Post Tension « AMSYSCO Post Tension Blog

Updated PTI Specification Requiring Encapsulated Tendons

By admin, on January 19th, 2012%

The following is a press release from the Post-Tensioning Institute which requires the use of Encapsulated Tendons for all unbonded PT buildings subject to ACI-318.

Release:

NEW PTI SPECIFICATION UPDATE REQUIRES THE USE OF ENCAPSULATED TENDONS

FARMINGTON HILLS, MI (January 16, 2012) – The Post-Tensioning Institute (PTI) has announced a new specification update approved by the PTI Board of Directors and the Technical Advisory Board. Addendum #3 to PTI M10.2-00 Specification for Unbonded Single Strand Tendons requires the following:

All unbonded single strand post-tensioning tendons used for structures designed in accordance with the Building Code Requirements for Structural Concrete (ACI 318) must be encapsulated. The encapsulation of the tendons shall be in accordance with the material requirements for aggressive environments in the PTI Specification for Unbonded Single Strand Tendons (PTI M10.2-00).

The intent is to augment the durability of the post-tensioning systems used for all applications governed by ACI 318. Encapsulated systems provide additional protection of the prestressing steel regardless of location of the structure or exposure to moisture intrusion from any source.

Addendum #3 to PTI M10.2-00 Specification for Unbonded Single Strand Tendons is available for free download on PTI’s website at http://post-tensioning.org/Uploads/Addendum3.pdf .

Source: http://www.post-tensioning.org/press_releases_details.php?id=21

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Missed Stressing Operation

By nkhosa, on December 18th, 2011%

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

By admin, on July 28th, 2011%

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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4 Questions to ask your Unbonded Post-Tension Supplier

By nkhosa, on July 2nd, 2011%

Hiring a quality Unbonded Post-Tension Supplier can be the difference between a smooth construction project and a problematic situation. The Post-Tension (PT) supplier will play a key role in your reinforced concrete project. Therefore, general contractors and concrete contractors need to choose wisely.

Traditionally, the Post-Tension Supplier is awarded their contract after other subcontractors in larger trades have been selected. Finding the right PT supplier to work with is essential to completing your concrete construction project on time and within budget.

Aside from checking whether the project has been bid per specifications, the following are some questions to ask PT suppliers before awarding the job:

What is your proposed pour sequence?

Unless the construction joints and pour numbers have been referenced on the structural drawings, a PT supplier will have to assume a pour sequence for bidding purposes. If you and your PT supplier had different pour sequences in mind, then time may be wasted in the shop drawing stage. It’s best to be on the same page prior to the contract award.

How many trucks (or deliveries) have you included in your proposal?

If your construction site has limited lay-down areas, then you may not want full truck-load deliveries from your PT Supplier. Your PT Supplier should provide the construction materials per your schedule, but it’s always good idea to let them know the plan.

What previous PT projects have has the PT Supplier worked on that are similar to this project?

Whereas having the lowest price is important during bidding, but hiring a Post-Tensioning Supplier with the right expertise could help you avoid losing money during construction. There may be unique challenges in your construction project that need to be overcome in a timely and cost-effective manner. As with other construction trades, experience matters.

How many sets of stressing equipment will you provide and for how long?

Most PT specifications require at least 2 sets of stressing equipment at the construction site. If one set goes down, then having a backup set can help maintain the schedule. For larger projects with multiple crews, a contractor may require more than 2 sets. As with other rented equipment, the rental period may vary depending on the duration of the project.

- Neel Khosa, Vice President, AMSYSCO
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