PT Repair Book 3

[MacGruber22]

Would like some recommendations for PT repair books, other than ICRI and PTI manuals.

I am having trouble with lift-off test pressures not correlating with measured elongations.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

At the risk of citing a document associated with both PTI and ICRI, I like this: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

There are not many PT repair books around for UNBONDED (nor BONDED PT), other than PTI and ICRI.

In an attempt to assist your immediate need, is your current lift-off pressure vs measured elongation "issues" related to old tendons, or new tendons where the elongations are coming up short?

Lift-off pressure values alone are not a true measure of effective prestress at the anchorage. The stressor needs to carefully watch the gauge as he/she approaches lift-off - he/she will see (and feel and hear) when the wedges pop, but the gauge will immediately jump back several "psi" to give a more accurate effective prestress force value. The drop in pressure immediately after the wedges pop is the result of the 'release' of additional mechanical force required to overcome the wedge seating which then 'drops' accordingly, upon lift-off.

Are your tendons especially short, or undergo large plan sweeps?

 

[MacGruber22]

Thanks, KootK. I do have that document as part of the ICRI concrete repair manual.

Ingenuity - I kind of figured as much.

It is related to old tendons (1969 - 1970) that are being locked-off for a new stair opening. The measured elongations seem to be in the ballpark. We were looking at about 2-inches total initial elongation, and I measured an average of 1/4" elongation after lock-off and re-pulling. Obviously, that tells me there was a reasonable amount of residual force in the existing tendons for which we only needed to bump up the jacking force equal to about 1/4". One of the tendons was broken (outside the lock-off) and when re-stressed, elongation was approx. 2".

However, the problem came when asking the operator to provide me a lift-off test and back-pressure. But, on nearly every tendon (that wasn't broken) the supposed lift-off pressure was around 1,000 psi. That didn't make sense after pulling a few tendons and measuring the elongations, so I pressed the operator/superintendent about the lift-off pressure. I got no good answer. The language barrier may have been a problem, or he didn't know what he was doing. I was watching the gauge myself, and we weren't getting that initial jump. The equipment was calibrated recently, so I am fairly convinced the operator doesn't understand what a lift-off test is or how to do it. It seems as pressure was building in the system, the wedge was never initially released, and therefore the build-up of pressure appeared to be smooth and uniform up to the 30-kip jack force.

Tendons from new lock-off to end are about 22-ft with no horizontal sweeps. Slab is 8-inches thick.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

I thought about it some more, and I am wondering if it is even possible to perform a lift-off test in the middle of a stressed strand. See sketch.

I am thinking that they would have needed to first seat the anchor with the ram, cut the tendon to engage the wedge and leave a tail that can accommodate the ram stroke length, THEN perform the lift-off test.

Thoughts?

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

Ingenuity,

Do you know of where there is there a written procedure to perform the lift-off test. I checked a few manufacturers' sheets but found nothing.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

Photos for your enjoyment....

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ingenuity]

I am thinking that they would have needed to first seat the anchor with the ram, cut the tendon to engage the wedge and leave a tail that can accommodate the ram stroke length, THEN perform the lift-off test.

Exactly.

After installing a trouble-shooting anchor and wedges and seating the anchor with the ram, if they tried to conduct a lift-off test WITHOUT severing the strand between the new trouble-shooting anchors it would internally stress the ram to full pressure, but it would be trying to compress the strand behind the new anchors and since there will be no displacement the wedges will not lift off.

Great photos!

Seems that you could now get them to conduct a true lift off test as the photos indicate stressing tails behind each trouble-shooting anchor. A few anchors seem to only have a few inches of tail - so if they need to be checked for lift off the contractor may have to use a stressing-tube and splice chuck (aka short-stressing equipment) with a ram.

Over the weekend I will write up a procedure/method for a Lift-off Test for monostrand tendons and post it here.

I did search my digital library and found a chapter of a text on PT repairs by Florian Barth that goes into some detail on field procedures that I will post over the weekend too - but it is a bit light on info when it comes to lift-off procedures.

 

[MacGruber22]

Thanks for confirming my assumptions, and the info you found!

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ingenuity]

MacGruber22

See attachment for Lift-Off test procedure.

Hope it helps.

I cannot post the reference text/chapter on PT repairs I wrote of above, as it is copyrighted material, but if you do a search for "CONCRETE CONSTRUCTION Engineering Handbook", Nawy, Editor-in-Chief, and specifically Chapter 12 "Unbonded Post-Tensioning System Technology in Building Construction" by Florian G. Barth, P.E." it may be available somewhere on the web, or maybe purchase the book from Amazon if it is something you can justify in your library.

 

[MacGruber22]

Very good! Star for you, Ingenuity.

"It is imperative Cunth doesn't get his hands on those codes."

 

[Ingenuity]

MacGruber22,

You are welcome.

Keep in mind that for older PT systems (especially paper-wrapped, and occasionally the heat-sealed HDPE sheaths), Lift Off tests can be problematic, and if those tendons are then de-tensioned (sometimes not possible, depending on stressing tail, access etc) are then re-stressed to check elongation, the 'new' elongation will come up very short due to dried grease, corrosion, etc... and sometimes the strands break too.