Fiber-reinforced concrete

Fiber-reinforced concrete (FRC) is concrete
containing fibrous material which increases its
structural integrity. It contains short discrete
fibers that are uniformly distributed and
randomly oriented. Fibers include steel fibers,
glass fibers , synthetic fibers and natural fibers.
Within these different fibers that character of
fiber-reinforced concrete changes with varying
concretes, fiber materials, geometries,
distribution, orientation and densities.
Historical perspective
The concept of using fibers as reinforcement is
not new. Fibers have been used as
reinforcement since ancient times. Historically,
horsehair was used in mortar and straw in mud
bricks. In the early 1900s, asbestos fibers were
used in concrete, and in the 1950s the concept
of composite materials came into being and
fiber-reinforced concrete was one of the topics
of interest. There was a need to find a
replacement for the asbestos used in concrete
and other building materials once the health
risks associated with the substance were
discovered. By the 1960s, steel , glass (GFRC ),
and synthetic fibers such as polypropylene fibers
were used in concrete, and research into new
fiber-reinforced concretes continues today.
Effect of fibers in concrete
Fibers are usually used in concrete to control
cracking due to both plastic shrinkage and
drying shrinkage. They also reduce the
permeability of concrete and thus reduce
bleeding of water . Some types of fibers produce
greater impact, abrasion and shatter resistance
in concrete. Generally fibers do not increase the
flexural strength of concrete, and so cannot
replace moment resisting or structural steel
reinforcement. Indeed, some fibers actually
reduce the strength of concrete. The amount of
fibers added to a concrete mix is expressed as a
percentage of the total volume of the composite
(concrete and fibers), termed volume fraction
(V f ). V f typically ranges from 0.1 to 3%. Aspect
ratio (l/d) is calculated by dividing fiber length (l)
by its diameter (d). Fibers with a non-circular
cross section use an equivalent diameter for the
calculation of aspect ratio. If the modulus of
elasticity of the fiber is higher than the matrix
(concrete or mortar binder), they help to carry
the load by increasing the tensile strength of the
material. Increase in the aspect ratio of the fiber
usually segments the flexural strength and
toughness of the matrix. However, fibers which
are too long tend to "ball" in the mix and create
workability problems.
Some recent research [where? ] indicated that
using fibers in concrete has limited effect on the
impact resistance of the materials[1 & 2]. This
finding is very important since traditionally,
people think that ductility increases when
concrete is reinforced with fibers. The results
also indicated out that the use of micro fibers
offers better impact resistance compared with
the longer fibers. [1]
The High Speed 1 tunnel linings incorporated
concrete containing 1 kg/m³ of polypropylene
fibers, of diameter 18 & 32 μm, giving the
benefits noted below. [1]
Benefits
Polypropylene and Nylon fibers can:
Improve mix cohesion, improving pumpability
over long distances
Improve freeze-thaw resistance
Improve resistance to explosive spalling in
case of a severe fire
Improve impact resistance
Increase resistance to plastic shrinkage during
curing
Steel fibers can:
Improve structural strength
Reduce steel reinforcement requirements
Improve ductility
Reduce crack widths and control the crack
widths tightly thus improve durability
Improve impact & abrasion resistance
Improve freeze-thaw resistance
Blends of both steel and polymeric fibers are
often used in construction projects in order to
combine the benefits of both products;
structural improvements provided by steel fibers
and the resistance to explosive spalling and
plastic shrinkage improvements provided by
polymeric fibers.
In certain specific circumstances, steel fiber can
entirely replace traditional steel reinforcement
bar in reinforced concrete. This is most common
in industrial flooring but also in some other
precasting applications. Typically, these are
corroborated with laboratory testing to confirm
performance requirements are met. Care should
be taken to ensure that local design code
requirements are also met which may impose
minimum quantities of steel reinforcement
within the concrete. There are increasing
numbers of tunnelling projects using precast
lining segments reinforced only with steel fibers.
Useful standards:
EN 14889-1:2006 Fibres for Concrete. Steel
Fibres. Definitions, specifications & conformity
EN 14889-2:2006 Fibres for Concrete.
Polymer Fibres. Definitions, specifications &
conformity
EN 14845-1:2007 Test methods for fibres in
concrete
ASTM A820-06 Standard Specification for
fibres in Fibre Reinforced Concrete
ASTM C1018-07 Standard test methods for
flexural toughness & first crack strength
Some developments in fiber-reinforced
concrete
An FRC sub-category named Engineered
Cementitious Composite (ECC) claims 500 times
more resistance to cracking and 40 percent
lighter than traditional
concrete. [ citation needed ] ECC claims it can
sustain strain-hardening up to several percent
strain, resulting in a material ductility of at least
two orders of magnitude higher when compared
to normal concrete or standard fiber-reinforced
concrete. ECC also claims a unique cracking
behavior. When loaded to beyond the elastic
range, ECC maintains crack width to below
100 µm, even when deformed to several
percent tensile strains. Field results with ECC and
The Michigan Department of Transportation
resulted in early-age cracking [2] .
Recent studies performed on a high-
performance fiber-reinforced concrete in a
bridge deck found that adding fibers provided
residual strength and controlled cracking [3] .
There were fewer and narrower cracks in the
FRC even though the FRC had more shrinkage
than the control. Residual strength is directly
proportional to the fiber content.
A new kind of natural fiber-reinforced concrete
(NFRC) made of cellulose fibers processed from
genetically modified slash pine trees is giving
good results[ citation needed ] . The cellulose
fibers are longer and greater in diameter than
other timber sources. Some studies were
performed using waste carpet fibers in concrete
as an environmentally friendly use of recycled
carpet waste [4] . A carpet typically consists of
two layers of backing (usually fabric from
polypropylene tape yarns), joined by CaCO 3
filled styrene-butadiene latex rubber (SBR), and
face fibers (majority being nylon 6 and nylon 66
textured yarns). Such nylon and polypropylene
fibers can be used for concrete reinforcement.
Other ideas are emerging to use recycled
materials as fibers [5] .
For statistical calculations there is a new
modelling in the book: B.Wietek,
Stahlfaserbeton, edited by Vieweg + Teubner,
2008, ISBN 978-3-8348-0592-8 .

Posted in: Concrete Technology