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Re: Fibrous Concrete

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We have used fiber reinforcing for slabs on grade as secondary reinforcing.  The reinforcing is only for temperature and shrinkage and not for primary reinforcing.
Admittedly, we did accept the use of the fibers approximately 5 years ago (due to contractor's request) and there were some problems.  Some cracks did develop and the contractor had to torch the top of the slab to remove the fibers that protruded above the concrete.  It turned out that the cracks developed due to the contractor not providing sawed control joints in a timely manner (as late as 1 month or more).
Within the last two years we were faced with making the decision to allow fiber reinforcing at the contractor's request.  Due to our past experience we rejected the request.  However, the fiber manufacturer was persistent and we set up a time to discuss his material.  It turns out that fiber reinforcing has come a long way.
We approved the use of fiber reinforcing from a company called Fibermesh.  Fibermesh uses fibrillated  fibers which helps with the finish work and increases the bond with the concrete.  Anyways, the fibers were used in a 12", 8", 6", and 4" slabs.  No cracks have been detected (the pours occurred one year ago).  Please keep in mind that the concrete contractor knew what he was doing and control joints were placed as specified.
Overall, if the correct fiber is specified and the contractor has experience with fiber reinforced concrete, then there should be no problems.
Remember how welded wire fabric is installed?  At least the fibers are through out the slab.  The concrete contractor prefers to use the fibers due to the decreased amount of labor.  However, due to the increased material cost, the overall cost to install a slab is the same as conventional methods.
Mike Brown, P.E.