Abstract use of fibers in brittle matrix

 

Abstract

This study deal with reviewing the
past researchers works and collecting information on toughness of different fibrous
concrete from various researches to determine how the different types of fiber
can influence concrete toughness. The collecting information from researches
shows that the fiber has great effect on improving toughness of concrete and fracture
resistance. This
increasing in toughness varies depending upon type of fiber, fiber content%,
fiber geometry, aspect ratio and w/c ratio of concrete.  Toughness of fiber reinforced concrete
increase by increasing fiber content% for all fiber types.

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Key words: Concrete, Fiber,
Fiber reinforced concrete, Toughness

 

 

 

 

 

 

 

1-1 History of using fiber

The use of fibers in brittle matrix materials
has a long history going back at least 3500 years during building the 57 m high hill of Aqar Quf
near Baghdad in which straw was used with sun-baked bricks as reinforcement. In
more recent times, asbestos fibers have been used to reinforce cement products
for about 100 years, cellulose fibers for at least 50 years, and steel,
polypropylene and glass fibers have been used for the same purpose for the past
30 years 1. First studies transaction with use of steel
fibers and glass fibers in concrete date back to the 1950, in the 1960 the
first studies concerning fiber reinforced concrete using synthetic fibers.

 

1-2 Introducing Fiber reinforced concrete

Concrete is weak in tension and has a brittle
character, concrete contains numerous microcracks due to low tensile strength
under stress. Originally, it was assumed that tensile as well as flexural
strengths of concrete can be considerably increased by introducing closely
spaced fibers that would prevent the dispersion of     microcracks1.
Fiber is a small piece of reinforcing material which increases structural integrity.
It contains short discrete fibers that are uniformly distributed and randomly
oriented. The main objectives of using fiber in concrete is modify the
properties of concrete (a) to improve the rheology or plastic cracking
characteristics of the concrete in the fresh state or up to about 6 hours after
casting, (b) to improve the tensile or flexural strength, (c) to improve the
impact strength and toughness ( as shown in figure 1-1), (d) to control
cracking and the mode of failure by means of post-cracking ductility,(e) to
improve durability2.

                     Figure 1-1 Relation between load and deflection 3   

 

Fiber reinforced concrete is a composite material consist of fiber
and cement matrix, its property depend upon the efficient transfer of stress
between matrix and fiber, which is dependent upon (type of fiber, fiber
content%, fiber geometry, aspect ratio, orientation and distribution of fiber,
size and shape of aggregate) 4. Important type of fiber used with concrete is
presented below:

a-Steel fibers:
Straight, crimped, twisted, hooked, ringed, and paddled ends. Diameter ranges from
0.25 to 0.76mm

b-Glass fibers : Straight. Diameter
ranges from 0.005 to 0.015mm

c-Natural fibers: Wood, asbestos, cotton,
bamboo, coir, hay and rock wool. They come in wide range of sizes.

d- Synthetic fiber: Polypropylene fibers (plain, twisted, fibrillated, and with buttoned
ends), kevlar,
nylon, and polyester.

Figure 1-2 shows some type
of fiber.

 Figure 1-2 Type
of fiber

1-3 Toughening
mechanism

·       
The
greatest advantage in fiber reinforcement of concrete is the improvement in
flexural toughness. Toughness is defined by the total energy absorbed prior to
complete separation of the specimen is given by the area under the
stress-strain curve. Toughness or energy absorption of concrete is increased
considerably by the addition of fibers ( as shown in figure 1-3) .( For FRC,
toughness is about 10 to 40 times that of plain concrete 5) . Toughness is
total area under the (stress – strain curve). It can also be defined as
resistance to fracture of a material when stressed.

Figure 1-3 Stress strain curve of different
concretes

·       
The
magnitude of improvement in toughness is strongly influenced by fiber
concentration and resistance of fibers to pull-out, which is influenced by
other factors such as shape or surface texture. The energy absorbed Gc
by the fiber pullout is given by 6.

Where :

Vf = fiber volume fraction

 Lf  =length of fiber

 df =
diameter, respectively,

t= interface parameters

 

The
addition of fiber increases the toughness index of hooked and straight fibers
up to 19.9 and 16.9, respectively 5.

Table
1-1 shows results of toughness index for different fibers volume.

 

Table 1-1 Effect of fiber content on the toughness index

 
Fiber content %

Toughness index

Hooked fiber

Straight fiber

0.0

1

1

0.5

11.4

9.2

1.0

18

11.1

1.5

19.9

13.6

2

16.7

16.9

 

 

The toughness index is calculated as the area under the load deflection
curve (Figure 1-4) up to the 1.9 mm deflection divided by the area up to the
first crack strength (proportional limit)

 

    Figure 1-4 Definition of toughness
index according to ACI committee 544

 

 

·       
Aspect ratio of fiber has great effect on toughness,
aspect ratio is ratio between length and diameter (L/d) of fiber. Aspect ratio up
to 75 increase in relative toughness, aspect ratio beyond 75 decrease in
relative toughness.(as summarized in Table1-2) 4.

 

 

 

Table 1-2 effect of
aspect ratio of fiber in relative toughness

 

Type of concrete

Aspect ratio

Relative toughness

Plain concrete

0

1

      With

25

2

Randomly

50

8

dispersed fiber

75

10.5

 

100

8.5

 

 

 

2-Objectives of this study

The main
purpose of this study is to investigate the
influence of different types of fiber that mixed with concrete on toughness
property of concrete. Available factors affecting the toughness of fiber
reinforced concrete studied by the past researchers are discussed.

 

3- Toughness
of different types of fibrous concrete

3-1 Toughness of Steel Fiber reinforced concrete

Steel fibers are short, discrete lengths of steel with an aspect
ratio (ratio of length to diameter) from about 20 to 100. Some steel fibers
have hooked ends to improve resistance to pullout from a cement-based matrix.
Steel fiber has great effect on increasing toughness of concrete under stress,
this increasing varies due to some factors that has effect on toughness of
steel fiber reinforced concrete (SFRC) including w/c ratio of concrete, steel
fiber content %, aspect ratio of steel fiber, shape of steel fiber. Increasing
toughness of SFRC at different condition is presented in the section

 

a)    Effect of 
steel fiber content % on toughness

Figure (3-1), (3-2) and (3-3) shows that further steel
fiber content lead to increasing fracture resistance via increasing toughness.

 

             

                         Figure 3-1 variation
of toughness index with fiber content % 7

 

 

 

Figure3-2 Load-deflection curves for fiber concrete 7

 

Figure 3-3 . Effects of Steel Fibers Content on
Compressive Stress- Strain Curve of FRC 8

 

 

 

b)   
Effect
of water cement ratio on toughness  of
SFRC

 

w/c
ratio in SFRC greatly affect the amount of energy required to
extend a crack, increasing w/c ratio of SFRC cause reducing toughness (as shown
in the figure 3-4).

 

Figure 3-4 Load- deflection curve 9

 

 

c)     Effect of various
types of steel fiber on toughness of SFRC .

 

Figure 3- 5 and   table 3-2 shows that the end-deformed fibers
F1 and F4 appear to be significantly more effective than fibers F2 and F3 on
toughness (energy absorbed).

 

 

 

 

Table 3-1 Detail of various type of steel fiber in figure (3-5)

Figure 3-5
Load deflection curve of various type of normal strength
fiber

                                     Concrete 10.

 

 

Table 3-2 Energy absorbed at 2mm
deflection of F1, F2,F3 and F410

 

Type of fiber

 Energy
absorbed at 2mm deflection(N-mm)

Plain concrete

0.44

F1 Hooked end(circular)

37.80

F2 Crimped (circular)

24.08

F3 Crimped (crescent)

18.69

F4 Twin code(circular)

32.79

 

 

3-2 Toughness of Glass Fiber
reinforced concrete

Glass fiber-reinforced concrete
consists of high-strength, alkali-resistant glass fiber embedded in a concrete matrix
11. In this form, both fibers and matrix keep their physical and chemical
properties, while by this mixing concrete can give properties that cannot be
achieved with either of the components alone.

Glass fiber is available in continuous
or chopped length, fiber length up to 35 mm lengths used in spray applications
and fiber length up to 25 mm lengths 
used in premix applications. Concrete with reinforced by glass fiber
more tough and better resistance to cracking than plain concrete, by increasing
glass fiber content % further fracture energy (toughness ) can obtain13.

 

Figure(3-6) and ( 3-7 )shows that concrete with further
toughness(resistance to fracture) can obtain by adding glass fiber, also by
increasing volume of fiber content fracture energy (resistance to fracture) increase.

         

               

Figure 3-6 Load and crack mouth opening
deflection (CMOD) curve 12.

          

 

Figure 3-7 Fracture energy
(J/m²) and fiber volume % 12.

 

R=concrete mixture (cement, fly ash, water, coarse aggregate, crushed sand
and river sand) without glass fiber

GF=concrete mixture(cement, fly ash, water, coarse aggregate, crushed sand
,river sand and glass fiber 1% by volume)

 

 

3-3 Toughness of
Natural Fiber reinforced concrete

Natural organic
and mineral fiber has some type like wood, asbestos, cotton, bamboo, coir, hay
and rock wool. Natural fibers offer  many
benefits for reinforcement (a) Low cost and abundant (b) Renewable (c)Non-hazardous
(d) Can improve some characteristics of concrete,  inside them toughness of concrete 13.

 

 

 

Figure 3-8 shows that adding natural fiber to concrete improve
toughness (area under the curve) of concrete, also the figure tell us that long
fibers are more effective than short fibers in increasing toughness.

                 

Figure 3-8 Typical
stress-strain curves for natural fiber concrete 14.

 

3-4 Toughness of
synthetic Fiber reinforced concrete.

 

Polypropylene fiber is a synthetic hydrocarbon polymer, Fibers in general and polypropylene fibers particular have gained
popularity in recent years for use in concrete, mainly to improve shrinkage
cracking resistance and toughness of plain concrete. Polypropylene fibers are
not expected to increase the strength of concrete, but to enhance its ductility
and toughness, and impact resistance 15.

 

 

 

Figure 3-9 shows that the existence
of polypropylene fiber enhances toughness of concrete. This improvement
increase by increasing fiber content %. The figure also shows that the
toughness of light weight concrete reinforced with polypropylene fiber is
better than normal weight reinforced concrete.

 

 

Figure 3-9 Typical load (crack mouth opening
deflection) curve of different volume of     Polypropylene
fiber AL-PE(all-light weight Polypropylene fiber reinforced concrete),                 N-PE (normal weight Polypropylene
fiber  reinforced concrete) 16

 

 

 

 

 

 

4-Conclusion

 

From
the data presented in this study the following conclusion can be drown.

1)    All types of Fiber improve the toughness (fracture
resistance of concrete)

2)    Aspect ratio of fiber (L/D) and fiber content%
for all type of fiber reinforced concrete has great effect on toughness of
concrete.

3)    In steel fiber reinforced concrete increasing
w/c ratio lead to reducing toughness.

4)    Geometrical shape of steel fiber in concrete
has influence on toughness. Hooked
end (circular) fiber is more effective one among all steel fiber that gives
high toughness to concrete.