According to major supplies, “Use of synthetic fibers for reinforcing concrete
is continuously, increasing. The increase has been considerable since 1980, but
slowed somewhat in 1990, a year of substantial construction cutbacks. Apparently
the construction community believes there are advantages in the use of synthetic
fibers in concrete.” (Schupack) Synthetic fibers are used to improve crack
control in concrete. Some reports say that synthetic fiber reinforced concrete (SFRC)
would replace welded wire fabric in many slab-on-grade applications. But in
reality if the welded wire fabric is placed properly it controls crack width
better than the synthetic fiber reinforced concrete. In a case study of the use
of synthetic fibers in reinforced concrete, the following conclusions were
obtained. No matter what concrete placing job is being done, there is no
substitute for good concrete practices. The use of a low fiber volume mix will
help control plastic shrinkage cracks and bleeding, but not give good cracking
control once the crack forms. The synthetic fibers running through a crack have
a poor bond providing no shear friction. Impact and toughness tests on synthetic
fiber reinforced concrete imply less edge spalling will occur. The American
Concrete Institute (ACI) conducted two studies on polypropylene fiber reinforced
concrete, one dealing with plastic shrinkage cracking and the other on
permeability characteristics. Plastic shrinkage cracking occurs when the surface
water on the concrete evaporates faster than the bleed water reaches the surface
of the concrete. It was determined by the plastic shrinking cracking study that
polypropylene fibers helped reduce the total plastic shrinkage crack area on
test panels. Also determined is that the screeding rate affects the total crack
area in polypropylene reinforced concrete, while finishing operations showed no
significant effects. This study also suggests the use of longer fibers (about
0.75in.) will produce less crack area. “Permeability plays an important role
in long-term durability of concrete materials. Permeability of concrete
generally refers to the rate at which particular aggressive substances (water,
sulfates, chloride ions, etc.) can flow through the concrete.” (Soroushian) As
discussed in the plastic shrinkage study that polypropylene fibers reduce
cracking. Less cracking in the concrete surface that surface would be less
permeable. In the permeability study, they concentrated on the effects of
chloride and the permeability of the concrete. The results of this study
concluded polypropylene fibers had little effects on chloride permeability of
concrete. The polypropylene fibers only help reduce plastic shrinkage cracks. In
residential construction, polypropylene fibers have been in use since the
mid-to-late 1970s. Polypropylene fibers are used for their high tensile strength
and low cost. These fibers fit into two categories, for early plastic shrinkage
and for improved long-term crack control. One of the first residential
applications of polypropylene fibers was in western Pennsylvania, in March of
1979. Fibers were used in all the flat work of the residence. Polypropylene
fibers have gained acceptance by residential home builders due to its
flexibility and its ability to conform to forms. Home builders say polypropylene
fibers help them sell more concrete by reducing the potential risks of cracking.
The use of polypropylene does not compensate for the lack of good design,
water/cement ratios, temperature and wind conditions. One important thing to
remember is nothing replaces good concrete practices.
Bibliography
Schupack, Morris, and William R. Stanley. “Seven Case Studies of Synthetic
Fiber Reinforced Slabs.” Concrete International Feb. 1992:50-56. Soroushian,
Parviz., Faiz Mirza, and Abdulraman Alhozaimy. “Permeability Characteristics
of Polypropylene Fiber Reinforced Concrete.” ACI Materials Journal
92(1995):291-295. Mirza, Faiz., Parviz Soroushian, and Abdulraman Alhozaimy.
“Plastic Shrinkage Cracking of Polypropylene Fiber Reinforced Concrete.” ACI
Materials Journal 92(1995):553-560. Biddle, Daniel T. “Fiber Reinforcement in
Residential Concrete.” Concrete International Feb. 1991:46-47.