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331© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
I. S. Dunmade etal. (eds.), Sustainable Engineering, Green Energy
and Technology, https://doi.org/10.1007/978-3-031-47215-2_19
Automated Shotcrete: AMore Sustainable
Construction Technology
GeoffIsaac, PaulNicholas, GavinPaul, NicoPietroni, TeresaVidalCalleja,
MikeXie, and TimSchork
1 Introduction
The most common building material is concrete making it a major contributor to the
ecological footprint of the construction sector, accounting for between 5 and 8% of
global CO2 emissions (Anton etal. 2021, pp.1–2). Global production of concrete
totalled 4.4 billion tons in 2021, with output projected to reach 5.5 billion tons by
2050. Traditional concreting techniques rely on formwork, a mould or cast in which
to pour concrete. Conventional formwork is often made using timber, but it is also
possible to use other materials. Developing formwork is labour and material intense
and accounts for between 35 and 60% of the total cost of concrete work (Bedarf
etal. 2021, p.3). It is also extremely wasteful, as most formwork is temporary and
discarded after use, often having been used once only.
G. Isaac (*)
School of Architecture, University of Technology Sydney, Sydney, NSW, Australia
e-mail: Geoffrey.isaac@uts.edu.au
P. Nicholas
Centre for Information Technology and Architecture, Royal Danish Academy of Fine Arts,
Copenhagen, Denmark
G. Paul · T. VidalCalleja
School of Mechanical and Mechatronic Engineering, University of Technology Sydney,
Sydney, NSW, Australia
N. Pietroni
School of Computer Science, University of Technology Sydney, Sydney, NSW, Australia
M. Xie
Centre for Innovative Structures and Materials, RMIT University, Melbourne, VIC, Australia
T. Schork
School of Architecture and Built Environment, Queensland University of Technology,
Brisbane, Australia
332
Conservative practices together with technological challenges make construction
one of the least digitised and least efcient industry sectors, with concrete construc-
tion still heavily reliant on manual processes (Anton etal. 2021, p.2). Construction
is accountable for 37% of CO2 emissions related to energy (UN Global Status
Report for Buildings and Construction 2021, p.15). Industry pressures including a
desire to improve worker safety, productivity and efciency, combined with market
pressures caused by urbanisation, increasing labour shortages and the challenge of
climate change, are driving the growing interest in advanced technologies such as
shotcreting to challenge established practices (ABB Robotics 2022, p.5). Extending
and accelerating the application of shotcrete has the potential to reduce carbon
emissions and increase the efciency of concrete construction by reducing the
quantity of used material, therefore supporting the construction industry in address-
ing UN Sustainable Development Goal 12.
2 3D Concrete Printing
Additive manufacturing (AM) processes offer potential efciency through lower
labour demands and reduced construction waste (Bedarf etal. 2021). AM also offers
the potential for leaner and more sustainable structures to be developed, reducing
waste by placing material only where it is needed to meet structural demands (Anton
etal. 2021, p. 1). However, cement materials suitable for 3D printing can exhibit
distinct mechanical properties compared with traditional concrete (Lu etal. 2019b,
p. 478). Signicant challenges to improve various characteristics of the material
used for 3D printing including shape retentivity and extrudability remain unresolved
(Nair etal. 2020). Other limitations of 3D printing with concrete (3DPC) are its
inability to build overhangs and the vertical building rate. Mixtures must remain
sufciently soft to be extrudable and to mix with the layer deposited previously, the
substrate, which needs to have cured sufciently to support itself together with the
weight of the most recently deposited material without signicantly sagging
(Wangler etal. 2016, p.70). Achieving a high-quality surface nish and bonding
weaknesses (cold joints) caused by the layering process are issues that limit current
applications and require further investigation (Neudecker etal. 2016, p.335).
Those limitations of 3D printing specically when applied to concrete have
motivated the investigation of alternative technologies and solutions with the aim of
reducing the GHG impact of the construction industry.
3 Shotcrete
Shotcrete was developed (from early in the twentieth century) to address the needs
of the tunnelling and mining industries. In contrast to conventional 3D printing,
shotcrete is not extruded in a bead but combined with pressurised air and sprayed to
G. Isaac etal.
333
create a 3D structure. Concrete is pumped through a tube to a nozzle through which
it is sprayed under pressure (and thereby compressed) on to newly blasted surfaces
to stabilise the excavation. ‘Conveying, compacting and application of concrete
material are performed in one operation, which is distinctive for this technique’
(Lindemann etal. 2017, p. 4). Manual application of shotcrete is an arduous task
with physical demands on the ‘nozzle man’ restricting productivity. The drive for
improved productivity combined with the hazardous nature of tunnelling and min-
ing automation of the shotcreting process has been an industry priority for many
decades.
Shotcrete has the potential to be signicantly more time and materially efcient
than 3DCP, and therefore more sustainable, for both new construction and repair,
thereby also delaying end-of-life impacts (Rispin etal. 2005). A brief history of the
automation of shotcrete is detailed in (Rispin etal. 2005). Early innovations were
restricted to nozzle-holding devices using cranes and lifts to move and direct the
application of concrete. Specialised shotcrete manipulators began to appear in the
1980s offering greater accuracy and extension possibilities. Within a decade spray-
ing manipulators became commonplace on all large construction projects where
shotcrete was used (Rispin etal. 2005, p.4).
As demands for increased productivity grew, the need to quickly and accurately
apply large volumes of shotcrete led to the development of remote controls, with
radio remote quickly becoming the industry standard. Autonomous spray mobiles
containing all the equipment needed to deliver quality shotcrete were tailored to
meet the needs of specialist construction tasks (Rispin etal. 2005, p.4). Computer-
controlled multi-axis robots capable of scanning and spraying an area have become
increasingly common since the start of the twenty-rst century.
Automation has delivered signicant productivity enhancements and offers the
potential to create complex geometries, a signicant advantage compared with
3DCP which is restricted to vertical builds. Hand-held nozzles can cover 7–9 cubic
metres per hour, while mechanised spraying can cover more than double of the out-
put up to 20 cubic metres per hour (Rispin etal. 2005, p.5). Automation has deliv-
ered improvements in safety and reduced set-up times, requiring less people in the
process.
Academic attention has recently focused on developing shotcrete as an alterna-
tive to conventional 3DCP, combining the benets of shotcrete with AM processes,
shotcrete-based 3D printing or SC3DP. Researchers at Technische Universität
Braunschweig led the development of ‘a robotically controlled additive manufactur-
ing process that builds concrete components layer by layer through the controlled
addition of compressed air’ (Kloft etal. 2020a, p.609). The concrete layer’s height
is inversely proportional to the speed of the robot-controlled nozzle, while width
can be determined through adjusting the distance of the nozzle from the surface.
Complex geometries including overhangs and the integration of built-in parts or
reinforcement become achievable (Kloft etal. 2020, p.2).
One of the main benets of SC3DP is that interlayer bonding is improved using
high air pressure which ‘tears up’ the concrete in the nozzle and creates a high con-
tact surface area with the air stream facilitating the intermixing of additives (Kloft
Automated Shotcrete: AMore Sustainable Construction Technology
334
etal. 2020, p.2). Layer interlocking and hence interlayer bond strength are enhanced
by SC3DP compared with traditional additive manufacturing due to the high kinetic
energy of the sprayed material, which reduces the prospect of cold joints forming.
‘In general, SC3DP specimens show better mechanical performance than extrusion-
based 3D printed materials and cast specimens of the same mixture design’ because
of improved interlaying bonding (Heidarnezhad and Zhang 2022, p.7). Performance
can be enhanced by precisely controlling layer thickness (Northcroft and
Ziegler 2008).
3.1 Formwork
Developing traditional formwork is labour and material intense accounting for
between 35 and 60% of the total concrete work cost (Bedarf etal. 2021, p.3). It is
also extremely wasteful, as most formwork is temporary and discarded after use,
often having been used once only. Reducing or eliminating formwork saves not only
the natural resources and labour required to build it but also the transport and dis-
posal impacts, highlighting a signicant sustainability advantage of shotcrete
(Schokker 2010).
Work at Braunschweig has led to the development of techniques to SC3DP with-
out conventional single-use formwork. A 2016 paper outlined their proposed exper-
iments progressing from a at wall with opposing formwork to creating complex
curved walls (Neudecker et al. 2016, p. 336). Among the many challenges, the
research team needed to develop simulation tools, an automated injection tool and a
control system. Long-range scanners supplemented with a 3D laser triangulation
scanner and a 3D vision system were used to monitor performance (Neudecker etal.
2016, p.335). Although experiments were not complete at the time of publishing,
the researchers identied ‘a water-to-cementitious ratio of 0.4, with a pressure of
6.5 bar in the pneumatic cylinder of the pumping system’ produced the best spray
(Neudecker etal. 2016, p.338). Pressure at the nozzle tip of 5.2 bars resulted in the
highest-quality results with a rebound rate of just 8%.
The Block Research Group combined a reusable cable net with a fabric overlay
to create formwork for an anticlastic mesh-reinforced sandwich shell roof (Block
etal. 2017). This lightweight solution eliminated the need for both falsework and
foundations for the formwork. An evolutionary design process was employed to
identify the most optimal geometry for the roof, allowing the thickness of the shells
to be reduced to between just 5cm and 3cm (Block Research Group 2018).
3.2 Reinforcing Agents
Combining the traditionally separate requirement of formwork and reinforcement
into a single robotic fabrication process has potential to ‘produce an additive
and waste-free, material-efcient, and geometrically unconstrained method of
G. Isaac etal.
335
fabricating complex non-standard concrete constructions’ (Hack and Lauer 2014,
p.52). Hack and Laurer report on using acrylonitrile butadiene styrene (ABS) to
print mesh mould formwork that can also act as a reinforcement for concrete struc-
tures. Concrete is sprayed and protrudes through the mesh mould with surfaces
manually trowelled to smooth and level (Hack and Lauer 2014, p.49). Using poly-
mers developed for conventional 3D printers permitted ‘precise control over the
material’s hardening behaviour. Pinpoint cooling during the extrusion process, for
example, gives such a high level of control that free spatial extrusions become pos-
sible and, consequently, the “knitting” of structures freely in space’ (Hack and
Lauer 2014, p. 49). Moving to spatial extrusion, in contrast to layer deposition,
signicantly reduces fabrication time and can be deployed at a large scale. However,
the mesh structure is not sufciently strong to resist structural loads, limiting its
application to non-structural components (Wu etal. 2022, p.13). Hack and Lauer
also highlight the potential for carbon, glass, bamboo or basalt to be co-extruded to
develop constructions that can withstand high tensile forces. Hack then continued to
experiment with the technique for his PhD dissertation, transitioning from polymer
to structurally superior steel meshes suitable as a loadbearing construction system,
which was used at the DFAB (NEST) house (see below) (Hack 2018).
Inspired by the ferrocement technique (developed in the 1940s) of manually
throwing concrete against a dense, self-supporting reinforcement mesh, researchers
at ETH Zurich investigated robotic spraying of glass bre-reinforced concrete on a
permeable reinforcement mesh made from carbon bre (Taha et al. 2019). This
approach allowed the researcher to move away from the limitation of only spraying
horizontal layers (as with the work at Braunschweig). Square, 38mm glass-bre
mesh was bent and stabilised into the desired shape. Glass bre was mixed with the
concrete in the nozzle of the spraying gun. Through experimentation the optimal
bre length (42.5mm) in relation to mesh opening size was identied as crucial to
ensure the material clogged the openings of the mesh and adhered to it, ‘while also
assuring that excess of material penetrating through the mesh during the fabrication
was minimized’ (Taha etal. 2019, p.248). A double-curved structure with an aver-
age thickness of just 3cm was successfully developed. The researchers conclude
that their approach could be compatible with mesh mould in the application of sur-
face nishing.
The DFAB (NEST) house was conceived as a multi-technology demonstrator of
digital fabrication techniques (Fig.1). A densely reinforced load-bearing concrete
wall was built in situ at the house. The steel reinforcing mesh was constructed more
densely than traditional steel rebar cages to prevent the concrete mix from owing
through the mesh. To build the mesh accurately, the fabricator sensed its position
within the construction site and continuously monitored the shape of the mesh. A
12m wall, 2.8m high, requiring over 20,000 weld points was constructed using
6mm steel rods over a period of 125hours.1 Undulations were incorporated into the
design to stiffen the wall to compensate for its relative thinness.
1 A video of this project can be viewed at: https://www.youtube.com/watch?v=Fi3SyfQ3hnc.
Automated Shotcrete: AMore Sustainable Construction Technology
336
Fig. 1 The in situ fabricator building the mesh mould at the NEST house. The mesh mould pro-
cess unies the reinforcement and formwork production into a single and robotically controlled
on-site fabrication system. (Imagecourtesy: NCCR Digital Fabrication)
In 2019, Hack etal., inspired by lattice structures traditionally constructed using
steel or aluminium, began to experiment with using reinforced concrete for geo-
metrically complex spatial structures suitable for long-spanning, column-free con-
struction (Hack etal. 2019). In this approach, the spatial structures were modularised
into planar components (Fig.2). Using identical, planar truss girders reduced mate-
rial use and eliminated the need for formwork. Planar components were 3D printed
using three layers of shotcrete reinforced with a (manually placed) carbon bre grid.
After milling the edges, the planar elements were cured and then assembled. It was
noted that more sophisticated module typologies need to be developed ‘to allow for
improved connectivity and multidirectional load transfer between neighbouring
modules’ (Hack etal. 2019, p.371).
In 2020, Kloft and Hack successfully produced a fully reinforced, double-curved
concrete wall over 5 square meters and 18cms thick using a 6-axes Stäubli robot
(Hack and Kloft 2020). In this process horizontal and vertical (10mm B500B steel)
rebars were positioned manually as the structure was printed (Fig.3 and described
in more detail in (Kloft etal. 2020b)). A second three-centimetre layer of concrete
was applied embedding the reinforcements, while creating a foundation for the sur-
face nishing, using a trowelling process (a 3-axes Omag milling application) (Hack
and Kloft 2020, p.1130).
Mike Xie, from RMIT in Melbourne, led a project to construct a 4.2m wall using
bre reinforced ultra-high-performance concrete sprayed over 80 moulds, 3D
G. Isaac etal.
337
Fig. 2 Assembled prototype element demonstrating proof of concept for a project at Technische
Universität Braunschweig. (Image courtesy of Norman Hack)
printed from PETG (Fig.4). After printing and assembly moulds were sprayed with
concrete in layers. The demoulded polished concrete components were transported
for on-site assembly (Centre for Innovative Structures and Materials 2021;
Dingwen 2022).
The German government-nanced Carbon Concrete Composite project is
approaching completion of the construction of The Cube, a building made, in part,
from precast panels with sections shotcreted onto a carbon bre mesh. The carbon
bre reinforcement allowed double-curved geometry walls just 4cm thick to be
built without the need for conventional formwork. It is claimed that the construction
will have four times the strength of a regular reinforced concrete building and con-
tain 70% less embodied carbon (the use of clinker has been avoided). The use of
exible (rust proof) carbon reinforcement mesh has allowed for new geometric
forms to be explored in the design of the building. The 220 square meter building on
the grounds of the Technische Universität Dresden has a predicted lifespan nearly
three time the standard 70–80years for concrete buildings reinforced convention-
ally (Cousins 2021; Fearson 2021). The Cube showcases the potential for shotcrete
to create longer-lasting buildings, a signicant sustainability advantage compared
with traditional construction techniques as end-of-life impacts are delayed.
Automated Shotcrete: AMore Sustainable Construction Technology
338
Fig. 3 Threading unbent vertical reinforcement into the shotcrete core structure. (Image courtesy
Norman Hack)
Fig. 4 Intelligent form. (Image courtesy: Dingwen ‘Nic’ Bao, Xin Yan, Yi Min ‘Mike’ Xie, Wei
Qiu and Jianan Peng)
G. Isaac etal.
339
Additionally, shotcrete can be used to economically repair or rehabilitate structures,
extending their lives and delaying potential impacts caused by demolition and
reconstruction.
3.3 Control Systems
The accuracy and consistency of the sprayed results is determined by the sophistica-
tion of the control system which determines the location and quantity of material
and calculated the desired compaction. Adaptive control algorithms can be used to
incorporate feedback from sensors monitoring material distribution in real time.
The ability to precisely control the thickness of a section to match its structural
requirements eliminates wasted material, one of the main sustainability advantages
of shotcrete. Traditional processes such as trowelling and milling can be used to
improve surface nish (Hack and Kloft 2020, p.1129).
Synchronising the measuring system with the robot-held nozzle to obtain consis-
tent results within acceptable tolerances remains a signicant challenge. Irregular
environmental characteristics and uctuations in the composition of the concrete
mix can result in ‘vast deviation between designated and printed geometry’
(Lachmayer et al. 2023). A 2018 state-of-the-art review of in situ measurement
technologies identied laser triangulation as more accurate and reliable compared
with 2D camera-based monitoring (Lindemann etal. 2019). Informed by this review,
the authors concluded that model-based ofine planning alone was insufcient to
deliver the accuracy required and implemented a series of monitoring and control
initiatives using a Beckhoff control system.
Layer width and height were controlled by two algorithms developed to compen-
sate for material displacements causing geometric inaccuracies. By controlling the
velocity of deposition and the distance from the nozzle to the surface, the required
layer height and width could be achieved, although the authors note that control of
the layer width measurement is indirect, which they planned to address by direct
measurement to deliver closed-loop control of layer width (Lindemann etal. 2019,
p.294). This project resulted in SC3DP constructing a wall with complex concrete
geometries, featuring a signicant overhang and integrated reinforcement for the
rst time (Lindemann etal. 2019, p.296).
Researchers at Cambridge University developed trajectory planning algorithms
to produce doubly curved ribbed concrete shells using the C# plug-in for
Grasshopper’s 3D visual programming environment (Nuh et al. 2022). Two
prototypes were produced using this process: a 4.5 m nine-segment shell and a
deep ribbed thin shell. The authors note that the benet of the Grasshopper plug-in
is that ‘it can be applied to any robotic assembly system with a concrete sprayer
attached’.
Automated Shotcrete: AMore Sustainable Construction Technology
340
3.4 Material Mixtures
Developing suitable cementitious materials specically for SC3DP is another area
of enquiry (Lu et al. 2019a, p. 1074). Incorporating supplementary cementitious
materials such as y ash or recycled aggregate into the concrete mix avoids landll,
highlighting another sustainability advantage of SC3DP.Material must be speci-
cally developed to ensure it is suitable to be pumped through the hose and propelled
through the nozzle. Coarse aggregates have not been used to date. Pumping perfor-
mance can be improved if the mixture has low plastic viscosity and low dynamic
yield, both of which can be manipulated by incorporating additives (Heidarnezhad
and Zhang 2022, p.6). Of particular note Yun etal. report that adding up to 4.5% of
silica fumes by weight improved pumpability by lowering viscosity (Yun etal.
2015). Yun also notes shootability is improved with higher yield stress and increased
viscosity resulting in improved build-up thickness while reducing rebound rates.
Neudecker etal. recommend a water to cement ratio of 0.4 applied with a pressure
of 6.5 bar to achieve the best shootability performance (Neudecker et al. 2016,
p.338). Researchers at Zurich found that the addition of steel bres led to signi-
cant improvements in ductility and strength (Pfändler etal. 2019).
To reduce the density of concrete and to improve the accuracy of spraying,
researchers at the Singapore Centre for 3D Printing experimented with adding an
air-entraining agent (AEA) and incorporating lightweight aggregate (y ash ceno-
sphere– FAC) in the mixture (Lu etal. 2019a). Adding AEA reduces yield stress,
improving pumpability (Lu etal. 2019a). Experiments showed that the introduction
of FAC and AEA lowered the spread diameter and slump values of freshly sprayed
cement, suggesting the material could better retain its shape, but negatively impacted
its pumpability (Lu etal. 2019a, p.1075). A mixture containing 100% FAC aggre-
gate and 0.1 grams/litre of AEA presented the lowest plastic viscosity and dynamic
yield stress requiring the lowest calculated pumping pressure, achieving the best
performance in delivery and deposition.
Computer-controlled dosage of accelerator creates a more uniform shotcrete
quality. Adding accelerator and silica fume to shotcrete means it hardens more
quickly, achieving high static yield stress more quickly (Heidarnezhad and Zhang
2022, p. 7). Material with 6% accelerator had a deformation modulus about 14
times higher than the material without accelerator (Dressler et al. 2020, p. 16).
However, increasing the accelerator dosage can reduce interlayer bonding, poten-
tially compensated by increasing the air volume ow (Lachmayer etal. 2023, p.13).
3.4.1 Functionally Graded Concrete
Functionally graded concrete is produced by adding a gas-releasing foaming agent,
such as aluminium powder, to react with alkaline hydration products or by mixing
wet foams with cement paste. The later approach enables different density levels to
be achieved, from 200 to 1900kg/m3, compared with ~2500 kg/m3 for standard
G. Isaac etal.
341
concrete. The ability to vary density creates the opportunity to develop monomate-
rial manufacturing processes that address demand for varying the mechanical prop-
erties of elements within a structure. Functionally graded materials offer the
potential for improvements in material usage, strength and functionality while
reducing weight when compared with their homogenous equivalents (Keating 2011,
p.1). Researchers at MIT produced a graded cylinder beam weighing 9% less than
a solid cylindrical beam of the same dimensions capable of supporting the same
load, illustrating the sustainability benets of this approach (Keating 2011, p.5).
Functionally graded concrete can contribute to improved material efciency in con-
struction. In addition, ‘foam concrete has lower thermal conductivity (~0.065 Wm−1
K−1 at ~250 kg/m3) compared to regular concrete (~0.5 Wm−1 K−1)’, decreasing the
need for insulation, yet another sustainability benet (Bedarf etal. 2021, pp.7, 10).
The internal composition of structural components can be aligned with their ‘spe-
cic structural and thermal performance requirements’ (Herrmann etal. 2018, p.54).
The researchers at Universität Stuttgart combined two concrete mixes using two
pumps and spray nozzles, continuously varying the quantities of mixtures: mixture
one is a high-density ne-aggregate concrete, and mixture two has a lower density
with a higher porosity. Signicantly the two concretes were not mixed before spray-
ing but applied at the same time through two separate nozzles. Topology optimisa-
tion algorithms were rened to achieve optimal material distribution. The pumps’
volumetric ow control allows for seamless gradation across a wide spectrum of
characteristics, ranging ‘from low to high strength, heavy to ultra-lightweight, and
low to high heat insulation properties’ (Herrmann and Sobek 2017, p. 57).
Experimental tests revealed that the bulk density of the specimen decreased pro-
gressively over its height, and this change was reected in the mean compressive
strength. Steel-reinforced beams with a mass reduction of 34% were successfully
produced using functionally graded components (Herrmann and Sobek 2017, p.62).
4 Future Research
Many topics require further investigation before SC3DP reaches the level of matu-
rity required to achieve widespread deployment by the construction industry.
Effectively guiding the development and deployment of in situ robotic fabrication
processes presents a complex challenge that spans multiple disciplines and domains,
requiring interdisciplinary collaboration and expertise (Buchli etal. 2018). Solutions
require the intense collaboration of architects, materials scientists, roboticists, civil
engineers and mechanical engineers, among others. Areas of research required are
summarised by Heidarnezhand and Shang under three headings: (1) Establishing
the correlation between the operational process parameters, material properties and
the resulting printed layer geometry is crucial to enhance printing precision. We
note that while it has been proven that laser triangulation is superior to 2D cameras
to monitor performance, investigation of other measuring techniques is needed. (2)
Automated Shotcrete: AMore Sustainable Construction Technology
342
Examine combining shotcrete with 3D extrusion printing, to help overcome the
shortcomings of both technologies (e.g. 3D printing formwork to be over sprayed
by shotcrete). (3) Innovate to develop superior printing mixtures, particularly with
the addition of bres, potentially as a replacement for reinforcement (Heidarnezhad
and Zhang 2022, p.9). Alternative reinforcement agents including plastics, carbon,
glass and hemp require further experimentation. The effects of combining these
reinforcement agents with other additives and accelerators remain an area ripe for
further inquiry (Ivanova etal. 2022). Continued investigation into reinforcement
structures that are more efcient than traditional steel rebar is needed. Offering
faster build times and more efcient use of resources shotcreting is a more sustain-
able construction technology, but more research is required to quantify these envi-
ronmental benets. In particular, Saade etal. note a lack of comparable life cycle
assessment studies (Saade etal. 2020).
Research priorities include varying the nozzle geometry to obtain more preci-
sion, as the layer geometry is determined by a variety of additional factors including
pumping speed, air pressure and stand-off distance. Increasing the nozzle diameter
decreases the spray velocity and results in lower compaction rates (Burak etal.
2018). Rebound rates are affected by nozzle positioning and speed, mix proportion,
additives and rheological properties. Clear relationships between viscosity and
yield stress and their impact on rebound rates have not yet been identied (Yun etal.
2015). Rebound not only wastes resources but also negatively impacts placement of
material and potentially mechanical properties. Investigations to reduce rebound or
compensate for its effects are needed. In their review of reinforcement technologies,
Wu etal. emphasise the need for further development of design standards specic
to printed concrete, as well as the establishment of code recognitions and/or guide-
lines that can verify equivalency to reinforced concrete through comprehensive test-
ing (Wu et al. 2022, p. 21). They also call for standard testing procedures for
safety-related performance to be established. SC3DP produces a rough textured
surface, and more efcient, fully automated post-processes need to be developed to
rene nished products.
Finally, the opportunity to continuously vary the density of the mixture (func-
tionally graded concrete) during construction, investigated by researchers at MIT
and Stuttgart, offers the possibility of achieving signicant material and energy ef-
ciencies and suggests a rich area for further investigation. The Stuttgart experiment
used two nozzles to mix concrete materials at the point of application. Alternatively,
more accurate methods to combine and deliver functionally graded concrete on
demand require further experimentation. SC3DP functionally graded concrete cre-
ates the potential to deliver signicant GHG reductions for both the concrete and
construction industries through optimising structural designs, minimising the use of
materials and signicantly reducing waste while improving the sustainability prole
of the construction industry and helping it meet the UN Sustainable Development
Goal 12.
G. Isaac etal.
343
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