How to verify if post-tensioned strands were yielded after stressing. 1

[struggle67]

Hi

Good Day To you

In a post-tensioned building, if the tendon elongations are more than the theoretical elongation by about 15%, is there a way/method to verify whether the strands were yielded.

Anyway if elongations fall out of tolerance, most of PT companies here will do this ...

But it seems to me that almost all of them do not know why they are doing it and it really disturbs me. I would like to know why.

When elongation is more, stress upto 80% to overcome higher friction?
When elongation is less, why stress 75% and hold the pressure?

Thanks

 

[rb1957]

can you take a sample of the strand in question (or maybe one from the same processing lot) and do a tension test with it ?

What other things (other than material properties) could be "messing" with your analytical prediction ?

another day in paradise, or is paradise one day closer ?

 

[jayrod12]

I think you've got that reversed, when they are seeing elongation exceeding the expected (i.e. more) than they are holding at 75% UTS. If the tendon has truly reached yield then elongation would continue until rupture if the stress is held constant, if the stress holds and the elongation doesn't increase substantially then it likely hasn't yielded yet. When they are seeing elongation below the tolerance (i.e. less) then they are increasing the stress to get to the expected extension.

 

[cooperDBM]

Prestressing strand doesn't have a physically defined yield point or a yield plateau where strain increases at constant stress. It almost plateaus near the ultimate stress (~98% UTS). 75% UTS should still be well within the elastic range. I don't know the background to the 75% hold but it might just be testing for system stability. For instance checking for crushing of bad concrete at the anchorages, anchorage slippage, or excess friction which may slowly dissipate, etc.

 

[struggle67]

rb1957,

We did strand test from same strand coil before we did the work. Nothing unusual! Our elongation calculation spread sheet should be alright. Anyway I will check again!

Jayrod12,

I was hoping that but there is no plateau so holding the constant pressure wont cause the strand to elongate further?

CooperDBM,

So does it mean that I only left one way to verity whether the strands are out of elastic range (i.e stress exceeded the so called 0.1% proof stress fp0.1k which is about 86% of UTS I think) is to hack off the concrete and get a strand sample and test it again?

Thanks all and really appreciated to have your replies.

 

[Ron]

Agree with Jayrod12....if the strand has actually yielded, then applying constant stress at 75% will cause more elongation without reaching ultimate and breaking the tendon. This is a safety measure.

If the tendon has not yielded, 75% is sufficient to show that the first assessment of elongation was probably incorrect.

 

[struggle67]

Hi Ron, Jay,

Thanks sorry late reply.

This is the stress strain curve of typical strand.

I have been thinking and wondering since there is no plateau, how applying constant stress will cause more elongation?