.5" diam post-tension strands 1

[v2]

does anyone knows how long does the grease in the strand sheathing remain in good condition in providing continued protection to corrosion in a residential building with post-tentioned slab construction that was build in the mid 70's ?
thank you V2

 

[Ingenuity]

v2,

That all depends. Depends on the sheath, its thickness, the exposure environment, service maintenance, water ingress, construction techniques...and others.

In the mid-70's was when extruded sheaths was introduced into the unbonded PT industry - actually it was patented by Lang in 1972 . Prior to extruded sheaths, you had kraft-paper, heat-sealed and "stuffed" sheath systems for strand. Both grease and wax compounds were used, and neither was to any particular quality standard.

I have seen strand tendons with no grease and in fine corrosion-free condition due to the lack of presence of water on the concrete structure and within the tendons.

But I have also seen tendons with grease voids, punctured sheaths, corroded end-anchorage pockets, zero concrete cover, with substantial corrosion.

You cannot answer you question with any practical significance. If you are in doubt of the corrosion protection, or the conditon of the tendons, do a Condition Survey of the strcuture. Check for water ingress through cracks, CJ's etc. Be aware that corrosion damage to unbonded PT structures often is not visible from external observations - ie won't see spalls as you will for rebar corrosoion, probably will not see failed tendons via loops or strands shooting from slab edges. If you suspect corrosion damage and it is not visible, I advise some selective invasive probing - chip and remove concrete cover and expose the tendons and selected locations. Bit like finding a needle in a hay stack, but is a start. Check out live end grout pockets too. You may see signs of grease stains on the slab soffits too co-inciding with crack locations.

There are some good ACI documents on this subject that you may wish to check out. Let me now if you have access to ACI document and I will list them for you.

Current monostrand tendons with extruded sheaths and greased strands are manufactured to significantly better quality than older systems.

HTH

 

[v2]

ingenuity,

thank you, you have been very helpful.
the structure i am investigating does have somes corroded cables mostly as you said due to poor sheathing protection quality.

however, I am still wondering if grease as a substanse does not degrade with time assuming if it is sealed and protected in its sheathing.
in other words, does the physical properties of grease (its capability to protect against corrosion) change or degrade with time if it was ketp protected from an open environment?
thank you again. V2

 

[Ingenuity]

v2,

If you mean grease used in the manufacture of unbonded tendons to current ACI and PTI specification, then probably there will be no degradation.

For older systems, that will depend on what actual PT coating materials were used.

Prior to 1985 when the PTI issued its first specification for unbonded single-strand tendons, the PT coating had no formal performance specifications to be judged/tested.

In older systems I have seen grease emulsify in the presence of water, and have seen others with a hardened, grease-like compound, with a thickened mass. The hardening was observed without the presence of water, so was the result of breakdown under a protected environment. But at these locations where hardening occurred is was providing corrosion protection - or at least there was no evidence of corrosion at that tendon test site, which are too different things.

Be aware that some older system did not use grease but wax
and asphaltic materials were sometimes used as the coating material.

Today, mineral oil-based grease is used almost exclusively in current systems. Certain petroleum derivatives react with polyethylene and polypropylene (common sheath materials) changing its physical properties to the point where they no longer a sheath.

The current ACI and PTI specifications require testing to ensure the stability of the grease at high temperatures; this is to prevent the grease from softening and flowing out of the sheathing during fabrication and installation. In addition, the flash point of the grease is required to be above 300 °F (149 °C). Too low a flash point indicates the oil is prone to evaporation; this may affect the long-term stability and consistency of the grease.

Not sure if I have answered your question very well.

HTH