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© September 2016 | IJIRT | Volume 3 Issue 4 | ISSN: 2349-6002
Experimental Study on Partial Replacement of Concrete
in and Below Neutral Axis of Beam
Er.Ima Mathew1,Er.Sneha M.Varghese2
1,2
Department of Civil Engineering, Saintgits College of Engineering, Kottayam
Abstract—In case of simply supported reinforced
concrete beam, the region below neutral axis is in
tension and the region above neutral axis is in
compression. The tension and compression in the
neutral axis is zero. In RC beams strength of concrete
lying in and near the neutral axis is not fully utilized.
The concrete below the neutral axis acts as a stress
transfer medium between the compression and tension
zone. In this thesis work, experiment is conducted to
partially replace the concrete both in and near the
neutral axis and that below the neutral axis by creating
air voids using waste plastic bottles. This helps in
reduction in concrete used, thereby reducing selfweight, cost, etc. Since waste plastic bottles areutilized
to create air voids, it add on to sustainability.
which reduces the greenhouse gases emissions. So it
is considered as environment friendly.
II.
Several studies are carried out on replacement of
concrete below neutral axis of beam. Also studies
have been separately conducted on beams with
hollow neutral axis and replacing concrete in the
zone below neutral axis by creating air voids. But
there is no study on the combined effect of both
hollow neutral axis and partial replacement below
neutral axis of beam. The scope of this thesis work is
to fill this gap in the literature by studying the
combined effect of hollow N.A. and partial
replacement of concrete below N.A. by creating air
voids with waste plastic bottles.
Index Terms—neutral axis, partial replacement of
concrete, air voids, tension zone, compression zone
I.
INTRODUCTION
Reinforced cement concrete is one of the most
important components in the construction industry. In
case of normal simply supported reinforced concrete
beam, the neutral axis divides the tension zone and
compression zone. The region below the neutral axis
is in tension and the region above neutral axis is in
compression. The concrete below the neutral axis act
as the medium for transferring stress from
compression zone to the tension zone. Lot of
researches were carried out for the investigation of
alternate materials that can be used in concrete like
fly ash, copper slag, rice husk etc. An alternate
method of replacing concrete in the neutral axis of
beam by PVC pipes was studied and studies were
conducted on replacing the concrete below neutral
axis of beam by polythene balls thereby reducing self
- weight. This thesis work aims at studying the
combined effects of partial replacement of concrete
in and below the neutral axis of beam by creating air
voids using light weight inert waste plastic bottles.
Sustainability can be achieved by using waste plastic
bottles. By saving concrete, we can save cement,
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SCOPE OF THE PROJECT
III.




OBJECTIVE OF THE PROJECT
The objectives of the project are as follows:
To study a new method by replacing some amount of
the concrete in and below neutral axis of beam by
creating air voids using waste plastic bottles.
To study the load versus deflection characteristics.
To study the load versus strain characteristics.
To study the ultimate load carrying capacity of the
beams.
IV.
1.
2.
3.
4.
5.
METHODOLOGY
The methodology of work includes :Basic tests on constituent materials of concrete
Design mix for M20 grade concrete
Preparation of beam specimens
Testing of beam specimens under two point loading
on loading frame
Results and discussion
V.
PRELIMINARY INVESTIGATION
Test results on cement, fine aggregate and
coarse aggregate are given in table 1
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© September 2016 | IJIRT | Volume 3 Issue 4 | ISSN: 2349-6002
Table 1 : Test results on constituent materials
Material
Cement
(OPC 53
grade)
Fine
aggregate
Coarse
aggregate
(max. size =
20mm)
Test
Specific Gravity
Standard Consistency
Fineness
Initial Setting Time
Final Setting Time
7day Compressive
Strength
Specific gravity
Sieve analysis
Specific gravity
Water absorption
Result
3.2
34%
2%
50 minutes
275 minutes
Crushing strength
28.58%
bottles placed below neutral
axis
B. Reinforcement Details
The beam was designed as singly reinforced
beam. The support conditions were simply supported
at both ends. Reinforcement was designed for a load
of 5 ton. Reinforcement details are given in figure 1.
The loading type was two point loading.
Reinforcement details were obtained as follows:
40 N/ mm2
2.92
Zone IV
2.96
0.10%
Mix design details are given in table 2
m




Ast = 2 nos. 12 mm dia bars
8 mm dia stirrups @ 100mm c/c
Hangar bars of 2 nos. 10 mm diameter
20 mm cover
Table 2: Quantity of materials for M20 mix
Material
Quantity
(kg/m3)
Cement
415.12
(1: 1.55 : 3.25)
Fine aggregate
641.52
(w/c ratio = 0.47,
Coarse aggregate
1350.65
Water
195.05
Grade of concrete
M20
Fig 1 : Reinforcement details of beam
Neutral axis depth is calculated as𝑥𝑢 =
0.87𝑓𝑦 𝐴 𝑠𝑡
0.36𝑓𝑐𝑘 𝑏
Slump = 91 mm)
= 68.329 mm
C. Mould Preparation
VI.
EXPERIMENTAL INVESTIGATION
A. Specimen Details
Normal and replaced RCC beams
of size 0.2m x 0.3m x 2m were casted.Specimen
details are given in table 3
Table 3: Details of beam specimens
Beam
Notation
Specimen Details
Dimensions
N0B0
Normal RCC beam or
control specimen
0.2 m x 0.3 m x 2
m
N10B0
RCC beams with 10 bottles
placed at neutral axis
0.2 m x 0.3 m x 2
m
N0B10
RCC beams with 10 bottles
placed below neutral axis
0.2 m x 0.3 m x 2
m
N10B10
RCC beams with 10 bottles
placed at neutral axis and 10
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0.2
m x 0.3 m x 2
Wooden moulds of size 0.2 m x 0.3 m x 2 m
were prepared and bottles were placed in the
reinforcement cage as per different variations. The
moulds were greased properly. Reinforcement cage
was then placed inside the moulds by providing
proper cover blocks. Steel moulds of size 0.2 m x 0.3
m x 2 m are also used for casting beam specimens.
The bottles of 250 ml capacity were placed in a
continuous line at the neutral axis and the bottles
below neutral axis were placed at an inclination with
respect to the bottles at neutral axis.
D. Casting of Beam Specimens
The beam specimens were casted with M20
mix concrete of mix ratio 1:1.55:3.25 and water
cement ratio of 0.47. The quantity of materials
(cement, water, fine aggregate, coarse aggregate and
water) required for casting beams of size 0.2 m x 0.3
m x 2 m were taken and mixed well. The concrete
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© September 2016 | IJIRT | Volume 3 Issue 4 | ISSN: 2349-6002
mix was poured into the moulds prepared and
compacted well. Compaction was given with the help
of vibrators. The surface of beams were finished to
get a level surface after concreting. The specimens
were demoulded after 24 hours. The specimens were
then subjected to curing for 28 days.
Fig 2 : Reinforcement cage placed inside the greased
mould
Fig 4: Beam specimen subjected to two point loading on
loading frame
VII.
E. Testing of Beam Specimens
Each of the beam specimens were tested
under two point loading on the loading frame of
capacity 50 ton. The flexural strength of beams were
tested. The beams were simply supported at the two
ends. The supports were provided at a distance of
0.15m from both the ends of the beam specimens.
Effective length of beam was 1.7m. The effective
span of beam is divided into three equal spans. The
two points of loading were fixed at the ends of
central span. Strain gauges readings were taken at
centre of each span. Dial gauges of 10mm capacity
were attached at the two points of loading and at the
centre of beam. The beam specimens were given two
point loading at an interval of 0.5 ton. The behaviour
of beams under loading was observed. The beam
specimens were loaded till failure. The dial gauge
and strain gauge readings were noted for each load
applied. The development of first crack and
propagation of cracks were observed. Figure 4 shows
the test set up of two point loading on loading frame.
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A. Load vs Deflection
The load versus deflection characteristics of
the beams were studied. The deflection is plotted
along the X – axis corresponding to the loads in the Y
– axis. The load is taken in kN and deflection in mm.
The load - deflection characteristics of all the beams
specimens show similar curves. The load – deflection
characteristics of all the beams are linear, i.e., the
deflection of all the beams is directly proportional to
load. The load – deflection characteristics of beams
are compared in figure 5
150
Load (kN)
Fig 3: Compaction given to concrete with the help of
vibrators
RESULTS AND DISCUSSIONS
Load - Deflection
100
N0B10
50
N10B0
N10B10
0
0
5
10
N0B0
Deflection (mm)
Fig 5 : Load versus deflection curves at centre of beam
specimens N0B0, N10B0, N0B10 and N10B10
B. Load Vs Strain
Strain values corresponding to each load
increment was noted. Load versus strain curves were
plotted. Load (kN)is taken in the Y – axis and strain
in the X – axis. Load versus strain at centre span of
beam was plotted. At left and right span of beams,
the shear strain was plotted for corresponding load
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© September 2016 | IJIRT | Volume 3 Issue 4 | ISSN: 2349-6002
increment. Initially, the load versus strain curves
show linear behaviour upto a certain load increment.
After that the load versus strain graphs show a nonlinear behaviour. Figure 7 and 8 shows the strain
curves of beam having the combined effect of partial
replacement of concrete in and below neutral axis.
Load (KN)
N10B10
Table 4 : Ultimate load carrying capacity and
first crack load of beams
Beam Specification
Max
Load
(ton)
Load at
first
crack
(ton)
N0B0
Normal Beam
15.5
5
N10B0
Neutral axis alone
15
4
N0B10
10 bottles below neutral
axis
13.5
3.5
N10B10
Neutral axis plus 10
bottles below neutral axis
13.5
2
Beam
Notation
200
100
0
0 0.0020.004
at
centre
Strain
Fig 7: Load versus strain curve at centre of beam
N10B10
N10B10
Load (KN)
150
Load (ton)
Ultimate load
16
15.5
15
14.5
14
13.5
13
12.5
ultimate load
100
50
at right
0
Fig 9 :Ultimate load carrying capacity of beams
0
0.002
Strain
Load at first crack
C. Ultimate Load Carrying Capacity of Beams
The ultimate load taken by each of the beams
was observed. Beyond this load the beam takes no
load and failure occur. The load at which first crack
is developed was also noted for each beam. Table 4
shows ultimate load carrying capacity and first crack
load of each beam. Figure 9 and 10 shows the
comparison of ultimate load carrying capacity and
first crack load of beams.The beams with concrete
replaced at neutral axis alone has 3% reduction in
ultimate load carrying capacity. The beams with
concrete replaced by 10 bottles below NA alone
(N0B10) and that with concrete replaced by 10
bottles at NA and 10 bottles below NA (N10B10) has
12.9 % reduction in load carrying capacity.
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Load (ton)
Fig 8: Load versus strain curve at right of beam
N10B10
6
5
4
3
2
1
0
Load at first
crack
Fig 10 :Load at first crack of beams
D. Weight Reduction
The reduction in weight of the replaced
beams compared to the normal RCC beam is given in
table 5. As the number of bottles increases there is
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© September 2016 | IJIRT | Volume 3 Issue 4 | ISSN: 2349-6002
reduction in weight of beams, i.e., as there is an
increase in concrete replacement, there is a reduction
in self - weight of beams. This reduction in concrete
helps in cost reduction to a great extent in case of
large construction works.

Table 5: Weight comparison of replaced beam
specimens with normal beam
Beam
Beam
specific
ation
Weig
ht of
concr
ete
(kg)
N0B0
Normal
Beam
312.2
8
N10B
0
N0B1
0
N10B
10
10
bottles
at
Neutral
Axis
alone
10
bottles
below
NA
10
bottles
at NA+
10
bottles
below
NA
Differe
nce in
weight
of
concret
e (kg)
%
redu
ction
in
weig
ht

REFERENCES
305.7
8
6.5
2.08
305.7
8
6.5
2.08
299.2
7
13.01
4.17
Comparing the ultimate load carrying
capacity and the reduction in self - weight, the beams
with concrete replaced at NA alone is more effective
than other replaced beams. However, the beams with
concrete replaced in and below NA are more
effective than the beams with concrete replaced
below NA alone.
VIII.
CONCLUSION
The flexural behaviour of beams with concrete
replaced in and below neutral axis was studied. Also
beams with concrete replaced at neutral axis alone
and beams with concrete replaced below neutral axis
alone were studied. The following conclusions were
made.
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The flexural behaviour was similar for all the beams.
Self - weight of beams can be reduced by replacing
the concrete by creating air voids with the use of
waste plastic bottles.
Since waste plastic bottles are used for concrete
replacement, it adds on to sustainability and eco friendly construction.
Comparing the ultimate load carrying capacity and
the reduction in self - weight, the beams with
concrete replaced at NA alone is more effective than
other replaced beams. However, the beams with
concrete replaced in and below NA are more
effective than the beams with concrete replaced
below NA alone.
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VanissornVimonsatit (2012) ― ―Reinforced
Concrete Beams With Lightweight Concrete
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pp. 2370-2379, 19 July, ISSN 1992-2248.
[2] Aswathy S Kumar, Anup Joy (2015) —
―Experimental
Investigation
on
Partial
Replacement of Concrete Below Neutral Axis of
Beam‖, International Journal of Science and
Research (IJSR), volume 4, Issue 8, August.
[3] B S Karthik, Dr.H.Eramma&Madhukaran (2014)
― ―Behaviour of Concrete Grade Variation in
Tension and Compression Zones of RCC
Beam‖s, International Journal of Advanced
Technology in Engineering and Science, Volume
No.02, Issue No. 07, ISSN 2348 – 7550 July.
[4] Dr. G. Hemalatha and W.GodwinJesudhason
(2013) ― ―Experimental Investigation on Beams
Partial Replacement Below the Neutral Axis‖,
International Journal of Civil and Structural
Engineering Research,Vol. 2, January.
[5] Jain Joy and Rajesh Rajeev (2014)— ―Effect of
Reinforced Concrete Beam with Hollow Neutral
Axis‖, International Journal For Scientific
Research And Development (2014), volume 3,
November.
[6] Patel Rakesh, Dubey S. K, Pathak K.K., (2012) ―Brick Filled Reinforced ConcreteComposite
Beam‖, Indian Concrete Institute Journal, page
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