Post-Tension Slab

Your typical Unbonded Post-Tensioned concrete slab will require the following items:

1. Post-Tensioning Tendon
2. Anchorage Devices (at stressing, intermediate, fixed ends)
3. Reusable Pocket Formers (at stressing ends)
4. Split Pocket Formers (at intermediate ends)
5. Wedges (at stressing, intermediate, fixed ends)
6. Stressing Equipment (jack, pump, gauge)

The encapsulated post-tension system requires additional corrosion-protection than the non-encapsulated post-tension system.

- Neel Khosa, Vice President, AMSYSCO

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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.

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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

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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

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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.

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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.

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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.

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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***

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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

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.