The use of welded wire mesh in residential slabs has been on
the decline for the last ten years as more contractors switch
to synthetic-fiber-reinforced concrete. Synthetic fibers
— which are often called Fibermesh, a well-known brand
— have been marketed as a substitute for wire mesh in
Most contractors are eager to avoid the hassle of wrestling
with wire mesh, and prefer the easier option of ordering
synthetic fibers from their ready-mix supplier. Jon Hanson is
the vice president and estimator at Walker Construction, a
concrete contractor in Stowe, Vt. "We used to buy bundles and
bundles of rolls of wire mesh, but we don't do that anymore,"
says Hanson. "Now we use Fibermesh in just about all our slabs,
unless it is a structural slab with a rebar mat."
Wire Mesh versus Fibers
Although often seen as a substitute for wire mesh, synthetic
fibers, at least at manufacturers' recommended dosages, don't
perform the same function. Properly placed wire mesh helps hold
together a hardened slab, even if the soil settles slightly.
Unlike most synthetic fibers, wire mesh also controls drying
shrinkage cracking — the type of cracking which can occur
during a period of several weeks after the pour.
At typical dosages, the benefits of synthetic fibers occur
only during the first few hours after the concrete is placed;
after that time, they provide no real benefits. Many studies
have verified that synthetic fibers, unlike wire mesh, reduce
plastic shrinkage cracking. Plastic shrinkage cracking occurs
as the concrete makes the transition from liquid to a hardened
slab. "Synthetic fibers help with avoiding plastic shrinkage
cracks, but they don't do much after the concrete hardens, in
terms of providing a lot of strength for the slab later on,"
admits Hal Payne, marketing services manager at Synthetic
Industries, the manufacturer of Fibermesh.
These bundles of fibrillated
polypropylene fibers open up as the concrete is mixed,
forming a more tenacious bond with the cement paste
than monofilament fibers.
Plastic shrinkage cracking occurs only in weather conditions
that encourage premature drying of the concrete surface.
Although it is definitely a concern on a hot, dry, windy day,
there is probably no danger of plastic shrinkage cracking when
concrete is poured in cool, cloudy, windless conditions.
The reason that synthetic fibers have been successfully
marketed as a substitute for wire mesh is that most
nonstructural residential concrete slabs don't need much
reinforcement. Barring unusual soil conditions, if a
residential slab is being placed on a well-compacted subbase,
wire mesh — or, for that matter, synthetic fibers —
can be safely omitted. Depending on whether you want to control
either plastic shrinkage cracking or drying shrinkage cracking,
either synthetic fibers or wire mesh may be used. And if a
residential slab actually needs reinforcement — either
because it is partially unsupported or because it is
load-bearing — then what you need is rebar, not wire mesh
or synthetic fibers.
Hairy slabs? Some concrete
contractors worry that fiber-reinforced concrete will produce a
difficult-to-finish, hairy slab. "When synthetic fibers first
came on to the market, we had these real long, real thick
fibers," says Payne. "Finishers didn't like them, because it
was hard to get the fibers mixed in and the concrete was hard
to finish. Since then the whole industry has changed. We now
use a mix of fiber lengths and diameters, and the product gets
mixed in easier."
Hanson, who has installed a lot of fiber-reinforced concrete,
hasn't had any finishing problems. "The power trowels go right
over it, and lay the fibers down," says Hanson. "Many of the
fibers will pop up later, but ultraviolet light burns them off,
as long as they are exposed to daylight. After a year you don't
see them anymore."
Polypropylene is the most commonly used synthetic fiber, but
other types of fiber, including nylon and polyolefin, are also
available. Polypropylene fibers come in two different formats:
fibrillated and monofilament. Fibrillated fibers have been
deformed and shredded to form a net-like mesh that engages more
tightly with cement paste than monofilament fibers. The main
disadvantage with fibrillated fibers is their tendency to
protrude from a slab.
Contractors who don't want to wait for protruding fibers to
wear off can burn them off with a torch, or specify
monofilament polypropylene. Although monofilament fibers are
less tenacious, they are less likely to stick up than
fibrillated polypropylene. Another alternative is to specify
nylon fibers, which also lay down better than fibrillated
Permeability. Although some
synthetic fiber manufacturers have claimed that fibers make
concrete less permeable, there is no evidence that this is
true, at least at common dosage levels. "A few years ago, we
claimed, along with other companies, that the use of synthetic
fibers reduced the permeability of concrete," says Fibermesh's
Payne. "We have changed that claim. Fibers really don't affect
Neal Berke, principal scientist at Grace Construction
Products, a manufacturer of polypropylene fibers, agrees. "In
good quality concrete, fibers have no effect on permeability,"
he says. "To reduce the permeability, you'd be better off
adding a water reducer and lowering the water-to-cement
What's the best dosage? The
recommended dosages provided by synthetic fiber manufacturers
are not necessarily the optimal dosages for fiber concrete.
Most ready-mix suppliers charge an extra $7 or $8 per cubic
yard to add 1 1/2 pounds of polypropylene fibers to their
concrete, amounting to an upcharge of about 10%. This dosage
level has been chosen by the fiber manufacturers to make
synthetic fiber's price competitive with wire mesh.
"Fiber concrete is a wonderful technology with many good uses,
at the right dosage," says Parviz Soroushian, professor of
civil and environmental engineering at Michigan State
University. "But the manufacturers' recommended dosages are not
really sufficient to improve the hardened concrete properties.
At 0.1% by volume, which is equivalent to the typical
manufacturers' recommendations of 1 1/2 pounds per cubic yard,
the benefits are marginal in hardened concrete. You need about
3 pounds of fiber per cubic yard. Once you double the dosage,
you have much to gain — improvements in impact
resistance, toughness, and control of drying shrinkage
Bob Cruso, president of Nycon, a manufacturer of synthetic
fibers, doesn't dispute Soroushian's points. "We would love to
be able to promote higher dosage levels, and we do for some
applications," says Cruso. "But we have to look at what we can
offer at a price that is somewhat equal to welded wire mesh,
because that's what we are competing against. That's
unfortunate, but that's the way it is."
Contractors interested in achieving the best possible
performance from synthetic fibers should consider the option of
doubling the normal fiber dosage recommended by the fiber
manufacturers. Before adjusting the dosage, though, be sure to
consult with your ready-mix supplier and the fiber
manufacturer's technical experts, since higher fiber dosages
may require less aggregate and more cement paste in the
In addition to Fibermesh (Synthetic Industries; 800/635-2308;
www.fiber-mesh.com), several other
manufacturers make polypropylene fibers, including Columbian
Fibersource (800/821-4391; www.fibersource.org), Forta (800/245-0306;
www.fortacorp.com), and Grace Construction
www.graceconstruction.com). Nylon fibers
are available from Forta and Nycon (800/456-9266;
www.nycon.com), while polyolefin fibers are
available from 3M (888/364-3577;