Behaviour of reinforced concrete beams with 50 percentage fly ash

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
ISSN 0976 – 6316(Online) Volume 4, Issue 2, March - April (2013), © IAEME
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 4, Issue 2, March - April (2013), pp. 36-48
IJCIET
© IAEME: www.iaeme.com/ijciet.asp
Journal Impact Factor (2013): 5.3277 (Calculated by GISI) © IAEME
www.jifactor.com

BEHAVIOUR OF REINFORCED CONCRETE BEAMS WITH 50
PERCENTAGE FLY ASH

1 2 3 4 5
P.S.Joanna , Jessy Rooby , Angeline Prabhavathy , R.Preetha , C.Sivathanu Pillai
1
Civil Engineering Department, Hindustan University,Padur- 603103, India.
2
Professor, Civil Engineering Department, Hindustan University,Padur- 603103, India.
3
Professor, Civil Engineering Department, Hindustan University, Padur-603103, India.
4
Scientific Officer,Civil Engineering Division,Indira Ghandhi Centre for Atomic
Research,Kalpakkam- 603 102, India.
5
Associate Director,Civil Engineering Division,Indira Ghandhi Centre for Atomic
Research, Kalpakkam- 603 102, India.

ABSTRACT

Fly ash has emerged as novel engineering materials which lead to global sustainable
development and lowest possible environmental impact with considerable promise as binders
in the manufacture of concrete. In this paper, the results of laboratory investigation conducted
on the structural behavior of reinforced concrete beam with high volume of low calcium
(class F) fly ash are presented. Experimental investigation included testing of nine reinforced
concrete beams with and without fly ash. Portland cement was replaced with 50% fly ash and
Conplast SP430 was used as superplastisizer for the casting of beams. Data presented include
the load-deflection characteristics, cracking behavior, ductility indices, moment- curvature
and end rotations of the reinforced concrete beams with and without fly ash when tested at 28
days, 56 days and 75 days. The investigation revealed that there is a significant improvement
in flexural strength of reinforced fly ash concrete beams beyond 28 days.

Key Words: Ordinary Portland Cement, Reinforced Fly Ash concrete beams, ductility,
moment- curvature.

36


1. INTRODUCTION

The Ordinary Portland Cement (OPC) is one of the main ingredients used for the
production of concrete and has no alternative in the construction industry. Unfortunately,
production of cement involves emission of large amounts of carbon-dioxide gas in to the
atmosphere, a major contributor for green house effect and the global warming. Hence it is
inevitable either to search for another material or partly replace it by some other material. Fly
ash is one such pozzolanic material which can be used in concrete as partial replacement of
cement.
Jiang and Malhotra (2000) found that the incorporation of 50% fly ash in concrete
gives good compressive strength at 91 days. Gopalakrishnan et al. (2001) showed that the fly
ash concretes have superior durability properties. Rafat Siddique (2004) studied the
compressive strength and flexural strength of High Volume Class-F Fly Ash concrete and
found that there is a significant improvement of strength properties beyond 28 days. They
also found that the strength of concrete with 40%, 45% and 50% fly ash content, even at 28
days is sufficient enough for use in reinforced cement concrete construction. Khatib (2008)
found that the concrete with 60% fly ash replacement for cement can produce self
compacting concrete with adequate strength. Dakshina Murthy and Sudheer Reddy (2010)
found that fly ash replacement up to 30% in concrete gives good improvement in flexural
strength at 28 days. Sunilaa et al. (2011) found that the addition of 40% fly ash gives better
resistance against shear at 28 days.
Extensive research has been done on the compressive strength and flexural strength of
High Volume Fly Ash Concrete (HVFAC). Hence in this investigation, behavior of
Reinforced Concrete (RC) beams with HVFAC was carried out. 50% of cement was replaced
with fly ash and Conplast (SP430) was used as superplastisizer for the casting of beams. A
total of nine reinforced concrete beams with and without fly ash were cast and tested. Out of
the nine specimens, four controlled specimens were cast without fly ash and the other five
specimens were cast with 50% fly ash. Data presented include the deflection characteristics,
cracking behavior, ductility indices, moment-curvature and end rotations of the specimens.

2. EXPERIMENTAL INVESTIGATIONS

2.1 Materials and mix proportions
In the current investigation 50% of cement was replaced with Fly Ash in the casting
of RC beams. The materials used in the mix were Ordinary Portland Cement (OPC), river
sand, low calcium Fly Ash (Class F), aggregate and potable water. Conplast (SP430)
superplastisizer was incorporated in the mix to increase the workability. Beams were made
with M30 grade concrete. Water-cement ratio of 0.45 and 0.75% Conplast superplastisizer
were used for reinforced OPC concrete beams. Water/cement & Fly ash ratio of 0.45 and
1.5% Conplast superplastisizer were used for 50% fly ash concrete beams. Fe 500 grade steel
was used for longitudinal reinforcement and for stirrups.

2.2 Test beam details
Nine numbers of reinforced concrete beams with and without fly ash were cast and
tested in the loading frame. The span of the beam was 2500 mm and of size 150mm x
250mm. The specimens were designed as per IS: 456-2000. Out of the nine specimens tested,

37


four specimens were cast without fly ash and four specimens were cast with 50% fly ash.
Two specimens were cast in each series. Four specimens were tested at 28th day, four
specimens were tested at 56th day and one specimen was tested at 75th day from the date of
casting. Reinforcement details for the beam and the details of the specimens tested are given
in Table 1. A five lettered designation is given to the specimens. First 2 letters represents the
beam with Conplast superplastisizer, 3rd one % of fly ash added, 4th one identity of specimen
in a particular series as two specimens were tested in each series and the last one indicates the
day on which the specimen is being tested.

Table 1: Test beam details

SL.No. Beam Testing Reinforcement in beams
Number of Longitudinal Stirrups
Beams
Nos. and Nos. and Diameter Spacing
(days)
size at top size at (mm) (mm)
bottom
1 CB0% 1-28
2 CB0% 2-28 28 2#10 2#12 + 1#16 8 120
3 CB50% 1-28
4 CB50% 2-28
5 CB0% 1-56
56 2#10 2#12 + 1#16 8 120
6 CB0% 2-56
7 CB50% 1-56
8 CB50% 2-56
9 CB50% 1-75 75 2#10 2#12 + 1#16 8 120

3. TEST SET-UP

The testing was carried out in a loading frame of 400 kN capacity. TML strain gauge
was fixed at the mid span of the tension bar and then protected using coating tape to avoid
accidental damage during pouring of concrete. Strain gauges were also attached to the
concrete surface in the central region of the beam to measure the strain at different depths.
The top surface of the beam was instrumented with strain gauge to measure the concrete
compressive strains in the pure bending region. Linear Voltage Displacement Transducers
(LVDTs) were used for measuring deflections at several locations, one at mid span, two
directly below the loading points and two near the end supports as shown in the Figure 1.
Strain gauges and LVDTs were connected to a data logger from which the readings were
captured by a computer at every load intervals until failure of the beam occurred. The beams
were subjected to two-point loads under a load control mode. The development of cracks
were observed and the crack widths were measured using a hand-held microscope with an
optical magnification of X50 and a sensitivity of 0.02 mm. Figure 2 shows the test set-up.

38


Strain Gauge
TCS (Extreme fibre concrete strain)

25 mm
FCS1 10 mm
25 mm
FCS2 10 mm
25 mm
FCS3 10 mm

Hydraulic Jack
FCS4 10 mm
50 mm
Load Cell
CSS ( Center Steel Strain)

Steel Support

DL2 DL1 DC DR1 DR2
LVDT

Figure 1: Position of LVDTs and Strain gauges

Figure 2: Test set-up

39


4. TEST RESULTS

4.1. General observations
Vertical flexural cracks were observed in the constant-moment region and final failure
occurred due to crushing of the compression concrete with significant amount of ultimate
deflection. When the maximum load was reached, the concrete cover on the compression
zone started to fall for the beams with and without fly ash. Figure 3. shows the failure pattern
of the test specimens. Crack formations were marked on the beam at every load interval at the
tension steel level. Initial cracking was formed at 17% and 20% of ultimate load for beams
with and without fly ash respectively at 28 days and it is 16% and 20.4% at 56 days. Initial
crack for fly ash concrete beam at 75 days is 18.6% of ultimate load. It was noticed that the
first crack always appears close to the mid span of the beam. The cracks formed on the
surface of the beams were mostly vertical, suggesting flexural failure of the beams. The crack
widths at service loads for fly ash concrete beams ranged between 0.18mm to 0.2mm and this
is within the maximum allowable value as stipulated by IS: 456-2000 for durability
requirements.

(a) CB0% 1-28 (b) CB50% 1-28

Figure 3: Failure Pattern of the beams with 50% fly ash and without fly ash

4.2. Load-Deflection curve
The experimental load-deflection curves of the RC beams with 50% fly ash and
without fly ash when tested at 28th day, 56th day and 75th day are shown in Figure 4 Figure 5
and Figure 6 respectively. The average ultimate loads for both the reinforced OPC concrete
beams and 50% fly ash concrete beams are 182 kN & 152 kN respectively at 28th day and it is
186 kN & 168 kN respectively at 56th day. The ultimate load for fly ash concrete beam at 75th
day is found to be 197 kN. Though the ultimate loads for the fly ash concrete beam is 16%
and 9.6% less than the OPC beams at 28th day and 56th day respectively, its ultimate load
increases at 75th day. The average span-deflection ratios under the design service loads for the
reinforced concrete fly ash beams are 290 at 28th day 258 at 56th day and 251 at 75th day,
which are within allowable limit as per IS: 456-2000.

40


200 200

180 180

160 160

140 140
DR1
120 120 DC

Load(kN)
Load (kN)

DR2
100 100 DR1
DL1 80 DR2
80
60 DL1
60 DL2
DL2
40 40
DC
20 20

0 0
0 5 10 15 20 25 0 5 10 15 20 25 30 35 40

Deflection (mm) Deflection (mm)

(a) CB0% 1-28 (b) CB0% 2-28

160
160
140
140
120
120
100
100
DC DC
Load(kN)

80
80
Load (kN)

DR1 DR1
60
DL1 60 DL1
40
DR2 40 DR2
20 DL2 DL2
20
0
0
0 5 10 15 20 25 30
0 5 10 15 20 25 30
Deflection (mm)
Deflection (mm)

(c) CB50% 1-28 (d) CB50% 2-28

Figure 4: Load- Deflection curves for the beams tested at 28 days

41


200 200
180 180
160 160
140 140
DC DC
Load (kN)

120 120

Load (kN)
100 DR1 100 DR1
80 80
DR2 DR2
60 60
40 DL1 40 DL1
20 DL2 20 DL2
0 0
0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30

(a) CB0% 1-56 (b) CB0% 2-56

180 180
160 160
140 140
120 120
Load (kN)

DC DC
Load (kN)

100 100
DR1 DR1
80 80
DR2 60 DL1
60
DL1 40 DR2
40
20
20 DL2 DL2
0
0
0 5 10 15 20 25 30
0 5 10 15 20 25 30

(c) CB50% 1-56 (d) CB50% 2-56
Figure 5: Load- Deflection curves for the beams tested at 56 days

200
180
160
140 DC
Load (kN)

120
100 DR1
80 DL1
60
DR2
40
20 DL2
0
0 5 10 15 20 25 30 35 40
Deflection (mm)

(e) CB50% 1-75
Figure 6: Load- Deflection curves for the beam tested at 75 days

42


4.3. Displacement Ductility
Displacement ductility is the ratio of ultimate to first yield deflection. In general high
ductility ratios indicate that a structural member is capable of undergoing large deflections
prior to failure. The average displacement ductility for fly ash concrete beams and OPC
concrete beams are 4.8 & 5.8 respectively when tested at 56th day. The displacement ductility
for fly ash concrete beams increases at 75 days. Table 2 shows the ductility of beams. Thus
the fly ash concrete beams shows adequate displacement ductility and can be considered for
structural members subjected to large displacement such as sudden forces caused by
earthquake.

Table 2: Displacement ductility of beams
Beam Deflection at yield (mm) Max. deflection Displacement ductility
Specification (mm)

CB0% 1-28 4.8 20.0 4.17
CB0% 2-28 5.4 27.5 5.09
CB50% 1-28 5.0 19.3 3.86
CB50% 2-28 5.0 22.2 4.44
CB0% 1-56 4.6 24.6 5.34
CB0% 2-56 3.5 22.0 6.28
CB50% 1-56 5.0 21.6 4.32
CB50% 2-56 4.0 20.5 5.13
CB50% 2-75 4.3 27.0 6.30

4.4. End rotation
The moment-end rotation curves of fly ash concrete beams and OPC concrete beams
are presented in Figure 7 when tested at 28,56 and 75 days respectively. It was observed that
the average end rotations of the fly ash concrete beams and OPC concrete beams at ultimate
loads are 1.70 & 1.80 respectively when tested at 56 days and it is 20 at 75 days. Thus the end
rotations of the beams with fly ash are comparable with OPC concrete beams.

80 80
70 70
Moment (kN.m)
Moment (kN.m)

60 60
50 50 CB0% 1-56
CB0% 1-28
40 40 CB0% 2-56
CB0% 2-28
30 30 CB50% 1-56
CB50% 1-28
20 20 CB50% 2-56
CB50% 2-28
10 10 CB50% 1-75

0 0
0 0.5 1 1.5 2 2.5 3 0 1 2 3
End rotation (0) End rotation (o)

Figure 7: Moment-Rotation curves for beams tested at 28, 56 and 75 days

43


4.5. Concrete and steel strain

The concrete and steel strains were measured at every load increments. The strain
distribution for the concrete and steel at 28th day 56th day and 75th day are presented in Figure
8 and Figure 9 respectively. The measured concrete strains at the top surface and steel strains
at ultimate load varied from 2167x10-6 to 3073 x10-6 and 12342x10-6 to 15320 x10-6
respectively for OPC beams and it is from 2919 x10-6 to 3816 x10-6 and 9385 x10-6 to 25986
x10-6 respectively for fly ash concrete beams when tested at 28th days. These results also
show that fly ash concrete is able to achieve its full strain capacity under flexural loading.
Figure 10.and Figure 11.shows the comparison of concrete strain at top surface and steel
strains for all beams at 28 and 56 and 75 days.

200 200
180 180
160 160
140 FCS1 FCS1
140
Load (kN)

120 TCS FCS2
Load (KN 120
100 FCS2 100 FCS4
80 FCS4 80 FCS3
60 60
FCS3 TCS
40 40
20 CSS CSS
20
0 0
-20000 -15000 -10000 -5000 0 5000 -30000 -20000 -10000 0 10000
Strain (x10-6) Strain (x10-6)

(a) CB0% 1-28 (b) CB0% 2-28

160 160
140 140
120 TCS 120 TCS

100
Load (kN)

Load(kN)

100 FCS1 FCS1

80 FCS2 80 FCS2
60 FCS3 60 FCS3
40 FCS4
40 FCS4
20 20
CSS CSS
0 0
-30000 -20000 -10000 0 10000 -15000 -10000 -5000 0 5000
Strain (x10-6)
Strain (x10-6 )

(c) CB50% 1-28 (d) CB50% 2-28

Figure 8: Load- Strain curves for beams tested at 28 days

44


200 200
180 180
160 160
TCS 140 FCS1
140

Load (kN)
Load(kN)

120 FCS1 120 FCS2
100 100
FCS2 FCS3
80 80
60 FCS3 60 TCS
40 FCS4 40 FCS4
20 20
CSS CSS
0 0
-25000 -20000 -15000 -10000 -5000 0 5000 -20000 -15000 -10000 -5000 0 5000
Strain (x10-6) Strain (x10-6)

(a) CB0% 1-56 (b) CB0% 2-56

200 180
160
150 TCS 140 FCS1
Load (kN)

FCS1 120 FCS2
Load (kN)

100 100
FCS2 FCS3
80
FCS3 60 FCS4
50
FCS4 40 TCS
CSS 20 CSS
0 0
-10000 -5000 0 5000 -20000 -15000 -10000 -5000 0 5000
Strain (x10-6) Strain (X10-6)

(c) CB50% 1-56 (d) CB50% 2-56
200
180
160
140
FCS1
Load(kN)

120
FCS2
100
80 FCS3
60 TCS
40 CSS
20
0
-20000 -15000 -10000 -5000 0 5000
Strain(x10-6)

(d) CB50% 1-75
Figure 9: Load- Strain curves for beams tested at 56 days and 75 days

45


200 200
180 180
160 160
140 140
Load (kN)

120 120

Load (kN)
100 100
80 80
60 60

40 40
20
20
0
0
-25000 -20000 -15000 -10000 -5000
-6
0 5000
-30000-25000-20000-15000-10000 -5000-6 0 5000 10000 Strain (x10 )
Strain (x10 )
CB0% 1-TCS CB0% 1-CSS CB0% 2-TCS CB0% 1-TCS-56 CB0% 1-CSS-56 CB0% 2-TCS-56 CB0% 2-CSS-56
CB50% 1-TCS-56 CB50% 1-CSS-56 CB50% 2-TCS-56 CB50% 2-CSS-56
CB0% 2-CSS CB50% 1-TCS CB50% 1-CSS CB50% 1-TCS-75 CB50% 1-CSS-75

a) Steel and Concrete Strain at 28 days a) Steel and Concrete Strain at 28 & 56 days

Figure 10: Comparison of Steel and Concrete Strain at 28, 56 and 75 days

4.6. Moment-curvature:
Moment-Curvature diagrams were generated for all the beams based on the concrete
strain and steel strain. The moment-curvature of the beams at 28, 56 and 75 days is shown in
figure 11. Thus the curvature of the beams with fly ash is comparable with OPC concrete
beams. Table 3 shows the overall performance of the OPC Concrete beams and Reinforced
Fly Ash Concrete beams.

80
70 70
60 60
Moment (kN-m)

50 50
Moment (kN-m)

CB0% 1-56
CB0% -28
40 40 CB0% 2-56
CBO% 2-28
30 CB50% 1-56
CB50% 1-28 30
CB50% 2-56
20 CB50% 2-28 20
CB50% 1-75
10 10
0 0
0 20 40 60 80 100 120 140
0 50 100 150
Curvature (x10-6) Curvature (x10-6)

Figure 11: Moment- Curvature for beams tested at 28, 56 and 75 days

46


Table 3: Performance details of reinforced fly ash concrete beams and OPC concrete beams

Beam Max. Deflection Max. Strain in Displace Strain Load Deflection
designation Load at max. Moment concrete ment in steel at first at service
(kN) Load (kN.m) at max. ductility crack loads
(mm) load (kN) (mm)

CB0% 1-28 177.5 20.0 65.1 0.0012 4.17 0.0103 32.92 7.1
CB0% 2-28 186.8 27.5 68.5 0.0023 5.09 0.0253 36.20 8.0
CB50% 1-28 151.1 19.3 55.4 0.0029 3.86 0.0042 29.30 7.2
CB50% 2-28 153.5 22.2 56.3 0.0036 4.44 0.0041 22.80 8.0
CB0% 1-56 182.2 28.6 66.8 0.0005 5.34 0.0111 36.80 7.8
CB0% 2-56 189.7 22.0 69.6 0.0016 6.28 0.0131 38.85 9.9
CB50% 1-56 166.6 21.6 61.1 0.0035 4.32 0.0049 29.00 8.9
CB50% 2-56 169.3 20.5 62.1 0.0021 5.13 0.0113 23.60 9.7
CB50% 1-75 197.2 27.0 72.3 0.0018 6.30 0.0031 32.60 8.8

5. CONCLUSIONS

The following observations and conclusions can be made on the basis of the
experiments conducted on the nine RC beam specimens. From the experimental
investigation, it is generally observed that the flexural behavior of RC beams with 50% fly
ash is comparable to that of OPC concrete beams.

1. The ultimate moment capacity of fly ash concrete beam is 16% less than the
ordinary concrete beam when tested at 28th day. But its moment capacity increases
with age. It increases by 23% at 75th day than at 28th day.
2. The deflections under the design service loads for fly ash concrete beams were
within the allowable limit provided by IS: 456-2000.
3. Reinforced fly ash concrete beams with 50% fly ash showed displacement
ductility in the range of 4 to 6 which is adequate for structural members subjected
to large displacement such as sudden forces caused by earthquake.
4. The crack widths at service loads for fly ash concrete beams ranged between
0.18mm to 0.2mm and this is within the maximum allowable value as stipulated
by IS: 456-2000 for durability requirements.
5. Results of this investigation suggest that concrete with 50% fly ash replacement
for cement could be used for RC beams.

ACKNOWLEDGEMENT

This project is funded by Department of Atomic Energy, under Board of Research in
Nuclear Science(BRNS) research grant No. 2011/36/05-BRNS/308.The experiments were
carried out in the research laboratories of Hindustan University, Tamil Nadu, India.

47


REFERENCES

[1]. Jiang .L.H and Malhotra .V.M (2000), “Reduction in water demand of non-air-
entrained concrete incorporating large volumes of fly ash”, Cement and Concrete Research,
Vol.30, pp. 1785- 1789.
[2]. Rafat Siddique (2004), “Performance characteristics of high volume Class-F fly ash
concrete”, Cement and Concrete Research, 34, pp. 487-493.
[3]. Khatib .J.M (2008), “Performance of Self-Compacting Concrete Containing Fly Ash”,
Construction and Building Materials, 22, pp. 1963-1971.
[4]. Dakshina Murthy and Sudheer Reddy (2010), “Moment-Curvature Characteristics of
ordinary grade Fly Ash Concrete beams”, International Journal of Civil and structural
Engineering, Vol.1, No 3.
[5]. Sunilaa George., et. al. (2011), “Experimental study on shear behavior of activated fly
ash concrete beams, Journal of structural engineering, Vol.37, No.6.
[6]. P.A. Ganeshwaran, Suji and S. Deepashri, “Evaluation of Mechanical Properties of
Self Compacting Concrete with Manufactured Sand and Fly Ash” International Journal of
Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 60 - 69, ISSN Print:
0976 – 6308, ISSN Online: 0976 – 6316, Published by IAEME.
[7]. Aravindkumar.B.Harwalkar and Dr.S.S.Awanti, “Fatigue Behavior of High Volume
Fly Ash Concrete Under Constant Amplitude and Compound Loading” International Journal
of Civil Engineering & Technology (IJCIET), Volume 3, Issue 2, 2012, pp. 404 - 414, ISSN
Print: 0976 – 6308, ISSN Online: 0976 – 6316, Published by IAEME.

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