IRJET- A Study on Reinforced Concrete Beams Infilled with Light-Weight Materials below Neutral Axis

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 06 Issue: 04 | Apr 2019 www.irjet.net p-ISSN: 2395-0072
© 2019, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 3723
A STUDY ON REINFORCED CONCRETE BEAMS INFILLED WITH LIGHT-
WEIGHT MATERIALS BELOW NEUTRAL AXIS
Agna Skaria1, Samithamol Salim2
1 P.G Student, Department of Civil Engineering, Sree Narayana Gurukulam Engineering college, Kadayirppu,
Kerala, India
2 Assistant Professor, Department of Civil Engineering, Sree Narayana Gurukulam Engineering college,
Kadayirppu, Kerala, India
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Abstract-In recent days the problem faced by the
construction industry is acute shortage of raw materials. In
case of normally simply supported reinforced concrete beam,
the region below the neutral axis is in tension and the region
above the neutral axis is in compression. The concrete below
the neutral axis acts as a stress transfer medium between the
compression and tension zone. As concrete is weak in tension,
steel reinforcement is provided in this region. Partial
replacement of concrete below the neutral axis using light
weight aggregate is an idea to reduce the weight of the
concrete structure. By doing so, we can save the material and
hence cost. This project deals with the experimental
investigation on the behavior of reinforced concrete beam
after partially replacing the concrete below the neutral axis
with light weight materials like brick and crumb rubber.
Key Words: Neutral axis, Reinforced concrete beam, light-
weight material, Stress transfer.
1.INTRODUCTION
In recent days one of the main problem facedbythe
construction industry is the acute shortage of rawmaterials.
Lots of researches were carried out for the investigation of
alternative materials that can be used in concrete. Some
locally available materials like fly ash, copper slag, rice husk
etc. are experimentally evaluated.
Reinforced concrete has established itself as a
widely used composite material forstructural elementssuch
as slabs, beams, column, wall, footing etc., Reinforced
concrete is a composite material comprising concrete and
steel reinforcements. Thesuccessful useofthesematerialsin
structural elements attributedtothebond betweensteel and
concrete which ensure the strain compatibility so that the
load on structural elements is shared by steel and concrete
without distribution of the composite material. The
reinforcing steel imparts ductility to a material that is
otherwise brittle [3].
In case of normal simply supported reinforced
concrete beam, the neutral axis divides the tension zoneand
compression zone. The region below the neutral axis is in
tension and the region above neutral axis is in compression.
Since concrete is weak in taking up tension, steel
reinforcements are provided atthetensionzoneofthebeam.
The concrete below the neutral axis acts as the medium for
transferring stress from compression zone to the tension
zone. The concrete provided below the neutral axisisknown
as sacrificial concrete. This sacrificial concrete can be
replaced by light weight material inorder to reduce the
weight of structure and to achieve economy. The
compressive force is acting in the top zone at a distance of
0.42 XU. XU is the neutral axis distance from top of section
[1,4,6].
There are methods for increasing the effectiveness
of concrete below neutral axis such as prestressing and
converting the beam into other shapes such as Tee beams.
But these methods cause change in the geometry of the
structure and increases the construction cost. An alternate
method of replacing the zone below the neutral axis with
inert weightless substances like polythene balls, brick and
crumb rubber will not greatly affect the strength and stress
characteristics of the beam. Also it will not affect the
geometry and shape [7,2].
This project deals with the experimental
investigation on the behaviour of reinforced concrete beam
after partially replacing the concrete below the neutral axis
with light weight materials like brick and crumb rubber.
2. EXPERIMENTAL PROGRAM
2.1 MATERIALS
Table 2 gives the details of material testing
Table-1 : Material Tests
Tests Materials Equipment
used
Value
obtained
Specific
gravity
Cement(PPC) Le chatelier
flask
3.1
Consistency
limit
Cement(PPC) Vicat
apparatus
33.5%
Fineness Cement(PPC) Sieve shaker 10%
Initial
setting time
Cement(PPC) Vicat
apparatus
45 min
Specific
gravity
Course
aggregate
Wire basket 2.8
Specific
gravity
Fine
aggregate
Pycnometer 2.7

2.2 MIX PROPORTION
Mix design for M25 concrete have been worked out as per IS
10262:2009. The same mix proportionswereusedforall the
specimens. In table 2 gives the details of mix proportion.
Table-2 : Mix proportion for M25 grade,Kg/mᵌ
Cement Fine
aggregate
Course
aggregate
Water
441Kg/mᵌ 703.93Kg/mᵌ 1220.61Kg/mᵌ 441Kg/mᵌ
2.3 MIXING AND CASTING OF BEAM
A total of eight concrete beam specimens werecasted. Outof
the 8 beams, one beam was designed as control beam, 4
beams were composite beams with 10%, 15%, 20% and
30% of coarse aggregate replaced by bricks, 3 beams were
composite beams with 10%, 15% and 20% of coarse
aggregate replaced by crumb rubber. The entire specimen
dimensions were 1000mm x 120mm x 140mm with an
effective span of 750mm. The beams are designed as singly
reinforced beam with 2 nos. of 8mm diameter bars at the
tension region. Table 3 gives the specimen details.
Table-3: Specimen details.
Sl.No Specimens Details
1 RCCB Reinforced concrete control beam
2 RCBB10 Reinforced concrete beam replaced
with 10% of Coarse Aggregate (C.A)
below neutral axis with brick.
with 15% of C.A below neutral axis
with brick.
with brick.
with brick.
6 RCBR10 Reinforced concrete beam replaced
with crumb rubber.
with crumb rubber.
with crumb rubber.
Beams are assumed to fail whentheconcretereachesfailure
compression strain. But in all cases of design, the steel need
not have reached its yield point at the same time, unless it is
so designed. For balanced or under-reinforced sections, the
steel also reaches yields at the time of concretefailure.Butin
over-reinforced beams, the steel stress at failure will be
below its yield strength. As equilibrium of forces in bending
requires that at all times tension be equal to compression,
We have Total tension, T =fstAst -------------------(1)
Total compression, C =0.36fckb(Xu)----------(2)
Where fst= actual tension in steel corresponding to the
strain in steel.
Equating the two expression, we obtain fstAst =0.36fck
b(xu)
i.e. Xu = fst Ast/ 0.36 fckb----(3)
For under reinforced beams, steel firstreachesyieldstressof
0.87fy. Substituting its value and dividing both sides by the
effective depth d (IS 456 Annexure G), we get
Xu /d = 0.87fstAst/ 0.36fckbd--------(4)
Xu=0.87∗500∗157.07/0.36∗25∗120
= 63.26mm
= 64mm
The zone below the neutral axis is made of light weight
concrete by replacing the coarse aggregate with
10%,15%,20% and 30% of low quality brick in four
specimens and 10%,15% and 20%crumb rubber in three
specimens.
Fig-1: Reinforcement cage for beam specimen
Fig-2: Casting of replaced beam
Fig-3: Casted beam specimen
2.4 TEST SETUP AND INSTRUMENTATION
The ultimate load carrying capacity of specimens were
tested in UTM with 1000KN loading frame. The tests were
carried out under three point loading conditionasshown in
Fig 4.

Fig-4: Test setup
3. RESULT AND DISCUSSIONS
A.LOAD CARRING CAPACITY
It is found that there is no much difference in the load
carrying capacity of control specimen and that of beams
with replacement below the neutral axis. Theloaddeflection
curve of tested specimens were shown in Fig.5 and Fig.6
Fig.7 shows the ultimate load carrying capacity of tested
specimens in the form of bar chart. Table.4 shows the
summary of test results.
Fig-5: Load v/s deflection curve of RCB, RCBB10, RCBB15,
RCBB20 and RCBB30
Fig-6: Load v/s deflection of RCCB, RCBR10, RCBR15 and
RCBR20
Table-4 summary of test results
Specimen Ultimate Load %
increment/decrement
RCCB 35 __
RCBB10 35 0
RCBB15 37 5.17%
RCBB20 34 -2.85%
RCBB30 32 -8.57%
RCBR10 37 5.71%
RCBR15 43 22.85%
RCBR20 42 17.41%
Fig -7: Ultimate load of tested beam specimens
The ultimate load of specimens RCBR10, RCBR15 RCBR20
were increased when compared to the control beam RCCB,
higher increment being observedforspecimenRCBR15with
15% replacement of CA by crumb rubber.
Beam specimens with 15% brick replacement (RCBB15)
shows slight increment in ultimate load carrying capacity
compared to control beam. But when it increased to 20%
and 30%, ultimate load carrying capacity is found to
decrease with respect to control beam. This may be due to
the reduced bonding between cement mortar and brick.
Beam specimen with 20% crumb rubber replacement
(RCBR20) shows increment in ultimate load carrying
capacity but a reduction in strength was observed with
respect to 15% replacement (RCBR15).Thismaybebecause
of reduced bonding forcebetweencrumb rubberandcement
due to the smooth surface of crumb rubber.
4. CONCLUSIONS
1.The ultimate load carrying capacity of control beam and
that of composite beam specimen and the replaced
specimens were found to be almost similar.
2.Beam specimens with 15% replacement of CA in tension
zone with brick and crumb rubber showed higher
increment in load carrying capacitywithrespecttocontrol
beam. Therefore 15% is found as the optimumpercentage

for replacement of concrete in tension zone with light
weight materials.
3.It has been observed that the replacement of concrete in
the tension zone by light weight materials does not
require any extra labour or time. Economy and reduction
of weight in beams depends on the percentage
replacement of concrete. The concrete savingwill bemore
effective as the length and depth of the beam increases.
Light weight composite reinforced concrete beams can be
used for sustainable and environment friendly
construction work as it saves concrete which reduces the
emission of carbon dioxide during the production of
cement.
ACKNOWLEDGEMENT
My sincere gratitude to my guide, Ms. Samithamol salim,
Assistant professor who has believed in me since the
beginning and accepted undertaking my research work.
Moreover, she has helped me from start to finish and I have
always counted on her assistance.
I’m thankful to all the teaching and non-teaching
staffs of department of Structural Engineering, Sree
narayana gurukulam College of Engineering for offering me
the opportunity to do this research work.
Finally, deep thanks to God for his unconditional
support, and also to my family and friends. Although I have
not had them near, they have made me feel like I had.
REFERENCES
1. Aswathy S Kumar et al. (2015). “Experimental
investigation on partial replacement of concrete
below neutral axis of beam” , International journal
of science and research,vol. 4 Issue8 .
2. Jain Joy et al. (2014). “Effect of reinforced concrete
beam with hollow neutral axis” International
journal for scientific research and
developmet,vol.2,Issue 10 pp.341-348.
3. M.R.Ali et al.(2016) .“Thermal-resistant lightweight
concrete with polyethylene beads as coarse
aggregates”, construction and building
materials,Elsevier,pp.739-749.
4. Patel Rakesh et al. (2013). “Analysis of RC brick
filled composite beams using MIF.” , Elsevier,pp.30-
34.
5. Patel Rakesh et al. (2013). “Brick infilledcomposite
beams .” , International journal of advanced
engineering technology ,vol.3,Issue 2.
6. V.P Roshan Ahammed et al. (2016). “Experimental
and analytical of flexural behavior of reinforced
concrete composite beams.”, International research
journal of engineering and technology , vol.3
Issue8,p p.765-769.
7. W.Godwin Jesudhason et al.(2014). “Exprimental
invstigation on beams partial replacement below
the neutral axis”, International journal for
Engineering research and general
science,vol.3,Issue 4.

IRJET- A Study on Reinforced Concrete Beams Infilled with Light-Weight Materials below Neutral Axis

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IRJET- A Study on Reinforced Concrete Beams Infilled with Light-Weight Materials below Neutral Axis