strength and ductility behaviour of concrete … · cube characteristic strength (mpa) f ck (28...

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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: [email protected]

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.

(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

Mahesh Kumar, S. Kaleem A. Zaidi, S. C. Jain, K. V. S. M. Krishna


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

mix proportions of specimens are shown in Table 1. The specimen configuration and

dimensions, and the pictorial representation of the




Table 1 Mix proportions of concrete specimens M

ix/

Sp

ecim

en

Cem

ent

(Kg/m

3)

Wa

ter

(l/

m3)

F A

(Kg/m

3)

C A

(Kg/m

3)

Sil

ica

Fu

me

(Kg/m

3)

Su

per

pla

stic

i

-zer

(K

g/m

3) 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

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

Pro

xim

ity

rati

o b

etw

een

inn

er/

ou

ter

core

s

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

kN

)

Axial Strain

DCCCN11 DCCCN12

DCCCN13 DCCCN14

0200400600800

10001200140016001800

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

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

Axia

l L

oa

d (

kN

)

Axial Strain

DCCCN2 DCCCN6 DCCCN7

0200400600800

10001200140016001800

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Axia

l L

oa

d (

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 60

0. 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

also

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


failure SCCCN10

Compression-shear

failure DCCCH17

Compression-

crush failure


failure DCCCN11

Compression-shear

failure DCCCH18

Compression-

shear failure


failure DCCCN12

Compression-

crush failure DCCCH19

Compression-

shear failure

DCCCN6 Compression-

crush failure DCCCN13

Compression-shear

failure DCCCH20

Compression-

shear failure


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)

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.

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strength and ductility behaviour of concrete … · cube characteristic strength (mpa) f ck (28...

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