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RE: Cracks in an existing composite floor deck; by "Stephen L. Fisher" <slf(--nospam--at)idgcorp.net>

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Dear Stephen:

Frankly, the width of these cracks worry me a little. A 1/8-in. wide
crack would indicate that the reinforcing steel has yielded; while this
may not have a significant effect on the strength of the section, it
does reduce the stiffness considerably and the factor of safety against
failure.  As you have noted yourself, the floor will rotate and deflect
much more once the new heavier live load is applied.

I am also puzzled by the repair of cracks via epoxy injection into
cracks which in my opinion, has been often used as a "cure all".
Obviously, if the structural element is exposed to the elements, then
epoxy injection may be a good way to seal the cracks and
reduce/eliminate the possibility of corrosion of the reinforcement.
Clearly, that is not the case here. The other obvious concern is the
quality of the workmanship of the injecting contractors and making sure
that the resin has penetrated through the full depth of the cracks.  The
problem is that engineers often ignore to address the "cause" of the
cracking which in most cases is lack or insufficent reinforcing steel
area.  Then, by sealing the crack, we will most likely invite the
formation of new cracks a short distance away from the existing cracks. 

If you are relying on the contribution of the negative moment
reinforcement in the deck, I suggest that you take a closer look at the
problem and consider reinforcing the deck.  This can be done very
efficiently (and at not much more cost than that of epoxy injection) by
use of carbon plates that are only 0.05 inch in thickness and have a
tensile strength of over 300 ksi.  We just completed such a project
about two weeks ago for the concrete floors of St. Joseph Hospital in
Phoenix, where they were installing a new heavy medical equipment.  If
you wish, you may contact me and I can send you slides of that project.

Cheers,

Mo Ehsani, Ph.D., S.E.
Professor of Structural Engineering, Univ. of Arizona, and
President, QuakeWrap, Inc.



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