J. Cent. South Univ. (2015) 22: 3159−3167
DOI: 10.1007/s11771-015-2853-7
Shear modulus and damping ratio of sand-granulated rubber mixtures
M. Ehsani1, N. Shariatmadari1, S. M. Mirhosseini2
1. School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran;
2. Department of Civil Engineering, Amirkabir University of Technology, Tehran, Iran
© Central South University Press and Springer-Verlag Berlin Heidelberg 2015
Abstract: Recycled waste tires when mixed with soil can play an important role as lightweight materials in retaining walls and
embankments, machine foundations and railroad track beds in seismic zones. Having high damping characteristic, rubbers can be
used as either soil alternative or mixed with soil to reduce vibration when seismic loads are of great concern. Therefore, the objective
of this work was to evaluate the dynamic properties of such mixtures prior to practical applications. To this reason, torsional resonant
column and dynamic triaxial experiments were carried out and the effect of the important parameters like rubber content and ratio of
mean grain size of rubber solids versus soil solids (D50,r/D50,s) on dynamic response of mixtures in a range of low to high shearing
strain amplitude from about 4×10−4% to 2.7% were investigated. Considering engineering applications, specimens were prepared
almost at the maximum dry density and optimum moisture content to model a mixture layer above the ground water table and in low
precipitation region. The results show that tire inclusion significantly reduces the shear modulus and increases the damping ratio of
the mixtures. Also decrease in D50,r/D50,s causes the mixture to exhibit more rubber-like behavior. Finally, normalized shear modulus
versus shearing strain amplitude curve was proposed for engineering practice.
Key words: sand−rubber mixture; shear modulus; damping ratio; low to high shear strain amplitude; cyclic triaxial test; torsional
resonant column test; granular rubber
1 Introduction
Every year, huge amount of waste tires are being
produced by the rate of 200 to 250 million tires in the
United States of America and about 85% of them are
stockpiled or illegally dumped. Waste tires stockpiles
may create severe geo-environmental problems since
tires do not decompose and may contaminate ground
water by discharging leachate [1]. Recycling of waste
tires plays beneficial role both environmentally and
economically. As consumption of conventional
construction materials like gravel and sand are growing
due to different construction projects, rubber can
represent as an alternative material in construction
activities since it is cheap and available in abundance.
Moreover, using soil−rubber mixture as an engineering
material may not only provide alternative means of
reusing tires but also improve mechanical properties of
soft and problematic soils, such as lack of strength, and
instability andlow damping.
Recently, rubbers have been used as lightweight
construction material [1−3], vibration isolation system in
foundations, retaining walls and buried pipes [4−9],
earthquake resistant material [10], drainageand thermal
insulator. Static response of sand−rubber mixtures are
widely evaluated via various static experiments. Many
researchers have studied compaction, compressibility
and strength parameters of pure rubber and soil−rubber
mixtures [2, 11]. Some researchers also studied the
compression and stress generation in tire/soil interface
using numerical studies [12]. On the other hand, limited
numbers of studies are performed to assess dynamic
properties of mentioned mixtures.
Previous researchers have investigated liquefaction
potential due to the increase in rubber content and the
effect of tire percentage on dynamic properties of
structures constructed with complex materials [5, 13−17].
Various experimental tests [4, 18] and numerical studies
[6−7, 9] show results of using soil−rubber mixtures as an
underground layer preventing liquefaction and isolation
material because of high attenuation potential of rubber.
Principal parameters which affect the soil−rubber
mixture dynamic response are tire content, D50,r/D50,s as
well as dynamic behavior of the pure soil mixture [15,
17]. KIM and SANTAMARINA [15] have performed
experiments on soil−rubber mixtures with Dr/Ds=10
using modified oedometer cell instrumented with bender
elements, and the results show that shear wave velocity
increases up to 20% tire inclusion by mixture volume.
PAMUKCU and AKBULUT [14] have stated the
simultaneous increase in shear modulus and damping of
Received date: 2014−09−28; Accepted date: 2015−02−02
Corresponding author: M. Ehsani, PhD; Tel: +98−9124858549; E-mail: [email protected]
3160
soil−rubber mixtures with Dr/Ds=1 up to specific
percentage of rubber content. However, FENG and
SUTTER [13] and ANASTASIADIS et al [16]
investigated dynamic properties of sand−rubber mixtures
with Dr/Ds ratio respectively equal to 4 and 5 via
resonant column tests and stated that by adding tire
content, shear modulus decreases at low strain level and
damping ratio increases.
To evaluate shear modulus and damping ratio of
mixtures in various shearing strain amplitudes,
experimental program considering different conditions of
specimen size, saturation and their effect on dynamic
response of sand−rubber mixtures have been conducted
and analytical relationships for small shear strain
modulus and damping prediction have been introduced
[19−20]. ANASTASIADIS et al [21] and SENETAKIS et
al [22] have performed torsional resonant column tests in
the range of low to medium shear strain amplitude
(2×10−4% to 3×10−1%) and confining pressure from 25 to
400 kPa on sand−rubber and gravel−rubber dry mixtures.
Soil− rubber mixtures were constructed using 0, 5%,
10%, 15%, 25% and 35% of rubber contents by mixture
weight and with the ratio of mean grain size of rubber
solids versus soil solids ranging from 1:10 to 5:1; finally
dynamic properties of pure soil and mixtures in terms of
shear modulus and damping ratio have been studied.
Also, NAKHAEI et al [23] conducted a series of
large-scale consolidated undrained cyclic triaxial tests on
soil-granular rubber saturated mixtures to evaluate
dynamic response of mixtures. They have proposed
models to predict dynamic properties of mixtures
containing various rubber contents for different
confining pressure and in the area of small shear strain
amplitude.
However, the aforementioned studies are mostly
limited on evaluation of the small-strain shear modulus
and damping ratio or dynamic properties in the region of
low to medium shear strain amplitude. Moreover,
previous studies focus only on mixtures that are
constructed in saturated or dry condition having
D50,r/D50,s either lower or higher than unity. The objective
of this work is to investigate the dynamic response of
sand−rubber mixtures in terms of D50,r/D50,s and granular
tire content in the in a range of low to high shearing
strain amplitude, so in addition to G0 one can predict the
dynamic properties in strong earthquake events where
high amplitude shear strains are applied to the mixtures.
In this work, a series of torsional resonant column and
cyclic triaxial experiments were carried out to evaluate
the dynamic behavior of sand−rubber mixtures. The
samples are composed of several different percentages of
granular rubber having D50,r/D50,s both lower and higher
than unity, and compacted to almost maximum dry
density and optimum moisture content to simulate the
J. Cent. South Univ. (2015) 22: 3159−3167
ordinary compaction effort being used in geotechnical
projects. The variation of shear modulus, damping ratio,
slope of shear modulus and proposed normalized shear
modulus versus shear strain are presented in the way to
fulfill the literature gap concerning the dynamic
characteristics of sand-rubber complex materials.
2 Materials and methods
2.1 Firoozkooh sand
Firoozkooh sandy soil (F161) with the mean grain
size of D50=0.22 mm, coefficient of uniformity of
Cu=2.14 and specific gravity of 2.66 was used in the
laboratory tests. All soil samples were taken from natural
round silica sand mines of Firoozkooh township in 130
km of northeast of Tehran, Iran.
2.2 Granular rubber
Granular rubber used in this work was shredded and
processed in local tire manufacturing plant. Two sizes of
rubber without steel belts were considered for
experimental program: coarse granular rubber (CR) with
the maximum particle size less than 4.75 mm (No. 4
sieve) and fine granular rubber (FR) with the maximum
particle size less than 1 mm (No. 16 sieve). Both
granular rubbers with specific gravity of 1.1 are
categorized as granulated or particulate rubber according
to ASTM [24]. Particle size analysis of Firoozkooh sand
161, fine and coarse granular rubber are shown in Fig. 1.
Fig. 1 Grain-size distribution curves of soil and rubber
materials
2.3 Sand−rubber mixture
Since dry density of rubber is about one third the
dry density of soil, it is more practical in the laboratory
to use volumetric portions of rubber to prepare the
soil−rubber mixtures. Two groups of sand-coarse rubber
mixture and sand-fine rubber mixture were considered,
namely F161-CR and F161-FR. In mixture group
F161-CR, three different combinations of soil and coarse
rubber with 10%, 15%, and 30% (mass fraction) of
J. Cent. South Univ. (2015) 22: 3159−3167
rubber by sand volume were prepared for experimental
investigation, namely F161-10%CR, F161-15%CR and
F161−30%CR. According to previous experiences of the
authors from static experiments and dynamic results of
coarse granular rubber-sand mixtures presented in this
work, it was concluded that 5% (mass fraction)
difference of fine tire content in the mixture probably
cannot make any significant change in dynamic
properties of the mixture hence in mixture group
F161-FR, and fine granular rubber−sand mixtures were
prepared using only 10% and 30% of rubber contents,
namely F161-10%FR and F161-30%FR and experiment
on mixture containing 15% of fine rubber was eliminated
from testing program.
2.4 Testing equipment
Two different dynamic apparatus were employed to
monitor shear modulus and damping ratio of the
mixtures in both low and high shear strain amplitudes. To
this reason, a fixed-free longitudinal torsional resonant
column device and a pneumatic cyclic triaxial apparatus
manufactured by Seiken Inc. Company were used to
apply vibrations with lowand high strain amplitudes
respectively. Cyclic triaxial tests were performed on
samples of 70 mm in diameter and 140 mm in height,
while resonant column specimens were constructed with
70 mm in diameter and 100 mm in height.
In torsional resonant column device, the top end of
the specimen is responsible for sinusoidal excitation,
whereas the bottom end is rigidly fixed on a base
pedestal with a mass greatly more than the corresponding
mass of the specimen. By adjusting the excitation
amplitude and frequency in each step, resonance of the
system may occur. In cyclic triaxial apparatus, the
loading−unloading device connected to the top of the cell
applies cyclic loads to the specimen. A conventional
linear variable differential transducer (LVDT) of the
triaxial cell and a non-contact sensor connected to the top
of the specimen were two different strain measurement
elements. To monitor pore water pressure variations at
the top and bottom of the specimen, two pressure
transducers were used. Using a load cell, the
experimental team was able to measure sinusoidal loads
applied to the top of the specimen.
2.5 Specimen preparation
Firstly, compaction conditions were established
based on standard proctor compaction tests performed on
mixtures to obtain moisture content representing
optimum moisture content corresponding to a maximum
dry density. Secondly, sand and rubber parts of the
mixture were mixed under dry condition based on
designated percentages of rubber in a range of 0 (pure
sand) to 30% (volume fraction) by sand volume. In next
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step water content increased to optimum moisture level
and sample was compacted in five layers at optimum
moisture content using wet tamping technique by a metal
rod tamper and under-compaction method. The
maximum dry density and optimum moisture values of
mixtures are listed in Table 1. All specimens were
prepared in the same manner in order to guarantee
approximately the same compaction energy applied to
the samples. This helps the authors for interpretation of
the results based on uniformity of the samples.
Table 1 Maximum dry density and optimum moisture of
mixtures
No. Mixture group
γmax /(kN·m−3)
ωopt /%
10%
15%
30%
10%
15%
30%
18
19.5
16.1
15.9
14.8
19.5
16.5
1
F161-CR
17.5
2
F161-FR
17.5
12.8
2.6 Testing program
Nine wet samples of sand−rubber mixtures with
rubber in range of 10% to 30% were investigated using
dynamic triaxial and torsional resonant column tests in
accordance with ASTM [25−26], as listed in Table 2.
Dynamic experiments were performed in a range of low
to high shear strain amplitudes from about 4×10−4% to
2.7% and under confining pressure of 300 kPa. The
selection of confining pressure was based on a normal
loading case in geotechnical projectsso that the soil
medium in which soil−rubber mixture layer is
overburdened by 4−5 m topsoil is loaded by a foundation
applying vertical load of about 200 kPa.
Table 2 Cyclic triaxial and torsional resonant column testing
program
Rubber content by
sand volume
Mixture
Test type
No.
D50,r/D50,s
(volume fraction)
group
10% 15% 30%
1
F161-CR
11.07
2
F161-FR
2.13
Cyclic triaxial
●
●
Resonant column
●
●
Cyclic triaxial
●
●
Resonant column
●
●
●
When sample was completely prepared, triaxial or
resonant column cell filled with water. To disconnect the
vacuum, cell water was pressurized to 30 kPa. After the
initial steps, cell pressure increased to designated value
to apply to the specimen for consolidation stage. This
part continued for 12−15 min allowed the specimen to
equilibrate. At final step in resonant column tests,
multi-stage procedure of resonant frequency process
started in which approximately sinusoidal seismic wave
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was transmitted into the specimen until the resonance
was reached. Then, the mentioned procedure repeated
using higher strain amplitude applied to the top of the
specimen until the specimen failed. Same as resonant
tests, in cyclic triaxial experiments dynamic vertical
loads were applied in multi-stage procedure. Based on
ASTM, the load was cycled until the number of load
cycles reached 40 in each stage, then next stages of
loading-unloading process started applying more vertical
load to the specimen to obtain dynamic properties in
higher shear strain range until the failure was observed.
Cyclic loading was applied in almost symmetrical trend
with testing frequency of 1 Hz.
3 Results and discussions
Figure 2 presents the variation of shear modulus and
damping ratio with shear strain for all mixtures of this
work. In addition to results obtained from performed
dynamic tests, dynamic curves obtained by KOKUSHO
[27] were used as parent granular soil diagrams. There
exist several results of dynamic tests performed on pure
sand in previous studies; hence only sand−rubber
mixtures were examined in this work. As mentioned,
Fig. 2 Results of experimental G−γ (a) and D−γ (b) values of
pure sand and tested mixtures
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dynamic properties of different pure sands were
extensively investigated by various researchers [28−30].
According to previous work [29−30], mean grain size,
moisture content, plasticity index, relative density,
confining pressure and specially coefficient of uniformity
of sand grading curve play critical role in dynamic
curves of pure sand. Hence, KOKUSHO’s dynamic
results [27] were considered representative of pure sand
dynamic diagrams since factors influencing shear
modulus and damping of sand were approximately the
same as present work except for moisture content.
Moreover, various investigators examined moisture
effect on shear modulus degradation and damping curve
of sands through experiments performed on specimens in
dry and saturated condition [16, 29]. They concluded that
moisture content has no significant effect on shear
modulus of pure sand specimens and can be ignored,
which allows the authors to use the KOKUSHO’s shear
modulus curve [27]. On the other hand, based on
previous studies on sands, damping ratio of pure sand is
different, when saturated; that is, in saturated
conditionviscous damping of water retained in voids of
the soil structure increases the total damping. Therefore,
damping results by KOKUSHO [27] were modified
using previous investigations and employed as pure sand
damping diagrams in this work, as shown in Fig. 2(b).
3.1 Effect of tire inclusion on shear modulus and
damping ratio
The effect of tire content on shear modulus of
sand−rubber mixture is illustrated in Figs. 3 and 4. As
shown in figures, shear modulus (G) is plotted versus
shear strain amplitude (γ) in log scale. Figure 3(a)
presents the G-variations of mixture group No. 1, while
Fig. 4(a) shows the same for mixture group No. 2. In
mixture group 1, torsional resonant column tests
performed on mixtures of 10% and 15% rubber content
show that shear modulus reduction curves of mentioned
mixtures are the same until the shear strain reaches about
0.02%. Above that, dynamic triaxial tests illustrate lower
shear modulus values for mixture F161-15%CR with
respect to mixture F161-10%CR. In other words, having
almost the same G0 and shear modulus in small shear
strains, mixture containing 15% of coarse rubber behave
differently from mixture with 10% of coarse rubber when
shear strain exceeds 0.02% due to rubber to rubber
interaction. It means that when coarse rubber material are
subjected to high amplitude shear strains, they become
more stretched and rubber-like behavior begin to
dominate the total response of the mixture. This can be
proven by studies to investigate the micro behavior of
sand-rubber mixtures through experimental tests such as
X-ray, magnetic resonance imaging and photo-imaging
J. Cent. South Univ. (2015) 22: 3159−3167
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[31]. According to Fig. 4(a), shear modulus and
specifically small-strain shear modulus decrease as fine
rubber content increases in mixture group No.2.
Figures 3(b) and 4(b) illustrate the variation of
damping ratio with shear strain for mixtures of
sand−coarse rubber, and sand−fine rubber, respectively.
It is shown that increase in rubber content from 10% to
30% by sand volume in mixtures of the same D50,r/D50,s
has no significant effect on damping properties, whereas
damping values of sand−rubber mixtures with both
D50,r/D50,s values are significantly higher than those for
pure sand of the same moisture and density
characteristics.
Fig. 3 Effect of rubber percentage on G−lgγ (a) and D−lgγ (b)
of mixture shaving D50,r/D50,s=11.07
Fig. 4 Effect of rubber percentage on G−lgγ (a) and D−lgγ (b)
of mixtures having D50,r/D50,s=2.13
3.2 Effect of D50,r/D50,s on shear modulus and damping
ratio
The effects of the rubber−sand particle size ratio
(D50,r/D50,s) on dynamic properties of various mixtures
are investigated while rubber content is constant. Based
on the results plotted in Figs. 5(a) and 6(a), increase in
D50,r/D50,s of the mixture results in higher small-strain
shear modulus (G0). This finding was observed in both
rubber contents of 10% and 30% by sand volume. As
shown in Fig. 5(a), shear modulus difference of mixtures
F161-10%CR and F161-10%FR is being decreased when
Fig. 5 Effect of ratio of mean grain size of rubber solids versus
soil solids (D50,r/D50,s) on G−lgγ (a) and D−lgγ (b) of mixtures
having 10% rubber by sand volume
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sand−rubber mixtures and pure sand is illustrated in
Fig. 7. This trend was observed in all specimens tested in
this work. It is also interesting to note that all G/G0-γ
data points of different mixtures with various rubber
contents and D50,r/D50,s even pure sand are located
between two approximate lower and upper bounds drawn
by the dashed line in Fig. 7. Therefore, it can be
concluded that G/G0−lgγ curves are independent of
rubber content and D50,r/D50,s and a representative
average relationship for all the test data which is shown
by the solid line, is likely to provide values of
normalized shear modulus with sufficient accuracy for
many practical purposes. Since the scatter of
experimental data is insignificant, the adoption of such
an average relationship is more justified. The proposed
average curve provide a basis to evaluate the shear
modulus for sand-rubber mixtures containing different
rubber contents and D50,r/D50,s for which limited test data
is available. The only parameter needed to obtain shear
modulus of mixtures for practical purposes, is
small-strain shear modulus, G0.
Fig. 6 Effect of ratio of mean grain size of rubber solids versus
soil solids (D50,r/D50,s) on G−lgγ (a) and D−lgγ (b) of mixtures
having 30% rubber by sand volume
shear strain amplitude exceeds 0.02%. When rubber
content increases to 30%, it seems that same as Fig. 5(a),
shear modulus of mixtures having rubber−sand particle
size ratio of 2.13 and 11.07 are getting closer as shear
strain amplitude increases (Fig. 6(a)). Therefore, it may
be concluded that at high strain levels, shear modulus
values of mixtures with constant rubber content and
various D50,r/D50,s tend to converge.
Unlike the damping curves of mixtures having
constant rubber-sand particle size ratio in which effect of
increasing rubber content was approximately negligible,
increase in D50,r/D50,s seems to relatively decrease the
mixture damping of the same rubber content especially
in high shear strain amplitudes. This last observation
could be justified using rubber to rubber interface which
is decreased in mixtures containing coarse rubbers due to
reduction of rubber contact areas. Figures 5(b) and 6(b)
present the damping results of mixtures with various
D50,r/D50,s and rubber contents of 10% and 30%,
respectively.
3.3 Effect of rubber content and D50,r/D50,s on
normalized shear modulus
The general trend of the effect of tire inclusion and
D50,r/D50,s on normalized shear modulus (G/G0) of
Fig. 7 Effect of rubber percentage and ratio of mean grain size
of rubber solids versus soil solids (D50,r/D50,s) on G/G0−lgγ of
pure sand and tested mixtures
3.4 Slope changes of shear modulus attenuation curve
To better understand the shear modulus changes of
soil-rubber mixture in different shear strain ranges; small,
medium and large strain amplitudes, it is suggested to
focus more precisely on slope variation of shear modulus
curve as shear strain increases. Figure 8 is plotted based
on this purpose. Figure 8(a) investigates the slope
variation of G−lgγ curve as D50,r/D50,s changes whereas
Fig. 8(b) illustrates the slope changes of mixtures with
constant D50,r/D50,s and various rubber contents. It was
observed that increase in rubber content of mixtures
having fine rubber, mainly affects the mixture dynamic
shear behavior in medium range of shear strain
amplitudes.
Figrue 8(a) illustrates the effect of D50,r/D50,s on
rate of changing shear modulus when rubber content is
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4 Conclusions and future work
Fig. 8 Effect of ratio of mean grain size of rubber solids versus
soil solids (D50,r/D50,s) having 10% rubber (a) and rubber
percentage having D50,r/D50,s=2.13 on rate of changing shear
modulus (b)
10% by sand volume. It seems decrease in D50,r/D50,s has
less effect on slope variation of modulus degradation
curve than addition of rubber content. Both F16110%CR and F161-10%FR mixtures experience dynamic
shear path with almost the same rate of modulus
reduction in different strain levels except for small shear
strain amplitudes. When shear strain is about 0.01%,
decrease in D50,r/D50,s leads to more severe modulus
attenuation which is expected since sand-fine rubber
mixtures exhibit more softening and rubber-like behavior
results in higher rate of modulus reduction compared to
mixtures with coarse rubber.
As it can be seen in Fig. 8(b), the rate of changing
shear modulus of mixture with 10% fine rubber is almost
constant in medium shear strains, which implies that
slope of modulus attenuation curve is unchanging and
shear modulus is being reduced with a constant rate.
However, mixture containing 30% fine rubber exhibit
different behavior with descending and ascending rate of
changing shear modulus in small and large shear strain
amplitudes, respectively.
1) The shear modulus of soil−rubber mixtures
composed of fine sand and both coarse and fine rubber is
influenced by the rubber content, mean grain size,
dynamic properties of intact sand and D50,r/D50,s.
Moisture content, relative density, confining pressure and
especially coefficient of uniformity also affect the
dynamic response of mixtures. In this work
KOKUSHO’s dynamic results are employed as pure sand
dynamic curves, so authors can estimate the effect of tire
inclusion on dynamic properties of sand.
2) Concerning the range of rubber content used,
between 10% and 30% by sand volume, all sand−rubber
mixtures exhibit lower shear stiffness than pure sand and
this is more pronounced as rubber content increases. This
can be explained by insignificant contribution of soft
rubber particleson the shear modulus of mixture solid
skeleton. Opposite trend is observed for D50,r/D50,s in
which increase in relative mean grain size of sand versus
rubber leads to higher shear modulus of mixture.
3) Slight increase in coarse rubber content of the
mixture leads to lower shear modulus in large strain
amplitudes, while in the area of small shear strain, shear
modulus is not significantly influenced by coarse rubber
content. It can be verified that using F161-10%CR and
F161-15%CR shear modulus results in 5% increase in
coarse rubber content, leading to lower modulus in large
strains and does not affect shear modulus in small strains.
In other words, coarse rubbers are more involved in high
strain amplitudes and hence mixture behavior transforms
from sand-like to rubber-like.
4) Despite the mixture shear modulus, higher
damping ratio is observed in mixtures compared to pure
sand. Because of deformability of rubber particles and
rubber to rubber and sand to rubber contacts, mixtures
exhibit more dampening characteristic. It is interesting to
note that increase in rubber content and D50,r/D50,s does
not affect the damping behavior significantly at least for
rubber percentage and D50,r/D50,s used in this work.
5) The normalization of shear modulus in terms of
G/G0−lgγ eliminates the effect of rubber content and
D50,r/D50,s on experimental results and all normalized
curves can be represented by an average curve. Thus,
design G/G0−lgγ curve is presented for considered
confining pressure. In order to accurate assessment of
dynamic behavior of mixtures, one can use the suggested
design curve for many practical purposes such as seismic
design of geotechnical structures constructed by soil−
rubber mixtures.
6) The effect of rubber inclusion and D50,r/D50,s on
slope changes of shear modulus curve is studied and it
can be concluded that decrease in D50,r/D50,s or using finer
J. Cent. South Univ. (2015) 22: 3159−3167
3166
rubber material leads to more severe modulus reduction
specifically in small shear strain amplitudes. Using low
percentages of rubber up to 10% by sand volume, leads
to smoother G−lgγ curves in the area of medium shear
strains, while increase in rubber content to 30% by sand
volume results in steeper diagrams.
7) The results obtained and the design curve
proposed on the framework for the evaluation of
dynamic response of sand-rubber mixtures are derived
from torsional resonant column and cyclic triaxial tests.
Further research is required to extend the experimental
results of this study to a wider range of D50,r/D50,s and
rubber content percentages. Moreover, results of this
work could be enriched with extra laboratory
experiments in order to investigate the effect of confining
pressure, coefficient of uniformity (Cu), moisture content,
specimen preparation method, frequency and number of
cycles on dynamic characteristics of sand-rubber
mixtures, studies not covered yet in the literature.
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgments
All torsional resonant column and cyclic triaxial
experiments described in this work were conducted at the
International Institute of Earthquake Engineering and
Seismology (IIEES). The authors would like to express
their appreciation to the technical staff of the
Geotechnical Engineering Laboratory of (IIEES) for
their excellent assistance.
[14]
[15]
[16]
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(Edited by DENG Lü-xiang)