Note: Descriptions are shown in the official language in which they were submitted.
Doc. No. 328-18 CA
Green Tech Patent
SHOTCRETE COMPOSITION
FIELD
This disclosure relates generally to shotcrete mixtures for use in fanning
underground
supports, repairing damaged concrete structures, encasing structural steel for
fireproofing,
etc., and more particularly to shotcrete mixtures that foun substantially
waterproof structures
after hardening.
BACKGROUND
Shotcrete is a method of applying concrete projected at high velocity
primarily onto a vertical
or overhead surface. The impact created by the application consolidates the
concrete.
Although the hardened properties of shotcrete are similar to those of
conventional cast-in-
place concrete, the nature of the placement process results in an excellent
bond with most
substrates, and rapid or instant capabilities, particularly on complex forms
or shapes. In the
shotcrete method, concrete is applied using a wet- or dry-mix process. The wet-
mix shotcrete
process mixes all ingredients, including water, before introduction into the
delivery hose. The
dry-mix shotcrete process adds water to the mix at the nozzle. The shotcrete
method of
applying concrete is used in new construction and repairs and is suitable for
curved and thin
elements.
Buildings and construction together account for more than 30% of global final
energy use and
almost 40% of energy-related carbon dioxide (CO2) emissions when upstream
power
generation is included. Concrete is one of the biggest contributors to the
carbon footprint of
buildings and infrastructure. Every year, more than 10 billion tons of
concrete are used,
which requires more than 4 billion tons of cement, accounting for around 8% of
all CO2
emissions worldwide. Despite improvements in processes and control measures,
the
manufacture of concrete still emits between 70 and 90 kg of CO2 per ton.
The main environmental impact of concrete occurs during manufacture,
especially the
production of cementitious binder, reinforcing steel, mining, and transport of
aggregates, and
the energy used to transport the concrete to the job site.
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Reducing the impact of concrete in the construction industry will become
increasingly
important in coming years as rapid urbanization and economic development
increases demand
for new buildings and, thus, for concrete and cement, including for use in the
shotcrete
process. One of the most important and attainable ways of reducing the carbon
footprint and
other environmental impacts of concrete construction is to extend the life of
the buildings,
roads and other structures that are built using concrete. Since water
permeation leads to
corrosion and frost-thaw damage, etc., providing waterproof concrete
structures that are
resistant to water permeation is an area of intense interest due to the
potential to significantly
extend the time before it becomes necessary to demolish existing structures
and build new
ones.
As is well known, concrete is a composite construction material composed
primarily of the
reaction products of hydraulic cement, aggregates, and water. Water is both a
reactant for the
cement component and is necessary to provide desired flow characteristics
(e.g., spread and/or
slump) and ensure consolidation of freshly mixed concrete to prevent formation
of strength-
reducing voids and other defects. In the shotcrete process, air is entrained
in the composition,
which must be tailored to provide acceptable workability/pumpability,
sprayability and to
have suitable rebound and compaction characteristics. Of course, chemical
admixtures may
be added to modify the characteristics of the composition to suit a particular
application using
the shotcrete process.
Crack formation at or near the surface of the applied concrete, due to
shrinkage of the
concrete during hydration and hardening, are a common occurrence and may
result in weaker
structure and poor aesthetics. Unfortunately, these cracks also provide a
pathway that allows
water to permeate into the concrete, which may lead to corrosion of internal
reinforcements,
leaching of the aggregates and binders, and ultimately result in premature
failure of the
concrete structure and the need to build a replacement. Different mechanisms
are known to
result in crack formation. For instance, plastic shrinkage occurs in a freshly
mixed concrete,
with loss of water by evaporation from its surface, after placing and before
hardening of the
concrete. This can lead to plastic shrinkage cracking if the rate of
evaporation is higher than
that of the bleeding water rising to the surface of the concrete. Drying
shrinkage occurs due
to the loss of moisture from concrete after it hardens. Several factors impact
shrinkage, for
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example: the cement and water content, size of the aggregates, aggregate to
cement ratio,
excessive fines, admixtures, cement composition, temperature, humidity, curing
process, etc.
In general, it is not uncommon for these effects to produce cracks up to 1 mm
or more in
width in the hardened concrete structure, which is unacceptable for
applications in which the
concrete structure may be exposed to water.
Various technologies have been used to reduce shrinkage, using chemicals or
fibers or mixes
thereof. For instance, the use of cellulose fibers, polyethylene fibers,
polypropylene fibers
etc., has been widely practiced in the concrete industry for many years.
However, current
technologies have thus far failed to achieve a reduction in crack size that is
necessary to
prevent water permeation and avoid premature failure of the concrete
structure.
It would therefore be beneficial to provide a solution that overcomes at least
some of the
above-mentioned drawbacks.
SUMMARY OF EMBODIMENTS
In accordance with an aspect of at least one embodiment, there is provided a
shotcrete
composition, comprising: a cement, a fine aggregate, a coarse aggregate and
water, wherein a
weight ratio of water to cement is between 0.38 and 0.44 and is sufficient for
hydraulic setting
of the cement; a magnesium aluminosilicate material; a colloidal silica
material; and a
MgO/CaO blend in an amount of between 2.5% and 5% of the amount of dry cement,
wherein
the MgO/CaO blend comprises between 2% and 6% CaO by weight.
In accordance with an aspect of at least one embodiment, there is provided a
shotcrete
composition, comprising: a cement, a fine aggregate, a coarse aggregate and
water, wherein a
weight ratio of water to cement is between 0.38 and 0.44 and is sufficient for
hydraulic setting
of the cement; a magnesium aluminosilicate material; a colloidal silica
material; and a
MgO/CaO blend, wherein cracks formed during hardening of the shotcrete
composition are
less than 0.01 mm in width.
In accordance with an aspect of at least one embodiment, there is provided a
hardened
shotcrete structure fabricated from a shotcrete composition, wherein prior to
hardening the
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shotcrete composition comprises: a cement, a fine aggregate, a coarse
aggregate and water,
wherein a weight ratio of water to cement is between 0.38 and 0.44 and is
sufficient for
hydraulic setting of the cement; a magnesium aluminosilicate material; a
colloidal silica
material; and a MgO/CaO blend in an amount of between 2.5% and 5% of the
amount of dry
cement, wherein the MgO/CaO blend comprises between 2% and 6% CaO by weight,
and
wherein a surface of the hardened shotcrete structure is free from cracks
having a width
greater than 0.01 mm.
DETAILED DESCRIPTION
While the present teachings are described in conjunction with various
embodiments and
examples, it is not intended that the present teachings be limited to such
embodiments. On
the contrary, the present teachings encompass various alternatives and
equivalents, as will be
appreciated by those of skill in the art. All statements herein reciting
principles, aspects, and
embodiments of this disclosure, as well as specific examples thereof, are
intended to
encompass both structural and functional equivalents thereof. Additionally, it
is intended that
such equivalents include both currently known equivalents as well as
equivalents developed
in the future, i.e., any elements developed that perfolm the same function,
regardless of
structure.
As used herein, the temis "first," "second," and so forth are not intended to
imply sequential
ordering, but rather are intended to distinguish one element from another,
unless explicitly
stated to the contrary. Similarly, sequential ordering of method steps does
not imply a
sequential order of their execution, unless explicitly stated.
As used herein, "Cement" refers to a binder that sets and hardens and brings
materials
together. The most common cement is Ordinary Portland Cement (OPC) and a
series of
Portland cements blended with other cementitious materials, such as Portland
Pozzolana
Cement (PPC) and their typical blends available in the market.
As used herein, "Ordinary Portland cement" refers to a hydraulic cement made
from grinding
clinker with gypsum. Portland cement contains calcium silicate, calcium
aluminate and
calcium ferroaluminate phases. These mineral phases react with water to
produce strength.
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As used herein, "Fibers" refers to a material used to increase shotcrete's
structural
performance. Fibers are also responsible for reducing cracking, abrasion
resistance, surface
finishes etc. For example, fibers include steel fibers, glass fibers,
synthetic fibers, and natural
fibers.
As used herein, "Admixture" refers to a chemical substance used to modify or
improve
shotcrete's properties in fresh and hardened state. These could be air
entrainers, water
reducers, set retarders, accelerators, stabilizers, superplasticizers, and
others.
As used herein, "Concrete" refers to a combination of cement, fine aggregates,
coarse
aggregates, and water. Admixture can also be added to provide specific
properties such as
flow, lower water content, acceleration.
As used herein, "Shotcrete" refers to a concrete composition specifically
formulated for high-
velocity application onto surfaces by spraying. "Shotcrete" is a specific type
of concrete that
can either be dry-mix or wet-mix. "Shotcrete" also refers to the method of
applying the
concrete composition using a nozzle to spray the concrete onto a surface, etc.
As used herein, "Structural applications" refers to a construction material
having a
compressive strength greater than 25 MPa.
As used herein, "Coarse aggregates" refers to a manufactured, natural or
recycled mineral
with a particle size typically of about 10 mm for shotcrete applications, but
more generally
may be in the range between about 9 mm and about 15 mm. Coarse aggregates may
also
include mineral with a particle size outside this range, such as for instance
5-10%.
As used herein, "Fine aggregates" refers to a manufactured, natural or
recycled minerals with
a particle size between 0.1 mm and 1 mm. Fine aggregates may also include
mineral with a
particle size outside this range.
As used herein, "Shrinkage" refers to the reduction in the volume of shotcrete
caused by the
loss of moisture as shotcrete hardens or dries. Because of the volume loss,
shotcrete
shrinkage can lead, for example, to cracking when base friction or other
restraint occurs.
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As used herein, the water to cement ratio "w/c" refers to the total free water
(w) mass in kg
divided by the total cement mass in kg.
As used herein "Batch total weight" refers to the combined weight of cement,
fine aggregate,
coarse aggregate, and water added to form the shotcrete composition but
excludes the weight
of all admixture components.
According to a first aspect, the present disclosure relates to a shotcrete
composition including
i) cement, ii) a fine aggregate, iii) a coarse aggregate, iv) a magnesium
aluminosilicate, v)
colloidal silica, vi) a blend of magnesium oxide (MgO) and calcium oxide
(CaO), and vii)
water. After curing, a compressive strength in the range between about 20 MPa
and 40 MPa,
determined using the standard 28-day lab test for compressive strength of a
concrete cylinder,
is observed. The specific compressive strength may be tailored to suit the
requirements of a
particular project in which the shotcrete composition is to be used.
Advantageously, the
disclosed shotcrete composition, after curing, exhibits reduced cracking and
improved
waterproof properties relative to known compositions.
The shotcrete composition of this disclosure provides improved performance,
primarily
against plastic shrinkage cracking, thermal cracking, and drying shrinkage
cracking. The
performance of the disclosed shotcrete composition is improved relative to
prior art
compositions by the inclusion of specific admixture components which exhibits
a useful and
unexpected synergy. The inclusion of an expansion agent (i.e., the MgO/CaO
blend) acts
against the tensile forces that develop in the early setting stage of
shotcrete hardening and that
are responsible for plastic shrinkage cracking. The inclusion of colloidal
silica reduces curing
requirements and eliminates wet curing, and develops shotcrete strength
earlier compared to
typical mixes, thus reducing schedule times. The inclusion of colloidal silica
furthermore
minimizes capillary formation, bleeding of water and hence, drying shrinkage
cracking
compared to typical mixes. The inclusion of magnesium aluminosilicate provides
improved
workability and enables reduced water content (w/c ratio).
A currently preferred composition will now be described as a specific and non-
limiting
embodiment. However, as discussed in more detail below, the relative amount of
the various
components may be varied depending on the requirements of a specific
application. The
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currently preferred composition includes a cement, a fine aggregate, a coarse
aggregate, and
water. The cement is preferably a hydraulic cement, preferably a
sulfoaluminous clinker,
preferably Portland cement. Portland cement refers to the most common type of
cement in
general use around the world, developed from types of hydraulic lime and
usually originating
from limestone. It is a fine powder produced by heating materials in a kiln to
foun what is
called clinker, grinding the clinker, and adding small amounts of other
materials. The Portland
cement is made by heating limestone (calcium carbonate) with other materials
(such as clay)
to >1400 C in a kiln, in a process known as calcination, whereby a molecule
of carbon
dioxide is liberated from the calcium carbonate to foun calcium oxide, or
quicklime, which is
then blended with the other materials that have been included in the mix to
from calcium
silicates and other cementitious compounds. The resulting hard substance,
called "clinker" is
then ground with a small amount of gypsum into a powder to make ordinary
Portland cement
(OPC). Several types of Portland cement are available with the most common
being called
ordinary Portland cement (OPC) which is grey in color. The low cost and
widespread
availability of the limestone, shales, and other naturally occurring materials
used in Portland
cement make it one of the low-cost materials widely used throughout the world.
Of course, as
will be apparent to a person having ordinary skill in the art, other types of
cement, such as for
instance Portland Pozzolana Cements (PPC) and their typical blends available
in the market,
may be used as the cement in the currently preferred composition. Portland
Pozzolana
Cements are produced by either inter-grinding of OPC clinker along with gypsum
and
pozzolanic materials in certain proportions or grinding the OPC clinker,
gypsum and
Pozzolanic materials separately and thoroughly blending them in certain
proportions.
The fine aggregate may be of natural or synthetic origin and may have a
particle size in the
range of 0.1 mm to 1.0 mm. The fine aggregate may be sand or another suitable
material
having a similar particle size, including manufactured, natural, or recycled
minerals. Using
manufactured sand is a more environmentally beneficial alternative to typical
sand. The
benefits arise from the fact that manufactured sand is a by-product of
crushing rock. This
leads to reduced processing needed compared to typical sand and reduced river
dredging for
obtaining sand. These factors also help making manufactured sand cheaper to
use than
typically sourced sand.
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The coarse aggregate also may be of natural or synthetic origin and may have a
particle size
typically of about 9-15 mm, the industry standard being about 10 mm, although
the particle
size may be either smaller or larger than these values as will be understood
by a person having
ordinary skill in the art (i.e., 5-10%). The coarse aggregate may be
limestone, pea stone,
standard crushed stone, or another suitable material. Aggregates, from
different sources, or
produced by different methods, may differ considerably in particle shape,
size, and texture.
Shape of the aggregates of the present disclosure may be cubical and
reasonably regular,
essentially rounded, or angular and irregular. Surface texture may range from
relatively
smooth with small, exposed pores to irregular with small to large, exposed
pores. Particle
shape and surface texture of both the fine aggregate and the coarse aggregate
influence
proportioning of mixtures in such factors as workability, pumpability, fine-to-
coarse
aggregate ratio, cement binder content, and water requirement.
A typical batch weight is 2400 kg producing 1 m3 of shotcrete, although the
total batch size
may be greater or less than this value to suit a particular job requirement.
In the currently
preferred composition, a weight ratio of the cement to the fine aggregate is
between about 1:2
and about 1:2.5. A weight ratio of the cement to the coarse aggregate is
between about 1:1.5
and about 1:2.5. A weight ratio of the fine aggregate to the coarse aggregate
is between about
1:0.54 and 1:1.5, preferably between about 1:0.6 and 1:1.25. Specific and non-
limiting
examples of suitable ratios of cement to fine aggregate to coarse aggregate
include 1:2.5:2,
1:2:2.5, 1:2.5:1.5. Of course, other ratios within the above-mentioned ranges
may be used. In
addition, the ratio of fine aggregate to coarse aggregate may be tailored
depending on the
specific requirements for a particular job (i.e., vertical, or curved
application and site
conditions) and the material quality (shape, size etc.). For some
applications, a higher weight
ratio of fine aggregate to coarse aggregate may be advantageous (i.e., about
1:0.54, which
means that about 65 % of the total aggregate weight is the fine aggregate).
The amount of water added to the dry mixture of cement, fine aggregate and
coarse aggregate
is sufficient for hydraulic setting of the cement. More specifically, the
water to cement
content (kg/kg) is between about 0.38 and about 0.44 in the currently
preferred composition.
The relative amounts of cement, fine aggregate, and coarse aggregate, with a
water content
between 0.38 and 0.44, yields a shotcrete mixture that is suitable for
application via a concrete
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gun in known fashion. The currently preferred shotcrete composition may be
prepared e.g., in
a ready-mix truck at an appropriate time prior to a scheduled delivery at a
work site. Suitable
amounts of each of the above-mentioned components may be within the following
ranges:
400-460 kg of cement; 800-1200 kg of fine aggregates; 650-1100 kg of coarse
aggregates;
and 150-200 L of water. The amount of each component is adjustable within the
above-
mentioned ranges to produce a batch total weight of shotcrete of 2400 kg and
yielding 1 m3 of
shotcrete.
The currently preferred composition includes chemical admixtures and mineral
admixtures to
improve the physical properties of the wet mix of the finished shotcrete
material. In
particular, the currently preferred composition includes i) a blend of
magnesium oxide (MgO)
and calcium oxide (CaO) (MgO/CaO blend), ii) a magnesium aluminosilicate such
as for
instance palygorskite and/or attapulgite, and iii) colloidal silica. The
currently preferred
composition optionally includes additional admixtures, such as for instance
iv)
plasticizers/water reducers, e.g., lignosulfonate-based additives, v)
micro/macro fibers, e.g.,
steel fibers or synthetic fibers (e.g., polypropylene, polyvinyl alcohol,
etc.), and/or vi)
supplementary cementitious materials (SCM) used to further enhance the
properties of the
concrete or to reduce use of cement in the mix (e.g., typically in the form of
silica fumes,
limestone, flyash, slag etc.). The above-mentioned admixtures is discussed in
greater detail in
the following paragraphs.
.. The MgO/CaO blend is added, in the dry state, to the mixture of cement,
fine aggregate and
coarse aggregate described above. More particularly, the MgO/CaO blend is an
expansion
agent that counteracts the tensile forces across the applied shotcrete during
the initial setting
state, which leads to a significant reduction in plastic shrinkage cracking
after the shotcrete
composition hardens. It has been found that a blend, containing the relative
amounts of MgO,
CaO and silica fume that are disclosed in the following paragraph, has a
synergistic effect
with the other admixtures resulting in the formation of cracks that are orders
of magnitude
smaller than the cracks that are formed using prior art shotcrete
compositions. For instance,
after hardening, the disclosed shotcrete composition may have cracks that are
no larger than
about 0.01 mm in width, preferably no larger than about 0.001 mm in width.
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Preferably, the MgO/CaO blend is added in an amount between about 2.5 wt% and
about 5
wt% of the cement, on a dry weight basis. For a typical 2400 kg batch
including water,
between about 10 kg and about 23 kg of the MgO/CaO blend is added. The
relative amount
of the two components of the blend may vary depending on the specific
application and/or
depending on the desired properties of the shotcrete. Preferably, the MgO/CaO
blend
contains between 2¨ 6% CaO by weight, with the balance being MgO.
The magnesium aluminosilicate, such as for instance palygorskite and/or
attapulgite, is added,
either as a slurry or in dry foun, to the currently preferred shotcrete
composition described
above. The magnesium aluminosilicate admixture acts as a binder, thixotrope,
reinforcement
additive, anti-settling agent and rheology modifier. The magnesium
aluminosilicate can be
introduced at any point in the process with similar performance. To avoid
making the
preferred shotcrete composition too cohesive, the magnesium aluminosilicate
dosage typically
is reduced relative to the dosage in prior art shotcrete compositions. In the
currently preferred
composition, the total combined amount of magnesium aluminosilicate is in the
range
between 0.03% and 0.1% of the batch total weight (i.e., the combined dry
weight of cement,
fine aggregate and coarse aggregate, and water), or between 0.5 kg and 2.5 kg
in a typical
2400 kg batch total weight including added water.
The colloidal silica is provided in slurry foun, such as for instance about 15
wt% to 30 wt%
amorphous silica in 70 wt% to 85 wt% water. The silica particles in the slurry
may have a
size between about 1 nm and about 100 nm and a surface area of between about
300 m2/g and
about 900 m2/g. A currently preferred colloidal silica slurry has silica
particle between about
1 nm and about 50 nm in size and a surface area between about 500 m2/g and
about 600 m2/g.
As noted above, the colloidal silica enables internal hydration and curing,
promotes early
strength acceleration, and increases workability by binding to the cement
particles. The
colloidal silica is added to the preferred shotcrete mix in slurry foim in an
amount of about
0.95 L to about 1.15 L in a typical 2400 kg batch total weight including water
(i.e., about 0.11
L per 45.35 kg dry weight of cement) and is the last admixture component
added. The
amount of colloidal silica slurry in a typical 2400 kg batch total weight
including water may,
however, be as much as 2.43 L. For instance, when the preferred shotcrete
mixture is being
prepared in a ready-mix truck, prior to adding the colloidal silica slurry the
mixer is switched
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to discharge mode, and the mixer is turned to the point where the shotcrete
mixture is on the
final spiral of the mixer and about to fall off the chute. The mixer is then
switched to mixing
mode. The colloidal silica slurry is added, in a controlled manner, avoiding
spillage onto the
chute and avoiding contact with the internal surface of the mixer. Finally,
the mixer spins at
minimum speed of 70 rpm for at least 4 minutes to ensure proper dispersion
across the
volume of the shotcrete mixture.
The presently preferred shotcrete composition may include additional but
optional
admixtures, which are in any case added prior to adding the colloidal silica
slurry. Optional
admixtures include at least plasticizers/water reducers (e.g., lignosulfonate-
based additives).
These additives are typically used to reduce water/cement ratio, provide
additional
fluidity/workability, strength and slow down the settling rates of shotcrete.
The presently
preferred shotcrete composition is compatible and consistent with the use of
plasticizers
falling in the low to mid-range capabilities. A plasticizer admixture
component may be added
in an amount of about 160 ml to about 1000 ml per 100kg of dry cement, or
about 600 ml to
about 4600 ml in a typical 2400 kg batch total weight including water.
Another optional admixture includes at least micro/macro fibers, such as for
instance glass,
steel, nylon or other synthetic fibers (e.g., polypropylene fibers). The
inclusion of steel and/or
synthetic macro/micro fibers is to avoid all fauns of internal cracking and
limit the width of
cracks when the presently preferred shotcrete composition is to be used in
extreme weather
conditions. Some specific and non-limiting examples of suitable synthetic
fibers include
polyvinyl alcohol (PVA) micro filament fibers with a fiber diameter in the
range between
about 24 microns to about 100 micron and a fiber length in the range between
about 6 mm to
about 50 mm. PVA fibers can be suggested as the most preferred option.
Alternatively,
polypropylene fibers (PPF) with a fiber diameter in the range between about 50
microns to
about 200 microns and a fiber length in the range between about 12 mm to about
65 mm may
be used. The type of fiber selected will depend at least partially upon the
specific application
for the shotcrete batch. A micro/macro fiber admixture component may be added
in an
amount of about 0.45 kg to about 2.5 kg in a typical 2400 kg batch total
weight including
water.
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In teims of the present disclosure, the term "composition" may refer to the
fresh state solid
cement or shotcrete mixture comprising the cement, the fine aggregate, the
coarse aggregate,
the magnesium aluminosilicate, the colloidal silica, the MgO/CaO blend before
the addition
of the water and/or additional chemical and/or mineral admixtures. The
"composition" may
refer to a sprayable fluid shotcrete mixture after the addition of all or a
portion of the water
and/or additional chemical and/or mineral admixtures. The "composition" may
refer to the
hardened matrix shotcrete after any period of setting once the hydration
process has started.
In a preferred embodiment, all components of the shotcrete composition of the
present
disclosure are homogeneously dispersed in the composition.
Throughout the description and claims of this specification, the words
"comprise",
"including", "having" and "contain" and variations of the words, for example
"comprising"
and "comprises" etc., mean "including but not limited to", and are not
intended to, and do not
exclude other components.
When a range is given between "x" and "y" the range is intended to include
both "x" and "y."
The term "about" means 10% and preferably 5% when applied to values in a
range or to
single values.
It will be appreciated that variations to the foregoing embodiments of the
disclosure can be
made while still falling within the scope of the disclosure. Each feature
disclosed in this
specification, unless stated otherwise, may be replaced by alternative
features serving the
same, equivalent or similar purpose. Thus, unless stated otherwise, each
feature disclosed is
one example only of a generic series of equivalent or similar features.
All of the features disclosed in this specification may be combined in any
combination, except
combinations where at least some of such features and/or steps are mutually
exclusive. In
particular, the preferred features of the disclosure are applicable to all
aspects of the
disclosure and may be used in any combination. Likewise, features described in
non-essential
combinations may be used separately (not in combination).
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