Civil Engineering Reference
In-Depth Information
Synthetic Fibers
Synthetic fibers are man-made fibers resulting from
research and development in the petrochemical and textile
industries. Fiber types that are used in portland cement
concrete are: acrylic, aramid, carbon, nylon, polyester,
polyethylene, and polypropylene. Table 7-1 summarizes
the range of physical properties of these fibers.
Synthetic fibers can reduce plastic shrinkage and sub-
sidence cracking and may help concrete after it is frac-
tured. Ultra-thin whitetopping often uses synthetic fibers
for potential containment properties to delay pothole
development. Problems associated with synthetic fibers
include: (1) low fiber-to-matrix bonding; (2) inconclusive
performance testing for low fiber-volume usage with
polypropylene, polyethylene, polyester and nylon; (3) a
low modulus of elasticity for polypropylene and polyeth-
ylene; and (4) the high cost of carbon and aramid fibers.
Polypropylene fibers (Fig. 7-6), the most popular of
the synthetics, are chemically inert, hydrophobic, and
lightweight. They are produced as continuous cylindrical
monofilaments that can be chopped to specified lengths or
cut as films and tapes and formed into fine fibrils of rec-
tangular cross section (Fig. 7-7).
Used at a rate of at least 0.1 percent by volume of con-
crete, polypropylene fibers reduce plastic shrinkage
cracking and subsidence cracking over steel reinforcement
(
Suprenant and Malisch 1999
). The presence of polypropy-
lene fibers in concrete may reduce settlement of aggregate
particles, thus reducing capillary bleed channels.
Polypropylene fibers can help reduce spalling of high-
strength, low-permeability concrete exposed to fire in a
moist condition.
New developments show that monofilament fibers
are able to fibrillate during mixing if produced with both,
polypropylene and polyethylene resins. The two poly-
Fig. 7-7. Polypropylene fibers are produced either as (left)
fine fibrils with rectangular cross section or (right) cylin-
drical monofilament. (69993)
mers are incompatible and tend to separate when manip-
ulated. Therefore, during the mixing process each fiber
turns into a unit with several fibrils at its end. The fibrils
provide better mechanical bonding than conventional
monofilaments. The high number of fine fibrils also
reduces plastic shrinkage cracking and may increase the
ductility and toughness of the concrete (
Trottier and
Mahoney 2001
).
Acrylic fibers have been found to be the most prom-
ising replacement for asbestos fibers. They are used in
cement board and roof-shingle production, where fiber
volumes of up to 3% can produce a composite with
mechanical properties similar to that of an asbestos-
cement composite. Acrylic-fiber concrete composites
exhibit high postcracking toughness and ductility.
Although lower than that of asbestos-cement composites,
acrylic-fiber-reinforced concrete's flexural strength is
ample for many building applications.
Aramid fibers have high tensile strength and a high
tensile modulus. Aramid fibers are two and a half times as
strong as E-glass fibers and five times as strong as steel
fibers. A comparison of mechanical properties of different
aramid fibers is provided in
PCA (1991)
. In addition to
excellent strength characteristics, aramid fibers also have
excellent strength retention up to 160°C (320°F), dimen-
sional stability up to 200°C (392°F), static and dynamic
fatigue resistance, and creep resistance. Aramid strand is
available in a wide range of diameters.
Carbon fibers were developed primarily for their high
strength and elastic modulus and stiffness properties for
applications within the aerospace industry. Compared
with most other synthetic fibers, the manufacture of
carbon fibers is expensive and this has limited commercial
development. Carbon fibers have high tensile strength
and modulus of elasticity (Table 7-1). They are also inert to
Fig. 7-6. Polypropylene fibers. (69796)
