Testing Post-Tensioned Anchors

[dik]

The following is part of a Technical Memorandum:

"...water has intruded on the existing post-tensioning strand anchorages. The amount and degree of water present is unknown, but the presence of water can cause corrosion of the post-tensioned strand and anchor material.

If water is present, it is only a matter of time before the strength of the post-tensioning strands is compromised. If a post-tensioning strand fails, then the anchorage and structure associated with the strand may also fail. To minimise an untimely failure of the trunnion anchorage, remedial work should be undertaken.

Two approaches are prescribed. The post-tensioned anchorages can be ‘proof loaded’ on a regularly scheduled basis. When anchorages are identified with insufficient load capacity, then they should be replaced with the solid bar system noted earlier."

Is there a technical resource that has a prescribed manner and schedule for testing? I was thinking of taking the strands up to 1.25 times the 'real' design service loading, but less than 0.8 X fpu...

Any suggestions or comments?

Dik

 

[PEinc]

You really should provide more information.
Are these ground anchors?
Are these post-tensioned strands through concrete?
Do the strands already have any corrosion protection? Grease and sheath? Grouted Encapsulation? Epoxy coating?
Are these bare, grouted strands?

http://www.PeirceEngineering.com

 

[wiktor]

Typical testing for post-tensioning strands will be to check the existing force on the strand, as corroding steel will be gradually loosing tension.
Stretching it later to x1.25 service load could be great idea, as this, in conjunction with measuring the elongation, will give some indication of section losses.

 

[dik]

The existing anchors are seven strands of 0.6" dia stranded cables. They are unbonded with a deadend anchor and in a 2-1/2" conduit.The space between the conduit and the strand is filled with a corrosion resisting grease. It is this grease that may be contaminated. When pumping grease into the assembly, small amounts of water and emulsified grease were found.

Dik

 

[BAretired]

This is a pretty specialized job. Is there enough strand projecting past the anchors to enable grabbing the end? And if the strand breaks while stressing to 1.25 times service load, there is a danger of a chunk of cable firing out like a bullet.

Some of the early greases used in the post tensioned systems emulsified after only a few years, providing little or no corrosion protection to the cable. I recall one case in Calgary where strands in an apartment slab were said to be lashing out of the floor when the cable broke. One lady was exercising in front of the TV, went away for a few minutes and a cable ripped up out of the floor in the exact spot where she had been exercising. So much for exercising that day.

BA

 

[PEinc]

To get further information, I would contact someone from DSI (Dywidag) or possibly VSL Structural. I'm sure they have addressed this issue before.

http://www.PeirceEngineering.com

 

[SkiisAndBikes]

It's been 10+ years since I've worked on a post tension rehab project so I may be a little rusty, however, a potential alternate approach may be to expose a small section of each tendon (or sample thereof) at a low point and perform screwdriver tests. There is an ASTM standard for screwdriver penetration tests that will determine if the tendon has previously failed. The grease can also be tested at this time. Acoustical monitoring, while expensive to install, can detect future tendon breaks and depending on the quality of the system installed can triangulate a relatively accurate location of the tendon break. Often when an 'event' is recorded, the location is reviewed and a decision is made whether or not it is critical, thus requiring immediate repair or a decision can be made to wait.

In my limited experience, tendon/anchor emergence is a relatively rare event that typically only occurs when concrete cover is low. However, it can be dangerous when it happens. I have been involved in the destressing of hundreds of strands and it was rare to see a strand emerge from the slab upon destressing .... although there was that one time the contractor had uncovered a live set of anchorages .... and yes, the anchorage did fly quite far when destressed. I learned never to stand behind a stressing jack during testing (lift offs) and/or tensioning the new tendons.

I can't help with any insight on the proof loading. Once the wedges were set and the strand was trimmed, there was never enough length left for the stressing jack to grab onto for any retests.

 

[dik]

BAR... there is about 1/2" strand projecting beyond the collette... Specialised job... I seem to luck into these things of late... I propose to machine the end cap (holds strand and collettes and have a fitting machined to fit the jack... I'm well aware of the dangers and the energy stored... fortunately the strand is only about 25' long and originally stressed to approx 325K. Energy of seven cables stretched about 2-1/2"... I still have to verify the

I've been in touch with Dywidag already and will be in contact with VSL after Easter... been using Dywidag for 40 years... neat product

thanks Dik