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ABSTRACT. Fiber reinforced concrete (FRC) is a new structural material
which is gaining increasing importance. Addition of fiber reinforcement in
discrete form improves many engineering properties of concrete.
Currently, very little research work is being conducted within the Kingdom
using this new material. This paper describes the different types of fibers
and the application of FRC in different areas. It also presents the result
of research about the mechanical properties of FRC using straight as well as
hooked steel fibers available in the region,
Introduction
Concrete is weak in tension and has a brittle character. The concept of using fibers to
improve the characteristics of construction materials is very old. Early applications
include addition of straw to mud bricks, horse hair to reinforce plaster and asbestos
to reinforce pottery. Use of continuous reinforcement in concrete (reinforced concrete)
increases strength and ductility, but requires careful placement and labour
skill. Alternatively, introduction offibers in discrete form in plain or reinforced concrete
may provide a better solution. The modern development of fiber reinforced
concrete (FRC) started in the early sixties(1J. Addition of fibers to concrete makes it
a homogeneous and isotropic material. When concrete cracks, the randomly
oriented fibers start functioning, arrest crack formation and propagation, and thus
improve strength and ductility. The failure modes of FRC are either bond failure between
fiber and matrix or material failure. In this paper, the state-of-the-art offiber
reinforced concrete is discussed and results of intensive tests made by the author on
the properties of fiber reinforced concrete using local materials are reported.
Fiber Types
Fibers are produced from different materials in various shapes and sizes. Typical
fiber materials are[2,31;
Steel Fibers
Straight, crimped, twisted, hooked, ringed, and paddled ends. Diameter range
from 0.25 to 0.76mm.
Glass Fibers
Straight. Diameter ranges from 0.005 to 0.015mm (may be bonded together to
form elements with diameters of 0.13 to 1.3mm).
Natural Organic and Mineral Fibers
Wood, asbestos, cotton, bamboo, and rockwool. They come in wide range of
sizes.
Polypropylene Fibers
Plain, twisted, fibrillated, and with buttoned ends.
Other Synthetic Fibers
Kevlar, nylon, and polyester. Diameter ranges from 0.02 to 0.38mm.
A convenient parameter describing a fiber is its aspect ratio (LID), defined as the
fiber length divided by an equivalent fiber diameter. Typical aspect ratio ranges from
about 30 to 150 for length of 6 to 75mm.
Mixture Compositions and Placing
Mixing of FRC can be accomplished by many methodsl21. The mix should have a
uniform dispersion of the fibers in order to prevent segregation or balling of the fibers
during mixing. Most balling occurs during the fiber addition process. Increase of
aspect ratio, volume percentage of fiber, and size and quantity of coarse aggregate
will intensify the balling tendencies and decrease the workability. To coat the large
surface area of the fibers with paste, experience indicated that a water cement ratio
between 0.4 and 0.6, and minimum cement content of 400 kg/m[3] are required. Compared
to conventional concrete, fiber reinforced concrete mixes are generally
characterized by higher cement factor, higher fine aggregate content, and smaller
size coarse aggregate.
A fiber mix generally requires more vibration to consolidate the mix. External vibration
is preferable to prevent fiber segregation. Metal trowels, tube floats, and
rotating power floats can be used to finish the surface.
Mechanical Properties of FRC
Addition of fibers to concrete influences its mechanical properties which significantly
depend on the type and percentage offiber[2-4J. Fibers with end anchorage and high aspect ratio were found to have improved effectiveness. It was shown that for
the same length and diameter, crimped-end fibers can achieve the same properties as
straight fibers using 40 percent less fibers[S]. In determining the mechanical properties
of FRC, the same equipment and procedure as used for conventional concrete
can also be used. Below are cited some properties of FRC determined by different
researchers.
Compressive Strength
The presence of fibers may alter the failure mode of cylinders, but the fiber effect
will be minor on the improvement of compressive strength values (0 to 15 percent).
Modulus ofElasticity
Modulus of elasticity of FRC increases slIghtly with an increase in the fibers content.
It was found that for each 1 percent increase in fiber content by volume there is
an increase of 3 percent in the modulus of elasticity.
Flexure
The flexural strength was reported[2j to be increased by 2.5 times using 4 percent
fibers.
Toughness
For FRC, toughness is about 10 to 40 times that of plain concrete.
Splitting Tensile Strength
The presence of 3 percent fiber by volume was reported to increase the splitting
tensile strength of mortar about 2.5 times that of the unreinforced one.
Fatigue Strength
The addition offibers increases fatigue strength of about 90 percent and 70 percent
of the static strength at 2 x 106 cycles for non-reverse and full reversal of loading, respectively.
Impact Resistance
The impact strength for fibrous concrete is generally 5 to 10 times that of plain concrete
depending on the volume of fiber uSl:d[2j.
Corrosion ofSteel Fibers
A lO-year exposure[2jofsteel fibrous mortar to outdoor weathering in an industrial
atmosphere showed no adverse effect on the strength properties. Corrosion was
found to be confined only to fibers actually exposed on the surface. Steel fibrous
mortar continuously immerse in seawater for 10 years exhibited a 15 percent loss
compared to 40 percent strength decrease of plain mortar.
Structural Behavior of FRC
Fibers combined with reinforcing bars in structural members will be widely used in
the future. The following are some of the structural behaviourI6.8l :
Flexure
The use of fibers in reinforced concrete flexure members increases ductility, tensile
strength, moment capacity, and stiffness. The fibers improve crack control and
preserve post cracking structural integrity of members.
Torsion
The use of fibers eliminate the sudden failure characteristic of plain concrete
beams. It increases stiffness, torsional strength, ductility, rotational capacity, and
the number of cracks with less crack width.
Shear
Addition of fibers increases shear capacity of reinforced concrete beams up to 100
percent. Addition of randomly distributed fibers increases shear-friction strength,
the first crack strength, and ultimate strength.
Column
The increase of fiber content slightly increases the ductility of axially loaded specimen.
The use of fibers helps in reducing the explosive type failure for columns.
High Strength Concrete
Fibers increases the ductility of high strength concrete. The use of high strength
concrete and steel produces slender members. Fiber addition will help in controlling
cracks and deflections.
Cracking and Deflection
Tests[9] have shown that fiber reinforcement effectively controls cracking and deflection,
in addition to strength improvement. In conventionally reinforced concrete
beams, fiber addition increases stiffness, and reduces deflection.
Applications
The uniform dispersion of fibers throughout the concrete mix provides isotropic
properties not common to conventionally reinforced concrete. The applications of
fibers in concrete industries depend on the designer and builder in taking advantage
of the static and dynamic characteristics of this new material. The main area of FRC
applications are1101 :
Runway, Aircraft Parking, and Pavements
For the same wheel load FRC slabs could be about one half the thickness of plain
concrete slab. Compared to a 375mm thickness' of conventionally reinforced concrete
slab, a 150mm thick crimped-end FRC slab was used to overlay an existing asphaltic-paved aircraft parking area. FRC pavements are now in service in severe and
mild environments.
Tunnel Lining and Slope Stabilization
Steel fiber reinforced shortcrete (SFRS) are being used to line underground openings
and rock slope stabilization. It eliminates the need for mesh reinforcement and
scaffolding.
Blast Resistant Structures
When plain concrete slabs are reinforced conventionally, tests showed(llj that
there is no reduction of fragment velocities or number of fragments under blast and
shock waves. Similarly, reinforced slabs of fibrous concrete, however, showed 20
percent reduction in velocities, and over 80 percent in fragmentations.
Thin Shell, Walls, Pipes, and Manholes
Fibrous concrete permits the use of thinner flat and curved structural elements.
Steel fibrous shortcrete is used in the construction of hemispherical domes using the
inflated membrane process. Glass fiber reinforced cement or concrete (GFRC) ,
made by the spray-up process, have been used to construct wall panels. Steel and
glass fibers addition in concrete pipes and manholes improves strength, reduces
thickness, and diminishes handling damages.
Dams and Hydraulic Structure
FRC is being used for the construction and repair of dams and other hydraulic
structures to provide resistance to cavitation and severe errosion caused by the impact
of large waterboro debris.
Other Applications
These include machine tool frames, lighting poles, water and oil tanks and concrete
repairs.