Ijciet 06 10_016

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 6, Issue 10, Oct 2015, pp. 185-190, Article ID: IJCIET_06_10_016
Available online at
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
BEHAVIOUR OF BEAMS WITH LOW
GRADE CONCRETE OR HOLLOW
NEUTRAL AXIS ZONE
Anuja Mary Kuriakose
Post Graduate Student, Department of Civil Engineering,
Mar Athansius College of Engineering, Kothamangalam, Kerala, India
Mathews M. Paul
Professor, Department of Civil Engineering,
Mar Athansius College of Engineering, Kothamangalam, Kerala, India
ABSTRACT
Concrete is widely used as one of the important construction material. It is
well recognized that prediction of concrete strength is important in modern
concrete constructions and in engineering judgments. In concrete beams,
strength of concrete lying near the neutral axis is not fully utilized. Hence in
this project, low grade concrete is placed near the neutral axis zone of the
beams and flexural behaviour of the PCC beams is studied. Also flexural test
of beams with hollow neutral axis is studied.
Key words: PCC, Low Grade Concrete, Hollow Neutral Axis, Flexural
Strength.
Cite this Article: Anuja Mary Kuriakose and Mathews M. Paul. Behaviour of
Beams with Low Grade Concrete or Hollow Neutral Axis Zone. International
Journal of Civil Engineering and Technology, 6(10), 2015, pp. 185-190.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
1. INTRODUCTION
Concrete is a composite material composed of coarse granular material (the aggregate
or filler) embedded in a hard matrix of material (the cement or binder) that fills the
space between the aggregate particles and glues them together. Concrete is also
considered as a composite material that consists essentially of a binding medium
within which are embedded particles or fragments of aggregates. According to the
type of binder used, there are many different kinds of concrete. For instance, portland
cement concrete, asphalt concrete, and epoxy concrete. In concrete construction, the
portland cement concrete is utilized the most.
The strength and deformation characteristics of concrete thus depend on the
proportion of materials, environmental conditions and curing. The increase of strength

2. Anuja Mary Kuriakose and Mathews M. Paul
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with its age during curing is considered to be marginal after 28 days. The strength of
concrete near the neutral axis is not fully utilized. Also the concrete just above the
neutral axis is less stressed where as the concrete below the neutral axis serves as a
shear transmitting media. Thus low grade concrete can be used in the neutral axis
zone. Flexural strength is one measure of the tensile strength of concrete. It is a measure
of an unreinforced concrete beam or slab to resist failure in bending. Flexural strength is
measured by testing beams under 2 point loading (also called 4 point loading
including the reactions)
Previous studies (2014) were carried out on reinforced concrete brick-filled
composite beams, with the view that the stresses in the beams are maximum at the top
and bottom and zero at the neutral axis. It was observed that the behaviour of
reinforced concrete brick-filled beams is similar to that of reinforced concrete beams.
It was also found that the presence of bricks in the low stressed zone has not caused
significant reduction in strength of reinforced concrete beams. Also experiments on
RCC beams with M20 and M25 grade concrete in tension and compression zones
respectively was studied (2014). Deflection and the crack patterns was noted down
carefully. It was found that the overall behaviour of partial beams used in the study
closely resembles to that of equivalent beam made with normal beam. Thus it was
noted that partial beam is more efficient and economical than normal beams.
This paper intends to study the flexural behaviour of PCC beams with low grade
concrete placed near the neutral axis zone and also for beams with hollow neutral
axis.
2. EXPERIMENTAL PROGRAMME
2.1. Materials
Cement used for project work is Ordinary Portland Cement of Coromandel King 53
grade. Table 1 and Table 2 shows the properties of cement and fine aggregate used.
M-Sand is used as the fine aggregate. Table 3 shows the properties of coarse
aggregate used in the work.
Table 1 Properties of Cement
Sl. No. Properties Value Codal Values
1 Specific gravity 3.14 3.12 – 3.14
2 Standard consistency 35 % 26 – 33 %
3 Initial setting time 65 min > 30 min
4 Final setting time More than 3 hrs < 600 min
5 Fineness < 10% < 10%
6
Average cube compressive
Strength (MPa)
53.5 53
Table 2 Physical Properties of Fine Aggregate
Sl. No Properties Value
1 Specific gravity 2.297
2 Water absorption 12%
3 Fineness modulus 3.787
4 Grading zone Zone II

3. Behaviour of Beams with Low Grade Concrete or Hollow Neutral Axis Zone
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Table 3 Physical Properties of Natural Coarse Aggregates (60% of 20mm down and 40% of
12mmdown aggregates)
Sl. No Properties Value
1 Water absorption 0.81 %
2 Aggregate crushing value 29.571 %
3 Specific gravity 2.76
2.2. Methodology
In the present investigation concrete mixes of M15, M20 and M25 grade were used
and design has done as per relevant Indian Standard specifications. The proportioning
is carried out to achieve workability of fresh concrete and durability requirements.
Relationship between strength and water cement ratio should be preferably
established for the materials actually to be used. The quantities of materials required
for M15, M20 and M25 grade are tabulated in the Table 4.
Table 4 Material Requirements for 1m3
Concrete for Designated Grade
Grade w/c Water (kg) Cement (kg) (kg) (kg)
M15 0.56 156.8 280 627.215 1272.276
M20 0.48 153.6 320 592.438 1287.736
M25 0.43 154.8 360 564.718 1282.572
2.3. Test Setup
Typical PCC beams of size 150 x 150 x 700 mm were used. Beams with hollow
neutral axis are made by PVC pipes of 20 mm ϕ and 25 mm ϕ. The length of the pipe
inside the beam neutral axis is 600mm and an anchorage length of 50mm on each side
is provided for the transfer of load. The depth of neutral axis is calculated by
considering the grade of concrete used. All the beams were subjected to 4-point
flexural test. Figure 1 shows the testing of beam specimens.
Figure 1 Testing of Beam Specimens
3. TEST RESULTS
3.1. Flexural Strength of PCC beams
In this test, plain concrete beam was subjected to flexure using symmetrical two point
loading until failure occurs. As the load point was placed at one third of the span, the
test was called as third point loading test. The theoretical maximum tensile stress
reached in the bottom fiber of the test beam is called modulus of rupture.

4. Anuja Mary Kuriakose and Mathews M. Paul
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The comparative study of flexural strength of control beams with beams having
low grade concrete near neutral axis zone is as shown in Fig. 2.
0 1
0
1
2
3
4
5
AVERAGEFLEXURALSTRENGTH(MPa)
BEAM DESIGNATION
M25
M20
M25+M15
M20+M15
Figure 2 Comparative study of flexural strength of control beams with beams having
low grade concrete near neutral axis zone
It is seen that there is not much difference in the flexural strength of control beams
and that of beams with low grade concrete near neutral axis zone. With the increase in
the grade of concrete, flexural strength of concrete also increases. It shows that the
flexural strength of M25+M15 beam is more than that of M20+M15 beam. It was also
found that there was an increase of 4.05% flexural strength for M25+M15 beam when
compared with that of M20+M15 beam. It was found that there was an increase of
4.72% flexural strength for M20+M15 beam when compared with that of M20
Control Beam. Also, for M25+M15 beam there was a decrease of 6.09% flexural
strength when compared to that of M25 Control Beam. This may be due to the
variation in the compaction between two beams.
The comparative study of flexural strength of control beams with beams having
hollow neutral axis is as shown in Fig. 3.
0 1
0
1
2
3
4
5
AVERAGEFLEXURALSTRENGTH(MPa)
BEAM DESIGNATION
M25
M20
M25+20dia
M25+25dia
M20+20dia
M20+25dia
Figure 3 Comparative study of flexural strength of control beams with beams having
hollow neutral axis

5. Behaviour of Beams with Low Grade Concrete or Hollow Neutral Axis Zone
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From Fig. 3, it is seen that there is not much difference in the flexural strength of
control beams with that of beams with hollow neutral axis. For M20 beam with 20mm
diameter pipe replaced as neutral axis, it was observed that the flexural strength
decreased by 2.17% and with 25mm diameter pipe replaced as neutral axis, the
flexural strength decreased by 0.71% when compared with that of M20 control beam.
Also, for M25 beam with 20 mm diameter pipe replaced as neutral axis, it was
observed that the flexural strength decreases by 3.8% when compared to that of M25
control beam. But for M25 beam with 25mm diameter pipe replaced as neutral axis,
an increase of 2.44% flexural strength was observed when compared with that of M25
control beam. The flexural strength of M20+M15 beam is more than that of
M20+20mm dia and M20+25mm dia beams. There was an increase of 6.9% and
5.46% flexural strength for M20+M15 beam when compared with M20+20mm dia
and M20+25mm dia beams respectively. Also the flexural strength of M25+M15
beam is found to be lower than that of beam with hollow neutral axis. The flexural
strength decreased by 2.6 % and 9.09% for M25+M15 beam when compared with that
of M25+20mm dia and M25+25mm dia beams respectively.
4. CONCLUSIONS
Based on the investigation, the following conclusions were drawn.
 It is seen that there is not much difference in the flexural strength of control beams
and that of beams with low grade concrete near neutral axis zone and hollow neutral
axis.
 The flexural strength of beams increases with the increase in grade of concrete used.
 It can also be seen that with the increase in size of pipe replaced as neutral axis, the
flexural strength also increases.
 It is also seen that the materials in the neutral axis zone is ineffective.
 Thus in the overall study, it can be concluded that behaviour of PCC beams with low
grade concrete near neutral axis or hollow neutral axis behaves in the same manner as
that of conventional concrete.
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