Automated construction by contour crafting—related robotics and information technologies Behrokh Khoshnevis*

Automated construction by contour crafting—related robotics and information technologies Behrokh Khoshnevis*
Automation in Construction 13 (2004) 5 – 19
www.elsevier.com/locate/autcon
Automated construction by contour crafting—related robotics
and information technologies
Behrokh Khoshnevis *
D.J. Epstein Department of Industrial & Systems Engineering, University of Southern California, Los Angels, CA 90089-0193, USA
Abstract
Although automation has advanced in manufacturing, the growth of automation in construction has been slow. Conventional
methods of manufacturing automation do not lend themselves to construction of large structures with internal features. This may
explain the slow rate of growth in construction automation. Contour crafting (CC) is a recent layered fabrication technology that
has a great potential in automated construction of whole structures as well as subcomponents. Using this process, a single house
or a colony of houses, each with possibly a different design, may be automatically constructed in a single run, imbedded in each
house all the conduits for electrical, plumbing and air-conditioning. Our research also addresses the application of CC in
building habitats on other planets. CC will most probably be one of the very few feasible approaches for building structures on
other planets, such as Moon and Mars, which are being targeted for human colonization before the end of the new century.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Contour crafting; Housing construction; Construction information technology; Construction on other planets
1. Introduction
Since the early years of the 20th century, automation has grown and prevailed in almost all production
domains other than construction of civil structures.
Implementation of automation in the construction
domain has been slow due to: (a) unsuitability of the
available automated fabrication technologies for large
scale products, (b) conventional design approaches
that are not suitable for automation, (c) significantly
smaller ratio of production quantity/type of final products as compared with other industries, (d) limitations
in the materials that could be employed by an auto-
* Tel.: +1-213-740-4889; fax: +1-213-740-1120.
E-mail address: khoshnev@usc.edu (B. Khoshnevis).
0926-5805/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.autcon.2003.08.012
mated system, (e) economic unattractiveness of expensive automated equipment and (f) managerial issues.
On the other hand, the following are reported to be
serious problems that the construction industry is
facing today [1]:
labor efficiency is alarmingly low,
accident rate at construction sites is high,
work quality is low and
control of the construction site is insufficient and
difficult, and skilled workforce is vanishing.
Automation of various parts and products has
evolved considerably in the last two centuries but
with the exception of a few successful attempts (see
for example [2]) construction of whole structures
remains largely as a manual practice. This is because
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B. Khoshnevis / Automation in Construction 13 (2004) 5–19
the various conventional methods of manufacturing
automation do not lend themselves to construction of
large structures. A promising new automation approach is layered fabrication, generally known as
rapid prototyping (RP) or solid free-form fabrication
(SFF). Although several methods of rapid prototyping
have been developed in the last two decades [3], and
successful applications of these methods have been
reported in a large variety of domains (including
industrial tooling, medical, toy making, etc.), most
current layered fabrication methods are limited by
their ability to deliver a wide variety of materials
applicable to construction. Additionally, they are
severely constrained by the low rates of material
deposition which makes them attractive only to the
fabrication of small industrial parts. Currently, contour
crafting (CC) seems to be the only layer fabrication
technology that is uniquely applicable to construction
of large structures such as houses [5].
2. Contour crafting
CC is an additive fabrication technology that uses
computer control to exploit the superior surface-forming capability of troweling to create smooth and
accurate planar and free-form surfaces [6 –8]. Some
of the important advantages of CC compared with
other layered fabrication processes are better surface
quality, higher fabrication speed and a wider choice of
materials.
The key feature of CC is the use of two trowels,
which in effect act as two solid planar surfaces, to
create surfaces on the object being fabricated that are
Fig. 1. Simple historical construction tools.
Fig. 2. Contour crafting process.
exceptionally smooth and accurate. Artists and craftsmen have effectively used simple tools such as
trowels, blades, sculpturing knives, and putty knives,
shown in Fig. 1, with one or two planar surfaces for
forming materials in paste form since ancient times.
Their versatility and effectiveness for fabricating
complex free-form as well as planar surfaces is
evidenced by ancient ceramic containers and sculptures with intricate or complex surface geometries as
well as detailed plaster work that have shapes as
complicated as flowers, on the walls of rooms. Surface shaping knives are used today for industrial
model making (e.g., for building clay models of car
bodies). However, despite the progress in process
mechanization with computer numerical control and
robotics, the method of using these simple but powerful tools is still manual, and their use is limited to
model building and plaster work in construction.
In CC, computer control is used to take advantage
of the superior surface forming capability of troweling
to create smooth and accurate, planar and free-form
surfaces. The layering approach enables the creation
of various surface shapes using fewer different troweling tools than in traditional plaster handwork and
sculpting. It is a hybrid method that combines an
extrusion process for forming the object surfaces and
a filling process (pouring or injection) to build the
object core. As shown in Fig. 2, the extrusion nozzle
has a top and a side trowel. As the material is
extruded, the traversal of the trowels creates smooth
outer and top surfaces on the layer. The side trowel
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
7
can be deflected to create non-orthogonal surfaces.
The extrusion process builds only the outside edges
(rims) of each layer of the object. After complete
extrusion of each closed section of a given layer, if
needed filler material such as concrete can be poured
to fill the area defined by the extruded rims.
3. Application in construction
Application of CC in building construction is
depicted in Fig. 3 where a gantry system carrying
the nozzle moves on two parallel lanes installed at the
construction site. A single house or a colony of
houses, each with possibly a different design, may
be automatically constructed in a single run. Conventional structures can be built by integrating the CC
machine with a support beam picking and positioning
arm, and adobe structures, such as the ones designed
by CalEarth (www.calearth.org) and depicted in the
Fig. 4, may be built without external support elements
using shape features such as domes and vaults.
Following are some interesting aspects of this automated construction concept.
3.1. Design flexibility
The process allows architects to design structures
with functional and exotic architectural geometries
Fig. 4. Construction of adobe buildings using CC.
that are difficult to realize using the current manual
construction practice.
3.2. Multiple materials
Various materials for outside surfaces and as fillers
between surfaces may be used in CC. Also, multiple
materials that chemically react with one another may
be fed through the CC nozzle system and mixed in the
nozzle barrel immediately before deposition. The
quantity of each material may be controlled by computer and correlated to various regions of the geometry
of the structure being built. This will make possible the
construction of structures that contain varying amounts
of different compounds in different regions.
3.3. Utility conduits
As shown in Fig. 5, utility conduits may be built
into the walls of a building structure precisely as
dictated by the CAD data. Sample sections made with
CC and filled with concrete as shown in Fig. 8 demonstrate this possibility.
3.4. Paint-ready surfaces
Fig. 3. Construction of conventional buildings using CC.
The quality of surface finish in CC is controlled by
the trowel surface and is independent of the size of the
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B. Khoshnevis / Automation in Construction 13 (2004) 5–19
this configuration the CC nozzle, the steel reinforcement module feeder, and the concrete filler feeder
could all be on the same gantry system. Such a system
can create shapes with smooth outer surfaces and
reinforced internal structure automatically and in one
setup.
As an alternative to traditional metal reinforcement, other advanced materials can be used, such as
Fig. 5. Complex wall section.
nozzle orifice. Consequently, various additives such
as sand, gravel, reinforcement fiber and other applicable materials available locally may be mixed and
extruded through the CC nozzle. Regardless of the
choice of materials, the surface quality in CC is such
that no further surface preparation would be needed
for painting surfaces. Indeed, an automated painting
system may be integrated with CC.
3.5. Smart materials
Since deposition in CC is controlled by computer,
accurate amounts of selected construction materials,
such as smart concrete, may be deposited precisely in
the intended locations. This way the electric resistance, for example, of a carbon filled concrete may be
accurately set as dictated by the design. Elements such
as strain sensors, floor and wall heaters can be built
into the structure in an integrated and fully automated
manner.
3.6. Automated reinforcement
Robotic modular imbedding of steel mesh reinforcement into each layer may be devised, as shown in
Fig. 6. The three simple modular components shown
in this figure may be delivered by an automated
feeding system that deposits and assembles them
between the two rims of each layer of walls built by
CC. A three-dimensional mesh may be similarly built
for columns. Concrete may then be poured after the
rims of the wall or column are built by CC. The mesh
can follow the geometry of the structure. Note that in
Fig. 6. Reinforcement components and assembly procedures for
walls and columns.
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
9
the fiber reinforced plastics (FRP). Since the nozzle
orifice in CC does not need to be very small, it is
possible to feed glass or carbon fiber tows through the
CC nozzle to form continuous reinforcement consolidated with the matrix materials to be deposited. In the
proposed study, deposition of the FRP reinforcement
by a parallel nozzle built into the CC nozzle assembly
will also be considered. Co-extrusion is further discussed in a later section.
Reinforcement can also be provided using the
post-tensioning system. Accurate ducts can be generated by the CC process. Similar to post-tensioned
concrete construction, metal or FRP wires can be fed
through the ducts and then post-tensioned to provide
reinforcement.
3.7. Automated tiling of floors and walls
Automated tiling of floors and walls may be
integrated by robotically delivering and spreading
the material for adhesion of tiles to floors or walls,
as shown in Fig. 7. Another robotic arm can then pick
the tiles from a stack and accurately place them over
the area treated with the adhesive material. These
robotic arms may be installed on the same structure
which moves the CC nozzle.
Fig. 8. Plumbing modules and grippers.
3.8. Automated plumbing
Fig. 7. Automated tiling.
Because of its layer by layer fabrication method, a
contour crafting based construction system has the
potential to build utility conduits within walls. This
makes automated construction of plumbing and electrical networks possible. For plumbing, after fabrication of several wall layers, a segment of copper (or
other material) pipe is attached through the constructed conduit onto the lower segment already
installed. The robotics system, shown on the upper
left side of Fig. 8, delivers the new pipe segment and
in case of copper pipes has a heater element (shown in
red) in the form of a ring. The inside (or outside) rim
of each pipe segment is pretreated with a layer of
solder. The heater ring heats the connection area,
melts the solder and, once the alignment is made,
bonds the two pipe segments. Other universal passive
(requiring no active opening or closing) robotic gripper and heater mechanism designs used for various
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B. Khoshnevis / Automation in Construction 13 (2004) 5–19
plumbing components are also shown in Fig. 4. The
needed components may be pre-arranged in a tray or
magazine for easy pick up by the robotic assembly
system. Using these components various plumbing
networks may be automatically imbedded in the
structure.
3.9. Automated electrical and communication line
wiring
A modular approach similar to industrial bus-bars
may be used for automating electrical and communication line wiring in the course of constructing the
structure by contour crafting. The modules, as shown
in Fig. 9, have conductive segments for power and
communication lines imbedded in electrically nonconductive materials such as a polymer, and connect
modularly, much like the case of plumbing. All
modules are capable of being robotically fed and
connected. A simple robotics gripper can perform
the task of grabbing the component from a delivery
tray or magazine and connecting it to the specified
component already installed. The automated construction system could properly position the outside
access modules behind the corresponding openings
on the walls, as specified by the plan. The only
manual part of the process is inserting fixtures
through wall openings into the automatically constructed network.
3.10. Automated painting
During or after layer-wise construction of walls, a
spray painting robotics manipulator attached to the
CC main structure may paint each wall according to
desired specifications. The painting mechanism may
be a spray nozzle or an inkjet printer head (such as
those used for printing large billboards). The latter
mechanism makes painting wall paper or other desired
patterns possible.
Fig. 9. Electrical modules and assembly process.
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
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4. State of development
4.1. Reinforcement
Several CC machines have been developed at USC
for research on fabrication with various materials
including thermoplastics, thermosets and various
types of ceramics. These machine include a XYZ
gantry system, a nozzle assembly with three motion
control components (extrusion, rotation and trowel
deflection) and a six-axis coordinated motion control
system. The machine developed for ceramics processing is shown in Fig. 10 and is capable of extruding a
wide variety of materials including clay and concrete.
The material is extruded by means of a cylinder/piston
system shown on the left side. The figure on the right
shows the mechanism for nozzle rotation and side
trowel deflection.
We have conducted extensive experiments to optimize the CC process to produce a variety of 2.5D and
3D parts with square, convex and concave features,
some filled with concrete, as shown in Fig. 11. The
scale has been of the samples made to date (the hand
in Fig. 11 is indicative of the scale). Details of the
related research may be found in [9] and [10].
Towards improving the strength of large housing
structures built through CC, we have investigated the
use of a variety of reinforcements. For example, Fig.
12 shows pictures from our experiments with coil
reinforcement. Owing to the high extrusion pressures
prevailing in CC compared to other layered free-form
fabrication techniques [11], the extrudate thoroughly
adheres itself around the coils without causing any
internal discontinuities. Similar results have been
observed from our experiments with sand impregnation. Thus the use of reinforcements seems promising
in our CC process.
4.2. Depositions with hollow cavities
Through extensive experimentation and a series of
design enhancements, we have developed the capability to build layers with hollow depositions using
CC as shown in Fig. 13. Mandrel of various shapes
may be used for creating hollows of various shapes.
Note that these hollow cavities result in lighter struc-
Fig. 10. The CC machine for ceramic paste extrusion.
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Fig. 11. CC in operation and representative 2.5D and 3D shapes and parts filled with concrete.
tures. It is also possible to extrude reinforcement
materials, such as epoxies or various concrete based
compounds through these cavities for added strength.
5. Future research plan
Under a new grant from the National Science
Foundation, we are currently working on the develop-
ment of new nozzle assemblies that are especially
designed for full scale construction application. With
the new nozzles, we intend to first fabricate full scale
sections of various building features such as sections
of walls with conduits built in, and supportless roofs
and perform various structural analysis and testing
using a wide variety of candidate materials. The new
nozzle design, shown in Fig. 14, has the capability of
full 6 axis positioning when mounted on a XYZ gantry
Fig. 12. Reinforcement process of CC: (a) metal coil placed on a top layer, (b) a fresh layer of extrudate covers the coil and (c) cross sections of
the fabricated part with the reinforcement coil showing a reasonable adhesion between layers.
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
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Fig. 13. Laying hollow sections through CC: (a) the ceramic material in the nozzle before extrusion, (b) hollow circle formed as the extrudate
emerges from the orifice and (c) cross section of the fabricated part revealing the hollow sections.
system. and can co-extrude both outer sides and filler
materials. The design incorporates rigid double-coaxial piped for material delivery. The nozzle design also
conforms with concurrent embedding of steel reinforcement modules discussed earlier. This nozzle assembly will be capable of building a wide variety of
curved structures as designed by architects. In devising
our construction control software, we will benefit from
the ancient body of knowledge that is currently being
harnessed by CalEarth (see http://www.calearth.org)
for building supportless closed structures. An example
of a clever and ancient manual method of constructing
such supportless structures is shown in Fig. 15 for
construction of domes and vaults. Our corresponding
deposition pattern, inspired by these ancient methods,
could be such as the one schematically illustrated in
Fig. 14. Six axis nozzle design for concurrent rim and filler material delivery and conformance to reinforcement imbedding.
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Fig. 15. Manual construction of adobe structures using bricks (source: Khalili, 2000 [4]).
Fig. 16 to be made possible with the new CC nozzle
which provides the maximum positioning flexibility.
We will analytically evaluate the performance characteristics of various deposition patterns and implement
the most desirable approaches.
5.1. Alternative robotics approach
The process was depicted in Figs. 3 and 4 as using
a gantry robot that has to be large enough to build an
entire house within its operating envelope and lays
one continuous bead for each layer. Such an approach
is not without its attractions, but it requires a large
amount of site preparation and a large robot structure.
An approach involving the coordinated action of
multiple mobile robots is to be preferred. The mobile
robotics approach depicted in Fig. 17 has several
advantages including ease of transportation and setup,
the possibility of concurrent construction where mul-
tiple robots work on various sections of the structure
to be constructed (as illustrated in Fig. 14), the
possibility of scalable deployment (in number) of
equipment and the possibility of construction of
structures with unlimited foot print.
A construction mobile robot may use a conventional joint structure and be equipped with material
tanks as well as material delivery pump and pipes.
The end effector of the robot could carry a CC nozzle
that can reach from ground level all the way to the top
of a wall. If the mobile robot arm could be made of a
rigid structure, position sensing at the end effector
may not be necessary. Instead, a position sensor (e.g.,
a laser tracker) may be mounted at a fixed location,
and the related retroreflectors may be installed on each
mobile robot base. In this configuration, the robot
does not engage in fabrication while moving. Once it
reaches a pre defined post (called mobile platform
post), it anchors itself by extending some solid rods
Fig.16. CC approach to fabricate supportless structures.
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
15
material tank and a special CC nozzle for roof
material delivery.
5.2. Related information technology research
Fig. 16 represents the IT components in our future
research directed at mobile robotics application in
construction by CC. The diagram depicts a planning
system, the output of which we plan to feed into a
virtual system (simulation and animation), and eventually to a real system, once the required hardware
becomes available. When connected to a hardware
system, the proposed planning system will receive
feedback at the implementation stage. Following is a
brief explanation of each of the components shown in
Fig. 19.
Fig. 17. Construction by mobile.
from its bottom. Then, it starts the fabrication from the
last point fabricated while at the previous post. This
arrangement is routinely practiced in some industrial
applications such as robotic welding of large parts,
such as in ship building.
Roof construction may or may not need support
beams. Supportless structures such as domes and
vaults may be built by mobile robots. For planar
roofs, beams may be used. Under each beam a thin
sheet may be attached. The beams may be picked and
positioned on the structure by two robots working
collaboratively, each being positioned on the opposite
sides outside of the structure. Delivery of roof material becomes challenging with mobile robots and may
be done by a robot inside the structure. This robot
may progressively deliver the material over the beam
panels as each beam is placed on the roof. For the last
few beams this robot could exit the structure and
perform the delivery from outside. An alternative
approach is to use the NIST RoboCrane system which
may be installed on a conventional crane as shown in
the lower part of Fig. 18. (The top part of Fig. 15
shows the RoboCrane moving a steel beam.) Besides
the gripper for beams, the RoboCrane may carry a
Fig. 18. RoboCrane for roof construction.
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B. Khoshnevis / Automation in Construction 13 (2004) 5–19
Fig. 19. IT components of future automated construction.
5.2.1. Analysis of CC feasibility
After possible refinements specific analysis is conducted for conformance to fabrication by the CC
process. This activity is supported by various engineering models and simulation programs. For example, to
verify the feasibility of constructing a certain section of
a curved roof, fluid dynamics and material science
models may be consulted to assure feasibility and
specifications of feasible materials and process param-
eters. The design requirements and these process specifications are then passed to the planning system.
Infeasible features are reported to the architect.
5.2.2. City inspection requirements generation
Design specifications are compared against local
construction codes and an inspection plan is generated
in accordance with the city inspection process, specifying various inspection types at various stages of
B. Khoshnevis / Automation in Construction 13 (2004) 5–19
construction. The inspection requirements are integrated with other construction requirements and are
submitted to the planning system. Due to variations in
local construction codes, the effort on this module will
be minimal in the proposed research.
5.2.3. High level partial plan
This is a representation of possible meaningful
sequences of high level activities (e.g., build living
room, dispatch concrete truck, build roof of kitchen,
etc.). A centralized planning system may generate the
plan in whole, or generate it partially upon demand.
The plan includes alternative sequences of activities
which may be fetched if downstream high level
planning runs into logistics conflicts or undesirable
high level schedules.
5.2.4. High level plan
These are generated by a central planner the output
of which includes items such as specification of
platform posts (various stationary points at which
robots anchor and perform their assigned operations)
for various progressive stages of construction, without
specific allocation of robots to posts. These high level
plans, which specify what needs to be done at which
post, are also sent to the multiple robot coordination
module.
5.2.5. Multi-robot coordination
This module performs a decentralized allocation of
tasks to the available robots based on various factors
such as: extent of suitability of robot for the task (for
example, a robot equipped with a plumbing assembly
gripper is less suitable for electrical assembly as it
must first change its gripper), closeness to the task
post point, amount of concrete left in the robot tank,
amount of batter charge left, etc. This module performs decentralized planning for maximum plan efficiency and agility. The module can change task
allocation on the fly if unaccounted events (e.g., robot
breakdown) take place.
5.2.6. Logistics planner
Details of the layout for resources (main concrete
tanks, reinforcement, plumbing, and electrical modules, paint and charging station) and possible palletizing schemes, as well as dispatching and delivery
schedules, are generated by this module, which oper-
17
ates in harmony with modules identifying the platform
posts and schedule of operation at each post.
5.2.7. Dynamics and control
This module is in charge of actual delivery of tasks
and assurance of successful performance. The module
uses robot dynamics modeling and devises control
schemes that incorporate objectives beyond mere task
performance. These include: determination of the best
position of the post with respect to the position of the
feature to be fabricated or assembled to assume
minimal power consumption, coordinated control of
the robot and material delivery system for fabrication,
range limits where deceleration and acceleration are
needed, and the like.
6. Extraterrestrial applications
The ability to construct supportless structures is
an ideal feature for building structures using in-situ
materials. Hence, we plan to explore the applicability of the CC technology for building habitats on
the Moon and Mars. In the recent years, there has
been growing interest in the idea of using these
planets as platforms for solar power generation,
science, industrialization, exploration of our Solar
System and beyond, and for human colonization. In
particular, the moon has been suggested as the ideal
location for solar power generation (and subsequent
microwave transmission to earth via satellite relay
stations). A conference on Space Solar Power sponsored by NASA and NSF (and organized by USC
faculty) included several papers on this topic (http://
robotics.edu/workshops/ssp2000).
Once solar power is available, it should be possible
to adapt the current contour crafting technology to the
lunar and other environments to use this power and insitu resources to build various forms of infrastructures
such as roads and buildings. The lunar regolith, for
example, may be used as the construction material.
Other researchers [12] have shown that lunar regolith
can be sintered using microwave to produce construction materials such as bricks. We envision a contour
crafting system that uses microwave power to turn the
lunar regolith into lava paste and extrude it through its
nozzle to create various structures. Alternatively, lunar
regolith may be premixed with a small amount of
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B. Khoshnevis / Automation in Construction 13 (2004) 5–19
polymer powder and moderately heated to melt the
polymer and then the mix be extruded by the CC
nozzle to build green state (uncured) depositions in
the desired forms. Post sintering of the deposition may
then be done using microwave power. Understanding
of the following is crucial for successful planetary
construction using contour crafting: (a) the fluid
dynamics and heat transfer characteristics of the
extrudate under partial-gravity levels, (b) processes
such as curing of the material under lunar or Martian
environmental conditions, (c) structural properties of
the end product as a function of gravity level and (d)
effects of extrudate material composition on the
mechanical properties of the constructed structure.
One of the ultimate goals of the Human Exploration and Development of Space (HEDS) program of
NASA is colonization, i.e., building habitats for long
term occupancy by humans. The proposed approach
has direct application to NASA’s mission of exploration, with the ultimate goal of in-situ resource utilization for automated construction of habitats in nonterrestrial environments. We believe that the contour
crafting technology is a very promising method for
such construction.
cation approach contour crafting could result in little
or no material waste. The CC method will be capable
of completing the construction of an entire house in a
matter of few hours (e.g., less than 2 days for a 200
m2 two-story building) instead of several months as
commonly practiced. This speed of operation results
in efficiency of construction logistics and management and hence favorably impacts the transportation
system and environment.
There are numerous research tasks that need to be
undertaken to bring the CC construction technology to
commercial use. The activities reported in this article
are the first few steps toward realization of actual full
scale construction by contour crafting. Readers may
obtain updated information on research progress and
view video clips and animations of construction by
CC at the author’s website: http://www-rcf.usc.edu/
~khoshnev.
Acknowledgements
This material is based upon work supported by the
National Science Foundation under Grants No.
9522982, 9615690 and 0230398, and by a grant from
Office of Naval Research.
7. Conclusion
Due to its speed and its ability to use in-situ
materials, contour crafting has the potential for immediate application in low income housing and
emergency shelter construction. Construction of luxury structures with exotic architectural designs involving complex curves and other geometries, which
are expensive to build using manual approach, is
another candidate application domain for CC. The
environmental impact of CC is also noteworthy.
According to various established statistics the construction industry accounts for a significant amount of
various harmful emissions and construction activities
generate an exorbitant amount of solid waste. Construction of a typical single-family home generates a
waste stream of about 3 –7 tons [13]. In terms of
resource consumption, more than 40% of all raw
materials used globally are consumed in the construction industry [14]. Construction machines built for
contour crafting may be fully electric and hence
emission free. Because of its accurate additive fabri-
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