PT Cable Strands look Stripped after Stressing 1

[SoleSteel]

Hi,

I'm working the construction administration on an elevated PT slab project as the EOR and I'm troubleshooting an issue in the elevated slab levels below grade. The elongations are coming out consistently low (between 10-17% low on cables between 50-125 feet). They are all single-end pulls. We've looked at several different causes and seem to be hitting dead ends everywhere (no pun intended). We visited the job site and the procedure they are using to stress the cables seems acceptable. We could still measure the stressing force output using a load cell to rule out the hydraulic rams being out of calibration.

One thing we noticed was that the oxidation from the stressed cables coming out of the slab was stripped on the top side of the cable to reveal the unoxidized metal. On some cables, the strands were stripped so much that their surfaces had actually gone from being round to being flat. I'm curious if anyone has ever seen this before. I'm hearing from a third party that this condition has occurred frequently on the project and I'm thinking that if something is providing additional friction for the PT cables as they are stressed, there is less force being put into the cables than is required and that can certainly lower the expected elongations. The only other thing I could think of was to take the the friction calculations to their limit. We ran our own friction calculations and we are able to bring the expected elongations lower, but only after maxing the wedge seating, angular coefficient, and wobble coefficient limits. I am not sure that it's the best way forward, but it's an idea.

I have a photo of the damage, but I unfortunately left my phone at home. I'll upload it as soon as I have a free moment.

I appreciate your experience and input! Please let me know if there's any additional information I can provide.

 

[AUCE98]

My response to a similar thread not to long ago:

The tendon elongation is a measure of the average force in a tendon of a given length. The methods for determining losses due to friction and wobble are based on approximate equations with assumed values of friction coefficient and wobble coefficient. Accordingly, the computed average force along the tendon is merely a reasonable approximation of the actual values. The force imparted by a properly calibrated is a far more precise method of measuring the force on a tendon, with the measured elongations serving as a check on the force indicated by the jack. Additionally, because the tendon elongation is a measure of the average force along a tendon and not the maximum or minimum, over or under elongations do not imply that the tendon is overstressed or under stressed, respectfully. It simply implies that the average force along the tendon is higher or lower than the assumed value used in the elongation calculations.

Whenever deviations outside the 7% target range occur, there are several possibilities which may cause such differences. One obvious cause of differences between measured field elongations and calculated elongations is the tendon is not placed in accordance with the drawings from which the elongations were calculated. Deviations of tendon placement from the drawings can account for large variations between calculated and measured elongations.

When excessive elongation occurs, there are several possibilities which may cause such differences between the calculated and measured elongations. The jack calibration should be checked and the tendons should be inspected for signs of wire breakage. Normally, wire breakage will be apparent to the jack operator during the stressing procedure. Provided there is no tendon slippage a the dead end or anchorage movement at either end due to honeycomb in the concrete, the most probable cause for differences would be wobble and friction coefficients lower than the assumed values. For tendon elongations with excessive elongations which show no signs of anchorage movement or tendon damage or breakage, there is no rational reason to reject the tendon based on the strength and serviceability of the structure, as explained in the following paragraph.

Assuming the worst case scenario, which is the tendon is actually overstressed by some amount, the resulting condition can be analyzed in detail. Regarding the service load stresses in the concrete, additional tendon force would serve to increase the precompression in the structure, thereby decreasing the service level tensile stresses. The additional precompression would also be beneficial in regard to the strength aspect of the structure. However, the higher the service load tensile stresses in the tendon would mean that the prestressing strand would enter the yield plateau slightly earlier under factored loads.

The maximum strain in a prestressing strand is 0.00758 in/in (216/28,500). This maximum strain occurs at the jack only, and is reduced significantly upon release of the strand and the onset of seating loss. The minimum specified strain at failure for strand fabricated per ASTM A-416 is 3.5% or 0.035. The ratio of maximum strain at strand failure to the maximum strain at stressing is about 4.5.Accordingly, the strand provides a great deal of reserve strain capacity regarding any stress that may exist in a tendon for which excessive elongation develops during stressing.

Additionally, the failure mechanism for a member prestressed with an unbonded tendon is nearly always precipitated by failure of the concrete in the compressive zone, not a failure of the prestressing strand. In an unbonded member subjected to loadings far in excess of the service loads, several large cracks tend to develop at the locations of maximum moment. Since the tendon is not bonded to the concrete, the strain the steel is experiencing due to these cracks is distributed over the entire length of the tendon. However the concrete strains a these cracks are concentrated at these crack locations, causing a compression failure at these locations. Because the strains in the prestressing steel are distributed over the entire length of the tendon, in many cases, the maximum stress in the prestressing steel upon failure of an unbonded prestressed member is actually less than the maximum stress in the prestressing steel during stressing. In essence, the highest force the tendon will experience during the life of the structure is during stressing.

For tendons that are lower than the 7% target range, the same possibilities for differences between the measured and calculated elongations as mentioned above exist. Tendons not placed in accordance with the drawings as well as differences in the assumed and actual friction and wobble coefficients can cause large variations in the measured elongation. When deficient elongation occurs (elongations that are below the calculated value by more than the stated tolerance) there would be no concern about the stress levels in the tendons, provided the tendons were stressed by a properly calibrated jack. Additionally, the deficiency in tendon stress for an individual tendon may have a negligible impact on the concrete under service loads. If the average tendon elongation in a particular beam or grouping of prestressing strands is within the stated target range, no additional consideration should be taken. The deficiency in force of one individual tendon at a section may be offset by somewhat excessive force in other tendons at the same section. For these reasons tendons with lower elongation values rarely have an effect on the service load stress in the concrete. Accordingly, there would be no impact on the strength of the structure, since the tendon would still reach or exceed the maximum design force prior to reaching the structural capacity of the member.

 

[Ingenuity]

One thing we noticed was that the oxidation from the stressed cables coming out of the slab was stripped on the top side of the cable to reveal the unoxidized metal.

Are these tendons part of a bonded (grouted) system, or typical US unbonded?

 

[MillR]

How come every time we start talking about PT my brain melts?

 

[rapt]

I have not seen the stressing jacks normally used in the USA but assume they have some mechanism to hold the wedges in as the stressing operation pulls the strand through. The force used to hold them in needs to be controlled so that it is sufficient to keep them tight enough to reduce draw-in to acceptable levels but not too much to cause significant damage to the strands pulling through them. I would suggest that someone has really jammed the wedges in during stressing and this has caused a lot of extra friction and damaged the strands at the same time! This could be sufficient to cause strands to snap during stressing, so it needs to be looked at urgently.

This will also damage the wedges and often they will slip. The damage to the wedges would be a combination of wear of the teeth and a build-up of junk between the teeth reducing their effectiveness. With un-bonded tendons this could not be accepted as they could slip at any time afterwards and you effectively have no tendon!!!

If this has happened the wedges would need to be replaced. While possible, the extra force required to remove the wedges to replace them could break the strand, especially if the strand diameter has been reduced significantly, so it will be dangerous!

It is not logical to play with the friction factors to make it all work. The prestressing contractor should have reliable figures on the factors they can achieve and these should have been used in the design. If the friction is a lot different, then you need to do something about it, not fudge the calculations to make the measured extensions work.

AUCE98
Answering the question might be useful!

MillR
You will probably need to go to a Medical forum to sort out your brain problems. While we might know something about PT, I doubt we could help you with affect of discussions about it on your brain!

 

[Ingenuity]

In light of the good info that rapt has posted, the stressing jack nose should be checked for excessive wear. If the wedges cannot slide free of the wedge cavity and enable the strand to freely elongate during stressing then such problems may occur.

I would recommend the following field check:

1. Choose a tendon with the problems you describe, preferable one that only requires a single pull (less than 100 foot for 8" stroke ram);
2. De-stress the tendon with experienced personnel;
3. Remove the wedges from the stressing-end anchorage;
4. Re-stress the tendons WITHOUT wedges, measuring the elongation on the strand from the ram stroke, at 33 kips;
5. Compare the measured elongation of strand (from Item 4 above) with calculated elongation at 33 kips;
6. Retract ram;
7. Install new wedges;
8. Re-stress tendon.

Preferable to do the above with EoR or experienced Special Inspector present.

 

[rapt]

In Ingenuity's list of steps, inspect the wedges after removal for scouring of the teeth from the strands pulling through, loose metal shavings and build-up of junk in remaining teeth. No matter what their condition, do not allow reuse of the wedges as they are designed for single use and will probably be weakened and deformed.

You could also watch closely while they are stressing another strand to see if the wedges are being released during the stressing and are not jammed in and causing damage and friction!

Or you could fly RAPT and Ingenuity over to supervise!

 

[AUCE98]

rapt..
I was responding to the first paragraph and the possible causes for the low elongations. Thank you for the flippant response.

 

[EngineeringEric]

Can one un-stress and re-stress a tendon? would it be like installing an already used tendon or a tendon from a different job.... Similar to bolt management, don't use old bolts

I don't do much with PT in this vain so thanks for also spreading wisdom.

(sorry for hijack)

 

[Ingenuity]

Can one un-stress and re-stress a tendon? would it be like installing an already used tendon or a tendon from a different job.... Similar to bolt management, don't use old bolts.

If the stressing tails have not been cut off, then, yes, the tendon can be de-stressed and same tendon re-stressed without issue.

Assuming UNbonded PT, or bonded PT BEFORE grouting.

De-stressing is best undertaken with experienced personnel - removing the wedges can be a tricky exercise. Sometimes special equipment is required, like a destressing bridge or stool.

If the tendon has to be de-stressed and fully removed for some reason, the the fixed-end anchorage needs to be accessed (concrete chipping from slab edge) and the wedges removed to enable the strand to be extracted and replaced with new. Such a removal and replacement process does not happen too often. Usually a broken tendon is repaired by a shorter strand segment repair, using splice chucks etc.

 

[rapt]

EngineeringEric

One thing Ingenuity missed (repeated from earlier) is that the wedges should not be reused. They would have to be replaced.

Re-gripping in the same area in the same area can reduce strength so it is not something you should do a lot.

Auce98
I did not realise I was being flippant! As I pointed out the last time you printed this piece, 2 of your last 3 paragraphs are misleading regarding the "reserve strain capacity" where overstressing has occurred. And the ACI code rules for stress increase under strain do not apply in this case as they assume an initial transfer prestress stress of less than 70% UTS.
Also, apparently some testing in USA has shown that un-bonded tendon assemblages are failing at about 2%, in tests, nowhere near the strand minimum of 3.5%.

 

[Ingenuity]

One thing Ingenuity missed (repeated from earlier) is that the wedges should not be reused. They would have to be replaced.

Agree, but I did I state above "7. Install new wedges;" so I implied that the old wedges should be replaced.

 

[rapt]

I only read the post immediately above, not the one from yesterday!

 

[SoleSteel]

Before I read the comments and reply, here is a photo of the condition I mentioned in my original post. These are ~120 ft. single live end pull unbonded cables with ~9.5 inches of calculated elongation. These cables are coming up consistently short by 3/4" to 1". Additionally, several cables throughout this pour are showing wear like the stressed cable shown below:

 

[rapt]

SoleSteel

Why is there so much cover to the top steel? And the anchorages so low in the slab?

Looks like the wedges are jammed in too tight and are scouring the strand as we suggested. There is a lot of gunk in the bottom strand from this and there will be a lot in the wedge teeth, if there are any teeth left!

 

[SoleSteel]

Ingenuity: Unbonded Cables

rapt: The wedges being jammed too hard sounds plausible. I will verify that the wedge-setting procedure is acceptable on the next pour. As for the current wedges, I will actually be able to see one of them after it's removed and will verify any damage to the wedge teeth, if they can remove it without yielding the cable. Depending on what we find out, the other PT cables will need to be examined.

Ingenuity: That's another sensible idea. I will certainly evaluate the jack nose. The field check would give me some good information I need to evaluate different causes. Could be the the wedge installation, could be the interaction between the wedge and the jack, could be the jack/pump (though the calibrations are acceptable).

rapt: How could you see the wedges during stressing? I've been under the impression that one should never stand anywhere behind the PT cable being tensioned. I mean, I'd do it with a flashlight and binoculars. I don't know about flying you helpful engineers out, but I will at some point in the future buy you each a beer!

EngineeringEric: Like Ingenuity and rapt said, tendons can be de-stressed and re-stressed given some conditions. The procedure is usually done by a specialist who has a tool where he can de-stress the cable without blocking the way to remove the wedges from the grout pocket. The cables have to be pulled past the force that was imparted on them since the wedges are seated in there pretty tightly. It's important not to yield the tendon in this process or else it needs to be replaced.

rapt: Those are great points. The grout pocket should be at mid-depth at the pour strip and the top reinforcing has been placed excessively low. It seems to have been installed incorrectly.

Looks like I need to look into some of these clues as you guys have suggested above. I will let you know what I find out.

 

[rapt]

If the extension is low because of the extra friction from the wedges, the stress in the strand will be low, giving you some leeway. Also, you can stress the strands up to 85% in de-tensioning to allow the wedges to be pulled out to remove them.

When they are de-tensioning, watch the gauge and wedges to try to determine the pressure the wedges start to move at and the strand past the anchor starts to increase in load. This will give you an idea of the stress in the tendons. The jack should slow appreciably when the whole strand starts to take force! After they have moved about 1/4" (6mm) that should make up for the draw-in and that gives an estimate of the level of stress that was put into the strands!

You can normally stand to the side of the jack and look at the strand and wedges from reasonably close range. Just make sure you have your hard hat and steel capped boots on so that your insurance will pay out in case you get hit . The person doing the de-tensioning is going to have to be close enough to get his hands in to get the old wedges out. Make sure they leave the wedges in place if they are going to need multiple bites to release all of the extension depending on the available jack travel!

You should be able to tell just by looking at the jack nose if there is sufficient space for the wedges to pull back and spread slightly to allow the strand to pull through without much resistance.

Is the scouring of the strand on all side or only on the top?

Is the jack they are using pulling straight out, or have they used a curved nose on the end. If a Curved nose it could be forcing the strand into the top of the anchorage causing scouring only on the top!