Ijciet 10 01_100

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 10, Issue 01, January 2019, pp. 1081–1096, Article ID: IJCIET_10_01_100
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=1
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
©IAEME Publication Scopus Indexed
STRENGTH AND DUCTILITY BEHAVIOUR OF
CONCRETE COLUMNS UNDER COMPRESSION
WITH DOUBLE LAYERED STIRRUPS: AN
EXPERIMENTAL STUDY
Mahesh Kumar*
Associate Professor, Department of Civil Engineering
Mangalayatan University, Aligarh, India
S. Kaleem A. Zaidi
Associate Professor, Civil Engineering Section
Aligarh Muslim University, Aligarh, India
S. C. Jain
Emeritus Professor, Department of Mechanical Engineering
Indian Institute of Technology, Mandi, India
K. V. S. M. Krishna
Professor, Institute of Business Management
Mangalayatan University, Aligarh, India
*Corresponding Author E-mail: mahesh.kumar@mangalayatan.edu.in
ABSTRACT
The strength and ductility of concrete ameliorated by providing appropriate
confinement has paved way for designing structures that would withstand loads of
extreme intensities. The behaviour of concrete confined by single layered transverse
reinforcement has already been construed substantially. This paper presents a
consistent experimental study conducted on a novel and recently proposed Reinforced
Concrete column consisting of two layers of confining reinforcement. The concrete
inside the column experiences three different levels of confinement, viz., doubly
confined concrete inside the inner layer of lateral reinforcement, singly confined
concrete between the two layers of the transverse reinforcement, and the unconfined
concrete cover. The variables contemplated to study the behaviour and amount of
confinement in double layered stirrup concrete column comprise: addition of inner
layer, variedness in the shape and form of the transverse reinforcement forming the
inner layer, grade of concrete, varying number and amount of longitudinal
reinforcement forming the outer layer, proximity ratio between the inner and outer
layers and the varied amount and spacing of transverse reinforcement encompassing
the inner layer. It has been ascertained that the confinement effects emerged from the

Strength and Ductility Behaviour of Concrete Columns Under Compression with Double Layered Stirrups:
An Experimental Study
use of single layered stirrup transverse reinforcement were deficient when compared
to that of double layered stirrup transverse reinforcement. Aside from enhanced
strength and ductility, this novel structural form of concrete column exhibited an
added advantage in terms of ease of construction over conventional single layered
stirrup column.
Key words: Single layered confined concrete, double layered confined concrete,
ductility, reinforced concrete column, confinement effectiveness
Cite this Article: Mahesh Kumar, S. Kaleem A. Zaidi, S. C. Jain, K. V. S. M. Krishna,
Strength and Ductility Behaviour of Concrete Columns Under Compression with
Double Layered Stirrups: An Experimental Study, International Journal of Civil
Engineering and Technology (IJCIET) 10(1), 2019, pp. 1081–1096.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=1
1. INTRODUCTION
Ductility is ability of the structure or its components to offer resistance in the inelastic domain
of response. It can only be obtained if the ingredients of the material itself are ductile. This
however is not the best characteristic that concrete possesses. (Esmerald Filaj, et. al. 2016).
Columns are considered to be the searing members of a moment resisting structural frame.
In seismically active regions it is essential to improve the ductility deformation capability of
columns. The desired capability in the RC structural components, especially in columns is by
and large achieved through proper confinement of core concrete. The effects of the various
key parameters of confinement on the strength and ductility of single layered confined
concrete are well documented [Sheikh, S. A., et. al. (1980), Mander, J. B., et. al. (1988),
Saatcioglu, M., et. al. (1992), Razvi, et. al. (1994), Sharma, U. K., et. al. (2005), Zaidi, K. A.,
et. al. (2011), D. H. Jing, et. al. (2016)]. Confinement in concrete is achieved by suitable
placement of transverse reinforcement. In principle, at low levels of stress, transverse
reinforcement is hardly stressed and concrete behaves similar to unconfined concrete. At
stress close to the axial crushing strength of concrete, formation and propagation of
longitudinal micro cracks take place, giving rise to development of high lateral tensile strains.
Transverse reinforcement in colligation with longitudinal reinforcement restrains the lateral
expansion of concrete, enabling higher compressive stresses and more importantly, much
higher compression strains to be substantiated by the compression zone before the failure
occurs (Esmerald Filaj, et. al. 2016).
It is well known that both strength and ductility of concrete are enhanced to improve the
seismic performance of R. C. Columns in conjunction with various other types of
confinements, including the use of short steel tubes, welded grids, welded wire fabric sheets,
continuous hoops, different types of fibres in combination with the lateral reinforcement and
fibre reinforced polymer jackets, etc. Furthermore, some studies have also been carried out to
improve the confinement effectiveness by exploring the optimization of column shape as well
as the transverse reinforcement configuration (Tanaka, H Park, Park R. (1993) and Maclean
D. I. (1994)). In more recent years, Yin, S., et. al. (2004), proposed the concept of
interlocking spiral or rectangular square column through several configurations of transverse
reinforcement (such as five spiral transverse reinforcement) that lead to enhance considerable
strength and deformability. Weng, et. al. (2010) demonstrated studies employing a set of five
spiral transverse reinforcement in concrete columns that manifested enhanced effectiveness.
Some other researchers along with Weng, et. al. (2010), D. H. Jing, et. al. (2016), Shih, et. al.

Mahesh Kumar, S. Kaleem A. Zaidi, S. C. Jain, K. V. S. M. Krishna
(2013) studied square column in which a circular spiral is interlocked with a star shaped spiral
for improving its confinement effectiveness. D. H., Jing, et. al. (2016) studied the transverse
reinforcement configuration (TRC) and multiple tied spiral transverse reinforcement
(MTSTR) for improving the seismic performance of Reinforced Concrete (RC) columns and
reported the advantages of using MTSTR in terms of excellent ductility as well as efficient
use of longitudinal bars with ease of construction.
Further, some more recent researchers, such as, Sun, L., et. al. (2016), Wu, D., et al.
(2016), D. H. Jing, et. al. (2016), Yang, F., et. al. (2015), Hui-Ding, Jie Chen and Li, Song
(2015) and Lin Zhu Sun, et. al. (2011) conducted studies on double layered confined concrete
columns. These studies affirmed mucho advantages over traditional single layered stirrup
columns, including enhanced strength and ductility along with the ease of construction.
R. C. column confined by two layers of hoops has been studied by Lin-Zhu Sun, et. al. in
2011. It featured two layers, one inner and other outer layer of hoop of square and circular
shaped columns under axial compression. Yang, et. al. (2015) studied three different high
strength circular columns confined using two layers of high strength steel spiral. Wu, et. al.
(2016) explored normal strength square Reinforced Concrete columns confined by using two
layers of normal strength steel hoops. Lin Zhu Sun (2011) has also studied the behaviour of
circular R. C. columns with two layers of spiral.
From the studies cited above, the mechanical behaviour of confined concrete is
characterized by the increase in strength and ductility of columns and heretofore a very
limited number of studies have been reported in the literature on double layered stirrup
columns. Further, there is an inescapable need to reckon at length, the amount of confinement
provided by double layer stirrup in the critical hinge region of columns. In order to attain this,
it becomes important to evaluate the effectiveness of confinement reinforcement in confined
core concrete, and to examine that by what amount the various parameters of confinement
affects the behaviour of a double layered stirrups reinforced columns.
Drawn by this imperative, this study sets an objective to present the
load-displacement behaviour of a double layered stirrup column apropos various parameters
such as: doubly confined concrete inside the inner layer of lateral reinforcement, singly
confined concrete between the two layers of the transverse reinforcement and the unconfined
concrete cover. The variables contemplated to study the behaviour and amount of
confinement in double layered stirrup concrete column are: addition of inner layer, variedness
in the shape and form of the transverse reinforcement forming the inner layer, grade of
concrete, varying number and amount of longitudinal reinforcement forming the outer layer,
proximity ratio between the inner and outer layers and the varied amount and spacing of
transverse reinforcement encompassing the inner layer.
2. EXPERIMENTAL PROGRAM
A total number of 63 RC short column prism specimens were casted and tested under the
present investigation. They included 54 numbers of double layered confined specimens and 9
numbers of single layered confined specimens. The specimens were casted and tested in
triplicate in order to get the average of three results thus making independent cases of 18
double layers confined concrete columns as well as 3 of single layer confined concrete. The

Table 1 Mix proportions of concrete specimensMix/
Specimen
Cement
(Kg/m3
)
Water
(l/m3
)
FA
(Kg/m3
)
CA
(Kg/m3
)
SilicaFume
(Kg/m3
)
Superplastici
-zer(Kg/m3
)
Cube Characteristic
Compressive
Strength (MPa)
fck
(28 days)
fck
(90 days)
SCCCN/
DCCCN
490.00 225.40 676.20 1029.00 - - 30.90 35.40
SCCCH/
DCCCH
580.00 185.60 638.00 1044.00 46.40 11.60 61.10 67.30
mix proportions of specimens are shown in Table 1. The specimen configuration and
dimensions, and the pictorial representation of the unconfined specimens along with specimen
details are shown in Figure 1 and 2, respectively. The double layered confined specimens
were of the same shape and size as single layered confined specimens.
Figure 1 Specimen configuration and dimension
Figure 2 Details of confined and unconfined specimens
The experimental variables included concrete strength, shape, pitch and amount of
transverse reinforcement forming the inner layer, number and amount of longitudinal
reinforcement forming the outer layer, proximity ratio between the inner and outer confining
layers. All the single and double layered confined specimens were cast in four different series,
viz., SCCCN, SCCCH, DCCCN and DCCCH. The first four letters in the abbreviations
(SCCC) and (DCCC) denote that it is singly or doubly confined concrete column,
respectively, and the last letter speaks of the type of concrete mix, i.e., normal grade concrete
mix (N) or higher grade concrete mix (H). Each series of the confined specimens consisted of
specimens with same concrete strength but with different attributes in terms of amount, pitch
and shape of inner transverse reinforcement, number and amount of longitudinal

reinforcement in outer layer, and c/c distance between the inner and outer layers of confining
reinforcement.
In addition to above, companion standard plain concrete cubes (150 mm x 150 mm x 150
mm) were also casted along with each series to determine the nominal strength of concrete on
the day of testing of test specimens.
A lateral concrete cover of 12.5 mm was provided in all the confined concrete specimens
along with a cover of 15 mm at the ends of the longitudinal bars at top and bottom surfaces of
the specimens to prevent the bars from direct loading. The specimens were cast using wooden
formwork in the laboratory following the prevalent practices in construction industry. After
24 hours, the specimens were taken out of the formworks and dipped in water tanks for
curing. The curing lasted for 28 days followed by another 62 days of air drying. Thus after 90
days of total ageing, the specimens were put to test in a compression testing machine under
uni-axial compression.
The spacing of the lateral ties and hoops were varied from approximately one third to half
of the core dimensions of the specimen in order to consider varying volumetric ratio of the
lateral confining reinforcement of the inner layer. For optimizing shape of the inner layer of
secondary reinforcement under uni-axial compressive load, various shapes such as square,
diamond, circular and spiral were provided. The concrete mixes were designed as per
specifications contained in IS-12620-2009, using Pozzolonic Portland Cement, natural river
sand, crushed lime stone aggregate of 12.5 mm nominal size, tap water, silica fume and super
plasticizer. Normal and high grade concretes with 28 days of characteristic compressive
strength of 30 MPa and 60 MPa were used to cast the test specimens as per test matrix
conceptualised in Table 2. Cube strengths along with Concrete mix proportions for the two
mixes are summarized in Table 1.
Figure 3 Loading and specimen testing under compression
Before mechanical testing of the specimens, a failure test region was forced into the
middle 300 mm length of the specimens by providing external confinement in the 75 mm end-
regions. The external confinement obtained by fastening the end-regions of the test specimens
using 18 mm thick steel collars prevented an undesirable premature end failure of test
specimens to happen. The test specimens were loaded onto a 3000 kN capacity Universal

Testing Machine (UTM) blessed with displacement controlled capabilities, stiff enough to
obtain a stable descending branch of the load-deformation curves. The loading and specimen
testing under compression was shown in Figure 3. The monotonic concentric compression
was applied at a very slow rate to capture clear and complete post peak behaviour of the load-
deformation curve. The axial shortening of the prism specimens was monitored by a linear
variable displacement transducer (LVDT) attached with the test specimen laterally. The mean
axial deformation of the 200 mm gage length in the central zone was measured and converted
into an average strain. An in-built load cell in the UTM was used to record the loads. A data
acquisition system was employed to feed and store the recorded data of the LVDT and the
load cell into the computer. Pictorial representation is shown in Figure 4 (a - b).
Figure 4 (a - b) Column specimen detail
3. RESULTS AND DISCUSSION
This experimental study details the results of the tests conducted on 63 square specimens.
Various arrangements apropos longitudinal and transverse reinforcement with Normal and
High Strength Concrete have been investigated. Experimental results of the parameters shown
in Table 2 are summed-up in Table 3.

Table 2 Properties of Square Test Specimens
Specim
en No.
fco f'co
Outer Layered
Reinforcement
Inner Layered
Reinforcement
Proximity
ratiobetween
inner/outer
cores
Longit
Bars
Transverse
Reinforcement
Longit
Bars
Transverse
Reinforcement
# #
ρs out
(%)
Shape # #
ρs in
(%) Shape
SCCCN1 35.4 30.09 4 10 10 6 0.57 Square - - - - - - -
DCCCN2 35.4 30.09 4 10 10 6 0.57 Square 4 10 6 6 1.16 Square 3
DCCCN3 35.4 30.09 4 10 10 6 0.57 Square 4 10 6 6 1.16 Diamond 3
DCCCN4 35.4 30.09 4 10 10 6 0.57 Square 4 10 6 6 1.16 Circular 3
DCCCN5 35.4 30.09 4 10 10 6 0.57 Square 4 10 6 6 1.16 Spiral 3
SCCCN10 35.4 30.09 8 10 10 6 0.57 Square - - - - - - -
DCCCN12 35.4 30.09 8 10 10 6 0.57 Square 4 10 6 6 1.16 Diamond 3
DCCCN13 35.4 30.09 8 10 10 6 0.57 Square 4 10 6 6 1.16 Circular 3
DCCCN14 35.4 30.09 8 10 10 6 0.57 Square 4 10 6 6 1.16 Spiral 3
SCCCH15 67.3 57.21 4 10 10 6 0.57 Square - - - - - - -
DCCCH16 67.3 57.21 4 10 10 6 0.57 Square 4 10 6 6 1.16 Square 3
DCCCH17 67.3 57.21 4 10 10 6 0.57 Square 4 10 6 6 1.16 Diamond 3
DCCCH18 67.3 57.21 4 10 10 6 0.57 Square 4 10 6 6 1.16 Circular 3
DCCCH19 67.3 57.21 4 10 10 6 0.57 Square 4 10 6 6 1.16 Spiral 3
3.1. Crack pattern and spalling mechanism of unconfined concrete cover
Failure pattern of unconfined concrete cover for the entire double layered stirrup specimens
was observed to be more or less identical. Until the application of 80% of the ultimate load,
there has been no emergence of any crack.
Table 3 Specimens Experimental Results
Specimen
No.
fco
(kN)
Po
(kN)
P'o
(kN)
P'sp
(kN)
P''sp
(kN)
Po/ P'o o 'o o/ 'o
SCCCN1 35.4 1109.00 729.78 889.69 1054.50 1.52 0.00218 0.00215 1.02
DCCCN2 35.4 1267.00 982.80 1001.67 1206.50 1.29 0.00328 0.00215 1.53
DCCCN3 35.4 1242.00 982.80 980.75 1178.00 1.26 0.00340 0.00215 1.58
DCCCN4 35.4 1202.00 982.80 945.07 1090.00 1.22 0.00370 0.00215 1.72
DCCCN5 35.4 1324.00 982.80 1059.25 1194.00 1.35 0.00378 0.00215 1.76
DCCCN6 35.4 1236.00 982.80 999.89 1094.20 1.26 0.00474 0.00215 2.20
DCCCN7 35.4 1259.00 982.80 1067.20 1196.05 1.28 0.00510 0.00215 2.37
DCCCN8 35.4 1293.00 982.80 1021.44 1228.35 1.32 0.00492 0.00215 2.29
DCCCN9 35.4 1157.00 982.80 945.05 1102.00 1.18 0.00464 0.00215 2.16
SCCCN10 35.4 1310.00 982.80 1067.26 1174.50 1.33 0.00368 0.00215 1.71
DCCCN11 35.4 1344.00 1135.69 1054.28 1196.80 1.18 0.00576 0.00215 2.68
DCCCN12 35.4 1347.00 1135.69 1057.68 1279.65 1.19 0.00352 0.00215 1.64
DCCCN13 35.4 1384.00 1135.69 1115.72 1214.80 1.22 0.00486 0.00215 2.26
DCCCN14 35.4 1396.00 1135.69 1126.80 1296.20 1.23 0.00528 0.00215 2.46
SCCCH15 67.3 1537.00 1431.49 1217.27 1457.30 1.07 0.00344 0.00215 1.60
DCCCH16 67.3 1661.00 1575.86 1337.09 1657.00 1.05 0.00499 0.00215 2.32
DCCCH17 67.3 1682.00 1575.86 1375.67 1297.90 1.07 0.00631 0.00215 2.93
DCCCH18 67.3 1764.00 1575.86 1431.26 1675.80 1.12 0.00660 0.00215 3.07
DCCCH19 67.3 1795.00 1575.86 1376.88 1541.20 1.14 0.00694 0.00215 3.23
DCCCH20 67.3 1716.00 1575.86 1392.88 1540.20 1.09 0.00572 0.00215 2.66
DCCCH21 67.3 1726.00 1575.86 1413.88 1459.70 1.10 0.00614 0.00215 2.86

When the intensity of applied load reached to about 85 to 95% of its ultimate value, few
vertical and diagonal cracks fine in nature, appeared over the outer surface as well as along
the upright edge of the specimens. The moment, load intensity arrived at 96% to 98% of its
ultimate load the unconfined concrete cover began to spall off from the surface and edge of
the specimen. It continued throughout the descending stage of the axial load-axial strain curve
giving rise to an oblique failure plane in the mid region of the test specimen. The crack
pattern and spalling mechanism of unconfined concrete cover has been shown in Figure 5.
Figure 5 Vertical crack emerged along the left corner of the column-DCCCN5
3.2. Load-strain behaviour
Following were the observations during testing:
3.2.1. Effect of inner layer
The axial load-axial strain behaviour of specimens SCCCN1 and DCCCN2 and SCCCH15
and DCCCH16 are shown in Figure 6 (a) and 6 (b). It can be seen that with the addition of an
inner layer into the specimen, the peak strength and corresponding peak strain increased by
14% and 50% as well as 8% and 45% for M30 and M60 grades of concrete, respectively.
Figure 6 (a)
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03
AxialLoad(kN)
Axial Strain
SCCCN1 DCCCN2

Figure 6 (b)
3.2.2. Effect of different shapes of inner transverse reinforcement
The axial load - axial strain curves of specimens DCCCN2 to DCCCN5 are compared in
Figure 6 (c) to examine the effect of the shape of the double layered stirrups. Po/ P'o and o/
'o values were found to be in the range of 1.22 - 1.35 and 1.52 - 1.76, respectively. It can be
assessed that specimen with spiral shaped inner transverse reinforcement showed better
performance when compared with its other shapes like square, diamond or circular, in terms
of both axial load and axial strain capabilities. This observation may be expected by virtue of
the significant contribution of the concrete, inside the circular spiral of double layered
concrete specimen. Figure 6 (d) shows a comparison among curves of specimens DCCCN11
to DCCCN14 where the amount of reinforcement in the outer layer has been doubled while
keeping other attributes similar to specimens DCCCN2 to DCCCN5. It can be noticed that Po/
P'o and o/ 'o values ranged between 1.18 to 1.23 and 1.64 to 2.68, respectively. Comparing
Figure 6 (c) and 6 (e), where only the grade of concrete is changed, it was observed that Po/
P'o and o/ 'o values were in the range of 1.05 to 1.14 and 2.32 to 3.23, respectively.
Figure 6 (c)
0
500
1000
1500
2000
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
SCCCH15 DCCCH16
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCN2 DCCCN3
DCCCN4 DCCCN5

Figure 6 (d)
Figure 6 (e)
3.2.3. Effect of concrete strength
To determine the influence of concrete strength on the load-deformation characteristics of
singly (SCCCN1 and SCCCH15) and doubly layered (DCCCN2 and DCCCH16) columns,
concrete grade M30 and M60 were analysed and other parameters such as reinforcement
properties, proximity ratio, and shape of transverse reinforcement, pitch of lateral steel and
yield strength of reinforcement were kept same.
Figure 6 (f)
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCN11 DCCCN12
DCCCN13 DCCCN14
0
200
400
600
800
1000
1200
1400
1600
1800
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCH16 DCCCH17
DCCCH18 DCCCH19
0
200
400
600
800
1000
1200
1400
1600
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
AxialLoad(kN)
Axial Strain
SCCCN1 SCCCH15

From Figure 6 (f) and 6 (g), it can be seen that peak strength and corresponding strain
rises by 38% and 44% with the increase in concrete strength in single layered columns.
Whereas for double layered specimens these values were enhanced by 31% and 52%.
Figure 6 (g)
3.2.4. Influence of longitudinal reinforcement
A comparison among the specimens where the amount of longitudinal reinforcement
increased to double while keeping other attributes, such as, concrete grade, shape of inner
transverse reinforcement, proximity ratio, pitch of lateral steel and yield strength of
reinforcement same, as shown in Figure 6 (h) & 6 (d). It can be accessed from the curves that
distribution of eight longitudinal bars circumscribing the column section influenced
significantly the axial load-axial strain behaviour of the test specimens. From the figure it can
be seen that the strength and the corresponding strain increased in the range of 5.44% -
15.14% and 3.53% - 75.6%, respectively.
Figure 6 (h)
3.2.5. Effect of proximity ratio
Fig. 6 (i) shows the test results of Specimens DCCCN2, 8 and 9. It can be inferred that when
the concrete strength and the properties of reinforcement are kept same, both peak load and
corresponding strain increased with the decrease in the distances between the confining
layers.
0
500
1000
1500
2000
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCN2 DCCCH16
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
SCCCN1 SCCCN10

Figure 6 (i)
3.2.6. Effect of pitch (Area ratio of transverse reinforcement)
The only variable kept constant among specimens DCCCN 2, 6, 7 and DCCCH16, 20, 21 was
area ratio of the transverse reinforcement forming the inner layer. Figure 6 (j) and 6 (k) show
the axial load-axial strain curves of the specimens DCCCN2, 6, 7 and DCCCH 16, 20, 21
respectively.
Figure 6 (j)
Figure 6 (k)
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCN2 DCCCN8 DCCCN9
0
200
400
600
800
1000
1200
1400
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCN2 DCCCN6 DCCCN7
0
200
400
600
800
1000
1200
1400
1600
1800
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
AxialLoad(kN)
Axial Strain
DCCCH16 DCCCH20 DCCCH21

The inner transverse reinforcement ratio ρs in considered for both the normal strength
concrete and higher grade concrete specimens has been 1.16%, 1.62% and 2.31%. The outer
transverse reinforcement ratio ρs out has been kept constant as 0.57%. With the increase in
transverse reinforcement ratio the strength capability has been marginally improved whereas
the deformation capability has improved immensely.
3.3. Failure mechanism of the core concrete
The failure pattern detailed in Table 4 elucidates the concrete inside the core of the specimens
failed either by shearing or crushing or both, under the applied axial compressive loads. The
pictorial representation is shown in Figure 7 (a), 7 (b) and 7 (c).
The intensity of the applied load kept continuously increased even after the specimen
yielded. It was observed that when the applied load reached its peak, a diagonal failure plane
emerged in the mid region of the specimen inclined to the horizontal by 400
to 600
. The
concrete in the mid region started cracking out. The steel stirrups at the middle height of the
specimens got over stressed due to increased intensity of applied load. Due to this rupturing of
stirrups along with cracking of concrete started occurring in the middle region. Harsh piercing
and cracking sound of high intensity, was heard repeatedly. The concrete in the mid region
was found severely cracked and crushed, forcing the longitudinal bars to buckle out. Further
Table 4 Failure Patterns
Specimen
No.
Failure
Pattern
Specimen
No.
Failure
Pattern
Specimen
No.
Failure
Pattern
SCCCN1
Compression-shear
failure
DCCCN8
Compression-
crush failure
SCCCH15
Compression-
shear failure
DCCCN2
Compression-shear
failure
DCCCN9
Compression-shear
failure
DCCCH16
Compression-
shear failure
DCCCN3
Compression-shear
failure
SCCCN10
Compression-shear
failure
DCCCH17
Compression-
crush failure
DCCCN4
Compression-shear
failure
DCCCN11
Compression-shear
failure
DCCCH18
Compression-
shear failure
DCCCN5
Compression-shear
failure
DCCCN12
Compression-
crush failure
DCCCH19
Compression-
shear failure
DCCCN6
Compression-
crush failure
DCCCN13
Compression-shear
failure
DCCCH20
Compression-
shear failure
DCCCN7
Compression-shear
failure
DCCCN14
Compression-
crush failure
DCCCH21
Compression-
shear failure
(a) Diagonal failure plane-DCCCN15 (b) Cracking and spalling of concrete in the
the middle region of the specimen-DCCC14

(c) Bowing out of longitudinal reinforcement-DCCCN8
Figure 7 (a - c)
also it was observed that the lateral steel reinforcement was exposed as well as the outer
longitudinal bars began bowing out between the ties of outer transverse reinforcement.
Thereafter the load bearing capacity of the specimens descended and the crushed concrete
started departing the specimen.
After buckling and bowing out of the longitudinal bars the applied load dropped to
substantially low and the test was stopped.
4. CONCLUSIONS
This study speaks of an experimental study on double layer stirrups reinforced concrete
columns with a cumulative corollary of several attributes in terms of strength of concrete and
tensile bar, using square, diamond, circular and spiral inner core sections. Based on the results
of this investigation, following conclusions could be drawn:
 Insertion of an additional inner layer enhanced the load-strain potentiality of the specimen
columns by substantial amounts.
 For the same axial compressive load, the performance of specimens in terms of confinement
efficiency for the spiral shaped inner core was found to be the best followed by circular,
diamond and square shaped inner cores, in that order.
 It is observed that the confinement efficiency in terms of axial strain is comparatively less
with a higher grade of concrete than with normal grade concrete. An interesting relation
between the quality of concrete and the confinement efficiency has emerged from this study. It
is seen that the improvement in the concrete quality has led to greater impact on the strength
of the column when single layered technique is used. With regard to deformability, greater
impact is seen when double layer confinement technology is used. This may mean that the
second/ inner layer has provided greater bonding when the concrete used is of relatively
higher grade. This observation bears implications for the choice of confinement technique
cum the quality of concrete. The findings on the impact on strength of the column remain
inconclusive as no plausible explanation could be arrived at. This opens up an area for future
research that may include greater number of samples and more contextual settings.
 The role of longitudinal reinforcement also played a vital role in the double layered confined
concrete specimens. The presence of greater number of longitudinal bars, evenly distributed

around the perimeter as well as suitably tied across the section enhanced the confinement
efficiency of the columns.
 Smaller the spacing distance between inner and outer layers the better is the load bearing
capacity of the specimen. This further validates that both inner and outer layers have
significant impact on the behaviour of the specimens.
 The increase in the transverse reinforcement ratio influenced significantly the load-strain
behaviour of the column specimens both for normal and high grade concrete.
These observations, the authors believe would form strong basis for initiating studies on
scaled-up structural activities and the relevant economics. While the study develops optimism
on enhancing compressive strength and strain through double layered stirrup concrete
structures, detailed estimates on the enhancement versus cost implications could lead to better
policy implications particularly for major works, such as, Metro Railways.
5. NOTATIONS
fco = Concrete strength obtained from standard cube test
f'co = Modified concrete strength taken as = 0.85 fco
# = Number of rebar
ø = Diameter of rebar
ρs out = Area ratio of outer transverse reinforcement
ρs in = Area ratio of inner transverse reinforcement
Po = Peak load
P'o = Theoretical peak load
P'sp = Load at the start of first crack appearance
P''sp = Load at the start of spalling of unconfined concrete cover
o = Strain at peak load
'o = Strain at theoretical peak load
ACKNOWLEDGEMENTS
I very fondly and gratefully remember a senior scholar Prof. Umesh Kumar Sharma,
Department of Civil Engineering, Indian Institute of Technology, Roorkee, U. K., India, for
igniting my interest and strengthening my will in this line of research.
The authors sincerely thank Prof. (Dr.) S. Aqueel Ahmad, Assoc. Prof. Mohd. Kasif Khan
and Assoc. Prof. Tabish Izhar, Department of Civil Engineering, Integral University,
Lucknow, U. P., India, for providing access to the relevant testing machinery of their
Laboratories.
REFERENCES
[1] Esmerald, F. Agim, S. and Erdit, L. (2016) Confined concrete behavior influencing
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confined concrete. Journal of Structural Engineering ASCE, 114(8): pp. 1804-1826.

[4] Saatcioglu, M. and Razvi, S. R. (1992) Strength and ductility of confined concrete.
Journal of Structural Engineering, ASCE, 118(6): pp. 1590-1607.
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concrete columns. ACI Structural Journal, 91(6): pp. 678-687.
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high strength concrete columns under axial compression. International Journal of
Advanced Concrete Technology, Japan, 3(2): pp. 267-281.
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compressive behavior of confined concrete. Fire Safety Journal, 48 pp. 58-68.
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for improved seismic resistance of rectangular Reinforced Concrete columns: Concept and
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columns with two layers of Spirals. KSCE Journal of Civil Engineering Structural
Engineering: pp. 1-9.
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behavior of square R. C. columns confined using two layers of stirrups’, Advances in
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Characteristics of Double Layered Stirrups Confined Concrete Square Columns. Advances
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10262, New Delhi, 2009, pp. 21

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