EXPERIMENTAL STUDIES ON THE BEHAVIOR OF CONFINED

REINFORCED CONCRETE COLUMNS

INTRODUCTION

The reinforced concrete columns are the main load-bearing elements in reinforced

concrete (RC) structure, as the column has to with stand the entire load and transfer it

to foundation .The studies on the behaviour of RC short columns subjected to axial and

eccentric loads were started in the early 1900s (Considère 1902, 1903; Talbot 1906; and

Withey 1911). The fundamental concept of confinement was pioneered in 1927. The

confinement of concrete by suitable arrangements of transverse reinforcement results in

a significant increase in both the strength and the ductility of compressed concrete.

Depending on how confining stresses are provided to concrete are categorized as being

active or passive. Confinement applied by pre-stressing on a concrete core is of active

type. On the other side, lateral reinforcements such as spirals or ties provide internal

passive confinement activated by the expansion of concrete. In the University of Illinois

under the guidance of Richart et al. 1929 introduced the term lateral confining stress

and proposed a model to predict the confined concrete strength. From this point

onwards many linear and non-linear models were developed to represent the stress-

strain behaviour of confined concrete.

CONFINEMENT

When reinforced concrete subjected to longitudinal compression is in a uniaxial state of

stress. Longitudinal strains generated by such loading give rise to transverse tensile
strains that may result in vertical cracking. As the material approaches its uniaxial
compressive capacity, the transverse strains become very high because of progressive
internal cracking and the concrete bears out against the transverse reinforcement.
Which then applies a confining pressure to the concrete. Combination of confining
pressure and axial compression results in a tri-axial state of stress. Transverse strains
caused by confining pressure counteract the tendency of material to expand laterally,
and result in increased strength and it is knows as confined concrete strength.
Confinement by transverse reinforcement considerably improve the stress –strain
characteristic of the concrete at high strain. (Richart et al., 1925)

The circular spiral confines concrete much more effectively then rectangular (or) square
hoops. Circular spiral because of their shape are in hoop tension provide a continuous
confining pressure around the circumference. But square hoops can apply only
confining reaction near the corners of the hoops because the pressure of the concrete
against the sides of the hoops tends to bend the side outwards. Therefore a considerable
portion of the concrete cross section may be unconfined. Because of internal arching
between the corners of the concrete is confined effectively only in the corners and
central region of the section. (Park and Paulay 1975 )

Figure 1 Confinement by square hoops and circular spirals.(Park and Paulay, 1975)
 Figure 2 Pressure Distributions Resulting from Different Reinforcemet
Arrangements. ( Saatcioglu and razvi, 1990)

OBJECTIVE

The objective of the research is to verify the effectiveness of modified ties for Reinforced
concrete column to enhance with internal confinement.

Figure 3 Pressure distribution expected due to modification of ties

SCOPE
The scope of the project is to study the Confined behaviour of axially loaded columns with
different

i. Column Sections ( Circular, Square and Rectangular)

ii. Grade of concrete (M20 and M30)

iii. Spacing of ties (100mm/200mm/300mm)

iv. Column Type (Short and Long column)

To study the the strength, failure mode, lateral and axial displacement of different types of
columns and behavior of column with modified ties during loading conditions.

CONFINEMENT MODELS REVIEW

A comprehensive review of confined models for concrete columns under concentric

axial compression that are available in the literature is conducted. In that most
important and reference model for all model was proposed by Richart et al. and
Mander et al. has given below and for rest of the few important models analytical
studies has been carried out based on some assumption and results were presented in
the form of graph and charts.

Richart et al. (1929) model was the first to capture the proportional relationship
between the lateral confined pressure and the ultimate compressive strength of
confined concrete. Found that the strength of concrete with active confinement from
lateral (fluid) pressure was approximately the same as for concrete with passive
confinement pressure from closely spaced circular steel spirals causing an equivalent
lateral pressure.
f 'cc = f 'uc + 4.1 fl
Where f 'cc -Confined concrete strength ,f 'uc-unconfined concrete strength and fl lateral
confining pressure.

Mander et al. (1984) have proposed a unified stress-strain approach for confined
concrete applicable to both circular and rectangular shaped transverse reinforcement.
The stress-strain model is and is based on an equation suggested by Popovics (1973).

Comparasion

80 62 63
56
Enhancement in stress

60 46 46
37 37
31 31
40

20 3
0
l. l. ly . er . . i l. .
a a u al d al al nt a al
et et Pa le
t
an e t e t e e t
re
t
rt'
s
m
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s s a l am a u
og i a m s
a u rk he A ad ku an
ic
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%

at S B hi M
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Sa as
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Author

Figure 4 Comparison of Enhancement In Confined Concrete Stress

 1.8
CONFINED STRESS VS STRAIN COMPARISON
1.6
ENHANCED STRESS RATIO

1.4
Richart
1.2
Mander
1 Saatcioglu
0.8 Assa et al.
0.6 Badalamenti
0.4 Hashikuma et al.

0.2
sheikh and
uzumeri
0
0 0 0 0.01 0.01 0.01 0.01 0.01 0.02 0.02 Fafitis and shah

CONFINED STRAIN

Figure 5 Comparison of Enhancement in confined concrete stress Vs Confined strain

STRESS STRAIN CURVE COMPARISON

40

35
COMPRESSIVE STRESS

30

25

20 MANDER
15 KENT AND
PARK
10
SCOTT PARK
5
PRIESTLY
0
0 5 10 15 20 25 30

COMPRESSIVE STRAIN

Figure 6 Comparison Of Stress Strain Curve Of Model

EXPERIMENTAL INVESTIGATION

Specimen details
An experimental program was going to be conducted to investigate the effectiveness of
columns with modified types of lateral ties with different column section specimens
consist of Rectangle, Circular & square columns of 150 mm x 250mm, 200 X 200 mm &
200 mm Dia. and height is 2000mm having different L/D ratio of 9-12 with two grades
of concrete M20 and M30 to study the difference in the effectiveness of confinement
due to variation of the concrete strength with three spacing of ties of 100mm,200mm
and 300mm. The test specimens is been divided into 16 categories and 3 specimen for
each set so totally 48 specimen were planned to be tested. Closer spacing of ties where
provided at top and bottom end of column to avoid bearing failure.

Figure 7 column cross section

Material properties

Properties of materials used in the experimental program were established from

standard tests. Standard concrete cube tests and reinforcement tests were conducted to
experimentally establish their characteristic strength. The concreting of the specimens
column was done in 16 batches 9 numbers of 150 mm cubes were going to be casted for
each batch. The strength of the concrete cube will be tested in compression testing
machine. The Columns were reinforced with 10 mm diameter tor steel bar as main
reinforcement and 4mm diameter alloy mild steel bar were used as ties. The steel bar
was tested for tension and for the percentage of elongation according to IS 1602:2005.

Testing setup

The test setup consists of hydraulic jack capacity of 2000 kN specimen on loading plate
attached to hydraulic jack thick rubber mate will be provided between the loading
plate and specimens to avoid non uniform distribution of load. Both top and bottom
end of the specimen surface where smoothened using hand machine to level surface of
the specimen. Plumb pop and sprit level will be used to check the vertical alignment of
the specimen.

Figure 8 Loading frame setup

Instrumentation

Three types of instruments were to be used in the test program four electrical resistance
strain gauges used to measure lateral and longitudinal strain values fixed at rebar's at
mid height of the column .Three Linear Variable Differential Transducers (LVDT) –
used to measure axial and lateral displacement placed at concrete surface at mid height
of the column and digital loading cell for measurement of applied loading.

WORKDONE

The Test specimens consist of six sets of square columns cross section of 200 mm x
200mm and height is 2000mm having L/r ratio of 10. All the three sets columns are
designed for axial loading condition of 0.8 % percentage of steel reinforcement. The
columns were reinforced with 10 mm diameter tor steel bar as main reinforcement and
4mm diameter mild steel bar were used as ties at 50 mm c/c spacing at the top and
bottom and spacing of 100mm,200mm,300mmand clear cover was given as 20 mm.

Figure 9 reinforcement cage

Casting and curing

Concrete is a composite material made up of several different constituents such as cement,

aggregate, water and admixture. The cement used was ordinary Portland cement of
43grade conforming to IS8112: 1989 .Two types of aggregate were used, coarse and fine.
The coarse aggregate of maximum size not exceeding 16 mm were used. The Fine
Aggregate (M-sand) passing through 4.75 mm IS sieve and having a fineness modulus
of 2.95 was used.
The characteristic strength test cubes were casted along with the casting of the
specimens. After casting the specimen was cured for about 28 days. The process of
curing was done by covering the columns using wet gunny bags daily .

CONCLUSION

In the 4th doctoral committee meeting the detailed review on the various internal
confinement models was presented. The experimental scheme for testing the
effectiveness of various internal confinement with the testing methodology was
explained. The time line for the pending work for the completion of the reseach
program was also presented.
 Proposed Time line for completion of works

 S.N TIME LINE WORK DESCRIPTIONS

O

1 I. Casting of specimen.
February 2020
II. Curing of Specimens for 28 days

2 I. Curing of Specimens for 28 days.

II. Testing of specimen 14 & 28 days

March 2020
III. Analytical work review Paper Publication writings

IV. Preparation for Thesis writing

3 I. Analysis of results and

April- May 2020
II. Preparation for Paper Publication

4 June 2020 I. Submission of Thesis