FIELD OF INVENTION
The invention relates to steel fibre providing high post crack strength and strain
to concrete.
BACKGROUND OF THE INVENTION
Concrete is a brittle material and is prone to cracking due to its inherent tensile
weakness. The tensile strength of concrete is typically 8% to 15% of its
compressive strength. To avoid or prolong its tending to crack and increase its
load bearing capacity, concrete has to be reinforced with wire mesh and bars or
by adding fibres.
Concrete primarily resists compressive stresses and re-bars resist tensile and
shear stresses. The longitudinal rebar in a beam resists flexural (tensile stress)
whereas the stirrups, wrapped around the longitudinal bar, resist shear stresses.
In a column, vertical bars resist compression and buckling stresses while ties
resist shear and provide confinement to vertical bars. Steel bars, however,
reinforce concrete against tension only locally. Cracks in reinforced concrete
members extend freely until encountering a rebar. This is a demand for
multidirectional and closely spaced reinforcement for concrete. Due to this
reason, steel fibres have been proposed as reinforcing agents. The fibres are
small and can be evenly distributed in the entire concrete layer. The concept is
to make the steel fibre reinforced concrete (SFRC) behave as a viscous material.
When the SFRC concrete matrix cracks, a number of fibres inside the matrix
bridge the crack. To further open the crack, these must be pulled out of the
concrete matrix. The matrix and the fibre dosages are determined so that the
load needed to pull out the steel fibres is higher than the cracking load. This

load needed to pull out the steel fibres is higher than the cracking load. This
automatically builds in a strain hardening effect and results in very fine
distributed cracking pattern instead of only one large crack.
At present steel fibres are being used in the concrete to increase the post
cracking strength of the concrete and tensile strength upto certains extent.
Performance of the concrete without steel fibres and with steel fibres can be
differentiated from (A),(B),(C) and (D) of Fig-1.
The initial stages of uni-axial tensile fracture characterized by micro-cracking and
appropriate schematic diagrams of the uni-axial tensile response as shown in Fig-
1 for,
A- Plain concrete with high aggregate quantity (circles represents
aggregates)
B- Plain concrete with low aggregate quantity
C- Fibre concrete with low quantity of thick fibres
D- Fibre concrete with high quantity of thin fibres
A,B,C and D show the tensile stress and displacement curve for plain concrete
and fibre concrete.
In plain concrete, the only resisting mechanism is the cement paste ( A and B of
Fig-1). Having somewhat higher tensile strength compared to the interface zone,
the cement paste can provide some resistance to further propagation of micro-
cracks, but this occurs to a relatively small extent. The higher the applied volume
quantity of aggregate, the smaller the distance between the aggregate grains
will be. Consequently, the micro cracks can propagate faster and easier in plain
concrete with a higher volume quantity of aggregate [Fig-1(A)] compared to the
concrete with lower volume of aggregate [Fig-1(B)].

In case of fibre reinforced concrete [Fig-1(C) & 1(D)] a greater density
corresponding to the minimal spacing and orientation of fibres control the
development of micro-cracks and their graduation to macro cracks. As a
consequence, the first macro crack develops at higher tensile stress (first
cracking stress) that is the higher tensile strength can be reached [Fig.1(D)].
Steel fibres are used in high end application across the world. Most of the
research done on steel fibres across the world is out of India and steel fibres
available in the market are designed based on those research findings.
The present invention of steel fibre using the Indian conditions gives better
performance compared to existing steel fibres.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a steel fibre which eliminates
the disadvantages of the prior art.
Another object of the invention is to propose a steel fibre which has high post
crack strength and strains in concrete along with tensile strength.
A still another object of the invention is to propose a steel fibre at low cost.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l- shows the initial stages of uni-axial tensile fracture characterized by
micro-cracking, and appropriate schematic diagrams of the uni-axial tensile
response, for: A) plain concrete with high aggregate quantity (circles represents

aggregates); B) plain concrete with low aggregate quantity; C) fibre concrete
with low quantity of thick fibres; D) fibre concrete-with high quantity of thin
fibres
Fig.2- shows improvement in the post crack behaviour of the concrete with the
addition of fibers at 1%,2%, and 3% volume fraction of fibres.
Fig.3- shows Load-displacement graph of new fibre
Fig.4- (A) & (B)- shows the steel fibre with straight bends and curved bands
Fig.5- shows Stress- Longitudinal strain graph
Fig.6- shows existing steel fibres
SUMMARY OF THE INVENTION
The present invention provides a steel fibre with better post crack strength and
strain of concrete. The steel fibre edges having deformation (bending) will bear
the load after the first crack. It is designed with very low range edges. A centre
bend is given to improve physical bonding with the concrete and two bends are
provided at the edges. All bends are given a radius in the range of 1.5-2mm
rather than sharp bending. The present invented steel wire having Mpa (Mega
pascals) strength in the range of 1000-1500 Mpa is of diameter in the range of
0.4 to 1.00 mm and of length in the range of 35-45 mm. As shown in Fig.4 (A) &
(B), the shape of edges provided to the present steel fibre is capable of arresting
the crack propagation better.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Accordingly, a steel fibre is made to increase the post cracking strength of
concrete along with tensile strength. Concrete starts cracking at various locations
at its critical load. Without steel fibres concrete collapses once it reaches critical
load. It takes negligible-load after reaching the critical load. With the addition of
steel fibre its post cracking behaviour changes as shown in Fig.l(c) & (d). Since
fibres are distributed all over the concrete fibres will arrest the propagation of
crack. Crack propagation will be arrested because of the shape of edges provided
to the steel fiber as presently invented. Deformation (bending) of the steel fibre
edges bears the load after the first crack. This type of behaviour is achieved with
the present invention by providing very low angle edges. To increase the
cracking length long edges are provided. The invented steel fibre is also provided
with a center bend to improve physical bonding with the concrete.
The present steel fibre consists of two bends at the edges and a central bend. All
bends are given certain radius rather than sharp bending. This steel wire is made
of low carbon steels with wire strength in the range of 1000-1500 Mpa (mega
pascals) imparting to concrete and of diameter in the range of 0.4 to 1.00 mm.
These datas have been achieved following extensive research work and
experiments which clearly shows that the present steel fibre has more post
cracking strength than the prior art.
Finite Element Analysis has been performed for single fibre pull out test. Single
fibre pull out test to access the performance of the steel fibers. As shown in
fig.3, during the pull out test concrete takes the load during post crack session
which cannot be observed in the plain concrete. Because of this, there are less
chances of spulling of the concrete which is normal case for traditional reinforced
concrete. Cylindrical compression test has been performed using steel fibres at

1.5% volume fraction of concrete as shown in fig.5 which shows good post
cracking behaviour.
The key considerations in the profiled design were, (a) providing high post
cracking strength to concrete (b) Good bonding between concrete and steel
fibre (c) decrease the steel requirement for a building.
The product design process included single fibre pull out analysis is followed by
experimental investigation including compression, split, flexural, workability and
pilot scale tests.
As shown in (A) and (B) the present steel fibre has been provided with a plurality
of bends (2,4,6,8) along its length to arrest crack propagation. The length of the
fibre is between 35 to 45 mm and preferably taken as 40 mm. The drawings
show the steel fibre with straight bends and curved bends. The shape of steel
fibre can be defined as trough shape. Steel fire is divided into 9 parts to
conveniently describe the drawing. Part 1 is kept at very low angle to part 3 with
high radius between these two parts at part 2 which will help in deforming the
steel fibre in concrete rather than breaking which will increase the post crack
resistance as well as decrease the balling effect or entangling between the steel
fibers during mixing. The same principal is involved between the part 3 and part
5 at part 4. Part 5 is a central portion of the steel fibre. Either side of part 5 is
similar to each other which will help in uniform behaviour of the steel fibre in the
concrete. Part 3 is the longest of all. Part 1, part 5 and part 9 are shorter than
1/3 of part 3 in length. The angle between part 1 and part 3, part 3 and part 5,
part 5 and part 7, part 7 and part 9 ranges between 120 and 160 degrees.

WE CLAIM
1. Steel fibre (A) providing high post crack strength and strain to concrete
characterised in that the fibre is shaped by providing a plurality of bends
(2,4,6,8) along its length to arrest crack propagation.
2. Steel fibre as claimed in claim 1, wherein the radius of each said bends
(2,4,6,8) is upto 2 mm and two bends (2,8) are positioned at the two
edges and two bends (4,6) are positioned at the centre.
3. Steel fibre as claimed in claim 1, wherein the bends (4,6) positioned at the
centre of the said fibre improve physical bonding with the concrete.
4. Steel fibre as claimed in claim 1, wherein the angle of the said bends
(2,4,6,8) between Part 1 and part 3, part 3 and part 5, part 5 and part 7
and part 7 and part 9 respectively lies in the range of 120° to 160°.
5. Steel fibre as claimed in claim 1, wherein the length of the said fibre lies
between 35 to 45 mm and is preferably 40 mm and diameter of the said
fibre lies between 0.4 to 1 mm.
6. Steel fibre as claimed in claim 1, which imparts high post crack strength in
the range of 1000-1500 Mpa to the concrete.
7. Steel fibre as claimed in claim 1, wherein the said fibre is of low carbon
steel.

Steel fibre (A) providing high post crack strength and strain to concrete is shaped
by providing a plurality of bends (2,4,6,8) along its length to arrest crack
propagation wherein the radius of said bends (2,4,6,8) is up to 2 mm. Two
bends (2,8) are positioned at the two edges while two other bends (4,6) are
positioned at the centre to improve the physical bonding with the concrete. The
length of said fibre is between 35-45 mm and is preferably 40 mm and diameter
of said fibre is between 0.4 mm to 1 mm.