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FRP Reinforcing

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Dave Handy wrote:
Has anyone used FRP reinforcement to strengthen existing structural slabs. I
am dealing with a slab with a significant deflection (3"+/-) and it is about
15 years old. I am concerned about the amount of jacking that could be done
prior to the installation of the FRP. I am thinking that we could only jack
the slab back up by the amount of the "immediate DL deflection" which could
be only about a 1/4 or 1/3 of the long term DL deflection. Any thoughts on
procedures? I am also not sure that this is the answer to the problem; it
was the suggested method. I seemed to remember reading that the stiffness
is not increased as much as the strength is. Is there any way to get the
slab back to level condition without the use of toppings?
 
 
I also have other concerns about the strength as the area where there is a
problem is the area where there are no drop panels. Has anyone on the list
ever added drops to an existing slab. I am mulling over what kind of
connection/joint there would have to be between the new and the old.
 
I write:
 
Dave,
I think you're right about the difficulty of obtaining large increases in stiffness with FRP.  Taking a step back, what is causing the large deflections you have?  Is the slab deteriorated?
 
And, is there a reason you can't jack the slab past level (at risk of some cracking), apply the FRP, let it settle to where it's going to settle to, and then apply a topping to level it and cover the cracks you just made?
 
One of the significant problems with FRP reinforcing, especially on the bottom of slabs, is fireproofing.  If the FRP installers start talking about fireproof paint, ask a lot of questions. 
 
On the shear strength, we have added capitals to existing columns.  The columns we had were round; we bush-hammered all around the top foot or so of the column to a depth of 1/4 inch or so (make sure this is more than just surface abrasion--break into the concrete a significant depth), put rebar hoops around the columns, and cast conical capitals to within about 3 inches of the underside of deck.  Then wait a week for cure and grout the 3 inch space.
 
The length of capital needed is determined by the shear force to be transferred (your judgement as to whether the capital takes all of it or the slab shares some) and the allowable stresses based on shear-friction.  Then the hoops are sized as if they were corbel reinforcing.  The hard part is to develop a hoop detail that can be installed, that gives you the strength you need.  I would suggest small rebar (#3 or #4) made up into spirals that the contractor then winds on the column, much like a key is put onto a keyring.
 
There is a reference article for what I describe, but I don't remember details (this was at a previous job).  Perhaps someone else remembers this, or it can be found by a search.
 
HTH,
Mike Hemstad
MBJ
Minneapolis, Minnesota