CHAPTER 4 COMPRESSIVE STRENGTH OF HYBRID ...shodhganga.inflibnet.ac.in/bitstream/10603/34576/7...70 CHAPTER 4 COMPRESSIVE STRENGTH OF HYBRID FIBRE REINFORCED CONCRETE 4.1 GENERAL High

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CHAPTER 4

COMPRESSIVE STRENGTH OF HYBRID FIBRE REINFORCED CONCRETE

4.1 GENERAL

High performance hybrid fibre-reinforced concrete (HyFRC) is a new class

of concrete that has been developed in recent years. When compared with high

performance concrete (HPC), HyFRC exhibits superior properties in terms of

compressive strength, shear strength, tensile strength and flexural strength and ductility.

The control specimens were cast without fibres and tested for determining the properties

of concrete. The test results such as compressive strength, elastic modulus, energy

absorption capacity and ductility of HyFRC are analysed in this chapter.

4.2 PARAMETERS STUDIED

This experiment was carried out, to observe the improvement in

compressive behaviour of the HyFRC by the addition of different volumes of fibres.

The specimen for compression tests consisted of 100 mm cubes and 100mm diameter

by 200mm high cylinders. Cylinders (100 mm diameter and 200 mm height) cast were

used to test for the determination of elastic modulus including the compressive strength.

The elastic modulus is determined as the slope of the tangent drawn at the initial portion

of the curve.

4.2.1 Compressive Strength

Compressive strength tested on cube specimens and cylindrical specimens at

the end of 28 days, strain at ultimate stress and strain at failure stress and modulus of

elasticity of M60, M80 and M100 control and hybrid fibre reinforced concrete

specimens are shown in Table 4.1. The stress strain behaviour of M60, M80 and M100

grade control and hybrid reinforced concrete mixtures are illustrated in Figures 4.1, 4.2

and 4.3 respectively.

80

Figure 4.5 Normalised Stress-Strain Curves for Hybrid Fibre Reinforced Concrete

(M60 Concrete)


(M80 Concrete)

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9

No

rma

lise

d S

tres

s (f

cy/f

1cy

)

Normalised Strain (Єo / Єof)

M60 Concrete

HPM1DF1 HPM1DF2 HPM1DF3 HPM1TF

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

No

rmali

sed

Str

ess

(fcy

/f1

cy)


M80 Concrete


81


(M100 Concrete)

Compressive strength:

RIff cycy 06.1' (4.1)

for strength between 60 – 110 MPa

where, fcy and f‘cy are the cylinder compressive strength of plain and HyFRC,

respectively in MPa.

Strain corresponding to peak stress:

RIof 0001.00

' (4.2)


where, Єo and Є‘of are the strain corresponding to peak stress for plain and

HyFRC, respectively.

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

No

rma

lise

d S

tres

s (f

cy/f

1cy

)


M100 Concrete


82

Toughness ratio:

RITRTF ccf 013.0 (4.3)


where, TFc and TRcf are the toughness of plain and HyFRC, respectively.

Derived relationships have high correlation and tend to represent the raw

data successfully. Observed trends are well emphasized on the derived relationships.

It has been pointed out by Carrasquillo et al. [102] that the shape of the

experimentally drawn stress-strain curve is affected by the length of the strain gauges.

As suggested by Ezeldin and Balaguru [91], a correction factor may be introduced in the

equation to generate the analytical curves for the middle third or the full length strain

measurements. Fanella and Naaman [101] have considered the deformation of the whole

length of the cylinder whereas, Ezeldin and Balaguru [91] have considered the middle

half of the cylinder for measuring the strain.

Thus, the proposed model can be used to obtain the complete stress-strain

curve of steel-fibre reinforced concrete containing hybrid fibres. The parameters needed

are the reinforcing index RI, the peak compressive strength of the fibre reinforced

concrete f‘cy and the corresponding strain Єof. Knowing the compressive strength of the

plain concrete, fcy and the reinforcing index, RI, all required parameters of the stress-

strain curve of the steel-fibre reinforced concrete can be evaluated using the proposed

equations.

4.2.4 Ductility

The term ductility in seismic design is used to explain the ability of a

structure to withstand large amplitude cyclic deformations in the inelastic range without

a considerable strength reduction [88, 89]. Ductility factor, which is defined as the

maximum deformation divided by the corresponding deformation when yielding occurs,

83

permits the maximum deformation to be expressed in nondimensional terms. This can

be used as indices of inelastic deformation for seismic design and analysis. The

displacement ductility factor (μ), which is usually determined in inelastic time history

dynamic analysis, may vary between 1 for elastically responding structures to as high as

7 for ductile structures. However, it is typically ranging from 3 to 6 [88]. In this

research the displacement has been replaced by the strain and the ductility factor can be

calculated using below mentioned equation;

ofu / (4.4)

where, Єu is the ultimate (maximum) strain and Єof is the yield strain.

The ductility factor ranges between 5.75 – 7.85 for various hybrid fibre

reinforced concrete specimens and the percentage increase in ductility is shown in Table

4.2. The percentage increase in ductility was higher (293%) when all fibres were used in

hybrid form in M60 grade of concrete. Similar trend of results were also observed by

Amin Noushini et al. using single fibre as reinforcement in concrete [89].

4.2.5 Energy absorption capacity

Energy absorption capacity of the concrete is calculated as the area under

the stress-strain curve up to the maximum strain. The strain energy stored is expressed

as N-mm per cubic millimeter. It can be seen that after adding fibres, the energy stored

in the concrete increased greatly as shown in column 7 of Table 4.1. Under the

conditions of adding hybrid fibres, the combination of (0.25% Micro-steel, 0.75%

hooked steel and 0.5% polyethylene fibres) in M100 grade of concrete showed the

highest energy stored (1.17) which were also greater than the values under the condition

of adding two fibres either micro-steel – hooked steel (0.863) or micro-steel-

polyethylene (0.768). From the results, it is evident that the ductility and energy

absorption capacity of fibre reinforced concrete depends primarily on the fibre‘s ability

to bridge the cracks at high levels of strain. The combination of micro-steel, hooked

steel and polyethylene fibre was tougher indicating a possible synergy in the action of

these fibres.

84

4.3 SUMMARY

This chapter described the experimental results of the compressive strength

of high performance concrete incorporating hybrid fibres. Results from the study

indicate that the use of fibres caused a marginal increase in the compressive strength of

concrete. The maximum enhancement in compressive strength was only as high as

11.3% in the case 0.25% micro-steel, 0.75% hooked steel and 0.5% polyethylene in

M60 grade of concrete. The same hybrid combination showed an increase in the order

of 4.3% and 2.8% for M80 and M100 grade of concrete respectively. There was a

significant enhancement in energy absorption capacity and ductility when hybrid fibres

were used. The maximum enhancement of ductility was as higher as 292% for 0.25%

micro-steel, 0.75% hooked steel and 0.5% polyethylene in M 60 grade of concrete.

Similar increase was also seen in M80 and M100 grade of concrete. Post peak

performance was significantly improved with fibre addition, which resulted in good

compressive toughness and ductility. The shear strength, flexural strength and direct

tensile strength of HyFRC is discussed in the subsequent chapters.

71

Table 4.1 Compressive Strength Properties of Various Hybrid Fibre Reinforced

Concrete Specimens

S.No Mix

Identification

fck

fcy

Strain Stress

at

Failure

Strain

at

Failure

(Єu)

Ductility

(µ)

Energy

Stored

Initial

Tangent

Modulus

(1) (2) (3) (4) (5) (6) (7) (8)

Concrete Grade (60 MPa)

1. HPM1 65.2 50.00 0.00175 41 0.0035 2 0.122 28.50

2. HPM1DF1 70.7 52.00 0.00192 35 0.0130 6.77 0.525 29.50

3. HPM1DF2 71.6 58.35 0.0019 33 0.0120 6.31 0.477 30.10

4. HPM1DF3 70.2 56.25 0.0019 32 0.0110 5.79 0.421 29.60

5. HPM1TF 72.6 58.90 0.0021 32 0.0165 7.85 0.710 31.60


1. HPM2 84.2 67.10 0.0017 62 0.0035 2.05 0.163 33.60

2. HPM2DF1 86.3 69.90 0.0022 51 0.0130 5.91 0.732 35.80

3. HPM2DF2 86.2 70.25 0.0021 49 0.0125 5.95 0.666 36.10

4. HPM2DF3 85.3 67.38 0.0020 46 0.0115 5.75 0.577 33.70

5. HPM2TF 87.9 70.64 0.0023 46 0.0165 7.17 0.915 37.10


1. HPM3 107.3 85.90 0.0017 79 0.0035 2.05 0.208 41.90

2. HPM3DF1 109.4 86.42 0.0021 69 0.0120 5.71 0.863 43.20

3. HPM3DF2 108.9 88.20 0.0020 65 0.0120 6 0.854 44.10

4. HPM3DF3 108.6 85.79 0.0020 64 0.0120 6 0.768 42.90

5. HPM3TF 110.4 87.10 0.0022 61 0.0160 7.27 1.17 46.20

Note:

(1) Cube Compressive Strength (MPa) (5) Strain at Failure

(2) Peak Compressive Stress and Cylinder

Compressive Strength (MPa)

(6) Ductility

(3) Strain at Peak Stress (7) Energy Stored (N-mm/mm3)

(4) Stress at Failure (MPa) (8) Initial Tangent Modulus (GPa)

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0

10

20

30

40

50

60

70

0 0.005 0.01 0.015 0.02

Co

mp

ress

ive S

tres

s (M

Pa)

Strain (mm/mm)

HPM1

HPM1DF1

HPM1DF2

HPM1DF3

HPM1TF

Figure 4.1 Compressive Stress Vs Strain for PC and HyFRC (M60)

0

10

20

30

40

50

60

70

80

0 0.005 0.01 0.015 0.02

Co

mp

resi

ve S

tress

(MP

a)

Strain (mm/mm)

HPM2

HPM2DF1

HPM2DF2

HPM2DF3

HPM2TF


73

0

10

20

30

40

50

60

70

80

90

100

0 0.005 0.01 0.015 0.02

Co

mp

resi

ve S

tress

(M

Pa

)

Strain (mm/mm)

HPM3

HPM3DF1

HPM3DF2

HPM3DF3

HPM3TF


All cylinders were tested according to ASTM C-39 standards after 28 days.

The test was conducted in a servo controlled 1000 kN compression testing machine.

Before testing, the cylinder was capped with a hard plaster on the cast face to ensure

parallel loading faces of the test specimens and constant height for all cylinders. A

compressometer equipped with dial gauge available in the laboratory was used to record

the deformation on the cylinder. The load was applied at a very slow rate and an initial

load of about 50 kN was applied and released. The testing head was lowered slowly to

bring it in contact with the specimen. At this stage, the dial gauge was set to zero in the

compressometer. Load was increased slowly and the deformations were recorded.

Deformations were taken approximately at every 40 kN load increment. There was no

difficulty in recording the load and deformation readings manually. Load indicator of

the machine was increasing slowly during the loading stage (ascending). However, the

movement was rather faster in the unloading (descending) stage. Efforts were made to

take as many readings as possible, to get considerable observations of post-peak portion

of the stress-strain curve. In the descending portion, readings were taken at random

intervals. Strains and corresponding stresses were calculated and the average readings

were reported.

74

A typical stress-strain curve of HyFRC consists of three distinct branches—

an ascending branch, a pseudo strain-hardening branch and a descending branch. The

ascending branch is very similar to that of conventional fibre reinforced concrete. In

both cases there is a loss of linearity before reaching the peak load, which indicates the

initiation of fracture processes. The pseudo strain-hardening branch begins near the

peak load. This branch is nearly horizontal and short. At this branch, cracks form in the

specimen due to lateral expansion of the cylinder. These cracks are directed nearly

parallel to the direction of loading. Fibres aligned normal to the loading direction will

therefore be intercepted by these cracks and offer some resistance to their growth. As a

result, the transverse strain and the longitudinal strain increase with the stress almost

maintaining constant. The descending branch of the stress-strain curve depends on the

post-peak response of the specimen when fibres are fully activated due to extensive

cracking and dilation of fibres. The load drops off gradually and a shearing crack

develops. Because fibre bridging effect has obviously improved the post-crack ductility

of the material, the failure mode was changed from brittle or semi-brittle failure to

ductile failure.

The actual compressive strength determination of controlled concrete and

concrete with hybrid fibres was tested after 28 days of curing. It is observed from

Table 4.1 that the cube compressive strength varies from 65.2 – 110.4 MPa for the three

mixes with and without fibres. Comparing mixes containing hybrid fibres 0.25% micro

steel, 0.75% hooked steel and 0.5% polyethylene, it is observed increase in compressive

strength is about 11.34% when compared to plain concrete specimens in M60 grade of

concrete. Changing the fibre composition in hybrid fibres from (0.25% micro steel &

0.75% hooked steel) to hybrid fibres (0.75% hooked steel & 0.5% polyethylene)

increases the compressive strength only by 1.21% for M100 grade of concrete. As seen

from Table 4.2, the compressive strength increases by 1.21 to 11.3% for all the three

mixes containing with and without fibres. The percentage increase in strength is

decreased by increasing the grade of concrete mixtures, may be because of higher

matrix strength needs more fibre content.

75

Table 4.2 Characteristics of Various Hybrid Fibre Reinforced Concrete Specimens

Using Stress – Strain Behaviour

S.No Mix

Identification

fck

%

Increase

Ductility

(µ)

%

Increase

Initial

Tangent

Modulus

%

Increase

(1) (2) (3) (4) (5) (6)


1. HPM1 65.2 - 2 - 28.50 -

2. HPM1DF1 70.7 8.40 6.77 239 29.50 3.5

3. HPM1DF2 71.6 9.80 6.31 216 30.10 5.6

4. HPM1DF3 70.2 7.60 5.79 189 29.60 3.9

5. HPM1TF 72.6 11.34 7.85 293 31.60 10.8


1. HPM2 84.2 - 2.05 - 33.60 -

2. HPM2DF1 86.3 2.49 5.91 188 35.80 6.5

3. HPM2DF2 86.2 2.37 5.95 190 36.10 7.4

4. HPM2DF3 85.3 1.30 5.75 180 33.70 0.3

5. HPM2TF 87.9 4.39 7.17 250 37.10 10.4


1. HPM3 107.3 - 2.05 - 41.90 -

2. HPM3DF1 109.4 2.10 5.71 179 43.20 3.1

3. HPM3DF2 108.9 1.49 6 193 44.10 5.3

4. HPM3DF3 108.6 1.21 6 193 42.90 2.4

5. HPM3TF 110.4 2.88 7.27 255 46.20 10.3

Note:

(1) Cube Compressive Strength (MPa) (4) % increase in ductility

compared to plain concrete

(2) % increase in compressive strength


(5) Initial Tangent Modulus (GPa)

(3) Ductility (6) % increase in elastic modulus


76

Stress-Strain curve varied linearly up to an elastic limit very close to peak

load for all the grades of concrete. It may be observed that the hybrid fibre reinforced

concrete showed a modest increase in peak strengths with the increase in volume

fraction of fibres and the difference in behaviour was found in the post peak range. In

these cases, the specimens failed in a stable manner and the post peak softening branch

was captured. When compared to plain concrete mix the strain softening behaviour is

increased in all the hybrid fibre reinforced concrete (the slope of the ascending to

descending) is increased enormously in hybrid fibre reinforced concrete. Also the

failure strain showed increase in hybrid reinforcement when compared to concrete

without reinforcement. By increasing the grades of concrete, the strain softening

behaviour get closer to each mixtures.

4.2.2 Elastic Modulus of HyFRC

The initial tangent modulus of hybrid mixes is found to be 9 percent to

10 percent higher than that of control concrete in all the mixes when all the fibres were

used in hybrid form as shown in Table 4.2. The highest elastic modulus recorded was

46.20 GPa when all the fibres were used in hybrid form in concrete in M100 grade of

concrete. Increase in compressive strength leads to the increase in elastic modulus and

the trend of results are similar to Sivakumar et al. [55].

Furthermore, the concrete specimens containing no fibres cracked and failed

in a brittle condition when it had reached the ultimate strain in the concrete. On the

other hand, fibre reinforced concrete also cracked at the ultimate strain but it was

capable of carrying the load well after the crack developed on the concrete. This

indicates that the fibre reinforced concrete has the ability (ductility) to hold the

fragments from falling apart when crack gets propagated.

4.2.3 Toughness of HyFRC in Compression

All results pertaining to compression are presented in Table 4.1. Discussions

are based on the average results of three samples. The addition of fibres increased the

strain corresponding to the peak stress. The strain capacity and the elastic deformation

capability of the concrete matrix in the prefailure zone are increased considerably with

77

the inclusion of fibres. Increase in peak strain, is maximum for fibres having higher

volume fraction and for higher aspect ratios. Both ascending and descending portion of

the stress-strain curves are affected by the addition of steel- fibres.

The area under the stress-strain curve is a measure of toughness of the

material. Researchers have used different definitions for the toughness using the area

under the curve. Fanella and Naaman [101] have defined the toughness of FRC as the

ratio of toughness of the fibre reinforced matrix to that of the unreinforced control

matrix. The toughness of the specimen was computed up to a strain of 0.0154, though

some of the specimens still had significant resistance left. Here, this ratio is greater than

unity. It can also be defined as the ratio of area of the descending part, to the area of the

ascending part of the stress-strain curve. Ezeldin and Balaguru [91], have proposed a

rigid plastic approach to define the toughness ratio. It is observed in the present work

that the area under the stress-strain curve increases with the increase in fibre content. As

this area increases with respect to both volume fraction and aspect ratio (l/d), a common

parameter, which combines these two effects, is used in the form of reinforcing index.

However, the reinforcing index RI (=wf x l/d), is defined in terms of weight fraction wf.

Weight fraction is approximately equal to 3.2 times the volume fraction for steel fibres

and 0.4 times the volume fraction for polyethylene fibres. In this work, the toughness is

measured as the total area under the stress-strain curve up to a strain of 0.015, which is

five times the ultimate concrete strain of 0.003 adopted in the ACI building code or the

maximum ultimate strain as obtained by the experimental investigation. Fenella and

Naaman [12], Ezeldin and Balaguru [13] have used this strain, as it is sufficient to

represent the trend of the post-peak behaviour. This toughness is compared to the

toughness of a rigid plastic material in the form of a Toughness Ratio (TR) as indicated

in Figure 4.4.

By this approach, the toughness of the material can be easily evaluated and

the results are presented in the Table 4.3. The toughness ratios (TR), presented in

column (3) are calculated from the experimental stress-strain curves. Normalized stress-

strain curves between fcy/f1

cy and Єc / Єof for hybrid fibre reinforced concrete are shown

in Figures 4.5 - 4.7.

78

Figure 4.4 Toughness Ratio for Compression

Table 4.3 Compressive Strength Properties of Various Hybrid Fibre Reinforced

Concrete Specimens

Sl.No Mix Identification

From Experiments From Proposed Equations

(1) (2) (3) (4) (5) (6) (7)

f’cy Єof TR RI f’

cy Єof TR


1. HPM1 50.00 0.00175 0.69 0 50.00 0.00175 0.69

2. HPM1DF1 52.00 0.00192 0.72 2.19 54.32 0.00197 0.72

3. HPM1DF2 58.35 0.0019 0.71 3.56 55.77 0.00211 0.74

4. HPM1DF3 56.25 0.0019 0.71 2.67 54.83 0.00202 0.72

5. HPM1TF 58.90 0.0021 0.74 4.21 56.46 0.00217 0.74


1. HPM2 67.10 0.0017 0.74 0 67.10 0.0017 0.74

2. HPM2DF1 69.90 0.0022 0.79 2.19 69.42 0.00192 0.77

3. HPM2DF2 70.25 0.0021 0.78 3.56 70.87 0.00206 0.79

4. HPM2DF3 67.38 0.0020 0.75 2.67 69.93 0.00197 0.77

5. HPM2TF 70.64 0.0023 0.75 4.21 71.56 0.00212 0.79

TR = Area OABC/f ' cy x 0.015

A

B

Com

pre

ssiv

e S

tren

gth

, M

Pa

MP

a

Strain, mm/mm

0.003 0.006 0.009 0.012 0.015

C O 0

20

40

60

80

100

120

f ' cy

79

Table 4.3 (Continued)

A square fitting line analysis was performed, to establish a possible

relationship between the fibre reinforcing index and the parameters of the stress-strain

curve, i.e. the peak compressive strength, the corresponding peak strain and the

toughness of the concrete. These equations are obtained using the regression analysis

performed using the experimental results. All these equations very well quantify the

effect of fibres on the compression behavior of the fibre reinforced concrete. Results of

these equations are presented in Table 4.3 (columns 5-7) which are based on Eqn. (4.1) -

(4.3). Different proposed equations, for concrete strength between 60 to 110 MPa are

given in equations 4.1, 4.2 and 4.3. Similar trend of equations were also obtained by

Nataraja et al. [90] using single fibres.

Sl.No Mix

Identification

From Experiments From Proposed

Equations

(1) (2) (3) (4) (5) (6) (7)

f’cy Єof TR RI f’

cy Єof TR


1. HPM3 85.90 0.0017 0.73 0 85.90 0.0017 0.73

2. HPM3DF1 86.42 0.0021 0.81 2.19 88.22 0.00192 0.76

3. HPM3DF2 88.20 0.0020 0.78 3.56 89.67 0.00206 0.78

4. HPM3DF3 85.79 0.0020 0.79 2.67 88.73 0.00197 0.76

5. HPM3TF 87.10 0.0022 0.78 4.21 90.36 0.00212 0.78

Note:

(1) Peak Compressive Stress and Cylinder

Compressive Strength (MPa)

(5) Peak stress in MPa from the

proposed equation (2).

(2) Strain corresponding to Peak Stress (6) Strain corresponding to peak stress

from proposed equation (3).

(3) Toughness ratio based on experimental

results

(7) Toughness ratio from the proposed

equation (4).

(4) Reinforcing index, RI = wf x l/d, where,

wf is the weight fraction of fibres.

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CHAPTER 4 COMPRESSIVE STRENGTH OF HYBRID ...shodhganga.inflibnet.ac.in/bitstream/10603/34576/7...70 CHAPTER 4 COMPRESSIVE STRENGTH OF HYBRID FIBRE REINFORCED CONCRETE 4.1 GENERAL High