Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SENSOR-ENABLED GEOSYNTHETIC MATERIAL AND
METHOD OF MAKING AND USING THE SAME
STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH OR DEVELOPMENT
[0001] Not Applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0003] The present invention relates to a sensor-enabled
geosynthetic material used to construct geosynthetic structures (i.e.
geotechnical structures involving geosynthetics) and a method of
making the sensor-enabled geosynthetic material. Additionally, the
mechanical strains of the sensor-enabled geosynthetic material can be
measured or monitored without the need for conventional
instrumentation.
2. Description of the Related Art
[0004] Geosynthetics are polymer-based products specifically
manufactured to serve a wide range of applications in civil and
environmental engineering including soil stabilization and
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reinforcement, separation and filtration, drainage and containment.
The emergence and development of geosynthetic technology has had a
significant impact on the capabilities and economics of civil engineering
design and construction. Increasing number of geotechnical projects
involve the applications of geosynthetics as modern solutions to
conventional problems with proven advantages in the construction and
retrofitting of infrastructure including: ease and speed of construction,
construction in difficult access locations, superior performance under
static and seismic loading conditions, lower costs, reducing the size of
structures and hence providing greater usable space, aesthetically
pleasing appearance and blending with the environment. In several
cases (e.g geomembranes in hazardous and municipal waste
containment), the use of geosynthetics is mandated by law.
[0005]
Geosynthetic engineering and the related manufacturing
and construction industries have experienced tremendous growth over
the past few decades and are now an established technology involving
billions of dollars of projects in the U.S. and worldwide. At the same
time, as geosynthetic-related structures and facilities become
ubiquitous, it becomes vital to ensure that these structures are not
only safe but also offer a satisfactory level of serviceability through
health monitoring and timely measures to prevent catastrophic failures
and costly repairs due to inadequate structural performance resulting
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from uncertainties in site conditions, material properties and behavior,
construction practice, environmental effects and loading conditions.
This is especially true where these structures support crucial
infrastructure in urban areas and along transportation corridors, or
protect the environment from hazardous waste, leaking fuel or other
contaminants. The
importance of instrumentation and health
monitoring of infrastructure is increasingly recognized in order to
address these challenges and uncertainties to ensure the success of
the project with respect to its safety and cost. An important aspect of
health monitoring for geosynthetic structures is to monitor
geosynthetic strain during service life and/or extreme (e.g. seismic)
events.
[0006]
Geosynthetics have become an indispensable part of the
infrastructure development and renewal enterprise. Unfortunately, a
vital aspect of sustainable development; namely, their instrumentation
and health monitoring has received comparatively little attention with
costly consequences. A
significant predicament in performance
monitoring of geosynthetic structures has been due to the fact that
installation of instruments (e.g. strain gauges) are typically tedious
and costly with rather unpredictable outcome. Current
design
guidelines for different geosynthetic structures are largely based on
empirical and conventional (e.g. limit-equilibrium) approaches without
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proper assessment and in-depth understanding of the influence of
important factors such as peak strain and in-soil properties of
geosynthetics. As a result, overly conservative design procedures and
reduction factors are typically imposed on the strength of the
geosynthetic material to address concerns related to their durability
and creep. This renders the cost of these structures in many instances
much greater than necessary, and counters their intrinsic cost-
effectiveness. Other
important applications which could benefit
significantly from a reliable health monitoring system include landfills
to detect geomembrane overstress at their trenched anchors, covers
or other locations (e.g. within geomembranes often buried under
hundreds of feet of waste) well before the occurrence of leakage under
service conditions or, e.g, following seismic events. Current
technology merely involves leak detection systems which could only
detect the problems in more advanced stages. Similar benefits could
be achieved in geopipes, geosynthetic platforms over sinkholes and
other soil stabilization, containment and storage applications.
[0007] In
addition, the existing technology currently employed to
measure strains in geosynthetics requires complex and expensive data
acquisition systems. The existing technology for the instrumentation
of geosynthetics primarily entails the attachment of strain gauges and
extensometers to a geosynthetic material which are calibrated against
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average strains from crosshead displacement in their in-isolation tests.
However, these calibration factors are not truly applicable to a
geosynthetic layer embedded in soil due to at least three important
reasons: 1) different in-soil mechanical properties (e.g. tensile
modulus) of geosynthetics compared to their in-isolation values due to
confining pressure and interlocking effects, 2) complications such as
soil arching due to the mechanical interference and interaction of
strain gauges and their bonding assembly (e.g. adhesive and
protective sleeve) with the local soil, 3) unknown local stiffening effect
of the bonding assembly. These factors can introduce significant
errors in measured strains in geosynthetics in field applications.
Applying in-isolation calibration factors to in-soil readout data could
lead to significant underestimation of reinforcement strain and axial
load with potential consequences with respect to stability and
performance. Recent studies include discussions on subjects such as
strain gauge calibration, local vs global strains, under-registration of
strain due to attachment technique and correction factors to estimate
global strains in geosynthetic reinforcement.
[0008] In
addition, geosynthetic strain is not routinely monitored
in the field due to the added costs and level of care and skills required
for proper installation of the instruments. Other impediments include
lack of reliable strain gauging techniques and proper training of
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contractors, durability and reliability of instruments and requirements
for time-consuming installation and protection measures. For these
reasons, geosynthetic instrumentation in the field has been primarily
limited to research and demonstration projects with a comparatively
insignificant footprint in their mainstream construction considering the
vast number of geosynthetic-related structures constructed in the U.S.
In those occasions where field structures have been instrumented, the
extent of instrumentation related to the geosynthetic strain data has
been fairly limited. As a result, important information on the extent
and distribution of strains and stresses in these structures is not
typically available.
[0009] Recent
attempts to measure soil strains and in-soil
reinforcement strains include those involving digital imaging, X-ray
and tomographical techniques in small-scale laboratory specimens.
However, the limited in-soil penetration range of these techniques
renders them impractical for field-scale structures. Another recent
development involves the attachment of fiber optic cables to
geotextiles or geornemIDranes. However, these techniques do not
incorporate the sensing capabilities within the geosynthetic materials,
and the added manufacturing and construction costs related to the
fiber optic material and their installation need further analysis.
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[0010]
Accordingly, there remains a need for a new generation of
geosynthetics having sensing capabilities embedded therein in order to
measure their mechanical strain without the need for conventional
instrumentation.
SUMMARY OF THE INVENTION
[0011] The present invention relates to a sensor-enabled
geosynthetic material for use in geosynthetic structures. The sensor-
enabled geosynthetic material is fabricated from a polymeric material
and an electrically conductive filler combined with the polymeric
material. The sensor-enabled geosynthetic material fabricated has a
predetermined concentration of the electrically conductive filler
contained therein so as to provide the sensor-enabled geosynthetic
material with electrical conductivity, as well as electrical conductivity
that changes with strain.
[0012] In
another embodiment of the present invention, a method
of making the sensor-enabled geosynthetic material is provided. The
sensor-enabled geosynthetic material is fabricated by providing a
polymeric material and an electrically conductive filler. Once the
polymeric material and the electrically conductive filler are provided,
the polymeric material and the electrically conductive filler are mixed
to provide the sensor-enabled geosynthetic material. The sensor-
enabled geosynthetic material having a predetermined concentration
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of the electrically conductive filler so as to provide the sensor-enabled
geosynthetic material with electrical conductivity, as well as electrical
conductivity that changes with strain.
[0013] In a
further embodiment of the present invention, a
method of measuring geometric strains of a geosynthetic product is
provided. To measure the geometric strains of a geosynthetic product
a geosynthetic product is provided from fabricating a sensor-enabled
geosynthetic material. Once the geosynthetic product is provided, a
measuring location of the geosynthetic product is selected to measure
the geometric strain. Then, the geometric strains of the geosynthetic
product are determined at the measuring location.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Fig. 1
is a graph view showing the percolation threshold
and the percolation region of a sensor-enabled geosynthetic material.
[0015] Fig, 2
is a pictorial representation of a geosynthetic
structure constructed in accordance with the present invention.
[0016] Fig.
3(a) is a is a test specimen in accordance with ASTM
D1708,
[0017] Fig.
3(b) is a typical specimen used in the strain-resistivity
tests.
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[0018] Figs.
4(a) - 4(d) show a panel of scanning electron
micrographs (SEM) showing sensor-enabled geosynthetic material
specimens constructed in accordance with the present invention.
[0019] Fig.
5(a) is a graph view showing Force vs. Time of a
loading regime for strain-sensitivity testing of a sensor-enabled
geosynthetic material constructed in accordance with the present
invention.
[0020] Fig.
5(b) is a graph view showing Strain vs. Time of a
loading regime for strain-sensitivity testing of a sensor-enabled
geosynthetic material constructed in accordance with the present
invention.
[0021] Fig.
5(c) is a graph view showing Force vs. Strain of a
loading regime for strain-sensitivity testing of a sensor-enabled
geosynthetic material constructed in accordance with the present
invention.
[0022] Fig.
6(a) is a graph view showing the volume conductivity
of a sensor-enabled geosynthetic material constructed in accordance
with the present invention.
[0023] Fig.
6(b) is a graph view showing the surface conductivity
of a sensor-enabled geosynthetic material constructed in accordance
with the present invention.
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[0024] Fig.
7(a) is a graph view showing the tensile strength of a
sensor-enabled geosynthetic material constructed in accordance with
the present invention.
[0025] Fig.
7(b) is a graph view showing the tensile strength of
another embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
[0026] Fig.
7(c) is a graph view showing the failure strain of a
sensor-enabled geosynthetic material constructed in accordance with
the present invention.
[0027] Fig.
7(d) is a graph view showing the failure strain of
another embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
[0028] Fig.
8(a) is a graph view showing strain-resistivity of a
sensor-enabled geosynthetic material constructed in accordance with
the present invention.
[0029] Fig.
8(b) is a graph view showing strain-resistivity of
another embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
[0030] Fig.
8(c) is a graph view showing strain-resistivity of yet
another embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
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[0031] Fig.
8(d) is a graph view showing strain-resistivity of a
further embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
[0032] Fig.
9(a) is a graph view showing tensile responses of a
sensor-enabled geosynthetic material constructed in accordance with
the present invention.
[0033] Fig.
9(b) is a graph view showing tensile responses of
another embodiment of the sensor-enabled geosynthetic material
constructed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The present invention relates to a sensor-enabled
geosynthetic (SEG) material used for constructing geosynthetic
structures, such as geogrids and geomembranes, which are used in
reinforcement and containment applications. The sensor-enabled
geosynthetic material is embedded with sensing capabilities in order to
measure the mechanical strains subjected on the geosynthetic
structures at any location of the geosynthetic structures. As the
mechanical strains on a geosynthetic structure change, the
conductivity of the sensor-enabled geosynthetic material is affected.
[0035] Generally, the sensor-enabled geosynthetic material
includes a polymeric material (or geosynthetic material) and an
electrically conductive filler. The
polymeric materials used in
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geosynthetic structures are typically electrically insulating materials.
An electrically conductive filler is added to the polymeric material in
sufficient amounts to transform the polymeric material from an
insulating material into a conductive material. The addition of the
electrically conductive filler to the polymeric material would transform
the polymeric material into the sensor-enabled geosynthetic material,
which allows the geosynthetic material to be self instrumented or
"sensor-enabled" and amenable to strain-monitoring at virtually any
location within the geosynthetic material. The
addition of the
electrically conductive filler to the polymeric material increases the
conductivity of the polymeric material substantially (e.g. by several
orders of magnitude).
[0036] The
polymeric material can be any polymer or combination
of polymers that provide the necessary structure requirements needed
for a given use of a geosynthetic structure. Examples of polymers that
can be used as polymeric material include, but are not limited to,
polyolefin polymers. Examples
of polyolefin polymers include
polyethylene (PE), high-density polyethylene (I-IDPE), polypropylene
(PP) and the like. In some instances, the polymeric material is
provided from commercial manufacturers having electrically
conductive filler contained therein.
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[0037] The
electrically conductive filler can be any material
capable of being mixed with the polymeric material to provide a
sensor-enabled geosynthetic material which is conductive. Examples
of electrically conductive fillers include, but are not limited to, metal
powders, conductive carbon black (CB), graphite fiber, carbon
nanotubes (NT), and the like. Further, specific examples of carbon
nanotubes include, but are not limited to, single-walled nanotubes and
multi-walled nanotubes. The
electrically conductive filler can be
provided in any form that can create a network of conductive linkages
throughout the polymeric material. For
example, the electrically
conductive filler can be provided in the form of particles, particulates,
aggregates, fibers, and the like.
(00381 As the
concentration of the electrically conductive filler
dispersed in the polymeric material is increased from zero (i.e. amount
of electrically conductive filler in a virgin polymeric material),
insulating gaps along pathways of the electrically conductive filler in
the polymeric material eventually disappear or become less than some
critical value (e.g. 10 rim). At a
critical concentration of the
electrically conductive filler in the polymeric material, the electrically
conductive fillers form a network of conductive linkages which allows
the electrons to be able to pass through the sensor-enabled
geosynthetic material.
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[0039]
Referring now to the drawings, and more particularly to
Fig. 1, shown therein is a graph depicting the relationship between the
concentration of the electrically conductive filler in the polymeric
material and the electric conductivity of the resulting sensor-enabled
geosynthetic material. The smallest concentration of the electrically
conductive filler in the polymeric material which allows the continuous
network of conductive pathways to be established within the sensor-
enabled geosynthetic material is called the percolation threshold.
Once the concentration of the electrically conductive filler in the
sensor-enabled geosynthetic material passes the percolation threshold,
the sensor-enabled geosynthetic material enters into a percolation
region. The conductivity of the sensor-enabled geosynthetic material
in the percolation region (a range of electrically conductive filler
concentrations) is highly sensitive to factors such as changes in the
electrically conductive filler placement or mechanical strain. In the
percolation region, small changes in the network structure of the
electrically conductive filler (e.g. due to tensile or mechanical strain)
can dramatically change the conductive pathways in the sensor-
enabled geosynthetic material which, in turn, can cause large changes
in conductivity. As the concentration of the electrically conductive
filler in the sensor-enabled geosynthetic material reaches
concentrations above the percolation region, the increased
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concentration of the filler in the sensor-enabled geosynthetic material
will ultimately result in relatively mild improvements in conductivity
but drastic reduction of strain sensitivity due to formation of denser
three-dimensional networks among the electrically conductive filler.
[0040] The range
of filler concentrations that defines the
percolation region varies with the specific type of the electrically
conductive filler that is used to construct the sensor-enabled
geosynthetic material, and to a lesser extent, with the type of
polymeric material. The
target concentration of the electrically
conductive filler in the sensor-enabled geosynthetic material is the
maximum concentration of the electrically conductive filler that still
places the sensor-enabled geosynthetic material within the percolation
region. When the sensor-enabled geosynthetic material is provided in
the upper concentration portion of the percolation region, both the
electrical conductivity and the strain sensitivity of the sensor-enabled
geosynthetic material are maximized.
[0041] The
electrically conductive filler is provided in the sensor-
enabled geosynthetic material in an amount sufficient to accomplish
the functionality of the sensor-enabled geosynthetic material described
herein. In one embodiment of the present invention, the electrically
conductive filler is provided in the sensor-enabled geosynthetic
material in a concentration of from about 0.01 wt% to about 30 wt%.
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In another embodiment of the present invention, the electrically
conductive filler is provided in the sensor-enabled geosynthetic
material at a concentration that places the piezoresistivity of the
sensor-enabled geosynthetic material in the percolation region. In a
further embodiment of the present invention, the electrically
conductive filler is provided in the sensor-enabled geosynthetic
material at a concentration that places the piezoresistivity of the
sensor-enabled geosynthetic material in the upper concentration
portion of the percolation region, or up to 5 wt% above the percolation
region.
[0042] Another embodiment of the present invention is directed to
a method of fabricating the sensor-enabled geosynthetic material.
Further, a method of fabricating a geosynthetic product from the
sensor-enabled geosynthetic material is provided herein. The method
of fabricating the sensor-enabled geosynthetic material starts with
providing the polymeric material and the electrically conductive filler.
The polymeric material and the electrically conductive filler are
provided in any amounts such that the concentration of the electrically
conductive filler in the sensor-enabled geosynthetic material places the
piezoresistivity of the sensor-enabled geosynthetic material in the
percolation region or slightly above it. Once the polymeric material
and the electrically conductive filler are provided, these materials are
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mixed to fabricate the sensor-enabled geosynthetic material. The
dispersion (or mixing) of the electrically conductive filler within the
polymeric material can be done in any manner that is suitable and
known in the art. For example, the mixing can be carried out utilizing
any melt-blending process (e.g. extrusive mixing) or compression
molding. Once the sensor-enabled geosynthetic material has been
fabricated from the polymeric material and the electrically conductive
filler, the sensor-enabled geosynthetic material is manipulated to form
a geosynthetic product, such as a geogrid or a geomembrane.
[0043] In another embodiment of the present invention, a method
of measuring and/or monitoring geometric (or mechanical) strains in a
geosynthetic product is provided. In the method of measuring and/or
monitoring geometric (or mechanical) strains in a geosynthetic
product, a geosynthetic product is constructed using a sensor-enabled
geosynthetic material described herein. During the construction of a
geosynthetic structure, e.g. a mechanically stabilized earth (MSE) wall,
selected locations (measuring sites) within the reinforced zone
constructed with the sensor-enabled geosynthetic reinforcement can
be wired to monitor and measure mechanical strain. The locations for
measuring strain can be any part of the layers of the geosynthetic
product (or geosynthetic layers). In addition, the method of measuring
and/or monitoring the geometric strain of the geosynthetic product can
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be done during different stages of the geosynthetic structure's
construction.
Furthermore, this method provides a geosynthetic
product that can have its mechanical strain simultaneously measured
and/or monitored at a plurality of measuring locations on the
geosynthetic product.
[0044] It should
be understood that the geometric strain can be
determined using any known method in the art. In one embodiment
of the present invention, conductive leads are attached to the
geosynthetic layers at the measuring locations to provide conductivity
data. The leads can be attached to the geosynthetic layers at various
predetermined gauge distances to provide the conductivity data. Once
the conductivity data is provided, the conductivity data is manipulated
to provide the geometric strain of the geosynthetic structure at the
measuring location. This method of measuring and/or monitoring the
response of geosynthetic structures enhances the ability to expand an
inventory of structural health monitoring data to gain advantages
which include: (1) developing a more accurate understanding of
geosynthetic structure's mechanical behavior during construction, at
service load levels and under extreme events such as earthquake and
blast loading, (2) creating a database of performance results that
could be used to validate analytical and computational models, and (3)
improving current design methodologies for geosynthetic structure's
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construction and retrofitting by developing performance-based design
approaches that would be more rational and economical.
[0045] In a further embodiment of the present invention, the
conductivity data and the geometric strain data obtained from the
measuring sites of the geosynthetic structure can be compiled and
stored. The conductivity data and the geometric strain data can be
compiled and stored for any purpose known in the art, such as those
described above.
[0046] Referring now to Fig. 2, shown therein is an example of a
geogrid 10 (geosynthetic product) setup to measure its geometric
strains. The geogrid 10 is comprised of a plurality of longitudinal ribs
12 and a plurality of cross ribs 14, only three of the longitudinal ribs
12 and the cross ribs 14 being labeled in Fig. 2 by reference numerals
12a and 12b and 14a, 14b, and 14c, respectively, for purposes of
clarity. Geogrid junctions 16 are created where the longitudinal ribs
12 and the cross ribs 14 intersect, only three of the geogrid sections
16 being labeled in Fig. 2 by reference numerals 16a, 16b, and 16c for
purposes of clarity. To measure the geogrid's 10 geometric strain, the
geogrid 10 is provided with wiring 18 and a plurality of pairs of electric
terminals 20 and 22 for attaching the conductive leads, only three of
the pairs of electronic terminals 20 and 22 being labeled in Fig. 2 by
reference numerals 20a and 22a, 20b and 22b, and 20c and 22c for
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purposes of clarity. The electronic terminals 20 and 22 are separated
by a gauge length 24. Across the gauge length 24 is where the
geometric strain of the geogrid 10 (or geosynthetic product) is
measured and/or monitored. It should be understood and appreciated
that while Fig. 2 shows a specific example of how to setup a
geosynthetic product to be measured and/or monitored, the
geosynthetic product can be setup to be measured and/or monitored
by any manner known in the art to measure the geometric strain
across a given gauge length of the geosynthetic structure.
PROCEDURE USED TO DEVELOP SENSOR-ENABLED GEOSYNTHETICS
Overview
[0047] The following four series of tests were carried out as
described in the following sections to develop the Sensor-Enabled
Geosynthetic (SEG) materials (or specimens) and determine their
conductive and tensile strength properties: 1) Test I: conductivity
tests to determine the percolation threshold of each polymer-filler
composite and to develop the corresponding SEG prototype (i.e. the
composite with optimal filler concentration), 2) Test II: tensile tests
according to the ASTM D1708 test protocol to examine the polymer-
filler compatibility, 3) Test III: strain-conductivity tests on the SEG
specimens to establish the proof-of-concept of measuring geosynthetic
strains subjected to tensile loads without the use of conventional
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instruments (e.g. strain gauges and extensorneters) and, 4) Test IV:
tensile tests on selected SEG geogricls according to the ASTM 1J6637
test protocol to examine their mechanical response.
Materials
[0048] Two main categories of SEG materials discussed in this
disclosure are the polymers with nanotube (NT) fillers and those with
carbon black (CB) fillers. The original polymers used to develop SEG
specimens included ethylene-vinyl acetate (EVA) filled with 25% by
weight multi-walled carbon nanotubes (NT), polypropylene (PP) filled
with 20% by weight NT, high-density polyethylene (HDPE) filled with
carbon black (CB) and polypropylene filled with CB. These materials
were supplied by commercial manufacturers and are recommended for
compounding (Le. diluting) with the corresponding virgin polymers
(i.e. polymers with no conductive fillers). The virgin polymers used
were PP and HDPE, which were also supplied by commercial
manufacturers.
Unlike the samples with NT fillers, weight compositions (i.e. filler
concentrations or loadings) for the CB-filled samples were not
disclosed to the authors by the supplying companies. In addition,
details regarding the dispersion technique of the fillers within the
rnasterbatches are proprietary. Therefore, the CB-filled samples in this
paper are identified by their "rnasterbatchil-to-"virgin polymer" mixing
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ratios instead of actual concentrations. The term masterbatch, as used
herein, refers to a polymer with either a known or undisclosed filler
concentration in as-supplied condition.
Test Series I: The Search for the Percolation Threshold
Mixing of Polymers and Conductive Fillers
[0049] In order to find the percolation threshold and the target
filler concentration for each type of polymer, several samples were
produced at different filler concentrations (NT) or mixing ratios (CB) as
presented later in Figures 6 and 7. These mixing ratios and
concentration values were chosen to give an adequate number of data
points within and near the percolation region so that the shape of this
region could be determined with reasonable accuracy.
[0050] Each masterbatch was first manually mixed with the
corresponding virgin polymer in a container until the polymer beads
appeared to be evenly distributed in the mix. The mix was
subsequently extruded using a Killion KL-100 single-screw extruder
(Killion/Davis-Standard 2008) with a mixing section. In addition, as is
commonly practiced when a single-screw extruder is used for
compounding, each composite batch produced in this study was
extruded twice to enhance the uniformity of the mix. Extrusive mixing
is known to be the most common method to disperse carbon black
(CB) within polymers and it has been shown that multiple extrusion of
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filled samples can help reduce the standard deviation of conductivity
measurements. However, over mixing is not desirable and can result
in reduced electrical conductivity and strain sensitivity in the filled
composites by increasing the gap between the conductive particles,
The temperatures in the four zones of the extruder were selected
based on the recommended values for injection molding of these
polymers and after preliminary investigation of the quality of the
extruded mix so that all polymer pellets in the batch would be pre-
heated and melted completely and uniformly. A heated die at the final
stage helped with the flow of the batch out of the extruder without the
risk of burning the material, Once extruded, the samples were cooled
in a cold water bath and were subsequently pelletized using a Wayne
pelletizer.
Fabrication of Specimens and Conductivity Tests
[0051.] Fifty-millimeter diameter, disk-shape polymeric specimens
were compression molded by subjecting them to a 44.5 kN
compressive force (equivalent to 22 MPa compressive stress) at 180
C for 10 minutes using a Carver Model M laboratory press and heated
plates. The compression molding technique has been used successfully
in the past to produce polymer/filler blends and has been reported to
result in relatively low percolation concentrations. For instance,
polyethylene (PE)/CB composites have been produced using
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compression molding. Compression molding has been used to blend
nylon powder and CB and has resulted in significantly lower
percolation concentration (i.e. 9% by weight) that would have been
expected using a melt-blending approach (i.e. 25% by weight).
[0052] Conductivity tests were carried out by taking resistivity
measurements using a Keithley 610C Electrometer equipped with a
Model 6105 Resistivity Adapter. Conductivity of each specimen was
readily calculated by inverting the value of its resistivity measured
from the test. Both types of surface and volume conductivity tests
were carried out on the specimens by setting up a voltage difference
across the specimen and measuring the electric current passing
through it using the electrometer.
[0053] In addition, the specimens' conductance values were
measured using an alternative method in which the voltage source was
disconnected and the electrometer was used to measure the electrical
resistance between the same two locations on the specimen. The first
method is the recommended method for measuring the conductivity of
the specimens because it directly provides the specimen conductivity
which is independent of the specimen size. However, electrical
resistance data from the second method (which unlike conductivity is a
size-dependent parameter) showed good corroboration with the first
method in detecting the percolation threshold of the tested specimens
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and provided additional confidence in the test results. Three
independent volume and surface resistance measurements were taken
on each side of every specimen in order to increase the accuracy of
the measured conductivity values. In addition, conductivity tests were
carried out on two nominally identical specimens for each of the five
filler concentration levels (i.e. 10 specimens in total) to increase
confidence in the measured data.
Test Series II: Fabrication and Tensile Testing of Polymer
Specimens According to ASTM D1708
[0054] The same compression molding procedure was used to
manufacture samples for both conductivity and tensile testing
procedures. A 1.5-ton manual expulsion die cutter was used to cut the
individual ASTM D1708 specimens (Figure 3a) from the disc using a
specially manufactured die. Serrated metal grips were attached to the
specimens with a slight amount of tab showing in accordance with the
ASTM D1708 test protocol. The position of the grips were adjusted at
very small increments until the force on the sample was approximately
zero, and then samples were strained at a rate of 10% per minute
until failure.
Test Series III: Fabrication and Proof-of -Concept Testing of
SEG Specimens
[0055] Prototype geogrids (both HDPE and PP) with filler
concentrations determined from the percolation diagrams (shown later
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in Figure 6) were manufactured using the mold described herein.
Strain-controlled tensile tests, as described later in this section, were
carried out on the 4.38% NT/1-1DPE, 2.8% NT/PP, 50/50 CB/HDPE and
33/67 C13/PP (i.e. SEG) specimens while their resistivity was measured
simultaneously. Data from these tests were used to develop a
relationship between electrical conductivity and strain for the SEG
specimens.
Molding Methodology to Manufacture SEG Specimens
[0056] Strain-conductivity measurements were carried out on
single-rib SEG specimens. These specimens were made by
compression molding of each polymer in an oven at 210 C for about
35 minutes. The mold consisted of a pair of 10 cm by 10 cm aluminum
plates and dead weights were used to apply normal load on the mold.
One of the plates had a 3x2-aperture latticework grooving on the
inside to mold the melted polymer in the shape of a geogrid with a
25.4 mm by 44.5 mm aperture size. The width of the grooving was
uniform and equal to 6.4 mm throughout the molding plate. Once the
specimens were completely melted and molded, the mold was
removed from the oven and cooled in a water bath. The plates were
then separated from each other and the geogrid specimen was
carefully removed.
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Scanning Electron Microscopy (SEM) Imagery
[0057] Samples of each SEG geogrid category (Le. NT/HDPE,
NT/PP, CB/HDPE and CB/PP) were frozen in liquid nitrogen and were
broken to obtain (brittle) fractured cross sections. Specimens of
fractured geogrid sections limited in size to 10 mm long by 3 mm wide
by 1.5 mm thick were cut, dried and gold coated (to make the
fractured surface conductive) for SEM imagery. Once the specimens
were ready, they were mounted on bent copper strips (so-called
boats) and placed inside the objective lens of a 3E01_ 3SM-880 High
Resolution Scanning Electron Microscope (SEM) with a maximum
resolution of X 300,000 (SRNEML 2008). Example SEM images of the
SEG specimens are presented in Fig, 4 at X 5000 magnification factor
with inset images magnified by a factor of X 25,000 or X 50,000. It
can be observed that both CB and NT fillers can be easily identified in
the SEM images and they are both well dispersed and incorporated
within the HDPE and PP polymer matrices,
Strain-Resistance Tensile Testing of SEG Specimens
[0058] Single-rib specimens, 44.5 mm long, 6.4 mm wide and
1.35 mm thick, were cut out from the geogrids produced in the mold
(Figure 3b) and were set up in the tensile testing machine. Conductive
leads were attached to the two ends of the specimen rib, and silver
epoxy was applied to obtain reliable and highly conductive (e.g.
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resistivity values in the order of 10 nem) electric connections
between the leads and the polymer. Tensile testing of the specimens
was carried out according to the following loading regime: The
specimens were stretched at 1% strain per minute for one minute and
then held at constant strain for 10 minutes (Figure 5a). This loading
regime was adopted in order to apply a relatively slow loading rate on
the SEG specimens (simulating field conditions) and at the same time,
minimize the testing time. The duration of the stress relaxation period
(i.e. t = 10 min) was decided after inspecting results from the
preliminary tests which indicated that there would be a diminished
return on the magnitude of stress relaxation beyond about 10 minutes.
Any further delay in the application of the next load increment would
only result in greater testing time during which the electrical
conductivity of the specimen would remain almost constant. The
loading pattern shown in Figure 5a was repeated until the specimen
strain reached 15%, or the specimen failed. Resistance measurements
were taken using the electrometer at the beginning of the test and at
the end of each 10-minute relaxation period. Figures 5b and 5c show
example strain-time and load-strain responses obtained for a CB/HDPE
specimen.
[0059] The electrical resistance of some SEG specimens was high
enough so that the conductivity of the testing apparatus was no longer
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negligible. Consequently, steps were taken to electrically isolate the
specimen from the testing apparatus. This was accomplished by
wrapping the SEG specimens in insolating tape before installing the
specimens at the clamps. Some trial attempts were needed to optimize
the thickness of the isolating tapes in order to minimize the slippage of
the specimens within the clamps. Similar attempts for electrical
insulation of specimens have been reported in the literature.
[0060] Several specimens were observed to break gradually
during testing rather than fail abruptly. When the micro-ruptures in
the specimen occurred in between the electric probes, the measured
resistance would show a spike in response. On the other hand, when
the failing of the specimen initiated outside the span between the
probes, the stress-strain curve showed a marked change while the
electrical resistivity remained practically unchanged. Both of these
occurrences might have contributed to the scatter in the data shown
herein.
Test Series IV: Fabrication and Tensile Testing of SEG Geogricls
According to ASTM D6637
[0061] A series of tensile tests were carried out to investigate the
mechanical response of the SEG geogrids according to the ASTM
D6637 test protocol. The geogrid specimens needed for these tests
were manufactured by pressing pellets using a Model TEG Baldwin-
Tate universal testing machine. The specimens were placed in a
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specially grooved aluminum-plate mold and pressed at 180 C with
4.45 kN force for 15 minutes. The mold contained a 7x5-apperture
grooved latticework that was 350 mm long and 250 mm wide with the
grooving width of 3 mm and aperture size of 41 mm by 25 mm.
Heating was provided by a Barnstead Thermolyne SRL12241 heating
mat with pieces of plywood placed on the top and bottom of the setup
to serve as insulation layers. A J-KEM Scientific Model 210 temperature
controller was used to control the specimen temperature. After the
mold was removed from the setup, it was immediately submerged in a
water bath for cooling. Single-rib SG specimens were cut out of the
fabricated grids according to Method A as described in the D6637 test
protocol and strained at a rate of 10% per minute until failure.
RESULTS AND DISCUSSION
Test Series I: Percolation Threshold Results
[0062] Figure 6 shows example volume and surface conductivity
results for the CB-filled and NT-filled polymers tested in this study.
Results shown in Figure 6 represent typical conductivity-concentration
plots in which the percolation regions are clearly identifiable. The
results obtained from the surface and volume conductivity tests were
consistent with each other for each of the CB-filled and NT-filled
polymers tested. Optimum concentration values (i.e. to simultaneously
maximize the strain sensitivity and magnitude of the specimen
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conductivity) for the NT/PP and NT/HDPE materials were found to be
2.8% and 4.38%, respectively. The optimum mixing ratio values for
the CB/PE and CB/PP specimens were found to be 50/50 and 33/67,
respectively. These optimum values (used to fabricate the SEG
specimens) are indicated with dashed lines on the plots in Figure 6. It
should be noted that the accuracy of optimum concentration and
mixing ratio values reported above corresponds to the number of data
points reported in Figure 6 and these optimum values could be fine
tuned by mixing the corresponding conductive fillers at slightly
different concentrations. However, the differences in conductivity due
to sample preparation procedures are believed to overshadow any
errors in the optimum values reported above.
Test Series II: Tensile Response of CB-filled and NT-Filled
Polymers (ASTM D1708)
[0063] Figure 7 shows the tensile test results obtained for the
CB-filled and NT-filled polymer specimens. Results shown in Figure 7a
indicate that the CB/PP specimens manufactured in this study failed at
significantly lower tensile loads (e.g. about 1/3) compared to the as-
supplied specimens (i.e. compared to both virgin and masterbatch
specimens). This observation suggests that the two PP polymers (i.e.
virgin and masterbatch) are not very compatible. On the other hand,
tensile strength values obtained for the corresponding CB/HDPE
specimens at different concentration levels were within the range of
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values obtained for the virgin and masterbatch specimens. Results
shown in Figures 7a and 7b indicate that filler concentration
influenced the tensile strength of the PP specimens more significantly
than the HDPE specimens. For instance, tensile strength values of NT-
filled PP specimens at the percolation threshold are approximately half
as large as those for virgin PP specimens.
[00641 Figures 7c and 7d show failure strains obtained for the
CB-filled and NT-filled specimens. Results shown in Figures 7c and
7d indicate that the presence of conductive fillers (Le. both CB and NT
fillers) could reduce the failure strain of the HDPE polymer
significantly. For instance, results shown in Figure 7c indicate that
CB-filled HDPE specimens with masterbatch-to-virgin ratio greater
than 50% are substantially more brittle than the original material. This
ratio for the NT-filled PP specimens is about 5% masterbatch or 1.25%
filler concentration (Figure 7d).
[0065] Comparison of results shown in Figures 7c and 7d
indicates that the addition of conductive fillers to virgin PP polymers
did not have the same magnitude of deleterious effects with respect to
failure strain as those observed for HDPE polymers. In fact, results
shown in Figure 7c indicate that failure strain of CB-filled PP specimens
increased with the amount of CB filler to an optimum value
corresponding to 50 /0 masterbatch-to-virgin polymer weight ratio. On
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the other hand, failure strain values of NT-filled PP specimens
gradually decreased from about 3% for virgin polymers to slightly
more than 1% for specimens with 20% NT filler by weight. The
reasons for the qualitatively different responses could be related to
one or more of the following factors: the differences in the
compatibility of masterbatch PP resin with the virgin polymer resin,
quality of dispersion in different conductive-filled materials (Figure 4),
or finally, quality of interfacial adhesion in the CB-filled materials as
compared to the NT-filled specimens. Complex trends in the
mechanical properties of filled polymers as a result of dispersing
carbon black have also been reported in previous studies.
Test Series III: Strain-Conductivity Response of SEG Specimens
[0066] Figure 8 shows strain-resistivity results obtained for the
SEG specimens with optimal conductive filler concentration values as
determined from Test Series I. The CB masterbatch-to-virgin polymer
ratio and NT filler concentration values for these specimens are
indicated on the plots. Results shown in Figures 8a and 8b indicate
that the resistivity (or conductivity) responses of both NT-filled and
CB-filled HDPE specimens are strain sensitive. The CB-filled HDPE
specimens show both greater strain sensitivity and slightly greater
scatter in data than the NT-filled specimens. One possible explanation
for the reduced strain sensitivity of NT-filled specimens compared to
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that of CB-filled specimens is that the NT particles have significantly
larger aspect ratios than the CB aggregates (see Figure 4). This
enables the long, rod-shape NT particles to maintain their contact with
each other once the composite specimen is subjected to tensile strain.
As a result, the resistivity of NT-filled polymer specimens undergoes
little change until the strain in the specimen reaches significantly
greater values compared to those of the CB-filled specimens. Results
shown in Figure Sc indicate that the resistivity (or conductivity)
responses of the CB-filled PP specimens are also strain sensitive.
However, the strain-sensitivity of the conductivity of NT-filled PP
specimens was found to be negligible (Figure 8d). These results
indicate that the NT aggregates developed a well-connected 3D
conductive network in the PP host that maintained a significant portion
of its conductive pathways over the strain magnitudes applied to the
specimens. As a result, the resistivity of the specimens was not
sensibly affected when they were subjected to tensile loading.
[0067] An interesting aspect of the results shown in Figure 8 is
that a sizeable magnitude of strain sensitivity was obtained in
polyolefins (i.e. PP and HDPE), which are used in geogrids, and at
relatively low strain values that are of interest in geosynthetic
engineering. For instance, except for the NT/PP specimens (Figure 8d),
the results shown in Figure 8 indicate that an increase in electrical
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resistance as great as -,100 A) (or greater) could be achieved for both
HDPE and PP specimens at strain c = 4%. These results indicate that
the SEG technology (with further refinements in the manufacturing
technique to reduce scatter in data) holds promise as a novel approach
to measuring geosynthetic strain in a variety of field applications. At
the Same time, the same results indicate that the sample-to-sample
reproducibility needs to be improved in order to improve the accuracy
and reliability of this technology.
Test Series XV: Tensile Response (ASTM D6637) of SEG
specimens
[0068] Tensile test results on HDPE SEG specimens according to
the ASTM D6637 test protocol are shown in Figure 9, which indicate a
fairly consistent mechanical response for both CB-filled and NT-filled
specimens prior to their peak strength. Most of the specimens
exhibited a ductile and strain-softening behavior beyond their peak
strength. Results shown in Figure 9 indicate that SEG specimens
developed in this study maintain mechanical properties (i.e. ductility
and tensile strength) that are suitable for reinforcement applications.
[0069]
