Session A11
2246
RECYCLED ALUMINUM IN MODERN INFRASTRUCTURE
Jim Brucker (jrb172@pitt.edu), Rob Gehris (rsg31@pitt.edu)
Abstract— Currently, humans are dealing with the issue of
using resources faster than the Earth can produce them. As
a result, there has been a significant push in recent years
toward “green” technology, processes which minimize
negative effects of production processes on natural systems
on Earth. This paper will delve into the uses and
composition of recycled aluminum. We will describe the uses
for aluminum in military, infrastructure and construction, as
well as the technology used to create each of these
applications. We will also discuss the implications of “green
technology.”
This paper’s main focus is on recycled aluminum, a
“green” technology. In order to be considered “green,” a
product is expected to have a minimal environmental impact.
One of the key measurements in this facet is the concept of
embodied carbon, or the amount of carbon dioxide
associated with the production of a material.
A crucial part of civil engineering is the materials used in
applications. Recycled building materials are used
regularly, and the military uses recycled plastics in a
majority of their bridges. Recently, more and more facets of
infrastructure have been built with the green movement in
mind, and recycled aluminum is a key component of this
movement. This paper discusses how the recycled aluminum
process works, as well as the physical and environmental
properties the product provides.
As modern consumers demand more environmentally
conscious technology, engineers must be able to meet these
demands. The widespread use of recycled aluminum is the
first step.
Key Words—Aluminum Composites, Embodied Carbon,
Recycled Aluminum, Recycled Dross, Salt Flux, Virgin
Aluminum.
MODERN VIEW ON ALUMINUM
Imagine a soda can. It’s about six inches tall, three inches
wide, filled with fizzy drink, and made out of aluminum.
That’s all it is, just your average, every day, run-of-the-mill
soda storage container. A can. Now imagine a building. Not
just any building though, a two-hundred foot tall behemoth
of modern construction, built to withstand earthquakes,
whipping winds, and just about everything else that Mother
Nature can throw at it. Now picture a metal baseball bat, or a
computer, fighter jet, car or a boat. Maybe even picture a
spaceship. What do these objects and a plethora of others
have in common? The basic material used to make them:
aluminum.
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Aluminum (or aluminium for users of the metric system),
is one of the most versatile metals thanks to its structural
properties which make it relatively lightweight, strong,
sturdy, and, most importantly, completely recyclable. This
last property is what make this element so amazing, and is
the topic of this paper. This paper will delve into the uses
and composition of recycled aluminum. It will describe the
uses for aluminum in business, infrastructure and
construction, as well as the technology used to create each of
these applications. We will also discuss what the term
“green technology” means.
Due to its high reactivity, aluminum is never found in its
pure elemental form in nature. Virgin aluminum is extracted
from bauxite ore and processed from there. The bauxite is
mined out of the ground in large geological formations. The
combination of all these processes to make useable
aluminum makes virgin aluminum a very inefficient metal.
However, aluminum’s ability to be completely recycled
more than makes up for this high initial energy toll, as the
energy to recycle aluminum is much less than the energy toll
to produce virgin aluminum. The degree of exactly how
much less energy will be discussed later into the paper.
WHAT IT MEANS TO BE “GREEN”
If a technology is considered “green,” it is commonly
understood that this technology is meant to be
environmentally friendly. However, this is where the
misconceptions start: what does environmentally friendly
mean? Does it have no environmental affects, or positive
environmental affects? Clearly, there is a grey area in the
common definition. There was no standard definition of
“green” until Underwriters Laboratories (UL) Environment
started evaluating products being marketed as eco-friendly,
environmentally friendly, and “green.” UL has been
ensuring the safety of household electronics for 115 years
[1], proving that they are the right company for this job. As
part of their verification of green products, UL requires
manufacturers to test the products to obtain an Energy
Efficiency Certification, an Environmental Claims
Validation, and a Sustainable Product Certification [1].
Companies want validation from UL Environment for one
main purpose: environmentally green products tend to make
companies financially green.
The word “green” is often used to sell products. Most
middle-class citizens are willing to spend a few extra cents
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on a purchase if they feel like they are doing a good deed for
the environment. The current trend throughout most of the
developed world is for products and processes to be
sustainable. Most people will equate sustainability and
greenness. It is the common belief that a product labeled as
“green” is a good product [2]. According to Tristan Roberts,
a columnist for the Brattleboro Reformer, this “green equals
good” mentality is driven by the media, and generally tend
to be true. However, this is not the case for all products,
which is why UL Environment provides validation. So,
instead of looking for the word “green,” consumers should
look for the UL stamp of approval, Figure 1.
removed using a chlorine flux [3]. During the addition of
flux, aluminum dross becomes a by-product of the aluminum
and the impurities. This dross is then crushed, filtered
through a screen, and then melted in a rotary furnace. The
aluminum from the dross is collected at the bottom of the
furnace and melted in with the rest of the pure molten
aluminum [3]. The molten aluminum is then ready to be
shaped into whatever shape it needs to be for it to be useable
for consumers. This type of aluminum smelting process –
one of a couple of d different styles of processing and
refining the metal – is the version preferred and outlined by
the Illinois Sustainable Technology Center at the University
of Illinois.
SALINE FLUX
FIGURE 1
THE UL STAMP OF APPROVAL FOR SUSTAINABLE PRODUCTS [1]
How can one measure the environmental friendliness (or
“greenness”) of recycled aluminum? The product itself
doesn’t emit gases or have any inherent side-effects on the
environment. One has to look at the production process. The
industry standard is to look at the embodied carbon of a
material. The embodied carbon of a product is measured
with an estimation of the amount of carbon dioxide, in
kilograms, associated with the production of one kilogram of
the material [2]. The embodied carbon of recycled material
tends to be much lower than the embodied carbon of virgin
material. This difference is incredibly noticeable in recycled
aluminum, as will be shown in the next section.
TECHNOLOGY INVOLVED
When one thinks of recycled aluminum, the simplest thought
is to imagine tossing an aluminum soda can into the
recycling bin and letting other people turn it into another
can. But how does that one can become a new can? Better
yet, how does that can turn into an aluminum bar used to
build a house, a bridge, or even a skyscraper? The simple
answer is that this process happens in an aluminum recycling
plant. What happens inside the plant though, is more
complicated. There are multiple processes used to recycle
aluminum; however, most plants use either one of or both of
the standard processes – saline flux technology and recovery
of dross. Prior to these specific technologies, aluminum
scrap is normally melted in a gas or oil fueled hearth
furnace. It is at this point in the process where impurities are
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Swanson School of Engineering
Saline (salt) flux is used in the melting of aluminum to filter
out impurities. Its main function is to become a barrier of
sorts between the molten aluminum and the oxygen in the
atmosphere of the furnace in order to, according to the
Handbook of Aluminum by George Totten and Scott
MacKenzie, diminish the oxidation of the top layer of
aluminum [4]. The salt flux also helps to remove the topmost
oxide layer which is sometimes formed in previous stages in
the melting process. As it has very specific tasks, the salt
fluxes must have certain characteristics. The flux must have
a melting point below the heat of the furnace – around
720OC – and have a low viscosity and vapor pressure at this
temperature. The flux cannot react with the molten
aluminum; if it did, it would add impurities to the metal,
risking its structural integrity. In the same vain as not
reacting with the metal, the fluxes must be easily detachable
from the liquid bath and able to be reused in future melting
of aluminum [4].
Several compounds have the properties needed to make a
good salt flux for melting down recycled aluminum. The
least expensive tend to be mixtures of sodium chloride
(NaCl) and potassium chloride (KCL), normally in a one to
one molar ratio mixture. To help create the barrier between
the aluminum and the oxygen, several fluorides are added to
the mix. The common fluorides are cryolite [5] (Na 3AlF6),
sodium fluoride (NaF), potassium fluoride (KF), calcium
fluoride (CaF2), and Magnesium Fluoride (MgF2) [4].
There are methods of protecting the aluminum from
forming oxides other than saline fluxes. The most prominent
of these technologies is the replacement of fossil fuels
heating the rotary furnace with plasma torch treatment [3].
Higher concentrations of aluminum are recovered using this
technique; however, the plasma torch treatment does not
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make sense economically, as it costs significantly more than
basic fossil fuels, and the aluminum output is not
significantly larger. Some aluminum melting plants prefer
this technology though because of the oddities that occurs in
the aluminum with the use of the salt fluxes. These oddities
include the formation of dross and saltcakes, as well as
fumes from the burning of fossil fuels [4]. The plasma
treatment generates some fumes, but significantly less than
the fossil fuel driven furnaces.
ALUMINUM DROSS RECOVERY
Dross is a consequence of the melting of aluminum. It is
formed on top of the molten aluminum as it reacts with the
atmosphere in the furnace. In general, “15-25 kilograms of
dross are produced per metric ton of molten aluminum” [4].
The three types of dross are white dross, black dross, and
saltcake. Each step in the metallurgy of aluminum produces
a different classification of dross. White dross comes from
the primary step in the refinement of recycled aluminum.
This step includes the extrusion and lamination plant, and
melting the processed aluminum in foundries. White dross
will contain anywhere from 15% to 70% of aluminum [4],
giving it a light gray color, hence the name “white” dross.
Black dross is a byproduct of the secondary metallurgy of
the recycled aluminum. This step in the process uses the
saline flux technology mentioned above. Black dross
contains significantly less aluminum than white dross,
causing it to have a darker color. Saltcake is produced from
the aluminum recovered from the white and black dross. It
has a very high salt concentration and a darkish hue.
Saltcake contains 1% to 7% of aluminum [4]. There is not
much difference between the saltcake and the black dross.
The combination of white dross, black dross, and saltcake is
then treated and the recovered aluminum is added to the
recycled material. Aluminum is extracted from the dross in
rotary furnaces in which salts are added to increase the
quantity of aluminum recovered [4].
ADVANTAGES OF RECYCLED ALUMINUM VERSUS
VIRGIN ALUMINUM
As previously mentioned, the industry standard of measuring
eco-friendliness, or “greenness,” is with the measurement of
the embodied carbon of the product. Using this as a scale,
recycled aluminum is much greener than virgin aluminum.
In a study referenced by Tristan Roberts in his article
“Recycled is Good… Right?” the average embodied carbon
associated with new aluminum is 12.79 kg CO2 while the
average embodied carbon of recycled aluminum is only 1.81
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Swanson School of Engineering
kg CO2 [2]. This equates to about 95% less energy being
used to produce recycled aluminum than used in producing
virgin aluminum. Recycled aluminum also produces about
95% less pollution than virgin aluminum in its production
process [6].
As well as being significantly greener, recycled aluminum
has comparable structural strength as the virgin material.
Less than 1% of aluminum’s structural ions are lost in the
recycling process, making it close to infinitely recyclable
with structural strength that barely deteriorates from the
initial strength of the virgin material [3]. This means that
recycled aluminum has the same properties as virgin
aluminum for less cost, financially and environmentally.
Also, as previously mentioned, virgin aluminum comes from
bauxite which comes from mines close to the Earth’s
surface. While there is plenty of bauxite down there, it is still
a nonrenewable resource, and there is a limited amount.
Additionally, bauxite mines take up large amount of space
and are very destructive to the environment around them.
Recycling aluminum can reduce the amount of mining,
aiding in the effort to reduce the environmental impact of
this product.
BUILDING AND CONSTRUCTION
While the technologies and processes behind the production
of recycled aluminum certainly are important, without an
effective way to apply this material to real-world
applications the production is pointless. We will discuss
specifically how recycled aluminum can be used both used
in and obtained from infrastructure and how it to useful to
the modern day building and construction industry.
One reason recycled aluminum should be used more in
industry today is because of how easily it can be recovered
after demolition processes. Based on a study in European
study that looked at nine different types of buildings in six
different countries, it was found that the average aluminum
collection rate after demolition was 95.7% [7].Even though
the content of aluminum in these buildings was less than 1%,
a substantial amount was able to be recovered for recycling
[7]. As we discussed earlier, sustainability is a key idea in
industry today. Not only can aluminum be used in the
production of buildings, but when it comes time for those
buildings to be demolished nearly all of the aluminum used
can be submitted for recycling and reused in further building
and construction applications.
A crucial concern when using this recycled aluminum in
construction is whether or not it is strong enough to
withstand stresses as a part of a building application. When
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recycled aluminum is taken recovered from building
demolition, it is split into two categories. Nearly equal
amounts of sheet products and extruded shapes, which
correspond to 5xxx and 6xxx alloys respectively, remain
after aluminum is recovered [7]. If these all alloys are
presorted during demolition, composites can be easily made
and directly reused in building and construction applications
without a significant loss of structural strength [7]. This is
of importance because it demonstrates the valuable benefits
of the use, and subsequent reuse, of aluminum in building
and construction applications. By simply sorting different
aluminum alloys in an appropriate manner, it is possible to
directly reapply the material to other building and
construction applications without much complication. This
is a major advantage over the use of unrecycled material,
which requires more complicated and arduous production.
The actual applications for recycled aluminum vary
greatly. While recycled aluminum typically is not the main
component of a structure, it still plays a major role in
building and construction. For instance the main structure of
a bridge may be made of steel, but, as suggested by the
Aluminum Extruders Council, recycled aluminum can be
effectively used for the deck of the bridge [8]. This can be
seen with the Corbin Bridge located in Pennsylvania, which
originally had a deck made out of steel and timber. After
being refurbished with an extruded aluminum deck, the
carrying capacity of the bridge increased from seven to
twenty-two tons [8]. This shows that while aluminum may
not be the main element of this bridge, its use in the deck of
the bridge can still have a major impact by improving the
strength of the structure. The fact that recycled aluminum
has the ability to improve facets of infrastructure while
exhibiting green characteristics is a major reason why its
widespread use would be beneficial.
Another prime example of the use of recycled aluminum
in building and construction is the Mirabella Portland, a
retirement community located in Portland, Oregon.
Aluminum was used for railing, trellis, and canopy parts,
reducing the risk for onsite injury as well as increasing the
speed of the instillation [8]. The project earned Leadership
in Energy and Environmental Design (LEED) Platinum
certification, largely due to the use of 25-percent recycled
aluminum [8]. The Mirabella Portland demonstrates the
myriad of possible uses for recycled aluminum in building
and construction design in addition to making construction
safer and more efficient. The recognition of LEED, a highly
respected organization, legitimizes the effectiveness and
positive impact of recycled aluminum in building and
construction.
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MILITARY APPLICATIONS
Recycled materials, specifically aluminum, have a place in
military usage as well. The military has already shown a
willingness to make the switch to recycled materials in order
to improve efficiency and performance. This is
demonstrated by plans to construct two railroad bridges
made out of nearly one hundred percent recycled material at
Fort Eustis, Virginia, the home of the US Army
Transportation Corps [9]. These bridges are set to be built
using a material known as Recycled Structural Composite,
or RSC, more quickly and less expensively than with
materials such as concrete or steel [9]. The usage of
recycled composites by the military is a very encouraging
sign. By using RSC in construction, the military is
recognizing the importance and effectiveness of recycled
materials in construction applications and showing openness
to further use. Perhaps most importantly, the use of recycled
materials in the military is a sign to other sectors of industry
that recycled materials can be just as effective as, if not
superior, to their non-recycled counterparts. This
reassurance from an entity as trusted and respected such as
that of the US military can be a key factor to the further
incorporation of recycled materials in society today.
Aluminum has proven time and time again to be of great
use in US military applications. In fact, its use goes back as
far as 1892, when it was a key component in military
torpedo boats [10]. An important military use for aluminum
is for armor, specifically on combat vehicles. Aluminum
offers advantages over other materials like steel for this
application because of its light weight, rigidity, and
temperature tolerance [10]. The use of aluminum in many
different types of vehicles is actually quite common.
Aluminum is used in the production of everything from
general purpose vehicles to amphibious combat vehicles
[10]. Almost everything on these vehicles is made at least in
part with aluminum, including the wheels, frame, and turbo
shaft engines [10].
Another military application of aluminum is in the
production of firearms and projectiles. According to
“Aluminum Alloys in Vehicles and Equipment,” aluminum
has been used to make ammunition and torpedo shells, as
well as components in the production of firearms, such as
the M-16, M-60 machine gun, and the M-72 recoilless rocket
launder [10]. Aluminum’s low cost, as well as light weight,
are the major advantages for these particular applications.
The low weight of aluminum is especially important for this
application because for soldiers the reduction of carrying
weight can make a big difference. Much of the aluminum
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used in military applications comes from the 5xxx alloy
[10]. As stated previously, the 5xxx alloy can be made
almost entirely from recycled materials without losing
anything in the way of strength or other properties. Because
all of the ways aluminum can be used in military
applications, an increase in recycled aluminum utilizations
would have quite an impact in increasing efficiency,
reducing cost, and improving renewability for the US
military and the country as a whole.
ADVANTAGES OF ALUMINUM OVER COMMON
CONSTRUCTION MATERIALS
Naturally, every material used in construction has a specific
task: concrete makes a great foundation, fiberglass makes for
great insulation. However, there are many structural aspects
of construction can be replaced by better materials. Just as
steel replaced timber as the main structural aspect of modern
buildings, aluminum can replace multiple facets of
architecture. As stated in an article on the website for The
Metal Initiative, aluminum is very malleable and ductile,
allowing it to be formed into a multitude of profiles and
shapes [11]. This property makes aluminum easy to use as
panels, siding, and shingling. This last one, shingling, is
where aluminum truly shines (literally). Aluminum roofing
provides a large degree of radiant heat reflectivity,
regardless of the slope of the roof. This ability to reflect
solar light results in less heat transfer to the building during
the summer, lowering energy costs to keep the building cool
[11]. Additionally, aluminum panels require fewer joints
than other paneling materials, providing for quick and
economical installation. Attaching aluminum panels to a
building is one of the quickest parts of building construction,
and one of the most important for the building to be energy
efficient and aesthetically pleasing.
Aluminum is not limited to being used as simple panels
in construction. Since it is so malleable, aluminum can be
used as lightweight aesthetic portions of buildings with, as
will be discussed later, very little maintenance work needed.
Due to its relatively low weight, aluminum can also be
retrofitted onto older buildings, either for functionality,
replacement necessity, or even just for the sake of the
building looking good [11]. Additionally, aluminum alloys
are very corrosion-resistant. For the majority of cases in
which aluminum is used in construction, aluminum does not
need protective finishes, unlike other common materials
such as steel or copper.
Recycled aluminum can also be used in applications
where it improves energy efficiency. Because of its high
strength to weight ratio, as shown in Figure 2, ability to
accept high-grade paint finish, and exceptional heat
qualities, extruded aluminum is an ideal material for the
panels of high-rise buildings [8]. Windows made of
FIGURE 2
STRENGTH TO WEIGHT RATIOS OF VARIOUS METALS [14].
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aluminum are also highly efficient due to favorable heating
properties. Once again, aluminum is shown to be a versatile
material with many favorable construction properties.
Perhaps most importantly, recycled aluminum is a
material that is able to perform at a high level with minimal
maintenance, a key factor when looking at building and
construction applications. Aluminum does not absorb
moisture, cannot rust, and its properties are unaffected by
weather and time [8]. In the case of aluminum windows
with a weather seal, the only maintenance required is a
cleaning with a wet cloth to maintain aesthetic value.
The ability for aluminum to be used in so many different
applications certainly makes an increased focus in the
production and usage of recycled aluminum attractive. Not
only does it have the potential to be energy efficient and
make industry more green; it also has the ability to be
extremely efficient and cost effective, an important concern
in industry today.
USES IN BUSINESS
When making the decision on what materials to use in
building and construction applications many factors play a
role. Everything from environmental factors to the visual
appeal of a material factors into the decision. However, with
today’s economic climate, the financial aspect of a project
moves to the top of the list in importance.
Firstly, the production of recycled aluminum is much
more energy efficient than making aluminum from scratch.
As previously stated in this paper and further confirmed by
The Aluminum Association, it takes 95 percent less energy to
produce aluminum from recycled materials than it does to
produce it from aluminum ore in bauxite [12]. The cost
benefit of using recycled material above production of new
aluminum is quite clear based on this data; the less energy
put into a process the more money is saved.
The cost effectiveness of recycled aluminum is favorable
when compared to other recycled materials as well. One ton
of recycled aluminum can lead to $1,000 in revenue, which
is five times more than that of steel [12]. This revenue is also
compounded with the energy – and money – saved in
recycling aluminum compared to other materials, making
aluminum even more economically beneficial. Such extreme
economic benefits are hard to ignore. Not only is the use of
recycled aluminum environmentally friendly, but it is able to
be extremely monetarily beneficial as well.
Aluminum production would not only be favorable
because of the energy efficiency, but also because of the
percent of the metals markets that it occupies. As discussed
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in “Recyclable Aluminum Rolled Products: Building Blocks
for a Sustainable World”, by John Green and Michael
Skillingberg, production of aluminum takes up close to 50%
of the metals market in North America [13]. To increase the
widespread use of aluminum would lead to a boom in an
already affluent industry, creating more jobs, better jobs, and
more profit. 60% of the aluminum production (that’s 30% of
the total market), comes from the recycling of aluminum.
Traditionally in North America, the trend has been an
increase in the tendency to rely on mostly recycled or
imported material [13]. Recycling aluminum would increase
the recycling aspect of this tendency, helping to reduce the
reliance on imported material
In fact, the importance of recycled aluminum is not just
limited to the scope of one particular corporation of industry;
it has a large impact on the American economy as a whole.
It is the largest component of domestic metal supply and
minimizes the need for imported metals. An increase in
recycled aluminum can collection of just one percent
corresponds to savings of twelve million dollars.
Additionally, recycling aluminum aids in the recycling of
other materials. Aluminum is one of the few products that,
when recycled, generates profit. Steel, glass, plastics, and
paper all have an average collection cost of $200 per ton,
and resale of the newly recycled products is nowhere close
to covering the collection cost [12]. Luckily though the
recycling of aluminum is very profitable, as previously
mentioned, and can often make up for the loss in revenue of
recycling the other products. If a company intends on
recycling paper, steel, and plastics, it can easily keep
producing profits and stay in the black by recycling
aluminum.
It is very clear that the financial benefits of recycled
aluminum are astronomical and are one of the main reasons
it is a process that should be looked into further and
expanded.
SUMMARY
The most prevalent method of recycling aluminum was
outlined in order to demonstrate the ease at which it is done.
In addition to basic smelting, other processes were
illustrated. The salt flux is added to molten aluminum to
keep the molten aluminum free of oxides. Dross and saltcake
are collected after the fact to maximize the amount of
aluminum collected from the process. The result is
aluminum which is nearly identical to the original virgin
product.
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The use of aluminum in industry was outlined in order to
show its versatility. Aluminum can be used in construction
for roofing, siding, and basic exterior or interior components
of buildings and other structures. The advantages of
aluminum over other various construction materials were
also outlined to further show its versatility. Finally, the cost
effectiveness of aluminum was discussed in order to show
the role recycled aluminum has in business. Recycling of
aluminum is profitable enough to keep all of the recycling
industry afloat in a down market and still allow corporations
which rely on aluminum to keep a profit.
CONCLUSION
It is the purpose of civil engineers to solve the problems that
the world will present. The two largest problems facing the
Earth as a whole are depletion of natural resources and the
struggle for the Earth to keep up with a constantly increasing
amount of human beings. As the human population
continues to increase, a greater need for improved
infrastructure will be highlighted. It is the job of engineers of
all disciplines, especially those in the fields of civil
engineering and materials engineering, to make these
improvements happen. This problem must be solved with
methods that reduce the depletion of our natural resources.
The answer to both of these issues can be found in the field
of recycled aluminum thanks to its value as a sustainable
product and its assets as a solid construction material. This
should, if nothing else, stress the importance of recycled
aluminum
in
modern
infrastructure.
.
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REFERENCES
ACKNOWLEDGEMENTS
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University of Pittsburgh
Swanson School of Engineering
April 14, 2012