AMSYSCO Post Tension Blog » Field Issues

Missed Stressing Operation

nkhosa — Sun, 18 Dec 2011 17:00:51 +0000

Q: What happens if you forgot to stress an unbonded post-tensioning tendon at the construction joint and have already poured the subsequent pour?

A: The most preferred option is to stress the tendon in the subsequent pour. However, the following items should be factored in:

a) The 7% Elongation Range must be revised.

Since you have a longer pull, the elongation calculation needs to be revised also. This should be noted in the elongation records from the PTI-Certified Inspector. A thumb-rule is to just add Min1+Min2 and Max1+Max2 together. For example,

First pull of 50′ would be Min1 = 3.72″ and Max1 = 4.28″
Second pull of 100′ would be Min1 = 7.44″ and Max1 = 8.56″
New elongation range for 150′ pull would be Min1 = 3.72″ + 7.44″ = 11.16″ and Max1 = 12.84″.
The new range is an close approximation. A more accurate range can be calculated with PT software. This is especially important for very long pulls.

b) The force in the tendon will likely be less.

Since the stressed tendon length is now longer, your angular and wobble friction have likely increased somewhat due to the increased number of spans. In other words, the final average force will drop in both pours for that particular tendon.

Previously, the 50′ pull may provide 27.5 kips of force in Pour #1.
Previously, the 100′ pull may provide 27.3 kips of force in Pour #2.
Now, the 150′ pull may provide only 26.9 kips of force in Pours #1 and #2.

c) The intermediate anchor has been abandoned.

The force within the 150′ tendon will be transmitted uniformly through the construction joint. Conversely, the 50′ tendon transmits force only within Pour #1 and the 100′ tendon transmits force only within Pour #2 (i.e. the construction joint “locks-off” the force within each pour). This is important should be there be any repair/renovation conducted in the future.

- Neel Khosa, Vice President, AMSYSCO

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Detensioning Unbonded Post-Tension Tendons

admin — Thu, 28 Jul 2011 15:31:33 +0000

On rare occasions, the ironworker/installer will need to detension an already-stressed post tensioning tendon. The reason may be due to a repair procedure or re-stressing due to out-of-range elongation readings.

Recommendations:

During the detensioning process, the post-tensioning tendon will have to be “over-stressed.” The PTI Field Manual recommends that the hydraulic jack and pump gauge pressure should not be taken more than 5% over the normal stressing gauge pressure (at 33 kips of 0.5″ tendon).
The jack should be opened at least 4 inches so that it does not get “hung up” in case the wedges fail.
The detensioning procedure be performed shortly after the normal stressing operation. Over time, the wedges deformed inside the anchor cavity and form a “lock” with the strand and anchor. (Refer to the PTI Field Manual 3rd Edition section 10.9).

The pictures below show an example of how to detension the tendons with a Detensioning Tool.

Place detensioning tool level with anchor cavity and strand. Secure hydraulic jack behind detensioning tool.

Extend Hydraulic Jack to unseat wedges. This follows the normal stressing procedure. Hold gauge pressure at 5% overstress.

Use pliers (and magnet) to take wedges out of anchor cavity. Do not use your fingers.

Failure to use the proper parts or equipment could result in damaging the anchor, strand, equipment or harming the operator/by-standers. This procedure should be carried out by a Post-Tensioning Institute Certified Ironworker (level 2). This post is not a complete procedure for detensioning PT tendons. Proper jobsite precautions should be taken to restrict personnel during stressing and detensioning processes.

- Neel Khosa, Vice President, AMSYSCO

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The importance of clean anchor cavities

nkhosa — Wed, 22 Jun 2011 16:26:20 +0000

Having clean anchor cavities is one of the most important part of getting proper elongations of unbonded post-tensioning tendons.

Post-Tension Stressing Tails - side

Post-Tension Tendon Stressing Tails

Since construction is not the cleanest of jobs, there may be debris inside the PT anchor cavity. A major cause of under-elongations and/or strand-slippage is due to having debris (concrete, dirt, mud, etc.) inside the PT anchor cavity. The debris doesn’t allow the wedges to seat properly (the wedges are inside the anchor cavity and hold the strand in place after the stressing operation). If enough tendons have under-elongations, then the structure may not have enough force within the concrete member. Therefore, it is important to have clean anchor cavities prior to stressing operations.

Post-Tension Anchor Cavity

Post-Tension Anchor Cavity - closeup

In order to clean the post-tension anchor cavities prior to stressing (and installation of wedges), we recommend spraying WD-40, or approved equal, into the anchor. This non-corrosive chemical should dislodge any loose debris. Afterwards, the strand and anchor cavities should be cleaned prior to the installation of the wedges and the stressing operations. (Note: The WD-40 should be compatible with the post-tension grouting used to fill the anchor cavity. The anchor cavity must be clean in order for the grout to bond properly.)

- Neel Khosa, Vice President, AMSYSCO

Pictures taken by AMSYSCO on the 31st Street Marina Harbor project. Structural Engineer: Desman Associates. General and Concrete Contractor: McHugh Construction.

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Video : Stressing Unbonded Post Tension Tendon

admin — Thu, 16 Jun 2011 23:31:35 +0000

This short video show the stressing an unbonded post-tensioning tendon with a hydraulic jack (8-inch stroke). This tendon was roughly 130 feet long so it required two strokes. The elongation for a 100-foot-long tendon is roughly 8 inches. Refer to the Post-Tensioning Institute’s “Field Procedures Manual for Unbonded Single-Strand Tendons” Chapter 6 for additional information.

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Video : Applying paint mark for Tendon Elongation

admin — Thu, 16 Jun 2011 23:17:34 +0000

This short clip shows how to apply paint-marks prior to stressing of post-tension tendons. It’s recommended to use a straight-edge, metal to help with elongation measurements. Refer to the Post-Tensioning Institute’s “Field Procedures Manual for Unbonded Single-Strand Tendons” Chapter 7 for additional information.

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How do I know if my Post-Tensioning Tendons have been stressed?

nkhosa — Thu, 16 Jun 2011 22:32:20 +0000

As a post-tensioning inspector or installer, have you ever been in a situation whether you are unsure whether some PT tendons have been stressed? By not having a PT tendon stressed, this could have detrimental effects on the structure.

Here are three pointers that may help you out:

1. PAINT-MARKS : Compare the location of the paint-marks to other tendons of the same length. If you see a major difference in one or more paint-marks, then the flag should go up. If the location of the paint marks are similar, then either ALL of tendon have been stressed or NONE of them have been stressed. If you have a deviation, then you may have an unstressed tendons or an elongation issue. (Related Video : Applying paint mark for post-tensioning tendon).

2. BITE-MARKS : Check whether there are “bite-marks” on the tendons. During the stressing operation, the hydraulic jack “grabs” the stressing tail of the PT strand and and creates a series of small bite-marks. The picture below shows two tendons that have been stressed (click picture to enlarge). This visual inspection helps if your paint-marks have been washed away.

Bitemarks into Post-Tensioning Strand

3. ELONGATION RECORDING : AMSYSCO highly recommends that the inspector (or testing agency) record elongation during the stressing operations…rather than several hours later. This step increases the quality control of the project and eliminates a potential unstressed tendon.

- Neel Khosa, Vice President, AMSYSCO

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Protecting exposed strand at Construction Joints

nkhosa — Wed, 20 Apr 2011 17:27:57 +0000

With some post-tension systems, the installer is required to strip the plastic-sheathing in order to install the intermediate anchors at the construction joints. One issue is how to protect the exposed strand until the second pour is made (pictured below – Pour #1 on left and Pour #2 on right).

Exposed strand at construction joint

One solution is to put visqueen or a tarp on top of the bulkhead until the second pour is made. This has been specified by some Structural Engineers in order to protect the exposed strand from corrosive elements such as rain and snow.

Another solution is to push the plastic tubes on to the backside of the bulkhead in the second pour (pictured below). This would provide added protection to the post-tension system at a lower cost than tarpping.

Strand protected by Tubing

- Neel Khosa, Vice President

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Sweeping beam tendons into the slab

nkhosa — Wed, 20 Apr 2011 16:59:12 +0000

Occassionally, the field will encounter a condition where the post-tensioned beam’s CGS (center of gravity of steel) requires several PT beams be installed in the slab. In other words, some PT anchors will not be installed within the web of the beam, but in the flange of the beam.

Below are several pictures showing how these tendons can be installed at the jobsite.

Beam tendons sweeping out of beam

Beam tendons swept into slab

Beam tendons swept into slab at Anchors

It is recommended to add some #3 hairpins where the tendons curve to prevent blowouts in the concrete. Without hairpins, the tendon may “slice” through the concrete during the stressing operations. The tendon has a natural tendency to straighten out during the stressing operations.

- Neel Khosa, Vice President

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Kink in a Tendon

nkhosa — Wed, 15 Dec 2010 18:10:04 +0000

On rare occasions, the installer will unroll a tendon that looks like this…

Kink in Tendon

WHAT WILL HAPPEN?

During the stressing operation, this tendon will probably break due to the eccentric force at the anchorage. Chances are that this tendon would break well before the required 33 kips is applied. Not only would this become a safety issue, it will eventually become a repair issue when the tendon breaks. That is time, money and aggravation that can be avoided.

HOW TO FIX IT?

I do not recommend stressing this tendon at a lower force. The question is what do you do to fix or repair a “kink” in an unbonded post-tensioning tendon?

Option #1 : Replace the tendon with a new tendon.
Option #2 : Cut off the dead anchor and field-seat a new anchor. Make sure you have enough stressing tail after cutting the anchor.

WHY DOES IT HAPPEN?

Why does this happen? During loading, transit or unloading, this anchor had some “extra” load applied to it and caused the tendon to bend or kink. When the kink is relatively far away from the anchorages, then the kink probably occurred due to a power surge in the manufacturing equipment. This issue can happen if the tendon bundles have the eye-to-the-sky or eye-to-the-side.

-Neel Khosa

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Informational Presentation about Post-Tensioned Concrete for Contractors

nkhosa — Sat, 23 Oct 2010 23:42:47 +0000

The following educational presentation is targeted towards General and Concrete Contractors interested in utilizing Post-Tensioned Concrete.

Please contact AMSYSCO at 630-296-8383 or at info@amsyscoinc.com to find out how we can help you with your construction project.

Post-Tension Concrete – Info session for Contractors

View more presentations from Amsysco, Inc..

Outline of Presentation:

Introduction to Post-Tension
Components of Post-Tension
Construction Team
Submittals
Pre-Installation
Installation Management
Post-Concrete Placement
Troubleshooting

Download slides here => AMSYSCO – Contractor info session (PDF, 3MB).

- Neel Khosa, Vice President, AMSYSCO

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