Steel fibers can be categorized
into five major categories, according to their method of production:
Group I: cold-drawn wire
The most used group in various
applications as it is the best type in performing with concrete.
Group II: cut sheet
Used in a considerable types of
applications.
Group III: melt extracted
Not used in much applications
Group IV: shaved cold drawn wire
Used in a considerable types of
applications.
Group V: milled from blocks
Not used in much applications.
This paper is focused on Group
I as it’s the most common type for reinforced concrete use. Steel fiber could
come in different shapes and sizes as follows:
-Shape: straight, hooked,
undulated, crimped, twisted, coned …
-Length: typically, from 30 mm
to 60 mm
-Diameter: typically, from 0.4
mm to a maximum of 1.3 mm
The performance of steel fiber
reinforced concrete is affected by the shape and the length of steel fibers.
Longer fibers and smaller diameters would have better performance as they have
more anchorage length. The ratio of fiber length to diameter could provide
ideal approximate calculation of fiber performance. Anchorage type change could
change the shape of the load deflection curve of the steel fiber concrete. The
product standards for steel fiber makes it easy to find a summary of fiber
properties quickly as well as performance from the mandatory CE-label. For
additional information.
Form Factors: The higher the
aspect ratio between length/diameter the better the performance of the mix.
However, this could cause balling which would limit the ability of mixing
higher dosage
As a result, a relationship
between high l/d rations and the great quantity of single fibers is derived.
Hence more attention is required to mix design. 20 kg/m³ or 15kg/m³ of high-
perform fibers could easily do the work of 40 Kg/m3 of easy-mix fibers (low
l/d). Systems have been developed to avoid balling and to insure optimal
distribution which can secure mixing 100 kg/m3 of high-performing fibers
perfectly with concrete.
(A) Loose Steel Fiber
With low aspect ratio loose
steel fiber should not face the problems of workability and balling.
Fig: Loose
Hooked End Steel Fiber
Example of Loose steel fiber:
The easy-mix type offers a
length of 50 mm at a theoretical l/d of 45. Since its low aspect ratio, its
performance is relatively low and its workability is relatively high.
Using blower blast equipment to
add fibers to concrete: For high aspect ratios like, extra ways should be used
so that can be effortlessly and efficiently added to the concrete like blower
blast. Blowing loose fibers with aspect ratios more than 70 usually tends to
cause issues like balling. Blowing is different from dosing as dosing requires
precise weight of fibers.
(B) Glued
Steel Fiber
Fig: Hooked End Bright Glued Steel Fiber
Glued fiber technology is
developed to prevent balling due to additional loose fiber of high aspect
ratio. Once the mixing begins, the glued steel fiber starts slowly to separate
to insure a homogeneous mix.
Example of glued steel fiber
bundles
It is possible to add glued
steel fiber bundles from the bag directly to the central mixer or mixing truck.
They can also be added indirectly through a conveyor belt. Automatic dosing is
likewise reachable. There are two methods to categorize fibers according to
their modulus of elasticity or their origin. In the view of modulus of
elasticity, fibers can be classified into two basic categories, namely, those
having a higher elastic modulus than concrete mix (called hard intrusion) and
those with lower elastic modulus than the concrete mix (called soft intrusion).
Steel, carbon and glass have higher elastic modulus than cement mortar matrix,
and polypropylene and vegetable fibers are classified as the low elastic
modulus fibers. High elastic modulus fibers simultaneously can improve both
flexural and impact resistance; whereas, low elastic modulus fibers can improve
the impact resistance of concrete but do not contribute much to its flexural
strength. According to the origin of
fibers, they are classified in three categories of metallic fibers (such as
steel, carbon steel, and stainless steel), mineral fibers (such as asbestos and
glass fibers), and organic fibers. Organic fibers can be further divided into
natural and man-made fibers. Natural fibers can be classified into vegetable
origin or sisal (such as wood fibers and leaf fibers), and animal origin (such
as hair fibers and silk). Man-made fibers can also be divided into two groups
as natural polymer (such as cellulose and protein fibers), and synthetic fibers
(such as nylon and polypropylene).
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