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Re: Concrete Fibers

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Jim,

I had a bad experience with fiber reinforcing several years ago. As a result I would only recommend using it in addition to the normally required steel reinforcing.

       My experience was the following.

About 15 years ago (+-) I designed a church which included a basement for class rooms. The church wanted to use a patented system for the suspended floor system which I agreed to use against my better judgment. This patented system consisted of: 1.) Cold formed steel C channel shapes, about 20 gage by 10 inches deep at about two feet on centre, or, perhaps a little less, covered by 2.) A nearly flat steel sheet of similar, or slightly less thickness, secured to the channels by 3.) Shear connectors which resembled cheap cigarette ash trays about 0.75 inch deep and 2.5 inches in diameter using two self tapping screws. 4.) The whole thing was covered with about 2 inches of concrete and was expected to act as a simple span composite slab. No reinforcing other than the fibers was used.

I was provided with a fairly large test report prepared by a respected local engineering company, otherwise I would not have accepted such a system.

The concrete was placed on the hottest day in the year (perhaps the hottest day in a few years) without using proper curing techniques (by volunteer labor, of course). When I saw the results next day I was horrified!! There was hardly a place to put your foot which was not an "island" surrounded by shrinkage cracks!

After much discussion (involving a few un churchly words) it was decided to not replace the concrete since it would be covered by carpet and the concrete was all in compression so the cracks shouldn't matter. To date there have been no reported problems.

Should the system be used again? To quote a famous American, I would have to say "Not on my watch!"

Regards,

H. Daryl Richardson


----- Original Message ----- From: "Jim Getaz" <jgetaz(--nospam--at)shockeyprecast.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, February 14, 2005 6:16 AM
Subject: Re: Concrete Fibers


Gail Kelley discussed various fiber types yesterday. Also yesterday,
I skimmed, though did not read, the last article in the Jan-Feb 2005 ACI
Materials Journal, "Influence of Different Fibers on Plastic Shrinkage
Cracking of Concrete." This was done at the University of Michigan and
funded by NSF. They poured 1 meter long strips of concrete on a surface that
looks like a square wave, obviously to try to make sure there would be
cracking if any shrinkage occurred. They used varying volume fractions of
polypropylene (both monofilament and fibrillated), high-density
polyethylene, flexible metallic, carbon and three kinds of polyvinyl acetate
fibers. They conclude that small fiber diameter (<40 microns), long aspect
ratio ( ~>200) and volume fractions => 0.2 -> 0.4%, any of these fibers
"should, for all practical purposes, eliminate shrinkage cracking in
concrete."
There has been discussion on this list of fibers successfully
keeping cracks from opening for some distance, then there is a large crack
that is essentially the sum of all the small cracks that might otherwise
occurred. Does anyone want to hazard a guess what would happen if these
researchers placed concrete many meters long with these fibers or does
anyone have any experience with such fiber loadings in a slab?
Thank you.
Jim Getaz
Winchester, Virginia

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