Materials and Design 160 (2018) 468–474
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Materials and Design
journal homepage: www.elsevier.com/locate/matdes
3D printing of asphalt and its effect on mechanical properties
Richard J. Jacksona,b, Adam Wojcika, Mark Miodownika,b,*
aMechanical Engineering Dept., UCL, London, UK
bInstitute of Making, UCL, London, UK
•We have created a technique to 3D
print asphalt — we believe it is the
ﬁrst of its kind.
•3D printed asphalt is more ductile
than cast asphalt.
•The changes in mechanical proper-
ties are related to the microstructural
changes in asphalt that occur during
•The mechanical properties of 3D
printed asphalt depend on process
conditions, this can be advantageous
allowing toughness to be tailored to
•The technique has the potential to
be used on autonomous vehicles or
drones to autonomously repair roads
and complex infrastructure.
Received 3 May 2018
Received in revised form 13 September 2018
Accepted 14 September 2018
Available online 20 September 2018
The paper describes work to design, build and test an asphalt 3D printer. The main diﬃculty encountered
is that asphalt behaves as a non-Newtonian liquid when moving through the extruder. Thus, the rheology
and pressure in relation to set temperature and other operational parameters showed highly non-linear
behaviour and made control of the extrusion process diﬃcult. This diﬃculty was overcome through an inno-
vative extruder design enabling 3D printing of asphalt at a variety of temperatures and process conditions.
We demonstrate the ability to extrude asphalt into complex geometries, and to repair cracks. The mechani-
cal properties of 3D printed asphalt are compared with cast asphalt over a range of process conditions. The
3D printed asphalt has different properties from cast, being signiﬁcantly more ductile under a deﬁned range
of process conditions. In particular, the enhanced mechanical properties are a function of process tempera-
ture and we believe this is due to microstructural changes in the asphalt resulting in crack-bridging ﬁbres
that increase toughness. The advantages and opportunities of using 3D printed asphalt to repair cracks and
potholes in roads are discussed.
© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
Asphalt (bitumen) composites are the most common material
used to surface roads, with 95% of UK roads paved with asphalt
E-mail address: M.firstname.lastname@example.org.
mixtures . Its success is due to a combination of factors that
have been widely studied: it creates a safe and robust road surface
for driving when combined with stone aggregates and appropri-
ate polymer binders [2,3]; road surfacing can be carried out rapidly
and without complex machinery; it has good acoustic properties
and so muﬄes the sounds of traﬃc ; it is robust, repairable and
indeed self-repairs [5-7]. However asphalt composites do degrade
0264-1275/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
R. Jackson et al. / Materials and Design 160 (2018) 468–474 469
over time due to the effects of road usage, oxidation, loss of volatiles,
moisture damage, and various other factors. This degradation leads
to increased stiffness of the road surface, cracks forming, stripping,
ravelling, loss of aggregate, and development of pot-holes [8,9].
Despite a stipulated minimum lifetime of 40 years, the re-
surfacing of roads is estimated to cost £2 billion per year in the UK
alone . Increasing the life of roads has the potential to reduce
environmental and ﬁnancial costs associated with road closures and
the congestion they cause. One approach taken to increase the life
of asphalt roads has been to enhance their self-healing properties.
For instance, mixing ferrous ﬁbres into the asphalt composite allows
the material to be heated by induction by the application of an alter-
nating magnetic ﬁeld . The heating of the ﬁbres locally heats the
asphalt and this has been shown in the laboratory to heal micro-
cracks and restore the strength of the road, as well as de-icing it
. There is currently an on-going trial being carried out in the
Netherlands in which a road section has been surfaced with such a
material and receives the heating through regular applications of a
magnetic ﬁeld via a specially adapted vehicle . Other approaches
to preventing road surface degradation are the inclusion of micro-
capsules of sunﬂower oil into the asphalt composite which burst
open in the presence of a crack and increase the ﬂuidity of the asphalt
allowing it to reﬂow and heal the crack .
Environmental considerations have led to much interest in the
use of recycled materials such as rubber and plastic in asphalt
composites [15,16]. These materials are often inexpensive, being cur-
rently considered as waste, but the asphalt mixing phase is energy
intensive [17,18] and the cost of transport is also a factor . So
when looking to increase the environmental sustainability of infras-
tructure maintenance, materials and repair processes should ideally
be optimised together. In the future, many of these demands could
be met by autonomous vehicles which repair locally on demand .
In the case of asphalt roads, this optimised preventative approach
to road maintenance would need to focus on the early stages of
road degradation when small cracks form on the road surface. These
cracks allow water ingress and grow rapidly during freeze-thaw
cycles through the de-bonding of aggregates to form potholes. Once
formed it is very hard to stop the growth of these potholes which
cause signiﬁcant vehicle damage and so lead to shortening of the
usable lifetime of the road.
This paper describes work to produce a 3D printing technology
that could be attached to an autonomous vehicle or drone, and used
to repair small cracks before they turn into potholes. 3D printing is a
method by which objects can be fabricated layer by layer from a CAD
model of the object. By scanning a road surface the negative shape of
the crack can be obtained and processed into a 3D model . This
information can then be processed and passed to a 3D printer, which
can then print exactly the correct amount of material to conform to
the crack shape and volume, thus repairing the crack. 3D printing
technology has previously been used to repair spall damage in con-
crete road surfaces . We show for the ﬁrst time that it is also
possible to 3D print asphalt into a crack to restore the road surface.
The focus of this paper is a description of the design and operation of
our asphalt 3D printer, a demonstration of its ability to repair cracks,
and an investigation of the mechanical properties of the 3D printed
2. Materials & methods
The 3D printer is designed as a three axis system in which the
extrusion nozzle is moved by individual stepper motors to print
onto a ﬂat bed, see Fig. 1 (a). The printer nozzle consists of an auger
screw, a stepper motor to drive the screw, and a pellet hopper to
take asphalt in the form of pellets. The pellets are softened as they
travel through the auger screw by an increase in temperature due to
the action of heating resistors, this results in a ﬂuid ﬂow of asphalt
out of the nozzle, as illustrated in Fig. 1 (b). The stepper motors,
temperature, temperature gradient, and auger screw rotation rate
are controlled by simple electronics interfaced to a PC, shown
schematically in Fig. 1 (c).
The 3D printer was constructed using an existing frame and
control system from a RepRap Mendel 90 3D printer, see Fig. 2
(a). The extrusion nozzle assembly was 3D printed using a Form 2
Stereolithography 3D printer using the Formlabs proprietary high
temperature acrylic based resin which allowed precise control of
the complex geometry of the extruder housing and auger screw
(attached to the stepper motor shaft via 5 mm grub screw), see
Fig. 2 (b). The extruder assembly had an inner heating sleeve
made from a 1 mm thick, 20 mm outer diameter aluminium pipe,
with the 15 W 20 YCaddock MP915 series TO-126 power resistors
attached radially, spaced 120◦apart, and connected in parallel. These
were attached to the outer part of the pipe with MG Chemicals
two-part silver epoxy/cold solder. The temperature was measured
using a 100 k EPCOS B57550G1104F thermistor. The thermistor was
attached to the aluminium pipe by drilling a 0.5 mm deep indenta-
tion into the pipe with a 2 mm drill bit approximately 5 mm away
from one of the power resistor contact points, and similarly ﬁxed
with silver epoxy. Heat conduction through the aluminium pipe to
the extrusion tip proved to be insuﬃcient to control the asphalt tem-
perature quickly and accurately enough and so a metal nozzle cap
was employed to improve heat conduction to the asphalt. This was
initially made from CNC machined aluminium but the same results
were obtained by concentrically stacking M8, M4 and M2 stainless
steel washers at the nozzle tip and ﬁxing to each other and the pipe
with silver epoxy, see Fig. 2 (b). Conventional fused deposition mod-
eling (FDM) via another, unmodiﬁed Mendel 90 RepRap printer was
used to print the stepper motor housing and PCB and serial port clip
in ABS plastic. Fig. 2 (c) shows a photo of the extrusion nozzle.
By selecting the hardest grade of bitumen, 10/20, we hoped to
match the in-use material properties as much as possible . The
asphalt pellets were formed from larger pieces of asphalt (Bitumen,
CAS 64742-93-4, 10/20 grade, material and data sheet supplied by
IKO PLC, UK ) by low temperature casting (below 150 ◦C) into
a machined mould to obtain millimeter scale pellets. Asphalt is a
substance made principally of long-chain hydrocarbon molecules in
a colloidal structure of aphaltenes and maltenes with complex rhe-
ological properties . Above a threshold temperature, typically
between the range of 30–70 ◦C, it behaves as a Newtonian ﬂuid.
Below this threshold, it undergoes shear-thinning. The rheology in
this regime has been studied in detail and shows an Arrhenius-like
behaviour . Much work has been done to study the effect on rhe-
ology of polymeric binders that are added to asphalt, these behave as
viscoelastic components . In the face of this complexity, we chose
not to try to model the regime of rheology under different shear
stresses as the asphalt pellets travelled through our extruder, but
instead we aimed to ﬁnd the optimum processing variables by carry-
ing out a systematic empirical investigation of the extrusion process.
We identiﬁed the important design parameters of the nozzle
through a number of design iterations, see Fig. 3 (a). Using infrared
cameras to identify thermal gradients allowed us to understand the
heat ﬂow within the extruder and so iterate the design towards
optimum parameters. The power limit (P= 45 W) and torque limit
(T= 44N cm) of the three power resistors and stepper motor respec-
tively, were self-imposed design constraints. A number of auger
screw designs were fabricated and systematically tested within the
printer framework. Chamber height, in combination with screw
height and metal insert height was found to be important as an
interim asphalt softening area was needed in between the hop-
per and screw, otherwise the screw would stall, or break, trying
to extrude asphalt which was too viscous. For this reason, the
ﬁnal design has zero unheated chamber height. We found that for
temperatures above 150 ◦C, the asphalt was so ﬂuid it ﬂowed out of
470 R. Jackson et al. / Materials and Design 160 (2018) 468–474
Fig. 1. System design: (a) 3D printer, (b) the extruder design, (c) the control electronics.
Fig. 2. Experimental: (a) photo of whole system, (b) photo of extruder components, (c) photo of complete extruder.
the extruder aperture under no screw rotation so was not useful for
The temperature range of 100–140 ◦C was explored to print
a range of test objects. Screw length and pitch affect the extru-
sion rate for a given rotations per minute (RPM) but this was not
investigated. Various extrusion diameters from 0.5 to 5 mm were
used with 2.5 mm found to give the best balance between relia-
bility and dimensional accuracy. Print failures were identiﬁed by
incomplete or poor ﬁrst layer adhesion, intermittent layer bonding,
incomplete printing, and partially hollow objects. Our ﬁnal design of
the extrusion nozzle is shown in Fig. 3 (b) with the following param-
eters: the chamber diameter (dc= 17.5 mm), metal insert height
(hi= 47.5 mm), the tip height (ht=2.5 mm), the unheated chamber
height (hu= 0mm), the extrusion diameter (de= 2.5mm), the auger
screw length (ls= 21 mm), pitch (ps= 7 mm) and the total thermal
input area (At= 270 mm2).
Fig. 3. Design parameters: (a) CAD of extruder, (b) design parameters, (c) process parameters.
R. Jackson et al. / Materials and Design 160 (2018) 468–474 471
Using this design, we found that the optimum operational param-
eters of the extrusion nozzle were the print speed (vp= 1 mm/s), the
z offset (Zo= 3 mm), the layer height (lh= 3mm), the auger screw
rotation speed (Ws= 5–10 RPM), the set temperature (tset = 125–
135 ◦C), see Fig. 3 (c).
We used open access Pronterface control software (version 3) 
and Slic3r slicing software  to generate g-code and print via stan-
dard STL ﬁles designed using the 3D CAD software Sketchup . The
Slic3r software is intended for use with fused deposition modelling
(FDM) printers which heat a ﬁlament of deﬁned diameter through
the heated extruder hole. As such, the ﬁlament diameter setting was
used as the extruder chamber diameter to represent the width of the
asphalt, however this is not completely accurate as the auger screw
takes up a signiﬁcant proportion of the chamber volume. Further-
more, the Slic3r “extrusion multiplier” setting is intended to increase
or decrease the ﬁlament feed speed to the extruder, whereas in our
case the ﬁlament feed motor was used as the auger screw motor
(100×extruder multiplier was found to be equivalent to 4.4 RPM).
An optimisation process was used to set these heat and print speed
A number of different shapes were printed including standard
mechanical test bars with dimensions 80 ×10 ×6 mm. These were
mechanically tested at room temperature measured as 22 ◦C using
a three point bend test rig on a Hounsﬁeld HK5 universal testing
machine one week after printing or moulding. The tests were carried
out with a 2500 N load cell at constant strain rate of 1 mm/min with
support bars spaced 25 mm apart. For each test condition, six sam-
ples were tested and the data aggregated. A number of cast asphalt
samples were also produced to compare with the 3D printed sam-
ples. These were cast into PDMS moulds with the same dimensions
as the mechanical test bars at 150 ◦C. The time between casting, 3D
printing and mechanical testing was 48 h.
Firstly, basic 3D printing experiments were carried out in which
three single lines of asphalt of length 100 mm were printed with
1 mm layer height using an aperture width of 2 mm. These exper-
iments were performed using a range of print-head temperatures
between 100 and 150 ◦C. Temperatures between 125 ◦C and 135 ◦C
were found to be optimal to create a continuous extrusion of asphalt
with a consistent line width. The rotation speed of the auger screw,
Ws, was found to be an important variable that determined the line
thickness. Fig. 4 shows the effect of Wson the line thickness of the
3D printed asphalt at 125 ◦C. Line printing was less successful and
reliable at lower rotation speeds (approximately 2–7 RPM) although
this was not an issue when printing objects, as subsequent layer
deposition and adhesion after the ﬁrst layer was helped by (and was
reliant on) the initial layer. Nonetheless above 5 RPM we were able
to reliably print single lines.
Fig. 5 (a) shows the ability of the 3D printer to fabricate an object
from a digital CAD ﬁle through the layering of lines of asphalt, in this
case the shape is a pyramid, but this technique is general, with the
same object geometry limitations as standard polymer prints. The
print layers are clearly visible and approximately 1 mm in height.
Some ﬂow and bleeding of the asphalt is visible which affects the
feature resolution of the object. Fig. 5 (b) shows the ability of the
system to 3D print a mechanical test sample of known proportions
for mechanical testing. A number of such samples were printed in
which a number of geometry and process variables were varied.
The temperature range of 125–135◦C at 1 mm/s print speed and
4.4 RPM (100×extrusion multiplier) gave the most accurate and
reliable prints in terms of desired dimensions and the success of
printing a fully formed object. These test samples were then mechan-
ically tested using a 3 point bend test together with cast asphalt test
Fig. 4. Results: Graph of effect of screw rotation speed on printed line thickness.
sample of identical dimensions. Fig. 5 (c) shows the ability of the
system to take as an input the inverted shape of a crack in an asphalt
sample and 3D print asphalt into the crack to ﬁll it.
Fig. 6 shows the stress/strain curves obtained from cast asphalt
and compares them to those from 3D printed asphalt at three dif-
ferent printing temperatures. It can be seen that the mechanical
properties are markedly different for the two fabrication methods.
For the cast samples there was, as expected, anisotropy observed
between those samples tested with their bottom or top surfaces
under compression, see Fig. 6 (a). The former showed a classic brit-
tle fracture while the latter showed some ductile behaviour before
fracture, see Fig. 6 (d). This anisotropy is likely due to the differences
in their surface roughnesses, porosity, and volatile content between
the top and bottom of the sample. There were no differences seen in
testing the 3D printed samples from top or bottom. Both sets of sam-
ples showed similar fracture strengths, see Fig. 6 (c). The 3D printed
specimens showed up to nine times the ductility of cast samples but
had similar fracture strengths of around 2 MPa, see Fig. 6 (b). The
toughness of the moulded and 130 ◦C printed samples were found
to be 10.2 ±7.1 and 24.1 ±7.2 J/cm2respectively.
The effect of extrusion temperature on mechanical properties is
small although there is some evidence that 130 ◦C is optimal, see
Fig. 6 (a). Observation of the fracture surfaces provides some expla-
nation for the difference in ductility between the cast asphalt and
the 3D printed asphalt. When the printed samples were cracked,
a brown substance was revealed which was dotted throughout the
sample cross section, that in many cases stretched out to bridge the
crack, see Fig. 7. This brown phase and crack bridging effect were not
observed in the cast test samples. Using X-ray photoelectron spec-
troscopy (XPS), the elemental composition of the brown phase was
compared to the bulk. No signiﬁcant differences were found in the
composition, both being hydrocarbons with trace amounts of silicon
4. Discussion & implications for design
We have successfully managed to design, build and test an asphalt
3D printer capable of printing small objects and repairing cracks
in asphalt. Since 3D printers are a mature technology this might
not seem remarkable, nevertheless it was not an easy task. The
main diﬃculty we encountered is that asphalt behaves as a rela-
tively low melting point non-Newtonian liquid when the material
is moving through the extruder as it is heated up, and then in
472 R. Jackson et al. / Materials and Design 160 (2018) 468–474
Fig. 5. Results: (a) photo of printed pyramid, (b) photo of three point bend test sample, (c) photo of moulded crack before (left) and after print ﬁll (right).
between the extruder tip and deposition surface, as it cools down.
Although polymers used in ﬁlament-fed 3D printers are generally
non-Newtonian too, their simpler extruder system makes ﬂow con-
trol much easier. Flow through our auger screw extruder created a
more complicated regime of rheology and pressure in relation to set
temperature and other operational parameters which showed highly
non-linear behaviour and made control of the extrusion process dif-
ﬁcult. The functional constraints of some of the process variables
affected our ability to print, for instance, the rotation speed of the
auger screw is linked to the print speed (the extrusion multiplier
is programmed to double the rotation speed if the print speed is
doubled in order to deposit material at the same rate). The print
speed was also limited by the materials properties of the auger screw
(we used the high temperature SLA resin), since the low fracture
strength of this resin limited the torque we could apply. The aperture
affects the resolution of the printer, but again, low fracture strength
of SLA resin limited our ability to reduce aperture size since it led
to high pressures and resulted in mechanical failure. It is hoped that
Fig. 6. Mechanical properties: 3 point bend tests. (a) Cast pieces average, tested from top (CT) and bottom (CB) compared to print averages at each temperature. (b) Comparison
of all cast and printed pieces. (c) Stress at fracture comparison. (d) Elongation at break comparison.
R. Jackson et al. / Materials and Design 160 (2018) 468–474 473
Fig. 7. Crack bridging: (a) bridging in printed test samples, (b) closeup, parallel to sample axis shows ﬁbrous crack bridging characteristic of ductile fracture, (c) 20×magniﬁcation
showing concentration of oily material throughout print layers. (For interpretation of the references to color in this ﬁgure, the reader is referred to the web version of this article.)
future designs with metal parts will allow us to explore a greater
range of extrusion rates and print resolutions.
The impact of 3D printing on mechanical properties is interest-
ing because it allows us to print a more ductile asphalt. There is a
signiﬁcant increase (up to 900%) in elongation to fracture for the
printed samples. A possible explanation of this increased ductility
lies with the appearance of a crack bridging component in the sam-
ples. It is hypothesised that the brown phase precipitated throughout
the sample is composed of a lighter saturated fraction of the asphalt
that has coalesced due to size dependent mobility conditions during
the heating, screw mixing and/or extrusion process. Small amounts
of softer components coalesce naturally in asphalt, but usually at
scales of around 1–10 lm. Here, the components are around
20–100 lm in diameter. This means that the 3D printing process at
this scale seems to avoid the degradation of its mechanical proper-
ties that can arise from leaving molten asphalt static for a long time
. The 3D printing process seems to create a composite struc-
ture comprising of large concentrations of the brown phase dotted
throughout the asphalt (as seen in Fig. 7 (c)), giving the material
a higher toughness than cast asphalt. This would be advantageous
to any crack repair scenario since sites of cracks on roads are often
areas of increased stress or wear, and so depositing material with
enhanced ductility could prolong the life of the repair.
Printed asphalt extruded at less than 120 ◦C often did not have
suﬃcient inter-layer bonding to avoid delamination, so were most
likely not fully bonded throughout the bulk of the material. Con-
versely, prints above 135 ◦C gave poor dimensional accuracy due
to the lower viscosity of the hotter deposited asphalt, as well as
showing mechanical properties similar to cast samples. This suggests
that there is a point between delamination and complete intermelt-
ing that gives superior mechanical properties, we found this to be
the optimum print temperature of 130 ◦C. Commonly printed poly-
mers such as ABS and PLA have lower speciﬁc heat capacities than
asphalt (1.8, 1.3, and 2.1 kJ/kg/◦C respectively) which may be the
reason why the initial printing temperature may occasionally be too
high as the print progresses due to heat build up in the printed
asphalt, giving melted edges and/or poor accuracy, and particularly
a “squashed” appearance with too low object height and too high
object width and length. Over the print time (approximately 20 min
for the test bar) this would be enough to deform layers previously
printed due to the excess heat in the upper layers. Although the
printer bed is heated for conventional polymer printing we did not
employ this method nor did we use a cooling fan normally used
with polymer 3D Printing, which may have exacerbated the issue.
With the correct parameters however, we can modulate proper-
ties through changing print temperature fairly rapidly over a small
10–15 ◦C range. Furthermore, the feed and pellet system make
it relatively straightforward to add other materials such as small
microaggregates or nanomaterials (initial test prints with 10%10 nm
diameter titanium dioxide nanoparticles have been successful), and
then vary the composition of the feedstock during printing to cre-
ate more complex, functionally graded infrastructure materials with
a wider range of properties.
We believe these improved and tunable material properties of 3D
printed asphalt, combined with the ﬂexibility and eﬃciency of the
printing platform, offers a compelling new approach not just to the
maintenance of road infrastructure, but by attaching it to a drone,
opens up a new way to repair hard to access structures such as the
ﬂat roofs of buildings and other complex structures. The advantage
of this is not only in being able to cut costs – other repair methods
often requiring the erection of scaffolding and the closure or shut-
ting down of infrastructure to gain access – but also the repair can
initiated earlier before large scale deterioration has occurred. The
development of such repair drones would have implications both
for the way in which city infrastructure is repaired but also for the
economic model that underpins it. At the moment, much of city
infrastructure is built to fail and then be replaced, with the capital
costs of construction dominating the design parameters. Infrastruc-
ture designed to be continually monitored and repaired by ﬂeet of
drones promises to be a different model which could have economic
beneﬁts to society.
For instance, such an approach has the potential to be used for
roads. If road degradation is continually monitored then small cracks
can be repaired before they turn into potholes. By intervening at this
early stage and repairing the crack autonomously using 3D print-
ing we believe the road surface might be preserved for longer. Such
approaches have been explored for concrete road surfaces for spall
damage repair . We have already demonstrated the 3D printing
of asphalt using a drone. The next stage of developing this technology
involves understanding the effect of environmental variables such
as road temperature, air temperature, the local chemistry, interface
with aggregate, as well as more comprehensive testing such as cyclic
loading of repaired crack roads.
The materials science of repair is not the only consideration in
the application of this technology. Identiﬁcation and detection of
crack morphology, especially in the case of complex-shaped cracks
will be an important challenge. Automated computer vision systems
are currently being explored to address this issue . The use of
gantry systems versus the employment of 6-axis robotic systems
is another design issue that is pertinent in the area of automated
construction and repair [32,33]. Although 6-axis systems have more
ﬂexibility; for the repair of sub-cm small cracks in a horizontal
road surface, a simple gantry system, such as ours, may well prove
474 R. Jackson et al. / Materials and Design 160 (2018) 468–474
We have designed and built 3D printer capable of printing
asphalt. We have shown that this technology can be used to 3D print
asphalt into complex geometries, and to repair cracks. The mechan-
ical properties of 3D printed asphalt are different from cast asphalt,
showing up to nine times the ductility of cast samples with similar
fracture strengths. The increased ductility is due to microstructural
changes in the asphalt which result in crack-bridging ﬁbres that
increase toughness. The material properties of 3D printed asphalt are
tunable, and combined with the ﬂexibility and eﬃciency of the print-
ing platform, this technique offers a compelling new design approach
to the maintenance of infrastructure.
This work was funded by the EPSRC (Balancing the Impact of
City Infrastructure Engineering on Natural systems using Robots —
EP/N010523/1). We would like to thank Professor Quentin Pankhurst
for access to laboratory facilities, and Dr Joseph Bear at Kingston
University for the XPS analysis. We would also like to thank all
members of Self-Repairing Cities network.
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