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RE: cored concrete beams

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The biggest problem that I have encountered with the use of carbon fiber and
fiberglass reinforcing materials is the ability to adequately fireproof
them.  The intumescent material provided by the manufacturer is not rated
for overhead use.  There are UL rated assemblies but these are not always
viable due to size constraints.  You should be aware that the epoxies used
are incredibly toxic once they start burning or vaporizing.  Since the fire
department is not likely to be aware of the presence of these toxic
materials, you should take every precaution to ensure adequate fire
resistance.  There is also a rapid loss of strength associated with elevated
temperatures. 

Since there is no bottom reinforcing left I would use a welded mild
reinforcing solution to repair the beam with lots of special inspection etc.

Good luck

Nick Blackburn, PE