Post-Tensioning Tendons

[ajk1]

Is it possible to get post tensioning 7-wire strands in stainless steel?

 

[Ron]

Only if you have a boatload of money.

 

[Ingenuity]

Georgia Tech and Sumiden Wire did some research on 7-wire stainless steel alloy strand back in 2009. Not sure if it made it to a commercial setting.

Link

And the research was based on pretensioned piles and so did not include any post-tensioning anchorages etc.

 

[ajk1]

To Ingenuity: Thanks for the link. Very interesting. Seems though like it is not readily available in stainless steel, it at all.

To Ron: Ye, right. All solutions are a lot of money, so that is not the issue in this case.

Thanks all.

 

[BridgeSmith]

Where a high level of corrosion resistance is required, typically "fully encapsulated" strands are specified. They are more expensive than bare strands or regular encapsulated strands, but not nearly as expensive as stainless steel strand, if you can even find it. Stainless steel threadbars are available for post-tensioning, if you're set on using stainless.

 

[Ingenuity]

HotRod10, based upon ajk1's numerous previous posts on related PT topics, I think he/she is referring to removal and replacement of existing corroded carbon-based strand with new strand, so the choice of encapsulation systems is not an option for an existing structure.

 

[Ingenuity]

ajk1:

I do notice that on Sumiden Wire's website that they are "advertising" stainless steel 7-wire strand. Made in the USA, too: Link

Some mechanical properties:

 

[BridgeSmith]

Thanks for the additional info, Ingenuity. Yes, if this is a rehabilitation project, the options become very limited, probably down to stainless steel, carbon fiber, or using standard steel and plan on replacing them again in a number of years.

I don't know how the costs would compare between the much more expensive material for longer-lasting solution and the higher labor costs of replacing the strands more often. That assumes that there's not changes being made this time around that precludes subsequent replacement. The useful life of the structure plays a role in that life-cycle cost analysis, as well. Also, if the strands weren't properly coated with a corrosion-inhibiting grease last time, using standard steel strands that are properly greased would increase the longevity significantly this time around.

 

[ajk1]

Hotrod has the right idea, namely it is rehab. Cost analysis is not a factor. Client just wants to do something that he/she does not have to worry about for the next 30 years, no matter what the cost. Period.

Has anyone used carbon fibre strand, and what are the fire protection requirements?

Is say 2.5" concrete cover at low point of tendon sufficient for 2 hour rating? If not, what fire rating would it have? Where can I find this info for carbon fibre strand?

 

[BridgeSmith]

Stainless steel isn't indestructible, either. Carbon fiber is unaffected by nearly all corrosive chemical agents, but I'm sure there are some. What caused the deterioration of the existing strands? Were they protected at all? How long have they been in place?

 

[ajk1]

To Ingenuity - thanks for the link. I will check it out.

 

[rapt]

If that stress/strain diagram for stainless steel is correct, you cannot use normal prestressing design rules to design it. It is basically an elastic material. There is no plastic region on the curve so very low ductility.

It's lack of ductility would mean it would have to be treated more like a carbon fibre design with a much lower capacity reduction factor (higher material factor).

 

[Ingenuity]

If that stress/strain diagram for stainless steel is correct...

It is, and the Masters thesis by Schuetz (Georgia Tech, 2013) has similar results: Link

I agree, with regards to lack of ductility, and similarity to the near-perfect elastic properties of FRP.

To date, all research on HSSS has been on precast pretensioned piles.

The thesis by Schuetz states the following with respect to strand rupture:

 

[ajk1]

To rapt and Ingenuity:

Very interesting and useful information. Sort of indicates I should not use it in this application.

Have you heard anything to the effect that the end anchors, used with unbonded carbon fibre tendons in beams (as replacement tendons for the corroding/breaking tendons) is still under development?

 

[Ingenuity]

Have you heard anything to the effect that the end anchors, used with unbonded carbon fibre tendons in beams (as replacement tendons for the corroding/breaking tendons) is still under development?

My understanding of CFRP strand tendons is that they are still very much still in research mode with some limited real-world projects undertaken, albeit on a very small scale by way of sort of validating the technology and use.

The issue - besides the near-perfect elastic mechanical properties (ie brittle!) - are the gripping and anchorage of the CFRP strands. Several manufacturers have proprietary/patented gripping systems that do not lend themselves to direct strand-for-strand replacement with existing monostrand anxhorages.

Prof Mark Green (Queens University) did a demonstration project to a Toronto garage using CFRP tendons. I will dig out info I have on this and post it.

 

[rapt]

Akt1,

Not saying you cannot use it.

Simplified methods cannot be used, Strains must be calculated and limited to a maximum of about .6 of the peak strain or less depending on the code and capacity factors appropriate to the use of elastic reinforcement must be used. PreStress to 70-80% would be a nono. The limit would be much less.

Design it by FRP rules, not normal PT rules if your country has any. Canada has an FRP code which covers elastic reinforcement! USA has recommendations in an ACI committee report.

 

[Ingenuity]

ajk1:

Here is a link to a thread from April this year where FRP tendons are discussed: thread507-437719

The discussions here are NOT FRP strands, but solid pultruded FRP bar - and you will notice that the splices and gripping devices are cumbersome to say the least - where it discusses the Toronto parking garage repairs as part of a study by Prof Mark Green and others.

 

[Ingenuity]

Here is a link to a paper by Green that may be of relevance to you: Link

 

[ajk1]

Ingenuity: Thanks. I will have a look.

 

[Prestressed Guy]

Could you use epoxy coated strand? I had a recent PC project that was all 7-wire epoxy with grit for prestressing but you can also get it without grit for unbonded post tension.