Triaxial Testing on Saturated Mixtures of Sand and Granulated Rubber

Abstract

The use of granulated recycled rubber as a lightweight material in civil engineering applications has been widely growing over the past two decades. Understanding the properties of sand-rubber mixtures is essential to evaluate its performance in geotechnical applications. However, limited experimental data are available on mixtures of sand and granulated-rubber. This paper presents the results of laboratory triaxial testing on saturated mixtures of sand and granulated-rubber having varying rubber content to assess static shear strength and deformability of the mixtures. The small size granulated-rubber used in this study was more effective in reducing the unit weight of the mixture compared to larger rubber sizes. As the rubber content in the mixture increased the shear strength and stiffness of the mixture decreased, and the deformability increased. Observations from shear strength and deformation characteristics indicate that rubber dominates the sand-rubber matrix and governs the behavior at rubber contents exceeding 20%. Thus, use of mixtures of sand and granulated-rubber must be accompanied with thorough assessment of the mechanical properties of the project-specific mixtures in order to optimize the rubber content and ensure that safety and serviceability standards are met.

Full text

TRIAXIAL TESTING ON SATURATED MIXTURES OF SAND
AND GRANULATED RUBBER
Rami El-Sherbiny
1
, PhD, M.ASCE; Ahmed Youssef
1
; and Hani Lotfy
1
, PhD
1
Department of Civil Engineering, Faculty of Engineering, Cairo University, Egypt, PH (+2010) 6669
-5600; FAX (+202) 3302-4009; email: rsherbiny@eng.cu.edu.eg
ABSRTACT
The use of granulated recycled rubber as a lightweight material in civil
engineering applications has been widely growing over the past two decades.
Understanding the properties of sand-rubber mixtures is essential to evaluate its
performance in geotechnical applications. However, limited experimental data are
available on mixtures of sand and granulated-rubber. This paper presents the results
of laboratory triaxial testing on saturated mixtures of sand and granulated-rubber
having varying rubber content to assess static shear strength and deformability of the
mixtures. The small size granulated-rubber used in this study was more effective in
reducing the unit weight of the mixture compared to larger rubber sizes. As the rubber
content in the mixture increased the shear strength and stiffness of the mixture
decreased, and the deformability increased. Observations from shear strength and
deformation characteristics indicate that rubber dominates the sand-rubber matrix and
governs the behavior at rubber contents exceeding 20%. Thus, use of mixtures of sand
and granulated-rubber must be accompanied with thorough assessment of the
mechanical properties of the project-specific mixtures in order to optimize the rubber
content and ensure that safety and serviceability standards are met.
INTRODUCTION
Waste material such as waste tires, rubbers, and plastic materials normally
generated in each society cause serious environmental issues. With the rapidly
increasing amounts of discarded vehicle tires generated each year, finding good
effective applications that may benefit from such waste materials is essential. From an
engineering perspective, using recycled tires can be advantageous because they are
lightweight resulting in low earth pressure, free draining and relatively durable. In
addition, recycling tires reduce tire stockpiles that are considered a health and fire
hazard, conserve materials, and eliminates need for disposal.
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Usage of tires in civil engineering has dramatically increased due to their
special properties. Tire waste can be mixed with soil resulting in a mixture known as
sand-rubber mixture that has several geotechnical applications. Feasibility of using
sand-rubber mixtures in geotechnical application has been studied in several research
and transportation centers. They have been considered for use as lightweight fill
material for embankments constructed on weak foundation soils (Edil and Bosscher,
1994), lightweight backfill material for retaining walls and bridge abutments (Tweedi
et al.1998, Humphery 2007), drainage layers for road and landfill applications
(GeoSyntec Consultants, 1998), and vibration damping layer beneath rail lines
(Humphrey, 2007).
BACKGROUND
Waste tires are processed to form tire shreds, tire chips, and tire crumbs
(referred to as granulated rubber). Processed tires are classified according to their
effective diameter into granulated (ground) having particle diameters less than 12
mm, tire chips having particle diameters ranging from 12 to 50 mm, and tire shreds
having particle diameters greater than 50mm.
Experimental studies have been conducted on mixtures of and with rubber
chips and shreds in both triaxial and direct shear devices. In general, mixing tire
shreds or chips with sand resulted in a reinforcing effect causing an increase in shear
strength over that of the host sand up to an optimum rubber content (among others:
Humphrey et al., 1993; Ahmed, 1993; Edil and Bosscher 1994; Foose et al., 1996;
Tatlisoz et al., 1998; Edinçliler et al., 2004; Zornberg et al. 2004, Gotteland et al.
2005; Attom, 2006). However, Balachowski and Gotteland (2007) suggested that the
increase in shear strength of the mixture is dependant on the confining pressure.
Strenk et al. (2007) evaluated the statistical variability of mechanical properties of
sand-rubber mixtures.
Less research has been conducted on properties of mixtures of sand and
granulated-rubber. Yang et al. (2002) studied the mechanical properties of rubber
having a diameter of 2 mm to 10 mm using direct shear and triaxial testing. Stress-
strain curves showed linear behavior up to 15% strain beyond which strain softening
occurred. They suggest that the failure envelope of rubber is non-linear for the range
of confining pressures of 50 to 350 kPa.
Youwai and Bergado (2003) conducted a series of triaxial tests on samples of
silty sand mixed with granulated rubber having a nominal diameter of 5 mm. They
observed that the shear strength decreased and deformations increased by increasing
the rubber content. Ghazafi (2004) carried out direct shear tests on sand mixed with
granulated-rubber having a diameter ranging from 2 mm to 8 mm. He concluded that
increasing the rubber content resulted in an increase in shear strength at constant
normal stress. He observed that a relatively clear peak stress is obtained at different
mixing ratios except for pure rubber samples, which exhibited a linear stress-strain
83Geo-Congress 2013 © ASCE 2013

curve. Ghazafi observed higher increase in shear strength in loose sand than in dense
sand. Mavroulidou and Etan (2009) conducted direct shear tests on mixtures of
granulated rubber having a particle size of 1 mm to 6 mm and two types of sand,
medium coarse and coarse. They observed that the shear strength did not change in a
regular manner with increasing rubber content. However, the shear strength of the
mixture was generally below that of the host sand except for one case. They also
observed that the deformations increased by increasing the rubber percentage.
The objective of this paper is to present experimental results on the shear
strength of sand-granulated rubber mixtures in order to provide insight for use of such
material in earthwork applications.
MATERIAL PROPERTIES
The host sand used in the testing
program consisted of coarse-to-medium
siliceous sand. The sand is classified as
poorly graded sand (SP) and the fines
content in the sand did not exceed 3%.
The grain-size distribution curve for the
host sand is presented in Figure 1. The
particle shape can be described as sub-
angular to angular. The specific gravity
(Gs) of the host sand was measured to be
2.67 (ASTM D854). The maximum void
ratio of the host sand was 0.67 and the
minimum void ratio was 0.43 (ASTM
D4253 and ASTM D4254, respectively). The corresponding minimum and maximum
unit weights were calculated to be 15.5 kN/m
3
and 18.2 kN/m
3
.
Granulated-rubber was obtained from a local processing plant in Cairo, Egypt,
where tires are stripped and shredded. The size of the rubber particles was
predominantly in the range of 1 mm to 2 mm. The grain size distribution of the rubber
particles are presented in Figure 1. The specific gravity of the granulated-rubber was
measured to be 0.91 (ASTM C127). The sand and rubber described above were mixed
in different proportions by weight in order to prepare the test specimens. The
specimens were prepared at a constant relative density. As such, the maximum and
minimum void ratios were evaluated for each mixture. Modified Proctor compaction
(ASTM D1557) was also performed to assess the relative compaction. The unit
weights obtained from the maximum and minimum voids ratio and from the Modified
Proctor are summarized in Table 2. In addition, the Unit weight of the specimen (at
80% relative density) and the relative compaction of the specimen are included in
Table 2. The relative compaction corresponding to the target relative density ranged
from 97% to 100%. The unit weight of the test specimens decreased from 17 kN/m
3
0
20
40
60
80
100
0.1110 Particle Diameter (mm)
Percentage Passing (%)
Hos t Sand
Granulated Rubber
Figure 1.
Grain Size Distribution of
Host Sand and Granulated-Rubber
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for sand to 11kN/m
3
for 30% rubber mixture. These values agree largely with values
reported in the literature as presented in Figure 2. However, the smaller size rubber
used herein (2 mm) had a larger impact on reducing the unit weight (Fig. 2).
Table 2.Unit Weight of samples with varying rubber content.
Rubber
Content
(%)
γ
dry max
Mod. Proctor
(kN/m
3
)
γ
dry
at e
min
(kN/m
3
) γ
dry
at e
max
(kN/m
3
)
γ
dry
of test
specimen
(kN/m
3
)
Relative
Compaction
(%)
0 17.2 18.5 16.1 17 98.8
5 17.1 17.5 14.9 16.9 98.8
10 15.9 16.3 14.11 15.8 99.4
20 13.3 14.6 13.14 13.3 100.0
30 11.4 11.9 11.09 11.2 98.3
100 5.17 5.2 3.9 5 96.7
EXPERIMENTAL PROGRAM
Consolidated drained triaxial
tests (ASTM D7181) were performed
on saturated samples of sand-
granulated rubber mixtures. The
samples were 50 mm in diameter and
100 mm in height. The tests were
carried out in a Wykeham Farrance
triaxial apparatus. An automatic data
acquisition system was used to collect
load, displacement, and volume
change.
Each sand-rubber mixture was
tested at three confining pressures covering the range of stresses that may be
encountered in typical applications: 50 kPa, 100 kPa, and 200 kPa. The host sand was
first tested, and then sand-rubber mixtures were tested with varying rubber content.
Mixtures having a rubber content of 5%, 10%, 20%, and 30% by weight were tested.
Calculations were made to determine the amount of the sand and the rubber in
each mixture to achieve the unit weight corresponding to 80% relative density
(approximately equivalent to 98% relative compaction). The portions were thoroughly
mixed in a container prior to sample preparation. The sample was prepared in a
stretched membrane within a split mold on the triaxial apparatus. The sand rubber
mixture was poured steadily into the split mold through a funnel to minimize particle
segregation. The sample was prepared in five lifts each mechanically compacted with
a wooden hammer. The method of undercompaction proposed by Ladd (1978) was
used to maximize sample homogeneity.
Samples were saturated in the triaxial apparatus and the confining pressure was
applied. The samples were sheared drained at a constant rate of 1 mm/min. Load,
displacement, and volume change were measured during shearing.
5
10
15
20
020406080
100
Rubber content (%)
Unit Weight
(
kN
/
m
3
)
This study-2mm
Ahmed ( 1993)-6mm
Tat liso z et al.(1996)
Edincliler (2004)-40mm
Youwai (2002)- 5 mm
Figure 2.
Variation of Unit Weight with
Rubber
Co
ntent
85Geo-Congress 2013 © ASCE 2013

RESULTS
The loads, displacements, and volume
changes measured during shearing were
reduced to obtain the deviatoric stress,
axial strain, and volumetric strains. The
deviatoric stress–strain and volumetric
strain behavior of the sand and sand-
rubber mixtures prepared at (Dr =80%)
are presented in Figures 3 through 5.
Figures 4 and 5 include sand-rubber
mixtures having a rubber content of 5%,
10%, 20%, and 30%.
Stress-Strain Behavior
The deviatoric stress–strain and
volumetric strain behavior of the host
sand specimens showed dilative
behavior, which is expected for the
relative density of the sample. A well-
defined peak was identified followed by
a reduction in shear stress for all sand-
rubber specimens. The shear strength
was defined at the peak deviatoric stress. The peak deviatoric stress generally
occurred at strains below 15%, with the exception of one specimen containing 30%
rubber tested at 200 kPa where the peak deviatoric stress occurred at approximately
16% strain. Increasing the confining pressure resulted in higher deviatoric stress and
axial strains at failure, larger stiffness, and lower volumetric strain at failure (i.e. more
contractive behavior).
The stress–strain responses shown in Figs. 3–5 indicate a significant impact
of the rubber content on the stress-strain behavior of the sand-rubber mixture. As the
rubber content increased the deviatoric stress at failure decreased, the axial strain at
failure increased, and the volumetric strain at failure decreased. In addition, the post-
peak strength loss was more significant for specimens of sand and low rubber content
than for specimens with higher rubber content.
Shear Strength
The shear strength envelopes obtained for sand and sand-rubber mixtures at
relative density (D
r
) of 80% are presented in Figure 6(a). Failure was defined at peak
strength or 15% strain. The strength envelope of sand is well characterized by a linear
envelope with no cohesion intercept for the range of confining pressures considered
herein. The angle of shearing resistance obtained for sand is 39°. This high friction
angle is due to the angularity of the sand particles as well as its high relative density.
Fi
gure 3
.
Stress-Strain and Volumetric Strain
Curves for Host Sand
0
200
400
600
800
051015
Axial strain, ε
a
(%)
Deviatoric stress, σ
d
(kPa)
-1.0
1.0
3.0
5.0
051015
Axial strain, ε
a
(%)
Volumetric strain, ε
v
(%)
50 kPa
100 kPa
200 kPa
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0
200
400
600
800
0 5 10 15 20
Axial strain, ε
a
(%)
Deviatoric stress, σ
d
(kPa)
-5.0
0.0
5.0
0 5 10 15 20
Axial strain, ε
a
(%)
Volumetric strain, ε
v
(%)
50 kPa
100 kPa
200 kPa
0
200
400
600
800
0 5 10 15 20
Axial strain, ε
a
(%)
Deviatoric stress, σ
d
(kPa)
-5.0
0.0
5.0
0 5 10 15 20
Axial strain, ε
a
(%)
Volumetric strain, ε
v
(%)
50 kPa
100 kPa
200 kPa
Figure 4. Stress-Strain and Volumetric Strain Curves for: (a) 5%, and (b) 10% rubber.
0
200
400
600
800
0 5 10 15 20
Axial strain, ε
a
(%)
Deviatoric stress, σ
d
(kPa)
-5.0
0.0
5.0
0 5 10 15 20
Axial strain, ε
a
(%)
Volumetric strain, ε
v
(%)
50 kPa
100 kPa
200 kPa
0
200
400
600
800
0 5 10 15 20
Axial strain, ε
a
(%)
Deviatoric stress, σ
d
(kPa)
-5.0
0.0
5.0
0 5 10 15 20
Axial strain, ε
a
(%)
Volumetric strain, ε
v
(%)
50 kPa
100 kPa
200 kPa
Figure 5. Stress-Strain and Volumetric Strain Curves for: (a) 20%, and (b) 30% rubber.
(a) (b)
(a) (b)
87Geo-Congress 2013 © ASCE 2013

0
100
200
300
0 100 200 300
Effective Normal Stress on Failure
Plane ,
σ
ff
(kPa)
Shear Stress, τ
ff
(kPa)
Sand
5%
10%
20%
30%
Rubber Content
0
10
20
30
40
0 5 10 15 20 25 30
Rubber Content, RC (%)
Friction Angle, φ (
o
)
0
10
20
30
40
Friction Angle
Apparent Cohesion
Apparent Cohesion, c (kPa)
Figure 6. Shear Strength of different Sand-Rubber Mixtures: a) Shear Strength
Envelopes, and b) Shear Strength Parameters: Friction Angle and Apparent Cohesion.
The effect of the rubber content on the shear strength envelope of the mixture
can be clearly identified from Figure 6(a). The shear strength envelopes of the sand-
rubber mixtures may be assumed linear over the range of stresses tested. As the
rubber content increases the shear strength of the mixture decreases over the range of
confining pressures tested. This indicates that the rubber particles tend to separate the
sand particles leading to decrease in interlocking and friction between the particles
with no reinforcement mechanism similar to that provided by tire chips and shreds
(Zornberg et al. 2004). The decrease in shear strength is less significant at rubber
contents of 20% to 30%, which further suggest that rubber governs the strength of the
mixture above this content. An increasing cohesion intercept is observed as the rubber
content increases. At effective stresses below 50 kPa, the cohesion intercepts result in
higher strengths for higher rubber content. However, this is considered apparent
cohesion because the rubber does not exhibit physical cohesion. This indicates that
the failure envelope is non-linear with increasing non-linearity as the rubber content
increases. The non-linearity of the failure envelope for granulated rubber has been
reported by Yang et al. (2002) who used a power function to describe the envelope.
The effect of rubber content on shear strength parameters (cohesion intercept,
c; and friction angle, φ) over the considered range of stresses is shown in Figure 6(b).
As the rubber content increases, the friction angle steadily decreases from 39° for the
host sand to approximately 19° at 30% rubber content. On the other hand, the
cohesion intercept increases rapidly to 20 kPa as the rubber content increases to 10%,
then remains nearly constant at higher rubber contents. This further justifies that this
apparent cohesion is an artifact of the non-linearity of the failure envelope.
The non-linearity of the failure envelope can be represented by the secant
friction angle (φ
secant
), which is presented in Figure 7(a) as a function of rubber
content for each confining pressure. The secant friction angle decreases as the
confining pressure increases, with higher effect at higher rubber content, which
highlights the effect of rubber on the non-linearity of the shear strength envelope. The
secant friction angle decreases as the rubber content increases. The decrease in φ
secant
(a) (b)
88Geo-Congress 2013 © ASCE 2013

is more significant for rubber contents
between 5% and 20%, beyond which the
effect is less pronounced especially at
low confining pressures.
Deformations
Generally, rubber has high
deformability that affects the behavior
of the tested samples, especially at high
rubber contents. Figure 7(b) shows the
effect of rubber content on the axial
strain at failure. The results show an
increasing axial strain at failure with
increasing rubber content and with
increasing confining stress. The axial
strain at failure for sand ranged between
3% and 5%, while for rubber content of
30% it ranged between 12% and 16%.
The larger strains at failure are affiliated
with a softer mixture response as rubber
dominates the matrix of the mixture.
The strains at failure obtained for sand-
rubber mixtures may exceed those
typically obtained for compacted soils
used in earth structures. It may be
appropriate in practical applications to
limit the shear strength to correspond to
a lower axial strain in order to control
deformability of the sand-rubber fill.
Volumetric strain of the sand-rubber mixtures are presented in Figures 3 to 5.
The behavior of the host sand was clearly dilative. As the rubber content increased,
dilatency decreases until the behavior became contractive at rubber contents above
20%. The specimen contraction increased with increasing the confining pressure.
The deformation modulus at 50% of the peak stress (E
50
) is plotted in Figure
7(c) against rubber content for different confining pressures. The modulus E
50
for
sand ranged between 20 MPa and 46 MPa, while for rubber content of 30% it ranged
between approximately 2 MPa and 3 Mpa. The modulus E
50
consistently decreases as
the rubber content and confining pressure increase. The change in rubber content and
confining pressure was observed to merely affect E
50
at rubber contents exceeding
20%. Combined with similar observations noted for the shear strength, it seems
reasonable to assume that rubber dominates the sand-rubber matrix and governs the
Figure 7. Effect of Rubber Content on: (a) Secant
Friction Angle (φ
secant
); (b) Axial Strain at Failure
(ε
f
); and (c) Modulus (E
50
)
20
25
30
35
40
45
0102030
Rubber Content, RC (%)
φ
secant
(Deg.)
0
5
10
15
20
0 102030
Rubber Content, RC (%)
Axial Strain at Failure (%)
50
100
200
Confining
Pressure (kPa)
0
10
20
30
40
50
0102030
Rubber Content, RC (%)
E
50
(MPa)
(a)
(c)
(b)
89Geo-Congress 2013 © ASCE 2013

behavior of the mixture at rubber contents exceeding 20%. At high rubber content, the
effect of confining pressure was more pronounced on the volumetric strains and axial
strains at failure than on the deformation modulus, which is contrary to the
observations for pure sand.
CONCLUSIONS
A series of triaxial tests were carried out on mixtures of sand and granulated
rubber prepared at D
r
of 80%. The unit weight of the test specimens ranged from 17
kN/m
3
for sand to 11kN/m
3
for 30% rubber mixture. The small size rubber used in
this study was more effective in reducing the unit weight of the mixture compared to
larger rubber sizes. A well-defined peak followed by a reduction in shear stress was
observed for all sand-rubber specimens. Increasing the confining pressure resulted in
higher deviatoric stress and axial strains at failure, larger stiffness, and lower
volumetric strain at failure (i.e. more contractive behavior). As the rubber content
increased the deviatoric stress at failure decreased, the axial strain at failure increased,
and the volumetric strain at failure decreased. Thus, it was observed that granulated
rubber reduces the shear strength when added to the host sand. This is because rubber
particles tend to separate sand particles leading to decrease in interlocking and friction
between the particles with no reinforcement effect.
The shear strength envelopes of the sand-rubber mixtures may be assumed
linear over the range of stresses tested. As the rubber content increases, the friction
angle steadily decreases from 39° for the host sand to 19° at 30% rubber content. On
the other hand, the cohesion intercept increases rapidly to 20 kPa as the rubber
content increases to 10%, then remains nearly constant at higher rubber contents. It is
however noted that the apparent cohesion is an artifact of the non-linearity of the
failure envelope. The failure criteria may better be described by a non-linear envelope
or secant friction angles (φ
secant
)
The deformation modulus (E
50
) consistently decreases as the rubber content
and confining pressure increase. The modulus E
50
for sand ranged between 20MPa
and 46MPa, while for rubber content of 30% it ranged between approximately 2MPa
and 3Mpa. Observations from shear strength and deformation characteristics indicate
that rubber dominates the sand-rubber matrix and governs the behavior at rubber
contents exceeding 20%.
In conclusion, use of granulated rubber may provide an effective method for
reducing the unit weight of sand rubber mixtures over larger size rubber. However,
use of mixtures of sand and granulated-rubber must be accompanied with thorough
assessment of the mechanical properties of the project-specific mixtures in order to
optimize the rubber content and ensure that safety and serviceability standards are
met.
90Geo-Congress 2013 © ASCE 2013

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