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.
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.
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.
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.
Our blog will announce AMSYSCO news and provide commentary about the Post-Tensioned Concrete and Commercial Construction industries. Our audience includes Architects, Structural Engineers, General Contractors, Concrete Contractors and Ironworkers. Please visit our main website for our Post-Tensioning reinforcement products and services.
www.amsyscoinc.com
1200 Windham Parkway
Romeoville, IL 60446 P: 630-296-8383
info@amsyscoinc.com
