASCE, FACI
    Consulting Structural Engineer
    Chalfont PA
    (215)8872838
    ------------------------------

    Original Message


  • 13.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-30-2018 04:54 PM

    I'm glad that you raised the issue of re post-tensioning.  It reminds me of a similar procedure in a different kind of structure: In 1978, I had a problem when the roof girders of Zayed Sports City deviated 11 mm (4 3/8") sideways preventing us from erecting the cylindrical shells above. Those segmental precast, counter-cast girders were post-tensioned with 200 tonnes/tendon by the top French firm in the field. So it was a surprise to see such an anomaly. It turned out that it was caused by differential EC between the two sides of those H-shaped girders, due to differential exposure to the sun. 

    To rectify it, I increased the PT on one side of the girder, just enough to fit the shells above, leaving enough tolerance gap for overshooting due to future creep. Thank God it wasn't grouted yet. 

    It was like directing a horse sideways by differential pull of the harness. 

    But I have a lingering concern: with repeated PT; wedges digging their teeth in the strands, any adverse long term effects?



    ------------------------------
    Neil Kazen, M.Eng., M.Sc., P.Eng.
    FASCE, FCPCI, FEC,
    Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
    Toronto, Ontario, Canada
    ------------------------------

    Original Message


  • 14.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-31-2018 10:54 AM
    Edited by Tirza Austin 08-31-2018 10:54 AM
    The pre-stressing forces in post-tensioning system is transferred at cable ends locations only where they are locked in beams/slabs for resisting tension in the section at given location along the member span. Examples:

    a) Bonded cables system (cable ducts are grouted with non-shrinkable cement grout). In bonded system cables are locked at one end termed as dead end if embedded in concrete or fixed end if cables are held in steel plate using steel conical wedges at fixing point. The cables profile are arranged within the section depth to resist the maximum design flexure and tension at given section. Then, cables are pulled from free end would be either one end or both ends to generate the required resisting tension. After attaining the desired forces cables are locked at ends called as anchorage end. The anchorage end/zone are designed for extremely high compression forces generated due to cables pull and locked at ends. The cables are allowed to locked during this stage, meanwhile different losses occurred like slip at wedge, friction offered to cables from ducts and cable profile, etc. Once, cables are locked ducts are grouted to prevent further losses in forces and to protect cables from corrosion. 

    b) Non-bonded system: cable ducts are not grouted, the forces get transferred to fixed ends where cables are locked. 
    In case, cables breakage, the provision of dummy cables (4% of total number of cables) is provided to compensate unexpected loss of forces in both bonded and non-bonded pre-stressing post-tension systems.

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    Nitin Baraskar A.M.ASCE
    Senior Engineer
    Bechtel India Private Limited
    New Delhi ND
    +919811850144
    ------------------------------

    Original Message


  • 15.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-02-2018 10:38 AM
    Edited by Tirza Austin 09-02-2018 10:39 AM
    One little addition: while in most cases it is as you described: a live end and a dead end, occasionally there might be a need for both ends to be live. That happens when the tendons are too long and/or the curvatures are too sharp (I.e. the pre-stressed losses due to friction are too high).

    ------------------------------
    Neil Kazen, M.Eng., M.Sc., P.Eng.
    FASCE, FCPCI, FEC,
    Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
    Toronto, Ontario, Canada
    ------------------------------

    Original Message


  • 16.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-05-2018 11:59 AM
    Edited by Tirza Austin 09-05-2018 11:58 AM
    Thanks a lot. Quite comprehensive answers.
    About the post-tensioning from both sides of a slab, I remember a PT practitioner told me generally it's preferable to stress from both ends (two live ends) had the tendon length exceeded 35 meters long. The longer the tendon, the more friction losses take place. One other reason to stress from both ends is the wobbling effect (as illustrated in the attached figure).

    ------------------------------
    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    86183 51933411
    ------------------------------

    Original Message


  • 17.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-06-2018 09:54 AM
    Edited by Tirza Austin 09-06-2018 09:53 AM
    Nice diagram; it shows it so clearly.
    Pre-stress losses, other than due to slippage at live ends, have two components: wobble and curvature.
    The wobble component is proportional to length (so the length effect and the wobble effect are one and the same).
    The curvature component is exponentially proportional to the angle alpha shown on your diagram.

    The need to two live ends increases when one or both of the above two components is high.

    ------------------------------
    Neil Kazen, M.Eng., M.Sc., P.Eng.
    FASCE, FCPCI, FEC,
    Retired Structural Engineering Manager, Transportation Division, SNC-Lavalin
    Toronto, Ontario, Canada
    ------------------------------

    Original Message


  • 18.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-29-2018 01:56 PM
    Edited by Tirza Austin 08-29-2018 01:56 PM
    Structural flexural members with bonded tendons general have a higher capacity than the same member with unbonded tendons as can be seen by comparing the stress in the pre-stressing at nominal strength, FPS, for bonded and unbonded tendons given in ACI 318-14 Equations 20.3.2.3.1 and Table 20.3.2.4.1.

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    Abieyuwa (Abi) Aghayere Ph.D., P.Eng, M.ASCE
    Avondale PA

    ------------------------------

    Original Message


  • 19.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-28-2018 10:03 AM
    Edited by Tirza Austin 08-28-2018 10:02 AM
    The function of post-tension and pre-tension is to increase the tensile strength of the reinforcing iron where in normal conditions, the reinforcing iron has a certain FY (+ -390 MPa for screw iron depending on the standard) so that the reinforcement does not return to its original state after the bond is removed, it is necessary to calculate the friction force between concrete and concrete. Different from the post-tension tendon, where there is a kind of stud at one end of the tendon, after fulfilling the fullness of the pull strength of the plan, the retractable tip will be given a stud as a support. This post-tensioned type is usually used on connections that do not make the tendon as a shear reinforcement because the tensile and shear forces that work together will result in a greater resultant force.

    ------------------------------
    Micha Priambodo A.M.ASCE
    Lead Civil Engineer
    PT. SOLUSI ENERGY NUSANTARA
    Jakarta Barat
    628193 8516903
    ------------------------------

    Original Message


  • 20.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 08-28-2018 01:28 PM
    Edited by Tirza Austin 08-28-2018 01:28 PM

    The question can be viewed as the Process of Force/Stress (localized) Transfer within a Solid medium.

    The Interface between concrete and the pre-stressing strands, serves as the force transfer medium which implies the stress at this point is expected to be equal for both concrete and steel, but their response to the applied stress in terms of strain and deformation are not equal, same as their respective elastic modulus. Therefore the bonded bars ensures equilibrium transfer of forces bearing the material properties and environmental condition.

    Pre-stressing compensates for the tensile stresses introduced when the element is loaded. Hence the concrete generally remains in compression. Pre-stressing, is the process by which a concrete element is compressed, generally by steel wires or strands. Pre-cast elements may be pre-stressed during the construction process (pre-tensioning) or structures may be stressed once completed (post-tensioning). 



    ------------------------------
    Olusegun Afolabi P.E., M.ASCE
    University of Lagos
    Lagos
    234803 4248600
    ------------------------------

    Original Message


  • 21.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-05-2018 12:12 PM
    Edited by Tirza Austin 09-05-2018 12:11 PM
    In both bonded and unbonded post-tensioning systems, the pre-stressing force is transmitted through the anchorages. Note that both systems
    during the jacking procedure are unbonded. Moreover if the tendon profile is curved, transverse forces the so called curvature loads are exerted on the beam. 

    In bonded systems after the jacking procedure has finished and the tendons are fully anchored, groute is injected into the tendon sheath, so bond is created. Since bond between tendons and the surrounding concrete exists, tendons now behave as ordinary reinforcement bars. The only difference between a rebar and a bonded tendon (apart from the obvious diffs in strength, yield, strain limits etc.) is that in reinf. member no external loads mean no stress in the rebars whereas in a pre-stressed member the tendons are stressed even if no external loads act on the member.    

    For the second part of your question, personally I have never used unbonded post-tensioning systems. One obvious reason for using unbonded post-tensioning systems is that since there is no bond, the tendon force can be adjusted during service and the tendons can be replaced.
    On the other hand, bonded systems, contribute to the flexural capacity of the member and they help in  cracking behavior.


    ------------------------------
    Dikaios Psarogiannis A.M.ASCE
    Consulting Engineer
    OMETE SA
    Holargos
    30210 6537644
    ------------------------------

    Original Message


  • 22.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-05-2018 05:27 PM
    Edited by Tirza Austin 09-05-2018 05:26 PM

    Thanks all again for the valuable comments. I started to believe that the grouting in post-tensioned strands do not transfer stresses. If we look at the process again, concrete is poured and cured till it reaches the required strength, strands are tensioned and the wedges get sucked in to the anchor (to keep hold of the stresses) and here where, I think, forces are getting transferred. Then after, comes the grouting to fill gabs around strands and to cover it. So grouting actually may transfer stresses bet I wouldn't count it in since it's been injected after we already make the pre-stressing. Isn't it more logical this way?

    I came across some scientific articles; they conclude that bonded tendons provide more ductility. One article entitled "Behavior of bonded and unbonded pre-stressed normal and high strength concrete beams" by O.F. Hussien, HBRC Journal(2012) 8, 239–251 concluded that beams with bonded tendons failed at higher deflection, which translated to higher ductility. In the attached figure, the load-deformation relationship is shown for two beams tested (B-bonded, U-unbonded). That's being said, it means grouting plays important rule in beam/slab ductility which further means, as I deduct from the mentioned article, grouting is transferring stresses. Interesting, huh?



    ------------------------------
    Muhannad Husain S.M.ASCE
    Researcher
    Hohai University
    Nanjing
    86183 51933411
    ------------------------------

    Original Message


  • 23.  RE: How does the pre-stressing force transmit in post-tensioning beams/slabs?

    Posted 09-07-2018 08:52 AM
    Edited by Tirza Austin 09-07-2018 08:51 AM
    @ Mr Muhannad Hussain; :from the referenced article, grouting is transferring stresses." Interesting, right?
    The Grout as explained in the article is the casing for the embedded prestressing tendons, Thereby providing the necessary bond for the transfer of stress between the prestressing strand and concrete. Grout is a mixture of water cement and sand, as employed in pressure grouting, embedded rebar, connecting sections of precast concrete, filling voids, and sealing joints etc.


    ------------------------------
    Olusegun Afolabi P.E., M.ASCE
    University of Lagos
    Lagos
    234803 4248600
    ------------------------------

    Original Message


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