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RE: Externally Bonded FRP Systems for Strengthening Concrete Structures

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-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com] 
Sent: Friday, June 03, 2005 9:28 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Externally Bonded FRP Systems for Strengthening Concrete
Structures

I will offer one caution regarding designing for FRP strengthening of
existing structures: ACI 440.2R and many manufacturer's recommend that
the existing structure be able to resist a minimum level of applied
loads without the FRP stiffening in place.  This is just in case the FRP
surface repair debonds.  I had a case where existing rebar had corroded
severely and could not be counted on for any appreciable strength.  I
was told that FRP stiffening to replace the missing reinforcing area was
not recommended in that case.  

-----/Original Message-----

You're right, that IS a very good point. In fact, I'd go so far as to
say that, in my experience, the [C][G][A]FRP strengthening approach
appears to be far more efficacious when the problem is SHEAR rather than
flexure.

The reason why is that flexural members typically have more "shear
reserve" capacity than "moment reserve." For example, a concrete member
such as a joist or P/S Double Tee has a good deal of "unused" shear
capacity, even when no shear reinforcing is present, that can be counted
upon for "residual strength." In the case where the member now has to be
analyzed for a significant concentrated load, you'll typically see that
even though the FRP strengthening system will add only a fraction of
capacity to the overall, it's enough to do the job.

At the current state of the technology, you can typically count on FRP
strengthening to give you not more than 15-20% increase over the
allowable moment strength, and that is often not enough to get you over
the hump, especially for long-span prestressed members. I think a lot of
research is being done at UM-R to try to improve the confidence in the
adhesive performance so that "debonding" and "delamination" are not as
likely.


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