Civil Engineering Reference
In-Depth Information
the equivalent strength of thicker conventionally rein-
forced concrete sections.
Steel fibers do not affect free shrinkage. Steel fibers
delay the fracture of restrained concrete during shrinkage
and they improve stress relaxation by creep mechanisms
(
Altoubat and Lange 2001
).
The durability of steel-fiber concrete is contingent on
the same factors as conventional concrete. Freeze-thaw
durability is not diminished by the addition of steel fibers
provided the mix is adjusted to accommodate the fibers,
the concrete is properly consolidated during placement,
and is air-entrained. With properly designed and placed
concrete, little or no corrosion of the fibers occurs. Any
surface corrosion of fibers is cosmetic as opposed to a
structural condition.
Steel fibers have a relatively high modulus of elas-
ticity (Table 7-1). Their bond to the cement matrix can be
enhanced by mechanical anchorage or surface roughness
and they are protected from corrosion by the alkaline
environment in the cement matrix (
ACI 544.1R-96
).
Steel fibers are most commonly used in airport pave-
ments and runway/taxi overlays. They are also used in
bridge decks (Fig. 7-3), industrial floors, and highway
pavements. Structures exposed to high-velocity water
flow have been shown to last about three times longer
than conventional concrete alternatives. Steel fiber con-
crete is also used for many precast concrete applications
that make use of the improved impact resistance or tough-
ness imparted by the fibers. In utility boxes and septic
tanks, steel fibers replace conventional reinforcement.
Steel fibers are also widely used with shotcrete in
thin-layer applications, especially rock-slope stabilization
and tunnel linings. Silica fume and accelerators have
enabled shotcrete to be placed in thicker layers. Silica
fume also reduces the permeability of the shotcrete mate-
rial (
Morgan 1987
). Steel-fiber shotcrete has been success-
fully applied with fiber volumes up to 2%.
Slurry-infiltrated concrete (SIFCON) with fiber vol-
umes up to 20% has been used since the late 1970s. Slurry-
TYPES AND PROPERTIES OF FIBERS
AND THEIR EFFECT ON CONCRETE
Steel Fibers
Steel fibers are short, discrete lengths of steel with an
aspect ratio (ratio of length to diameter) from about 20 to
100, and with any of several cross sections. Some steel
fibers have hooked ends to improve resistance to pullout
from a cement-based matrix (Fig. 7-2).
ASTM A 820 classifies four different types based on
their manufacture. Type I - Cold-drawn wire fibers are the
most commercially available, manufactured from drawn
steel wire. Type II - Cut sheet fibers are manufactured as
the name implies: steel fibers are laterally sheared off steel
sheets. Type III - Melt-extracted fibers are manufactured
with a relatively complicated technique where a rotating
wheel is used to lift liquid metal from a molten metal sur-
face by capillary action. The extracted molten metal is then
rapidly frozen into fibers and thrown off the wheel by cen-
trifugal force. The resulting fibers have a crescent-shaped
cross section. Type IV - Other fibers. For tolerances for
length, diameter, and aspect ratio, as well as minimum ten-
sile strength and bending requirement, see ASTM A 820.
Steel-fiber volumes used in concrete typically range
from 0.25% to 2%. Volumes of more than 2% generally
reduce workability and fiber dispersion and require spe-
cial mix design or concrete placement techniques.
The compressive strength of concrete is only slightly
affected by the presence of fibers. The addition of 1.5% by
volume of steel fibers can increase the direct tensile
strength by up to 40% and the flexural strength up to 150%.
Fig. 7-2. Steel fibers with hooked ends are collated into
bundles to facilitate handling and mixing. During mixing
the bundles separate into individual fibers. (69992)
Fig. 7-3. Bridge deck with steel fibers. (70007)
