Stress Corrosion Cracking and HE to tensioned high strength 7-wire carbon steel strands

[Ingenuity]

I recently extracted 4 failed 7-wire steel strands from a post-tensioned concrete parking/plaza deck.

Strand tendons are the typical MUTS 270 ksi, 1/2" diameter, greased and sheathed, and the structure is 20 years old, and otherwise in excellent condition with meticulous maintenance etc. The strand tendons are typically under a tensile stress of approx. 70% of MUTS.

At each of four locations there was about 8" of strand tail that exited the building slab edge, so we extracted the strands to determine probable cause.

We have done this many, many times, with often the reason for failure is due to someone installing post-installed fasteners (rotary-percussion drilling), but for older structures (>30 years old) it is often corrosion related, usually significant pitting over several feet of strand in close proximity to the failure location.

But with the 4 extracted strands to this project the corrosion was very localized (less than 12" length), and whilst there was the characteristic pitting, some of the wires with brittle fracture have what looks like longitudinal cracking along the length of the wire over about 5 wires diameters.

Is it possible to identify stress corrosion cracking and/or hydrogen embrittlement based off visual review, or is full metallurgical testing/analysis required?

Here are some sample photographs. I partially uncoiled the 6 outer wires to show the damage across all wires.

I am assuming that HE is less likely, unless environmental hydrogen caused the embrittlement, but that would require a breach through the tendon sheath AND a breach through the slab waterproofing too.

Thanks for your input.

 

[BenMacier]

Were these strands under any possible cyclic loading? So they may have failed under fatigue?
If there is no sign of water ingress, how can corrosion have occurred?

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[EdStainless]

I have seen the long breaks in spring before. Usually the combination of prior surface damage and HE (from localized corrosion).
That you had multiple failures so close together makes me suspect that the damage happened after the cable was manufactured.
Such as in handling or installation.
You need some lab work on this.

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P.E. Metallurgy, consulting work welcomed

 

[Ingenuity]

Thanks Ben and Ed for the replies.

There is no fatigue of significance - one of the benefits of prestressing steel in prestressed concrete - under cyclic applied loads to the slabs does not change the tensile force in the strands by any magnitude.

There are however, changes in slope/angle of the tendons along their length - they are placed in parabolic profiles along the slab spans - although the angle changes are very small in this case.

There had to have been a local breach in the concrete and protective sheath to the strand for corrosion to have taken place - we just have to find it.

We shall get the failed segments to a lab and get some microscopic/metallurgical analysis done.

Thank you.