Unbonded Post Tensioning Checklist for Corrosion Prevention

Due to recent events in the Post-Tensioning industry, AMSYSCO has taken the initiative to develop a checklist to address corrosion of unbonded post-tensioning.  The checklist relies on existing industry documentation.  (Disclaimer(s):  AMSYSCO, Inc. was not, or is, involved with the referenced project.   This checklist does not address bonded post-tensioned concrete structures.)

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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