Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study
The stress-strain model of concrete depends on the degree of frictional resistance across the loading surfaces of a test specimen depending on the antifriction medium used during testing. This article presents a comparative study of platen restraint on the behavior of concrete under uniaxial compres...
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Інститут проблем міцності ім. Г.С. Писаренко НАН України
2016
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| Цитувати: | Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study / S. Kumar, T. Mukhopadhyay, S.A. Waseem, B. Singh, M.A. Iqbala // Проблемы прочности. — 2016. — № 4. — С. 139-150. — Бібліогр.: 22 назв. — англ. |
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irk-123456789-1735152020-12-08T01:26:45Z Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study Kumar, S. Mukhopadhyay, T. Waseem, S.A. Singh, B. Iqbala, M.A. Производственный раздел The stress-strain model of concrete depends on the degree of frictional resistance across the loading surfaces of a test specimen depending on the antifriction medium used during testing. This article presents a comparative study of platen restraint on the behavior of concrete under uniaxial compression based on an experimental investigation. The effect of four commercially available antifriction media (neoprene, polyvinyl chloride, teflon, and grease) with different layer thicknesses on platen restraint have been studied for a normal strength concrete and a relatively high-strength concrete. Subsequently, the effect of platen restraint has been quantified using the analogy of toughness. The experimental results indicate that post-ultimate response of concrete is significantly affected by platen restraint. It is shown that the stress–strain curve obtained from a conventional uniaxial compression test not only describes specimen/material behavior but also represents interaction between specimen and loading platen. Among the four antifriction media used in this investigation, grease is the most effective in reducing frictional resistance. Failure patterns of the concrete specimens for different antifriction media are also subsequently analyzed. 2016 Article Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study / S. Kumar, T. Mukhopadhyay, S.A. Waseem, B. Singh, M.A. Iqbala // Проблемы прочности. — 2016. — № 4. — С. 139-150. — Бібліогр.: 22 назв. — англ. 0556-171X http://dspace.nbuv.gov.ua/handle/123456789/173515 539.4 en Проблемы прочности Інститут проблем міцності ім. Г.С. Писаренко НАН України |
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Производственный раздел Производственный раздел |
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Производственный раздел Производственный раздел Kumar, S. Mukhopadhyay, T. Waseem, S.A. Singh, B. Iqbala, M.A. Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study Проблемы прочности |
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The stress-strain model of concrete depends on the degree of frictional resistance across the loading surfaces of a test specimen depending on the antifriction medium used during testing. This article presents a comparative study of platen restraint on the behavior of concrete under uniaxial compression based on an experimental investigation. The effect of four commercially available antifriction media (neoprene, polyvinyl chloride, teflon, and grease) with different layer thicknesses on platen restraint have been studied for a normal strength concrete and a relatively high-strength concrete. Subsequently, the effect of platen restraint has been quantified using the analogy of toughness. The experimental results indicate that post-ultimate response of concrete is significantly affected by platen restraint. It is shown that the stress–strain curve obtained from a conventional uniaxial compression test not only describes specimen/material behavior but also represents interaction between specimen and loading platen. Among the four antifriction media used in this investigation, grease is the most effective in reducing frictional resistance. Failure patterns of the concrete specimens for different antifriction media are also subsequently analyzed. |
| format |
Article |
| author |
Kumar, S. Mukhopadhyay, T. Waseem, S.A. Singh, B. Iqbala, M.A. |
| author_facet |
Kumar, S. Mukhopadhyay, T. Waseem, S.A. Singh, B. Iqbala, M.A. |
| author_sort |
Kumar, S. |
| title |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study |
| title_short |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study |
| title_full |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study |
| title_fullStr |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study |
| title_full_unstemmed |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study |
| title_sort |
effect of platen restraint on stress–strain behavior of concrete under uniaxial compression: a comparative study |
| publisher |
Інститут проблем міцності ім. Г.С. Писаренко НАН України |
| publishDate |
2016 |
| topic_facet |
Производственный раздел |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/173515 |
| citation_txt |
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under Uniaxial Compression: A Comparative Study / S. Kumar, T. Mukhopadhyay, S.A. Waseem, B. Singh, M.A. Iqbala // Проблемы прочности. — 2016. — № 4. — С. 139-150. — Бібліогр.: 22 назв. — англ. |
| series |
Проблемы прочности |
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| first_indexed |
2025-07-15T10:16:02Z |
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2025-07-15T10:16:02Z |
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1837707624418639872 |
| fulltext |
PRODUCTION SECTION
UDC 539.4
Effect of Platen Restraint on Stress–Strain Behavior of Concrete under
Uniaxial Compression: A Comparative Study
S. Kumar,
a
T. Mukhopadhyay,
a,b,1
S. A. Waseem,
a
B. Singh,
a
and M. A. Iqbal
a
a Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India
b College of Engineering, Swansee University, UK
1 800712@swansea.ac.uk; URL: www.tmukhopadhyay.com
The stress-strain model of concrete depends on the degree of frictional resistance across the loading
surfaces of a test specimen depending on the antifriction medium used during testing. This article
presents a comparative study of platen restraint on the behavior of concrete under uniaxial
compression based on an experimental investigation. The effect of four commercially available
antifriction media (neoprene, polyvinyl chloride, teflon, and grease) with different layer thicknesses
on platen restraint have been studied for a normal strength concrete and a relatively high-strength
concrete. Subsequently, the effect of platen restraint has been quantified using the analogy of
toughness. The experimental results indicate that post-ultimate response of concrete is significantly
affected by platen restraint. It is shown that the stress–strain curve obtained from a conventional
uniaxial compression test not only describes specimen/material behavior but also represents interaction
between specimen and loading platen. Among the four antifriction media used in this investigation,
grease is the most effective in reducing frictional resistance. Failure patterns of the concrete
specimens for different antifriction media are also subsequently analyzed.
Keywords: uniaxial compression test, platen resistant, antifriction medium, stress–strain
behavior of concrete, failure pattern.
Introduction. Concrete has a wide range of applications as a modern construction
material because of its capability of meeting high performance engineering requirements.
The common ingredients for making concrete such as cement, aggregate and water are
easily available in most parts of the world. A broad familiarity with the procedure for the
batching, mixing, casting and curing of concrete has been naturally developed over the
years. Concrete can be moulded into virtually any shape and size according to requirement.
In structural applications, concrete is designed primarily to resist compressive loads since
this material is very strong in compression as compared to tension. The compressive and
flexural behavior of concrete is usually characterized by its strength in uniaxial compression
and by its stress–strain behavior under such loading [1, 2]. For safe and economic design of
concrete structures, it is of utmost importance to understand the stress–strain behavior of
concrete precisely. During compression tests, the contact between metal end platens and the
concrete specimen creates a frictional resistance at the ends of the specimen (platen restraint),
which affects the stress–strain behavior of concrete. A brief review concerning the studies
of platen restraint on the constitutive model of concrete is presented in the next paragraph.
Early researchers (Bresler and Pister [3], Iyengar et al. [4]) used a thrust bearing
assembly by means of ball bearings to reduce friction in concrete cylinder testing. Hughes
and Bahramian [5] used M.G.A. pad to reduce the friction for determining uniaxial
compressive strength of concrete. An M.G.A. pad consists of a melinex polyester film, a
layer of molyslip grease and a hardened aluminum sheet. The reason behind using
© S. KUMAR, T. MUKHOPADHYAY, S. A. WASEEM, B. SINGH, M. A. IQBAL, 2016
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4 139
aluminium sheet is that it has a similar lateral expansion to that of concrete under
compression. Kupfer et al. [6] used brush bearing platens consisting of a series of small
steel bars meant to flex with the deformation of concrete without buckling. Later comblike
platens were used by many researchers (Buyukozturk et al. [7], Atan and Slate [8], Tasuji et
al. [9]) to minimize the end friction. Several studies are also found in the literature to
address the influence of other factors involved in concrete testing on the material model of
concrete. Dhir and Sangha [10] studied the effect of several factors related to testing
techniques on the stress–strain behavior of concrete such as specimen slenderness ratio,
diameter of test platen used to transmit load from the testing machine to the test specimen,
ratio of specimen diameter to maximum size of aggregate and the methods employed for
strain measurement. Recent studies include the effect of reversed cyclic loads on bond
strength deterioration of concrete members (Choi et al. [11]), hygrothermic effect on
mechanical stresses and crack analysis of concrete (Dahmani [12]), effect of external bar
retrofitting on the bearing capacity of RC beams (Vasudevan and Kothandaraman [13]),
effect of AFRP bar reinforcement on flexural behavior of concrete beam (Buyukkaragoz et
al. [14]) and effect of curing material (Yilmaz and Turken [15]) on the constitutive
relationship of concrete. Comparative studies of different techniques to reduce platen
restraint are discussed next. Gerstle et al. [16] carried out a comparative investigation using
many different friction reduction methods such as cushion system, platens cured with
febcure, combination of polyethylene sheets and grease, aluminum bearing platens and
brush bearing platens. Kotsovos [17] experimentally investigated the effect of platen
restraint on stress–strain relationship of concrete under uniaxial compression through a
comparative assessment using neoprene rubber, MGA pad and brush platen. They
suggested that as the end friction is reduced, the descending branch of stress–strain curve of
concrete becomes steeper. Tschegg et al. [18] compared the use of brush platens and Teflon
film to reduce frictional resistance and ventured that the ideal correction can be realized by
a mechanism that is capable of following the lateral deformation of test specimen under
compressive load and allows a homogeneous and constant force initiation in the specimen.
Of late, Lee et al. [19] have used Teflon to reduce surface friction and they have concluded
that Teflon is more effective than brush platens.
Most of the investigations carried out so far concerning the issue of platen resistant on
stress–strain behavior of concrete are concentrated on biaxial or multiaxial stresses.
Comparative studies on platen restraint using different antifriction media for uniaxial
compression are very scarce in available literature, though such comparative assessments
are quite useful for practical purposes. Since the effect of platen restraint has a considerable
influence on post-ultimate material model of concrete, it is desirable to comprehensively
investigate its effect to propose more precise constitutive models for concrete, which will
enable more effective utilization of the strength after concrete reaches the ultimate strength
with the adequate level of confidence. Moreover, brush platens, which are considered to be
the most effective in reducing platen restraint, are relatively difficult to fabricate and hence
there is the need to explore other antifriction media, which are readily available and easy to
use in the routine testing of concrete in uniaxial compression. Objective of the present
study is to experimentally investigate the effect of platen restraint on stress–strain behavior
of concrete under uniaxial compression using four different commercially available antifriction
media (neoprene, polyvinyl chloride (PVC), teflon, and grease) for normal as well as
relatively high-strength concrete. The effect of layer thickness of different antifriction
media has also been explored. The comparative assessment of the aforementioned four
antifriction media with different layer thicknesses to minimize the effect of platen restraint,
in case of normal as well as high strength concrete is first attempted in this study.
1. Post-Ultimate Material Behavior of Concrete: Effect of Platen Restraint. The
effect of platen restraint on post-ultimate behavior of concrete is briefly discussed in this
section for the convenience of common readers. Complete stress–strain behavior of
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
140 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
concrete under uniaxial compression is normally obtained by loading cylinder specimens at
a constant rate of displacement, wherein the applied uniaxial load is transferred through
steel platen as shown in Fig. 1a. The central zone of the cylinders under such uniaxial
compression is subjected to a near-uniform uniaxial compressive stress (Fig. 1b), while a
complex state of compressive stress (Fig. 1c) is imposed on the end zones due to the
frictional resistance generated from the interaction between the specimen and steel platen.
As the effect of platen restraint reduces away from the two ends of the specimen, the
value of � c approaches to zero in the central zone (Fig. 1). During the test, cracks parallel
to the direction of loading appear in the central zone of the specimen at a load level close to
the ultimate load-carrying capacity (Stage 3). At this stage, stresses in end zones remain
below ultimate strength and thus it remains uncracked, whereas the stresses in the central
zone attain the ultimate strength value. Due to the presence of cracks in the central zone,
the lateral expansion of the central zone becomes much higher, as compared to that of end
zones resulting in the development of internal forces at the interfaces of these two
incompatible zones. If the stresses in the end zone of a specimen are considered, two lateral
stresses act there: the outward stresses exerted by the central zone due to incompatible
lateral expansion of the two zones and the inward stresses exerted by the steel platen due to
frictional resistance. With increasing lateral deformation of the central zone, the internal
outward stresses acting in the end zones progressively neutralize the effects of the end
friction and, eventually, at least one of the principle stresses in the end zone reaches the
ultimate strength value resulting in the formation of cracks in this zone and the consequent
failure of the total specimen.
The above discussion on the failure mechanism of a concrete specimen subjected to
uniaxial compression implies that the extent of frictional resistance at the boundaries of the
specimen plays an important role in the post-ultimate stress–strain behavior of concrete.
With the increase in compressive stress, the concrete specimen reaches gradually to Stage 3
from Stage 1 when the stresses in central zone reach to its ultimate strengths. Up to Stage 3,
the end zones remain uncracked. If the frictional resistance at the boundaries are reduced,
the height of central zone will increase, resulting a more rapid collapse of the specimen
after it reaches to the ultimate strength level. Thus it is of outmost importance to minimize
the effect of platen restraint for understanding the behavior of concrete under pure
Effect of Platen Restraint on Stress–Strain Behavior ...
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4 141
Fig. 1. Illustration of platen restraint in a typical uniaxial cylinder compression test (a); stress state of
the elements of central (b) and end (c) zones; typical stress–strain behavior of concrete (d).
a d
b c
compression. The present study focuses on the comparative performance of four different
antifriction media in reducing the platen restraint.
2. Experimental Details. In the present investigation stress–strain relationships (for a
normal strength concrete of grade M25 and a relatively high-strength concrete of grade
M50) have been obtained by testing the standard cylindrical specimens with the antifriction
medium sandwiched between the specimens and the platen in a stiff close-loop servo-
controlled INSTRON make universal testing machine of 2500 kN capacity under a
displacement rate of 0.0375 mm/min. Tests for each specimen have been terminated when
the stress level reached to 0.4 times of the ultimate strength in the descending branch of
stress–strain curve. Stresses have been calculated from applied loads and strains have been
calculated using platen-to-platen displacement as well as displacement measured by LVDT
(linear variable differential transformer) over a gauge length of 150 mm at the mid-height.
The outputs of the LVDT have been fed to an automatic data acquisition system. At the end
of each test, the mode of failure and crack patterns have been noted.
An extensive test program was undertaken for the present study with total 90
cylindrical concrete specimens having 150 mm diameter and 300 mm height (see Table 1).
The test specimens were cast in cylindrical moulds of 150 mm diameter and 300 mm height
and demoulded 24 h after casting. The cylinders were moist cured by complete immersion
in a water tank till the date of testing. The curing tank water was changed every week and
the compression tests were carried out after a nominal interval of 28 days from the date of
casting. Five nominally identical companion specimens were tested for each parameter
142 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
T a b l e 1
Test Program for Investigating the Effect of Platen Restraint
on Stress–Strain Behavior of Concrete
Grade
of concrete
Antifriction media Layer thickness/weight
of antifriction media
No. of test
specimens
M25 No antifriction medium – 5
Neoprene rubber 1 mm thickness
3 mm thickness
5
5
PVC 0.5 mm thickness
1 mm thickness
5
5
Teflon 1 mm thickness
2 mm thickness
5
5
Grease 5 g
10g
5
5
M50 No antifriction medium – 5
Neoprene rubber 1 mm thickness
3 mm thickness
5
5
PVC 0.5 mm thickness
1 mm thickness
5
5
Teflon 1 mm thickness
2 mm thickness
5
5
Grease 5 g
10g
5
5
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
under investigation. Since in all cases the coefficient of variation in the measured
compressive strengths of a group of five specimens was less than 5%, the average values of
the measured strengths and stress–strain relationships has been reported here. The test
cylinders were capped with a thin layer of stiff, neat Portland cement paste about 4 h after
moulding by which time it was assumed that concrete had ceased settling in the moulds.
Care was taken to ensure that the cement used for capping was nursed to a stiff paste for
about 2 h before it was to be used in order to minimize the possibility of shrinkage of the
cap. The planeness of the cap was checked by means of a straight edge and feeler gauge by
taking three measurements on different diameters. ACI guidelines [20] have been followed
for mix design. The mix proportions used in the present study are shown in Table 2 and
other test results of the materials used for two grades of concrete are presented in Tables 3–7.
A high range water reducing admixture (HRWRA) Glenium 51, based on high molecular
weight polymers and sulphonated melamine formaldehyde has been used for achieving
adequate workability. The nominal dosage of the HRWRA has been found to vary in the
range 0.5 and 2.25% of the weight of cement, according to the workability requirements of
the fresh concrete. All the antifriction media except grease have been cut into the size of
150 150� mm and placed in between the loading platens and concrete specimen at both
ends. In case of grease, it has been uniformly applied on the steel platen and then a piece of
thick paper has been placed between specimen and loading platens to prevent the direct
contact of grease with specimen.
3. Results and Discussions. In this section, the experimental results obtained from the
series of tests performed in this study are presented. Subsequently the results have been
analyzed to investigate the effect of platen restraint. Stress–strain relationships obtained
from the tests have been normalized with respect to their corresponding ultimate strength.
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4 143
T a b l e 2
Details of Concrete Mix
Grade
of concrete
Ordinary
Portland cement
Fine
aggregate
Coarse
aggregate
Water Water
cement ratio
M25 1 1.73 3.14 0.46 0.46
M50 1 1.98 2.99 0.33 0.32
T a b l e 3
Physical Properties of the Ordinary Portland Cement
Property Unit Test
result
Limiting values
as per IS 8112: 1989 [21]
Fineness by Blaine’s air permeability test
Soundness
Specific gravity
Autoclave expansion
Normal consistency
Initial setting time
Final setting time
72 1� h compressive strength
168 2� h compressive strength
672 4� h compressive strength
m2/kg
mm
–
%
%
min
min
MPa
MPa
MPa
268
2.8
3.14
0.06
26.5
90
180
25.5
35.0
44.3
�225
�10
3.14
�0.80
30
�30
�600
�23
�33
�43
Effect of Platen Restraint on Stress–Strain Behavior ...
144 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
T a b l e 4
Chemical Analysis of the Ordinary Portland Cement
Oxide composition Test result
(%)
Limiting values (%)
as per IS 8112: 1989 [21]
Silica (SiO2)
Ferric oxide (Fe2O3)
Alumina (Al2O3)
Calcium oxide (CaO)
Magnesia (MgO)
Sulphuric anhydride (SO3)
Total alkali in terms of sodium oxide (Na2O)
Insoluble residue
Loss on ignition
21.0
4.15
7.03
61.68
1.81
0.92
0.45
1.1
1.0
19–24
1–4
3–6
59–64
�6
�3
�0.6
�2
�5
T a b l e 5
Physical Properties of Fine and Coarse Aggregates (ASTM C33)
Physical property Fine aggregates Coarse aggregates
Fineness modulus
Specific gravity
Dry rodded mass (kg/m3)
Water absorption (%)
Moisture content (%)
3.02
2.61
2017
1.0
0–0.4
3.00
2.67
1620
0.50
Zero
T a b l e 6
Sieve Analysis Results of the Fine Aggregates
IS sieve
size
(mm)
Weight
retained
(g)
Percentage
retained
Cumulative
percentage
retained
Percentage
passing
Percentage passing
for Grading as per
ASTM C33 [22]
4.750
2.360
1.180
0.600
0.300
0.150
0.075
6.0
82.5
136.0
92.5
91.0
61.0
29.0
1.2
16.5
27.2
18.5
18.2
12.2
5.8
1.2
17.7
44.9
63.4
81.6
93.8
99.6
98.8
82.3
55.1
36.6
18.4
6.2
0.4
95–100
80–100
50–85
25–60
15–30
0–10
–
T a b l e 7
Sieve Analysis Results of the Coarse Aggregates
IS sieve
size
(mm)
Weight
retained
(g)
Percentage
retained
Cumulative
percentage
retained
Percentage
passing
Percentage passing
for Grading as per
ASTM C33 [22]
20.00
19.00
12.50
9.50
4.75
2.36
0
1.00
6.89
1.07
0.91
0.02
0
10.50
69.32
10.76
9.15
0.20
0
10.50
79.82
90.58
99.73
99.93
100.00
89.50
20.18
9.45
0.27
0.07
100
90–100
20–55
0–15
0–5
–
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
The stress–strain curves depicting the effect of various antifriction media for two different
grades of concrete are presented in Fig. 2, from which it is evident that the effect of platen
restraint has no effect on the ascending branch of stress–strain curve, while the descending
branches show considerable variations in their behavior for different antifriction media.
Thus the ascending branch of stress–strain curve for concrete is independent of technique
used for test and an intrinsic material property, whereas descending branch shows
dependency on the technique used for the tests. It can also be observed that slope of
descending branch of the stress–strain curve is the least when no antifriction medium is
used, and the slope progressively increases as degree of frictional restraint decreases.
Grease shows the highest slope in all the cases implying that it is most efficient in reducing
frictional restraint among the four tested antifriction media. In the case of Neoprene rubber,
peak strain is found to be higher for a relatively thick sheet (3 mm) because of large
compressibility of a thick Neoprene sheet.
Noteworthy is the fact that Fig. 2 exhibits a continuous increase of strain during the
entire loading path. This is in contrast to the observation of Kotsovos [17] during the
course of his investigation of platen restraint, in which he mentions a continuous decrease
in strain below 80 to 90% of the peak-stress in the descending branch of the stress–strain
curves. This phenomenon has been called ‘elastic recovery’ and has been attributed to the
occurrence of continuous axial cracks which is supposed to cause unloading within the
outer layers of the cylinders, the applied load being predominantly resisted by the core.
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4 145
a b
c d
Fig. 2. Normalized stress–strain curve for concrete using different antifriction media (� and �u
denote the stress and ultimate strength, respectively): (a) M25 grade concrete (lower antifriction
medium thicknesses); (b) M25 grade concrete (higher antifriction medium thicknesses); (c) M50
grade concrete (lower antifriction medium thicknesses); (d) M50 grade concrete (higher antifriction
medium thicknesses).
Effect of Platen Restraint on Stress–Strain Behavior ...
Interestingly, the load–displacement relationships furnished by Kotsovos [17] show a
continuous increase of axial displacement during the entire loading history. It is reckoned in
this investigation that the ‘elastic recovery’ identified by Kotsovos [17] is due to the
relatively flexible loading frame and the strain reversal in the ‘elastic recovery’ will not
take place if a stiff loading frame had been used. With the stiff loading frame of the present
investigation, the effect of testing technique on displacement measurements has been
minimized and the measured stress–strain relationships give a realistic description of
specimen behavior. According to Kotsovos [17], the descending branch of the stress–strain
curve describes the interaction between specimen and the loading platens and is not
representative of the specimen behavior. This observation is supported by the trends noted
in this investigation. Figure 2 shows that when grease is used as the antifriction media the
descending branch becomes almost vertical implying a complete loss of load-carrying
capacity as soon as the peak stress was attained. This 90� slope of the descending branch is
indicative of the fact that platen restraint had been practically eliminated by the use of
grease and for this case the descending branch of the stress–strain curve for the normal and
the medium-strength concrete was similar.
The effect of platen resistant in concrete cylinder testing can be explained using the
analogy of energy absorbed by specimen subjected to different degree of end resistance,
which can be quantified by calculating the area under the stress–strain curve of concrete for
a particular antifriction medium. For purpose of comparison a term ‘Toughness’ in this
article has been defined as the area under normalized stress–strain curve, where the
descending branch is considered up to 0.4� u (Fig. 3). Thus, toughness indicates the energy
absorbed by a specimen until its failure and it is dependent on the slope of the descending
branch of the stress–strain curve. As degree of frictional restraint increases, failure of
specimen is delayed and, consequently, the toughness increases. Slope of the descending
branch of stress–strain curve increases with the decrease of frictional restraint, implying the
reduction in load-carrying capacity of the specimen after reaching its ultimate strength.
146 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
a
b
Fig. 3. Schematic representation of the stress–strain behavior of concrete with different degree of end
friction (a); area under stress–strain curve (b). (Toughness in axial compression.)
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4 147
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Effect of Platen Restraint on Stress–Strain Behavior ...
Figure 4 shows the variation of toughness and average slope of the descending branch of
stress–strain curve with different antifriction media. From Fig. 4 it is evident that as the
toughness increases, the average slope of descending branch decreases implying a higher
end friction for the particular antifriction medium. Grease shows the least value of
toughness and highest value of average slope for both M25 and M50 grades of concrete.
For higher thickness of antifriction medium, the toughness is lower, while the average slope
of descending branch is higher. Observing the variation of toughness and average slope of
descending branch for different cases of antifriction media with respect to those of no
medium case, it can be concluded that the effect of platen restraint is more prominent for
the lower grades of concrete.
Typical failure patterns for M25 grade of concrete specimens tested under uniaxial
compression using different antifrictional media are shown in Fig. 5. For the case of no
antifriction medium, failure occurs with bulging of cylinder in central region, while the
bulging becomes lesser when platen resistant is reduced by using different antifriction
media. In case of grease, which shows the least frictional resistance (Fig. 2), such bulging
148 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
a b c
d e
Fig. 5. Typical failure patterns of concrete specimens (M25 grade) for different antifrictional media:
(a) no antifriction medium; (b) teflon (1 mm thickness); (c) neoprene (1 mm thickness); (d) PVC (0.5
mm thickness); (e) grease (5 g).
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
has been found to be almost absent. Cracks throughout the height of specimens have been
observed to appear when the end resistances are reduced. This observation can be attributed
to the fact that the height of two end zones (as discussed in Section 1) of the specimen
decreases with reduction of the end friction. Thus cracks appear almost throughout the
entire height of specimen when antifriction media grease is used. In case of the test
involving no antifriction media, the central zone of the specimen attains its ultimate
strength when the stresses in the end zones remain below the ultimate strength, resulting
bulging to appear in the central zone of the specimen as shown in Fig. 5a.
Conclusions. This article presents a comparative study on effect of platen restraint in
the stress–strain behavior of concrete under uniaxial loading using four different
commercially available antifriction media (neoprene, PVC, teflon, and grease). A stiff
loading frame has been utilized in the present investigation to minimize the effect of testing
technique on displacement measurements resulting more realistic description of specimen
behavior. The results obtained indicate that the ascending branch of stress–strain curve of
concrete is insensitive to platen restraint, while the descending branch is significantly
affected. The descending branch becomes considerably steeper as the frictional restraint
imposed by the end platens decreases, resulting more rapid loss of load-carrying capacity of
the specimen. This sudden reduction in load-carrying capacity of concrete will have a
considerable influence on the subsequent analysis and design process and should be
accounted with adequate importance for safe design of concrete structures. Contribution of
the present study is to access the comparative performance of different commercially
available antifriction media to reduce the effect of platen restraint and thereby making it
possible to test concrete under pure uniaxial compression. The effect of platen restraint has
been quantified using a novel analogy of toughness in this article. Experimental results
show that grease is the most efficient option for reducing the effect of platen restraint
among the four tested antifriction media. Effect of platen restraint is found to be more
prominent in case of lower grades of concrete. Failure patterns corresponding to different
antifriction media have also been studied. The following general tendency is revealed: with
a higher efficacy of the antifriction media in mitigating the platen restraint, the longitudinal
cracks in the specimens increasingly penetrate the end zones of the cylinders. Considering
the importance of studying the constitutive model of concrete under uniaxial compression
with the least influence of platen restraint, more comparative investigations are needed to
be carried out using other techniques. The results presented in this article can serve as a
valuable reference for such future research in this field and the inferences drawn from this
investigation can be useful for practicing engineers to choose proper antifriction media in
concrete testing with the least influence of platen restraint.
Acknowledgments. The authors would like to acknowledge the financial support
received from MHRD, India during the period of this research work.
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Received 14. 05. 2015
150 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 4
S. Kumar, T. Mukhopadhyay, S. A. Waseem, et al.
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/ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.)
>>
/Namespace [
(Adobe)
(Common)
(1.0)
]
/OtherNamespaces [
<<
/AsReaderSpreads false
/CropImagesToFrames true
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(InDesign)
(4.0)
]
/OmitPlacedBitmaps false
/OmitPlacedEPS false
/OmitPlacedPDF false
/SimulateOverprint /Legacy
>>
<<
/AddBleedMarks false
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/ConvertColors /ConvertToCMYK
/DestinationProfileName ()
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/FlattenerPreset <<
/PresetSelector /MediumResolution
>>
/FormElements false
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/PreserveEditing true
/UntaggedCMYKHandling /LeaveUntagged
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/UseDocumentBleed false
>>
]
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [612.000 792.000]
>> setpagedevice
|