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Re: Concrete Fibers

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I did not read the article either but I will note it is referring to plastic shrinkage cracking, which are short, shallow cracks that are typically not a problem unless the floor has stringent aesthetic requirements.   Using an evaporation retarder or fogging the concrete surface will also prevent plastic surface cracking.
 
Plastic shrinkage cracking has nothing to drying shrinkage cracking.
 
I personally find it rather annoying that NSF keeps wasting my tax money on worthless research like this.
 
Gail Kelley
 
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In a message dated 2/14/2005 8:19:17 AM Eastern Standard Time, jgetaz(--nospam--at)shockeyprecast.com writes:
    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