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Re: Concrete Fibers

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In a message dated 2/9/2005 10:02:30 PM Eastern Standard Time, CanitzCF(--nospam--at) writes:
When placing/finishing concrete slabs that have concrete fibers distributed throughout, what's the technique to insure that the fibers do not reflect through the top surface? Earlier today, I saw a sidewalk panel where the top surface was "hairy" due to the amount of visible fibers.
If you are asking because of  a project,  rather just curiosity,  you might want to consider nylon fibers rather polypropylene.  Polypropylene is lighter than water (its specific gravity is something like .91), so fibers tend to float to the top surface.  Nylon is slightly heavier than water (specific gravity 1.16)  so they don't float (as much).  In addition,  they are hydrophillic (they absorb water),  which according to the people that sell nylon fibers means they get coated with mortar,  so you don't see them on the surface.
They are equally useless in shrinkage crack control,  and equally useful for plastic shrinkage cracking,  so I'm not sure it matters which one you go with.
A few companies are now peddling PVA (polyvinyl acetate) fibers.  They are also non-floaters.
As a side note - I heard an interesting presentation at TRB by Prof. Abdeldjelil Belarbi of the University of Missouri-Rolla.  The presentation was titled
"Bond Splitting Behavior of FRP Reinforcing Bars Embedded in Fiber Reinforced Concrete"
They did some pull-out testing of FRP bars in concrete reinforced with Fibermesh (polypropylene fibers.)  They found that that the fibermesh did all kinds of wonderful things for the bond strength.  During the question period following the presentation,  I noted that the results were somewhat odd,  since this was hardened concrete, and the elastic modulus of polypropylene is so low, it has no effect on hardened concrete.
His answer - "We did not investigate the properties of the fibers".
So I asked, "Did you find any other research that supported your findings?"
His answer - "We did not investigate any other research."
So I  asked,  "Do you think anyone who isn't being paid by Fibermesh would agree with your findings?"
His answer,  "Well actually, we wanted to use steel fibers,  but this research was to look at the feasability of a totally steel-free bridge."
Ah, the great state of Missouri,  and its top-notch engineering research.  The University of Missouri-Rolla has done quite a bit of fiber-reinforced concrete testing.  Send them your problems,  they'll get the answers you need. 
Gail Kelley