The key input parameters for “Long-Term Losses” are as follows:

• Post-Tensioning System (Unbonded or Bonded)
• Type of Strand (usually Low-Lax)
• Ultimate Strength of Strand (usually 270 ksi)
• Modulus of Elasticity of Strand (usually 28,500 ksi)
• Estimate of initial average compression (depends on concrete member)
• Concrete Strength at 28 days (varies, but typically 5000 psi)
• Average Weight of Concrete (Normal or Light-weight)
• Estimate Age of Concrete at Stressing (usually 3 days)
• Modulus of Elasticity of Concrete at Stressing (ex. 57 x sqrt of specified 3-day compressive strength of concrete of 3000 psi = 3122 ksi)
• Modulus of Elasticity of Concrete at 28 days (ex. 57 x sqrt of specified 28-day compressive strength of concrete of 5000 psi = 4030 ksi)
• Estimate of Average Relative Humidity (varies by geographical region)
• Volume to Surface Ratio of member (depends on concrete member)

The key input parameters for “Friction & Elongation” are as follows:

• Coefficient of angular friction (varies by PT supplier)
• Coefficient of wobble friction (varies by PT supplier)
• Ultimate Strength of Strand (usually 270 ksi)
• Ratio of Jacking Stress to Strand’s Ultimate Strength (0.80 per ACI-318)
• Anchor Set (usually 0.25″)
• Cross-Sectional Area of Strand (varies, but usually 0.153 sq.in. for 0.5″ diameter strand)
• Total Number of Strand per Tendon (1 for unbonded, varies for bonded)
• Stressing Configuration (Single-End or Double-End)
• Length and Drape of Tendon per Span (depends on design of concrete member)

The output will include the following:

• Long-Term Losses
  • Elastic Shortening
  • Shrinkage
  • Creep
  • Relaxation
• Final Average Force in Tendon
• Total Elongation
• Critical Stress Ratios

ACI-318 and PTI Manuals go into further detail about analyzing the results of the friction-loss calculations.

- Neel Khosa, Vice President, AMSYSCO

Related Articles:

Video on Field Friction Test

Material Properties of Post-Tension Strand

____________________________________________

SHARE THIS

Download here: White Paper – Encapsulated Post Tensioning (PDF, 1MB)

- Neel Khosa

____________________________________________

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)

____________________________________________

The highlights include:

• Heights savings with Post-Tensioned Concrete in lieu of Structural Steel.
• Cost comparison between Post-Tensioning and Conventionally Reinforced Concrete.
• Reduction of construction materials.
• Potential USGBC LEED points gained with Post-Tension.

Download Here:  PTI Sustainability Case Study (2.5MB PDF file, 5 pages.)
____________________________________________

Barrier Cable is comprised of a 0.5 inch diameter, 7-wire high-strength steel strand.  This strand should meet the requirements of ASTM-A416 and Post-Tensioning Institute’s ‘Specification for 7-Wire Steel Strand Barrier Cable Applications.’

Typically, Barrier Cable is exposed to the elements and therefore requires a corrosion-protection system to prevent the steel from rusting.  There are several different types of barrier cable that Structural Engineers and Architects can specify for their structure.  The pictures and descriptions show the most common types of Barrier Cable used in the United States.

• Plastic-Coated: Plastic-Coated Barrier Cable has an extruded plastic HDPE sheathing around the steel strand to prevent rusting.  The protective system is similar to Unbonded Post-Tension strand except the Plastic-Coated Barrier Cable typically does not contain Post-Tension coating (“grease”) inside the sheathing.  Therefore, the extruded sheathing has a tighter bond to the steel strand as the interstitial spaces are filled.
• Galvanized Barrier Cable:  Galvanized Barrier Cable is zinc-plated per ASTM-A475 to prevent rusting.  It is recommended to use galvanized wedges to hold the cable within the corrosion-protected anchorage device.
• Epoxy-Coated:  Epoxy-Coated Barrier Cable has thermosetting plastic polymer coating around the steel strand to prevent rusting.  Special wedges may be required to hold the cable within the anchorage device.

In terms of costs, the Plastic-Coated system is the cheapest and the Epoxy-Coated is the most expensive.  In terms of performance and maintenance, opinions differ and greatly depend on the installation and the end-user.  In terms of aesthetics, beauty is in the eye of the beholder!!!

Architects and/or Structural Engineers should also consider the following when designing the Barrier Cable system:

1. Is the Barrier Cable anchorage device embedded in the column, or will a steel angle iron be required to attach to the column? If the anchors are embedded, then the Contractor will have to install the anchors as the columns are erected.  If steel angle iron is used, then the sides of the column cannot have any obstruction.  Several different anchorage devices are used in either case.
2. What type of anchorage device should be specified? Among the most common types include a galvanized donut/barrel anchor, encapsulated post-tension anchor, “Grabbit”/adjustable anchor, and spring-loaded anchor.
3. What type of wedges should be specified? Depending on the anchorage devices, galvanized 2-piece and 3-piece wedge sets are the most common.  Some of the specialized anchorage devices have the wedges included.
4. What areas have restrictions to stressing the Barrier Cable? Again, different anchorage devices would be required.
5. What force should the Barrier Cable be stressed at? Typically, Barrier Cable is stressed to 3 to 7 kips (and back-stressed to 28 kips).  Most Barrier Cable is Grade 230, 250 or 270.
6. How many rows of Barrier Cable are required to protect pedestrian and restrain vehicles? The PTI Barrier Cable Specification defines the requirements per UBC Building Code.  Local Codes or special circumstances should be factored.

Visit the AMSYSCO Inc. website for more information about Barrier Cable and other Post-Tensioning products.

- Neel Khosa, AMSYSCO, Inc.
____________________________________________

_______________________________

Assume 0.5″ diameter strand has cross-sectional area of 0.153 sq.in. and weight of 0.525 lbs/ft.

Assume 0.6″ diameter strand has cross-sectional area of 0.217 sq.in. and weight of 0.740 lbs/ft.

_______________________________

Minimum Ultimate Tensile Strength (MUTS) = (Grade of Steel) x (Cross-Sectional Area)

0.5″ inch diameter = (270 ksi) x (0.153 sq.in.) = 41.3 kips

0.6″ inch diameter = (270 ksi) x (0.217 sq.in.) = 58.6 kips

_______________________________

Minimum Yield Strength = 90% of MUTS = MUTS x 0.90 (per ASTM-A416)

0.5″ inch diameter = (41.3 kips) x (0.90) = 37.2 kips

0.6″ inch diameter = (58.6 kips) x (0.90) = 52.7 kips

_______________________________

Jacking Force = 80% of MUTS = MUTS x 0.80 (per ACI Code)

0.5″ inch diameter = (41.3 kips) x (0.80) = 33.0 kips

0.6″ inch diameter = (58.6 kips) x (0.80) = 46.9 kips

“Jacking Force” is the force that tendons are stressed to.

_______________________________

Allowable Initial Force = (Jacking Force) minus (Short-Term Losses) = 70% of MUTS = MUTS x 0.70 (per ACI-318)

Short-Term Losses include:

1. Angular Profile of Tendon
2. Horizontal sweeps in Tendon
3. Wedge-Seating (typically 0.25 inch)
4. Wobble due to installation (CLICK HERE to view the video on how to calculate Angular and Wobble Coefficients in unbonded post-tensioning tendons.)

0.5″ inch diameter = (41.3 kips) x (0.70) = 28.9 kips

0.6″ inch diameter = (58.6 kips) x (0.70) = 41.0 kips

“Initial Force” is the force at the anchorage after the wedges are seated and stressing jack is removed.  The calculated values above are approximate since the actual short-term losses may differ from the theoretical values.

_______________________________

Final Force = (Initial Force) minus (Long-Term Losses)

Long-Term Losses include:

1. Creep of concrete (permanent deflection due application of constant load)
2. Elastic Shortening of concrete
3. Relaxation of steel prestressing strand
4. Shrinkage of concrete during curing

0.5″ inch diameter = approx 26.9 kips

0.6″ inch diameter = approx. 38.1 kips

“Final Force” is the force at the anchorage after the long-term losses are accounted for.  The calculated values above are approximate since the actual long-term losses may differ from the theoretical values.

_______________________________

Average Tendon Elongation (approx.) = (P x L) / (A x E)

P = Prestress jacking force (70% of MUTS)

L = Length of steel (inches)

A = Cross-Sectional Area of steel (sq.in.) on mill certificates

E = Modulus of Elasticity of steel (ksi) on mill certificates

For example, using a 100-foot tendon (L = 100 x 12 inches) with Modulus of Elasticity of 28,500 ksi.

0.5″ inch diameter = (28.9 kips x 1,200 inches) / (0.153 sq.in. x 28,500 ksi) = 7.95 inches

0.6″ inch diameter = (41.0 kips x 1,200 inches) / (0.217 sq.in. x 28,500 ksi) = 7.95 inches

***Notice that the 0.5″ and 0.6″ have the same Avg. Elongation***

_______________________________

Post-Tensioning Institute recommends an allowable elongation range of plus/minus 7% of the Average Tendon Elongation for unbonded post-tensioning tendons.

Min. Allowable Elongation = 93% of Avg. Elongation = 0.93 x (Avg.El.)

Max. Allowable Elongation = 107% of Avg. Elongation = 1.07 x (Avg.El.)

If we use the same 100-foot tendon with average elongation of 7.95 inches, then Min.El. = 0.93 x 7.95 inches = 7.40 inches and Max.El. = 1.07 x 7.95 inches = 8.51 inches.

- Rattan Khosa, President, AMSYSCO

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.