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We hope that this checklist will help Structural Engineers and Concrete Contractors mitigate potential issues on future PT projects.  This checklist is not all-inclusive and we welcome feedback in order to improve this checklist.

REFERENCE MATERIALS:

1. Specification for Unbonded Single-Strand Tendons (2^nd Edition, 2000, Post-Tensioning Institute).  Addenda#1 issued Nov. 2003.  Addenda#2 issued Nov. March 2007.
2. Field Procedures Manual for Unbonded Single-Strand Tendons (3^rd Edition, 2000, Post-Tensioning Institute)
3. Ten-Year Marine Atmosphere Exposure Test of Unbonded Prestressed Concrete Prisms (2000, Post-Tensioning Institute)
4. Proper Filling of Single-Strand Tendon Stressing Pockets (Post-Tensioning Institute, FAQ #11)
5. ACI-318-08 Building Code Requirements for Structural Concrete and Commentary, Chapters 7 and 18 (American Concrete Institute)
6. ACI 423.4R-98 ‘Corrosion and Repair of Unbonded Single Strand Tendons’ (1998, American Concrete Institute, ACI/ASCE Committee 423)
7. ACI 423.6R-01 ‘Specification for Unbonded Single-Strand Tendons and Commentary’ (2001, American Concrete Institute, ACI Committee 423)
8. ACI 423.3R-05 ‘Recommendations for Concrete Members Prestressed with Unbonded Tendons’ (2005, American Concrete Institute, ACI Committee 423)
9. ACI 423.7-07 ‘Specification for Unbonded Single-Strand Tendon Materials and Commentary’ (2005, American Concrete Institute, ACI Committee 423)

AGGRESSIVE ENVIRONMENTS (minimum requirements per PTI):

1. De-icing Chemicals
2. Seawater / Brackish Water / Salt-spray
3. Direct contact with soil
4. Areas with planters, balconies, swimming pools

ENCAPSULATED SYSTEM for Unbonded Post-Tensioned Concrete consists of the following (per PTI):

1. STEEL STRAND - Dry steel strand with rust grading of:
   1. “A” (no visible rust)
   2. “B” (light surface rust that can be removed by vifous rubbing with a cloth.  No pitting noticeable to the unaided eye.  Discoloration in steel surface in affected area is permitted)
   3. “C” (Surface rust, removed with a fine steel wool pad, which leaves small pits on the steel surface of not more than 0.002 in. (0.05 mm) diameter or length)
2. PLASTIC SHEATHING – 50ML minimum thickness (extruded HDPE or HDPP)
3. PT COATING – Corrosion-inhibitor inside sheathing (refer to Table 1 in ‘Specification for Unbonded Single-Strand Tendons’)
4. ANCHORAGE DEVICES – Plastic-coated (free of sand, blowholes, voids and other defects).  Designed to attain watertight encapsulation of prestressing steel and all connections shall have demonstrated the ability to remain watertight when subject to hydrostatic pressure of 1.25psi (0.0086 MPa) for a period of 24 hours.
5. WEDGES – Dry (during installation and storage)
6. SLEEVES – Grease in translucent sleeves at all anchorages with Positive-mechanical connection at all anchorages.
7. ANCHOR CAPS – Grease-filled watertight cap at all stressing anchorages
8. ACCESSORIES – Keep accessories in a dry storage area.
9. Note:  Project specifications and codes can overrule the PTI specifications.

RECOMMENDATIONS:

1. SHIPPING:  PT material should be shipped in plastic-shrink wrap on a tarpped/enclosed truck.  Other approved methods may be acceptable.
2. STORAGE AT JOBSITE:  PT material should be stored on the jobsite in an enclosed area (refer to Chapter 3 of Field Manual).
3. STRESSING ANCHORAGES:
   1. Prior to stressing, spray WD-40 to clean anchor cavity of dirt, concrete, etc.  Afterwards, wipe WD-40 from all surfaces.
   2. Cap and grout anchorages 1 day after elongation approval.  (PTI Field Manual 9.7)
   3. Sometimes, contractors will wait until the end of the entire project to grout the stressing anchorage to save setup and mobilize costs.  Short of enclosing the entire building in visqueen, this delay in capping and grouting could expose the stressing anchorages to the corrosive elements (rain, snow, etc.)
4. TEMPORARY PROTECTION AT CONSTRUCTION JOINTS:
   1. After “Pour 1” is cast, the intermediate anchors are temporarily exposed to the elements until “Pour 2” is cast.
   2. In the interim (typically 2-3 days), install a plastic tarp (visqueen) over the construction joint to prevent water intrusion at the intermediate anchorages.
5. TEMPORARY PROTECTION AT POUR STRIPS:
   1. Similar to the construction joint, the stressing anchorages are temporarily exposed to the elements until the pour strip is cast.
   2. Option 1(preferred):  After the poured concrete reaches the required strength, the contractor should stress the tendon and install the grout cap.  After the elongations are approved, the stressing pockets should be grouted shortly thereafter.
   3. Option 2:  If grout cannot be installed immediately due to procurement or schedule, the grout cap should still be installed.  However, the stressing pocket will remain exposed for typically 28 days.  In the interim, install a plastic tarp over the pour strip to prevent water intrusion at the intermediate anchorages.
6. PT GROUT:
   1. Should be non-shrink, no chlorides or corrosive chemicals.  Must be reach required strength, consolidation and bonding properties.
   2. Coat/spray the pocket-formed surface with a resin bonding agent to product a better grout cap.  (Recommended by Ian McFarlane, P.E., Magnusson Klemencic Associates)
7. PT INSPECTOR and INSTALLER:  PTI Certified personnel should document that all grout caps were installed and all grouting was completed.  (PTI Field Manual 9.7)
8. PT SUPPLIER:  Recommend that supplier is PTI Certified (or approved equal).
9. TRAFFIC COATING:  If an urethane traffic coating is applied to concrete that is subject to an aggressive environment, then the product testing should be approved by the Structural Environment during the pre-construction stage.

Additional information can found on a previous post about Unbonded Post Tensioning – Protection.  During the past 10 years, most structural engineering PT specifications have improved greatly and the risk of corroded post-tensioning has declined.

- Neel Khosa, Vice President, AMSYSCO

(Updated 12/18/2010)

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The PTI inspection is unannounced twice a year by an independent certifying agency.  The following is an excerpt from the Post-Tensioning Institute’s website regarding the program:

Launched in 1989, the first program, Unbonded Single Strand Tendon Fabrication Facilities, is for facilities that fabricate unbonded single strand tendons.  Since its inception, participation in the program has grown steadily.  63 plants are certified, which represents approximately 95% of the industry’s production.

An abridged outline for the Manual for Certification of Plants Producing Unbonded Single Strand Tendons(5th Edition, May 2007, Post-Tensioning Institute) is located below for the grading criteria.

2.0 Post-Tensioning Institute Certification Program Criteria

2.1 General

2.2 Prestressing Steel

2.3 Anchorages and Couplers

2.4 Sheathing

2.5 Post-Tension Coating

2.6 Fabricating Process

2.7 Storage and Shipping

2.8 Record Keeping

2.9 Stressing Equipment

2.10 Quality Control Program

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- Rattan Khosa, President, AMSYSCO

Related Posts:  Certified Plant (AMSYSCO)

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This form is for Inspectors and Installers certified by the Post Tensioning Institute.  Check out our VIDEO for the basics in stressing post tensioning tendons.

*** Updated (10/21/2011) – In response to industry demand, we have updated our elongation report to include.  The updates include columns to track whether the tendon tail has been cut properly, the encapsulated anchor has been capped (if applicable) and the stressing pocket has been grouted.

Copyright © 2009 by AMSYSCO, Inc. All rights reserved.

POST TENSIONING SUPPLIER

While the protection of tendons is mostly under the control of the jobsite, the PT supplier must be involved from the beginning.  It starts with preventing the steel raw material and finished products from being exposed to corrosive elements (rain, snow, de-icing chemicals, salt-spray, etc.)  The exposure to these corrosive elements will greatly depend on where the Extrusion and Fabrication plants are located.  Ideally, the post tensioning tendons should be stored indoors in a climate-controlled warehouse.

Additionally, it is more beneficial to have the Extrusion and Fabrication plants under the same roof.  This minimizes the products exposure to elements during transportation.  It is a good idea to ask your PT supplier how their operations are set up and to visit the plants.

SHIPPING

Once the PT material is ready to ship, the PT supplier will typically use a flat-bed truck due to heavy-trucking requirements.  The question is whether to “shrink-wrap the encapsulated tendon bundles” and “tarp/cover the truck.”  The best Structural Engineering specifications incorporate both of these items.  Both are cheap insurance to minimize corrosion in the long run since it is hard to control the weather during shipping.

JOBSITE STORAGE

Finally, the material arrives at the jobsite.  During our jobsite visits, we have seen our shrink-wrapped PT bundles and boxes of PT material sitting in a pile of mud and water.  This is not good.  Again, the best Engineering specifications require that the post-tensioning material be stored as follows:  1) Stored on elevated dunnage in a dry-area, 2) Tarpped in a tent-like fashion to allow air to circulate, 3) Maintain shrink-wrapping until installation, 4) Long-term storage issues should be addressed.

The PT material includes tendons, loose anchors, wedges, miscellaneous accessories and the stressing equipment.

JOBSITE INSTALLATION

Protection must continue ever after the tendons are installed on the deck.  Some of the better Engineering specifications require “temporary protection for exposed strands/anchors at construction joints.”  The tendon system will be exposed for around 2-3 days until the concrete is poured and reaches the required strength for stressing.  On parking garages, this is very important if the ramp slopes downhill into the anchor cavity (i.e. the next pour is above the first pour).  Rain, mud and dirt all travel downhill and get lodged into the anchor cavity.  This debris will affect the seating of the wedges during stressing.

Another item that needs attention from the jobsite personnel is cutting and capping the tendon tails.  The tendon tails should be cut and capped soon after the Engineer gives the approval.  If anchor cavities can be grouted shortly thereafter, then the steel will have less exposure to corrosive elements.  At times, we have noticed that tendon tails have not been cut or capped on the lower levels, but construction is almost complete on the upper levels.  Granted, this method is a labor-saver for the contractor but it puts the whole PT system at risk.

- Rattan Khosa, AMSYSCO

- Neel Khosa, AMSYSCO

Copyright © 2009 by AMSYSCO, Inc. All rights reserved.

Post Tensioning Institute (definitions):

• Friction Loss:  the loss of stress (force) in a prestressing tendon resulting from friction created between the strand and sheathing during stressing.
• Wobble:  the friction caused by the unintended horizontal deviation of the tendon.
• Angular Curvature:  the friction caused by the profile of the tendon.

American Concrete Institute (ACI-318 Chapter 18, Table R18.6.2) gives ranges for Bonded and Unbonded post tensioning tendons.  The lower the Wobble and Angular Curvature friction coefficients are, the higher the force per tendon. A higher force per tendon can help solve field issues such as broken tendons or under-elongations.

AMSYSCO, Inc. recently conducted a field friction test in 2009 for our 0.5″ diameter Unbonded Post Tension Tendon.  We used the “load-cell” method (shown in the video) and our friction coefficients outperformed the recommended values given by American Concrete Institute.  To request a copy of our results, please email us at info@amsyscoinc.com.

Copyright © 2009 by AMSYSCO, Inc. All rights reserved.

Checklist – BEFORE Post Tensioning shop drawings are started

1. Does the Post Tensioning Supplier have the latest and complete set of Structural Drawings and/or Architectural elevations/floor Plans?

2. Does the Post Tensioning Supplier have the latest Post Tensioning specifications and/or Barrier Cable specifications?

3. Has the pour sequence been finalized showing pour numbers, construction joints and/or pour strips?

4. Are there any restrictions to stressing tendons due to existing buildings, shear/elevator/stair walls or space limitations?

5. Have the major slab openings been coordinated and approved by the Structural Engineer?

6. Are structural CAD Files available to save time on detailing shop drawings?

7. For residential/office buildings:  Are MEP CAD Files available for coordination with Post Tensioning drawings?

Checklist – BEFORE Post Tensioning is awarded

1. Does the jobsite have a copy of the Post Tensioning Institute’s “Field Procedures Manual for Unbonded Single Strand Tendons” (3^rd Edition is current as of 2009)?

2. Does the Post Tensioning Installer have an individual who is currently certified under the Post Tensioning Institute’s “Level 2 Field Certification for Superstructure Ironworkers” program (or approved equal certification program)?  Are the other members of the installation crew certified under the Post-Tensioning Institute’s “Level 1 Field Fundamentals of Installation” program?

3. Does the Post Tensioning Inspector have an individual who is currently certified under the Post Tensioning Institute’s “Level 2 Field Certification for Inspector” program (or approved equal certification program)?

4. Is the Post Tensioning Supplier certified under the Post Tensioning Institute’s “Plant Certification” program?

5. Does the Post Tensioning Installer have an individual with experience with hydraulic stressing equipment for Post Tensioning?  This individual should have a current PTI-Certification (Level 2 Ironworker), be onsite and have a proven track record of working with stressing equipment.  Safety is priority one.

- Rattan Khosa, AMSYSCO

Copyright © 2009 by AMSYSCO, Inc. All rights reserved.

Copyright © 2009 by AMSYSCO, Inc. All rights reserved.