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[SEAOC] [SEAOC] Precast Prestressed Concrete in a Marine Environment

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I do not see how the marine environment would be much worse than other P/C P/T uses. 
The box and I-beam type of P/C construction is used for many bridges over coastal
water ways.  The normal design for post tensioned structures keeps the beam
under compression in the unloaded case, which would be typical of the RO-RO dock.
(For those who have not seen this term - "roll on/ roll off" of heavy cargo from ships)
The compressive stress would limit infiltration of salts.  The RO-RO service would be 
similar to highway bridges with occasional heavy rolling loads.  You might make the
argument that RO-RO is done so slow that it is milder service than trucks at 80 mph.

Take these comments with a grain of salt (sorry) I am not a bridge designer.

Ed Haninger  
---------- Forwarded Message ----------
From:	Bill Polhemus, INTERNET:poly(--nospam--at)flash.net

We are looking at building a RO-RO wharf using Precast prestressed box 
beams of the type you see in bridge design.

The concern has been raised that this would not be an appropriate 
application of this type construction, due to the corrosive nature of said
environment.

Primarily, the concerns are these:

1) Any cracking would expose prestressing strands to the harshly corrosive
environment.

2) Prestressing strands are "small" and thus would represent more of a 
potential for catastrophic failure should corrosion be present.

3) Wharves of this type take a "pounding" and this is not the kind of 
loading situation that prestressed construction handles well.

I would like some other input.  If you could, please address the concerns 
above, and also answer these questions for me:

1) Have you any experience with the use of precast prestressed concrete in
a marine (or at least, estuarine) environment?

2) Do you have any feel for the life expectancy of such structures?

3) Are there any measures you would recommend, such as materials or 
additives to the concrete, to counter the anticipated "harsh marine 
environment?"

Thanks in advance for any and all input.

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