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RE: Post-tensioning tendon corrosion

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Rob,

Be careful on your assessment of corrosion in the PT strands.  High
strength, hard steels are prone to hydrogen embrittlement.  It is not as bad
as galvanized strand, but the potential is still there.  Strands that have
hydrogen embrittlement don't exhibit the same necking down and flaking type
of corrosion as observed in conventional steel.  They can look fine with
some minor pitting and just snap one day.  That is why strand will often
erupt in a corroded PT deck.  If the steel was conventional, the corrosion
would cause a necking down and reduction in section that would gradually
release the PT force as it yielded.  You need to consult with a metallurgist
familiar with hydrogen embrittlement.  

Check out: http://www.clihouston.com/hec.html

If you remove a couple of strands, make sure that you shore.  

The strands will not be easy to remove.  They can get bound up in the
sheaths.  I presume that the garage is in an area where no road salts are
used.  In areas with road salt use, the intermediate decks degrade faster
than the roof deck (no chloride wash off).  

Early 1980's construction indicates that you could probably have "button
headed" anchorages, which were particularly prone to corrosion damage.  Even
if the strand is good, your anchorages may not be good.

You probably have PT in 2 directions in the slab (slab strand parallel to
girders for shrinkage only).  Be careful in the corners, they can split
catastrophically.  It doesn't happen often, but it also was a problem in the
70's and early 80's.

Regards,
Harold Sprague


> -----Original Message-----
> From:	Rob Ward [SMTP:robward(--nospam--at)cwistructural.com]
> Sent:	Thursday, December 30, 1999 8:00 PM
> To:	'seaint(--nospam--at)seaint.org'
> Subject:	Post-tensioning tendon corrosion
> 
> We are evaluating the condition of a 4 story parking garage with deep 
> post-tensioned girders supporting one way post-tensioned slabs. 
> (Constructed in the early 80's.)
> 
> A number of girders have grease dripping from cracks in the bottom of the 
> girders at mid-span.  All of the girders with this condition are under the
> 
> roof parking deck.  At some girders where this condition occurs, the
> tendon 
> anchorages are protected from the weather, at others they are exposed.
> 
> Because we haven't experienced this condition before, we propose to 
> investigate the girders as follows:
> 
> Phase 1:  Open the bottoms of 2 girders where the most leakage and
> cracking 
> has occurred, look for moisture, extract grease samples for testing, and 
> visually inspect the tendons for corrosion.
> 
> Phase 2:  If corrosion is indicated, remove one or two tendons to allow
> for 
> inspection over their entire length.
> 
> Does anyone have experience with a post-tensioned structure in this 
> condition? Any insight would be appreciated, as well as comments about our
> 
> proposed investigation method.
> 
> TIA,
> 
> Rob Ward
> Copple & Ward, Inc.
> San Rafael, CA
>