Civil Engineering Reference
In-Depth Information
most chemicals. Carbon fiber is typically produced in
strands that may contain up to 12,000 individual fila-
ments. The strands are commonly prespread prior to
incorporation in concrete to facilitate cement matrix pene-
tration and to maximize fiber effectiveness.
Nylon fibers exist in various types in the marketplace
for use in apparel, home furnishing, industrial, and textile
applications. Only two types of nylon fiber are currently
marketed for use in concrete, nylon 6 and nylon 66. Nylon
fibers are spun from nylon polymer and transformed
through extrusion, stretching, and heating to form an ori-
ented, crystalline, fiber structure. For concrete applica-
tions, high tenacity (high tensile strength) heat and light
stable yarn is spun and subsequently cut into shorter
length. Nylon fibers exhibit good tenacity, toughness, and
elastic recovery. Nylon is hydrophilic, with moisture
retention of 4.5 percent, which increases the water
demand of concrete. However, this does not affect con-
crete hydration or workability at low prescribed contents
ranging from 0.1 to 0.2 percent by volume, but should be
considered at higher fiber volume contents. This compar-
atively small dosage has potentially greater reinforcing
value than low volumes of polypropylene or polyester
fiber. Nylon is relatively inert and resistant to a wide
variety of organic and inorganic materials including
strong alkalis.
Synthetic fibers are also used in stucco and mortar.
For this use the fibers are shorter than synthetic fibers
used in concrete. Usually small amounts of 13-mm (
1
⁄
2
-in.)
long alkali-resistant fibers are added to base coat plaster
mixtures. They can be used in small line stucco and
mortar pumps and spray guns. They should be added
to the mix in accordance with manufacturer's recom-
mendation.
For further details about chemical and physical
properties of synthetic fibers and properties of synthetic
fiber concrete, see
ACI 544.1R-96
. ASTM C 1116 classifies
Steel, Glass, and Synthetic Fiber Concrete or Shotcrete.
The technology of interground fiber cement takes
advantage of the fact that some synthetic fibers are not
destroyed or pulverized in the cement finishing mill. The
fibers are mixed with dry cement during grinding where
they are uniformly distributed; the surface of the fibers is
roughened during grinding, which offers a better mechan-
ical bond to the cement paste (
Vondran 1995
).
ufacture of low-fiber-content concrete and occasionally
have been used in thin-sheet concrete with high-fiber con-
tent. For typical properties of natural fibers see Table 7-1.
Unprocessed Natural Fibers.
In the late 1960s, research
on the engineering properties of natural fibers, and con-
crete made with these fibers was undertaken; the result
was these fibers can be used successfully to make thin
sheets for walls and roofs. Products were made with port-
land cement and unprocessed natural fibers such as
coconut coir, sisal, bamboo, jute, wood, and vegetable
fibers. Although the concretes made with unprocessed
natural fibers show good mechanical properties, they
have some deficiencies in durability. Many of the natural
fibers are highly susceptible to volume changes due to
variations in fiber moisture content. Fiber volumetric
changes that accompany variations in fiber moisture con-
tent can drastically affect the bond strength between the
fiber and cement matrix.
Wood Fibers (Processed Natural Fibers).
The proper-
ties of wood cellulose fibers are greatly influenced by the
method by which the fibers are extracted and the refining
processes involved. The process by which wood is
reduced to a fibrous mass is called pulping. The kraft
process is the one most commonly used for producing
wood cellulose fibers. This process involves cooking
wood chips in a solution of sodium hydroxide, sodium
carbonate, and sodium sulfide. Wood cellulose fibers have
relatively good mechanical properties compared to many
manmade fibers such as polypropylene, polyethylene,
polyester, and acrylic. Delignified cellulose fibers (lignin
removed) can be produced with a tensile strength of up to
approximately 2000 MPa (290 ksi) for selected grades of
wood and pulping processes. Fiber tensile strength of
approximately 500 MPa (73 ksi) can be routinely achieved
using a chemical pulping process and the more common,
less expensive grades of wood.
MULTIPLE FIBER SYSTEMS
For a multiple fiber system, two or more fibers are
blended into one system. The hybrid-fiber concrete com-
bines macro- and microsteel fibers. A common macrofiber
blended with a newly developed microfiber, which is less
than 10 mm (0.4 in.) long and less than 100 micrometer
(0.004 in.) in diameter, leads to a closer fiber-to-fiber
spacing, which reduces microcracking and increases ten-
sile strength. The intended applications include thin
repairs and patching (
Banthia and Bindiganavile 2001
). A
blend of steel and polypropylene fibers has also been used
for some applications. This system is supposed to com-
bine the toughness and impact-resistance of steel fiber
concrete with the reduced plastic cracking of polypropy-
lene fiber concrete. For a project in the Chicago area
(
Wojtysiak and others 2001
), a blend of 30 kg/m
3
(50 lb/yd
3
) of steel fibers and 0.9 kg/m
3
(1
1
⁄
2
lb/yd
3
) of fib-
Natural Fibers
Natural fibers were used as a form of reinforcement long
before the advent of conventional reinforced concrete.
Mud bricks reinforced with straw and mortars reinforced
with horsehair are just a few examples of how natural
fibers were used long ago as a form of reinforcement.
Many natural reinforcing materials can be obtained at low
levels of cost and energy using locally available manpower
and technical know-how. Such fibers are used in the man-
