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Additive Fabrication of Large-Scale Customizable
Formwork Using Robotic Fiber-Reinforced
Polymer Winding
Ya Ou,
1
Ding-Wen Bao,
2,3
Guan-Qi Zhu,
4
and Dan Luo
4,i
Abstract
Based on the established system of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) in civil engi-
neering and construction industry, this research presents a novel fabrication method for freeform FRP formwork
through an additive process of winding FRP fabric with industrial robots. Different from the filament winding
or fused deposition modeling process in additive manufacture, large-scale formwork is fabricated with layered
winding of FRP fabric and simultaneously applying fast cure epoxy resin in the proposed methods. It increases
the fabrication speed and material efficiency compared with the typical fabrication process of FRP formworks,
and achieved the geometry flexibility from the numerically controlled additive process. The fabrication methods
are developed through a series of preliminary tests, exploring the appropriate fabrication parameters, such as the
overlapping height of each layer, winding speed, and epoxy resin type. Two additional prototypes addressing
geometrical flexibility are also fabricated. Based on the feasibility studies, the article discussed the potential
application of this system on a double-skin tubular arch (DSTA) bridge and a tree-like topological optimized
column as the future outlook of this method. As developed based on the established construction systems such
as CFFTs and DSTAs, not only the proposed system is compatible with current structure and construction sys-
tem, but it also benefits from combining an off-shelf material with a flexible and accurate programmable robotic
process. This research contributes to the scope of additive manufacturing system by targeting the fabrication of
nonuniform optimized large-scale structures.
Keywords: fiber-reinforced polymer (FRP), additive manufacture, robotic fabrication, dynamic winding,
topological optimization
Introduction
With the development in structural design and topology
optimization,
1
there is an increasing need to fabricate the
customizable large-scale structural components with non-
standard geometry, while the fabrication process of these
structures in a rapid and cost-efficient manner remains a
challenge. The application of concrete structure components
with nonuniform sections in construction is restricted by its
steep cost, mainly due to the extra cost and time on the
customized formwork.
2
The development in additive manufacture (AM, also
known as three-dimensional [3D] printing) makes mass
customized concrete structures possible without extra
1
School of Civil Engineering, Central South University of Forestry and Technology, Changsha, China.
2
Center for Innovative Structures and Materials, School of Engineering, RMIT University, Melbourne, Australia.
3
School of Architecture and Urban Design, RMIT University, Melbourne, Australia.
4
School of Architecture, University of Queensland, St Lucia, Australia.
i
ORCID ID (https://orcid.org/0000-0003-1760-0451).
Opposite page: Prototype C, a tubular formwork with changing direction.
Image Credit: Guan-Qi Zhu and Dan Luo.
3D PRINTING AND ADDITIVE MANUFACTURING
Volume 00, Number 00, 2021
ªMary Ann Liebert, Inc.
DOI: 10.1089/3dp.2020.0358
1
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formwork.
3,4
Although a variety of AM technologies are
developed for small-scale rapid prototyping, the majority of
large-scale AM for building industry is based on 3D concrete
printing (3DCP). However, as the material composition of
3DCP is different from the conventional concrete, for ex-
ample, more additives and no coarse aggregates in the 3DCP,
and the challenge to integrate it with typical connection and
reinforcement system, there are significant challenges to in-
corporate the 3DCP into the established construction process,
thus restricting the wide application of AM to on-
site constructions.
5
This article proposes a fabrication system for nonstandard
curve-shaped formwork for concrete casting using a novel
AM method. This new fabrication method was inspired by
the concept of the fiber-reinforced polymer (FRP)-confined
concrete structure,
6,7
in which the tubular FRP structure ser-
ves both as stay-in-place formwork and as confinement for
the concrete inside. Using the FRP tubes as the concrete
formwork also saves the falsework, which significantly re-
duces the interruption of traffic and the on-site construction
waste. In previous applications, the FRP tubes are usually
prefabricated using filament winding or wet layup method
based on standard base formworks, while in this research,
industrial robot and AM method is used for the fabrication
of FRP tube with customizable geometry. It explores the
potential of utilizing industrial robotic systems for on-site
fabrication of optimized structural members with higher geo-
metrical flexibility and material efficiency, while signifi-
cantly reducing the transportation cost and the subsequent
energy consumption.
The main advantages and innovation of this new fabrica-
tion method are as follows:
The shape of the fabricated tube is numerically con-
trolled by the robotic arm with high accuracy, allowing
the components to be fabricated with curvature and non-
uniform sections, without significantly increasing the
cost and time.
The fabrication method is proposed to be applied on the
FRP-confined concrete structures, which provides a
feasible industrial application for it and increases the
compatibility of the fabricated component with an es-
tablished construction system.
The formwork fabricated serves both as the stay-in-place
formwork and as confinement for the concrete, which re-
duces the construction waste and increases the concrete
material efficiency, meanwhile the FRP layer can also
protect the concrete inside from the corrosion environment.
FRP fabric, instead of FRP filament, is used for the
tube in the proposed method. It helps increase the fab-
rication speed by covering larger areas with the same
winding runs compared with filament winding or fused
deposition modeling (FDM).
With the application of industrial robot, the current
system can accommodate components up to 2.2 m tall,
and expandable with additional external linear axis,
making its application in the fabrication of the large-
scale structural components possible.
The proposed fabrication process can be conducted on-
site where the robots are accessible, which can signif-
icantly reduce the transportation cost of prefabricated
formworks.
This article presents a novel fabrication method for FRP
tubular sections as the concrete formwork using fabric
winding by robotic arms (Fig. 1). The fabrication process is
first presented, then three prototypes with different geom-
etries are fabricated, followed by the application outlook
of this fabrication concept. This application broadens the
boundary of AM, and provides an efficient and automatic
way of fabricating large-scale formwork for the FRP-confined
concrete structures.
Background and Relevant Projects
FRP-confined concrete structures
FRP has been widely applied on the reinforcement of ex-
isting structures, by externally binding the FRP laminates
on the structure surfaces.
8–10
More recently, concrete-filled
FRP tubes (CFFTs) have also been developed, in which the
tubular FRP profiles are used as the stay-in-place formwork
for concrete filling,
11–13
as shown in Figure 2. In CFFTs, the
FRP section confines the concrete core and acts as structural
reinforcement, when loaded, the concrete is acting under
a three-axial compression load, and the FRP laminate is
under tensile load so that both materials can be efficiently
utilized. The FRP tube also acts as a durable and corrosion-
resistant protector for the concrete core, which makes this
type of structure ideal for the bridge piers, marine piers, and
girders.
14,15
A 12.5 m span bridge system based on the CFFT called the
hybrid double-skin tubular arch (DSTA) bridge was devel-
oped in the University of Queensland (Fig. 2).
13
In DSTA
section, an additional steel tube is used inside the FRP tube,
so the concrete is confined by the outside FRP skin and
the inside steel skin, much higher load carrying capacity can
be achieved than the CFFTs without significant self-weight
increase.
13
Currently, the construction process of the DSTA
bridge is divided into five main steps: (1) fabricate the FRP
and steel tube segments and transport to the factory con-
struction site, (2) put the steel and FRP tubular segments in
position in the factory, (3) weld the steel sections and con-
nect the FRP segments by FRP wrapping, (4) concrete casting
and hardening, and (5) transport the bridge to the construc-
tion site and install the bridge. In step (1), the FRP tubes
were fabricated overseas, and it was time- and cost con-
suming to transport them to the factory in Australia; and in
step (5), significant extra cost is needed for transportation and
traffic control to transport the whole bridge to the construc-
tion site.
Fabrication methods for FRP tubes
In CFFTs and DSTAs, tubular FRP sections are used as the
formwork and confinement for the concrete inside. Filament
winding, wet layup, and pultrusion are the most commonly
used methods for FRP tubular section manufacture. A wide
range of sections, such as circular section, rectangular sec-
tion, and elliptical section, can be manufactured using these
methods.
12,13,16
Filament winding is currently the most
commonly used method for the FRP tube in the CFFT and
DSTA sections; it creates tubular composite sections by
winding the filament under tension over a rotating man-
drel.
17–19
Fiber strength can be effectively utilized by care-
fully designing the fiber layouts in filament winding method,
2 OU ET AL.
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FIG. 2. Left: cross-section of CFFT; middle: cross-section of DSTA; right: a 12.5 m span DSTA bridge. CFFT, concrete-
filled FRP tube; DSTA, double-skin tubular arch.
FIG. 1. Concrete formwork fabricated with robotic FRP winding. FRP, fiber-reinforced polymer.
3
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and various sections and sizes can also be fabricated; how-
ever, its application on concave sections is very limited. A
project called BUGA Fiber Pavilion 2019
20
tried to solve this
problem by exploring a coreless filament winding method,
which winds the FRP filaments between two rotating winding
scaffolds, and builds the predefined concave shapes from the
interaction of the filaments. Wet layup method, or the so-
called hand layup method, puts layers of FRP fabrics into
or against the mold, and shapes the sections by the resin
applied between the layers; both concave and convex sec-
tions can be manufactured using this method.
21,22
Wet layup
method is also widely applied for the repair or reinforce-
ment of concrete structures by externally wrapping the FRP
mat on the beam or columns, which can effectively enhance
the mechanical behavior of the reinforced structure.
23,24
Pultrusion is a highly industrialized method for FRP sec-
tions; the resin-saturated FRP fiber is first guided and hard-
ened to the designed shape in a heated steel-forming die,
and then the profile is pulled out and trimmed into the de-
sired length.
25,26
Limited by the pultrusion process, fiber in
the section is always parallel to the section length, while in
the FRP-confined concrete structures the FRP fiber should
be orientated along the hoop direction to provide confinement
for the concrete. Thus, pultrusion is not suitable for the FRP-
confined concrete.
The above-mentioned traditional fabrication methods
for FRP profiles are usually well suited for factory produc-
tion, but not for on-site application owing to the limitations
of the accessibility to highly controlled manufacture facil-
ities. Also applying the current industrial prefabricated
FRP profiles on-site demands additional cost in storage and
transportation.
Topology optimization
Recent studies on topology optimization have found that
material efficiency can be significantly improved by using
irregular sections to replace the conventional sections in the
design of structural members.
27
Within the last decade, a
dynamic approach using the finite element analysis has been
developed for structural optimization. This technique seeks
the most efficient way of using the material by altering the
shape, topology, and geometry of the buildings and its struc-
tural components.
28
The optimized structures are usually
featured with changing cross-sections along the member span
or height, such as the tree-like structure exhibited at 2019
IASS Form and Force Expo.
29
The proposed fabrication
method has a strong outlook in fabricating large-scale topo-
logically optimized structure.
Additive manufacturing method for concrete formwork
AM
30,31
method makes it possible to fabricate the topol-
ogy optimized structures in an accurate manner. AM rep-
resents the process of fabricating 3D objects by adding
layer-by-layer material; materials commonly used for AM
including plastic,
32
metal,
33
and concrete.
4,34,35
Tradition-
ally, AM is mostly applied for fabricating the solid compo-
nents defined by computer, so that optimized, nonstandard,
and esthetic construction components or even structures can
be manufactured without using extra formwork, and mean-
while the construction waste can be significantly redu-
ced.
4,35,36
However, the application of the AM technology
on building-scale structures is still limited by some factors,
such as the steep cost of printing large components and
the difficulty of integrating the AM components with steel
reinforcement.
2,37,38
Recent trials of using AM for fabricating the concrete
formwork or tubular sections provide a way to solve these
problems. Kayser et al.
39
in Massachusetts Institute of Tech-
nology proposed a multirobot system called ‘‘FIBERBOTS’’
to manufacture a series of large-scale FRP tubular structures.
In this project, a fleet of robots worked parallel from the
ground up to enable collaborative and site-specific con-
struction of the tube structures using fiber winding. Peters
40
in Kent State University investigated a method of fabricat-
ing the formwork for concrete casting using FDM, while
they found that it is challenging to apply this method on
structural scale due to the concrete hydrostatic pressure.
‘‘Eggshell’’ developed in ETH Zurich
41
proposed a fabrica-
tion process for large-scale tubular formwork using robotic
FDM and can simultaneously cast the concrete inside.
FIG. 3. (A) Spatial winding of nonuniform FRP profile for large-scale structure components; (B) component with uniform
profile; (C) component with variation in section; (D) components with variation in direction; and (E) components with
variation in both section and direction.
4 OU ET AL.
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Eggshell solved the problem of concrete hydrostatic pres-
sure, while the formwork fabricated only works as con-
crete formwork without increasing the concrete material
efficiency, which can be further optimized. Another pro-
ject called ‘‘Mesh Mould’’ in ETH Zurich
42
explores a ro-
botic fabrication process for the nonstandard curved
reinforcement, which also acts as the formwork for the con-
crete. Besides, a range of research studies on dynamic form
casting have been conducted through dynamic fabrica-
tion with a numerically controlled formwork, achieving a
balance between shape customization and fabrication
efficiency.
43,44
Method and System Design
In this project, the glass fiber-reinforced polymer (GFRP)
fabric is winded over a designed path by the robotic arm, and
then hardened by the resin applied on it. GFRP fabric was
chosen as the material in this system because it is flexible
when no resin is applied, so it can easily be fabricated to
curved shapes with the guide of the robotic arm; and when the
resin applied is hardened, it will hold its shape and provide
high confinement to the concrete inside because of its high
strength. A workflow is carefully designed to control the
moving path of the arm and guide the GFRP fabrics to fab-
ricate the designated form. The additive fabrication system
starts from the bottom of the structure, where an initial sec-
tion with GFRP fabric is fabricated on a short tubular base,
and the section height is extended by winding fabric layers
over the lower fabricated part. Resin is applied alongside the
winding process, and the speed is carefully calibrated to en-
sure that the lower layers of the extruded fabric would be
hardened enough to hold its shape while the top layers are still
malleable to be shaped by the position roller. The moving
FIG. 5. Design of the end-effector and path planning. (A) Robotic operation path; (B) the sixth axis is set to limitless
spindle mode; (C) epoxy resin dispenser; (D) support frame; (E) position roller; (F) supporting roller with damper; (G)
spring damper; (H) GFRP roll; (I) head of the end-effector, connected to KUKA robot. GFRP, glass fiber-reinforced
polymer.
Digital Design
•Create Geometry
•Extract Geometry Surface
Information
•Path Planning Algorithm
•Script Generation
Robotic Fabrication
•Install Base and End Effector
•Dry Run
•Connect FRP Fabric to the
Base
•Epoxy Protection Measures
•Mix Epoxy Resin
•Wet Run Process
•Remove Structure from the
Base
Fabric
Winding
Apply
Epoxy
Resin
Epoxy
Cure
FIG. 4. Design and fabrication workflow of the proposed method.
ADDITIVE FABRICATION OF FORMWORK BY FRP WINDING 5
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path and speed of the robotic arm are investigated so the
shape of the fabricated structure can be accurately controlled.
Because of the high flexibility of the FRP fabric, the fabri-
cated tube can change its shape and direction progressively as
the winding process develops (Fig. 3). After one segment is
finished, the fabricated formwork will be removed from the
base before mounting into place for concrete casting. The
fabrication process of the proposed method is as illustrated in
Figure 4.
End-effector
An end-effector is designed to guide the fabric winding
and carry the GFRP fabric roll (Fig. 5). Three passive rollers
are designed in the end-effector, including one position roller
(Fig. 5E) and two supporting rollers (Fig. 5F). While wind-
ing, the free end of the GFRP is first attached to the fabricated
tube, then set between the position roller (Fig. 5E) and sup-
porting roller (Fig. 5F.a), followed by attaching to the other
supporting roller (Fig. 5F.b), and finally connected to the
GFRP roll (Fig. 5H). In this preliminary design, the size of the
end-effector is designed to carry a 150 mm wide, 50 m long
GFRP fabric roll, which is enough to fabricate an 800 mm tall
tube with an average diameter of 250 mm.
The position roller (Fig. 5F.a) sits inside the fabricated
profile to ensure the accurate position of the fabricated sec-
tion. The supporting rollers are designed to hold and ensure
the GFRP fabric is driven in tension. A spring damper is
connected to the supporting roller (Fig. 5F.a) to apply pres-
sure on the newly winded fabric, so it can be fully appended
to the previous winded layers. Another spring damper is
connected to supporting roller (Fig. 5F.b) to tighten the
fabric. The current end-effector requires a minimum 120 mm
clearance on the inner side of the tube to operate, which
restricts the minimum diameter of the fabricated tube to be
120 mm. In the original design, there was a resin dispersing
mechanism (Fig. 5C) designed in the end-effector beside the
position roller and supporting roller (Fig. 5F.a). However,
FIG. 7. Left: samples of success and failure version of testing prototypes; right: the inner surface of a successful
prototype.
FIG. 6. Fabrication time-lapse photographs of Prototype A.
6 OU ET AL.
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owing to the difficulties in balancing the curing time and
dispersing speed of the resin, in the preliminary explora-
tions, the resin is applied on the fabricated tube by brush
manually.
Robot settings
The head of the end-effector (Fig. 5I) was connected to
the Kuka KR16 robot arm to drive the GFRP fabric. During
the fabrication process, the end-effector operated in a spiral
path around the fabricated tube; therefore, the sixth axis
was set on a limitless mood in the robotic arm. The data
configuration file was set, so the robotic arm can rotate
beyond the default –360in a limitless spindle mode. The
path planning of the robotic arm was generated by the
KUKAjprc
45
and the Grasshopper plugin
46
of the 3D mod-
eling software Rhino 3D.
47
Material Properties
The section is composed of woven FRP fabric and epoxy
resin. The FRP fabric used in the prototype was plain-woven
GFRP fabric with a 150 mm width and 155 gsm density. The
adhesive used in the prototype was a two-part epoxy resin
Ampreg 22 with fast hardener; the mix ratio of the resin and
hardener was 100:28 by weight. The prototype was fabricated
at the indoor environment with a temperature of 25–30C. A
100 g resin was used in each mix, and the pot life (working
time) was *15 min before the viscosity of the mix was too
large to be applied by brush. The tensile strength of the cured
epoxy resin is 70.3 MPa according to the supplier.
48
Prototype Fabrications
To validate the feasibility of the proposed method, three
prototypes have been fabricated with it. An FRP hollow
segment with a height of 800 mm and cross-section changing
from a circular base with a 250 mm diameter to an elliptical
top with 280 and 180 mm axial lengths is first fabricated to
validate the proposed fabrication concept (Fig. 1). Following
the success of the initial prototype, a complementary exper-
iment was carried out by fabricating two additional proto-
types, validating the capability of such system in fabricating
formworks with more flexible geometries such as changing
section and developing along a curved path.
Prototype A: a straight hollow section
A straight hollow section with a height of 800 mm and
cross-section changing from a circular base with a 250 mm
diameter to an elliptical top with 280 and 180 mm axial
lengths was first fabricated. The prototype tube was fabri-
cated with the following workflow (as shown in Fig. 6).
Before the fabrication, one end of the FRP fabric was fixed to
the circular cardboard base at three points using paper fas-
teners. Then, the first layer of GFRP fabric was applied over
the base following the guide of the robotic arm. Right after
the fabric layer was applied, the mixed adhesive was applied
by brush on the outer surface of the tube. Then, the second
fiber layer was applied on top of the first one, with an overlap
of 140 mm (i.e., section height increases 10 mm after each
layer), and then resin was applied again. Each mix of 128 g
resin was enough for five layers of fabric, then the manu-
facturing process was paused for 30 min for the resin to be
FIG. 8. The section of FRP formwork and its condition during the winding process. (A) Self-supporting formwork;
(B) base; (C) fabric infiltrated by resin; (D) fabric clean from resin; (E) 15 layers of FRP fabric; (H) 150 mm.
Table 1. Summary of the Four Tests
for Prototype A
Hardener
Height
increases, each
layer (mm)
Fabrication
speed
(mm/15 min)
Test 1 Slow 50 250
a
Test 2 Fast 50 250
a
Test 3 Fast 30 150
a
Test 4 Fast 10 50
a
These values are theoretical numbers.
ADDITIVE FABRICATION OF FORMWORK BY FRP WINDING 7
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hardened. Afterward, the GFRP fabric was winded directly
over the hardened GFRP tube, and then repeated the process
until the full height is finished.
Three failed experiments were conducted before the first
success prototype was built (Fig. 7 and Table 1). The fol-
lowing technical skills were concluded from the trial tests in
Prototype A to provide a reliable fabrication quality:
(a) Proper hardener should be used in the fabrication. In Test
1, slow cure hardener was used, the fabrication process
stopped after *1 h because it took only a few minutes to
wind the firstfew fabric layers while taking >1 h for the resin
to cure. In Tests 2–4, fast cure hardener was used instead.
(b) The overlapping height of fabric layers is essential for
the success of the fabrication process. When the over-
lapping height is small, the section height increases fast
after each spiral run; however, the section may not be
strong enough to hold the later layers. To reduce the GFRP
material usage and increase the fabrication speed, 100 and
120 mm overlaps between two adjacent GFRP layers
(which means the section height increased 50 and 30 mm
with each spiral run) were used in Tests 2 and 3. But the
resultant section stiffness was not enough to maintain the
section shape when applying the upper layers. In Test 4, an
overlap of 140 mm between adjacent layers was found to
provide enough stiffness for the tube to hold its shape, so
the tube height increased 10mm with each spiral run and
there were 15 layers of fabric in each standard tube section.
(c) The winding speed of each spiral run was controlled
to be 1 min for the robotic arm and 2 min for resin ap-
plication. After each resin mix (five layers of fabric), the
process should be on hold for 30 min for the resin to be
hardened. As a result, the inner layers supporting the
bottom part of newly winded fabric should be cured for
*45 min, so it is hard enough to keep its shape during the
passive winding of FRP fabric; and the outer layers of the
wrapped fabric should be cured for *30 min to ensure it
is malleable and can be adapted to the section change dur-
ing the fabrication process.
(d) Excessive resin should be avoided at the outer surface of
the tube, so the supporting roller inside the tube (Fig. 5F.a) is
clean from the resin. As the resin creates adhesive force be-
tween the roller and the fabric during the rolling process, if
excessive resin permeate through the fabric layers, the inner
layers of fabric will stick to the position roller, when the
position roller move forward, the fabric will move with it and
the shape of the fabricated tube will be changed. (Fig. 7, Tests
2 and 3). To avoid this undesired deformation, the top freshly
winded 3–5 cm of GFRP fabric should be kept clean from
resin when applying the adhesive.
(e) At the end of the fabrication, there was a 150 mm high
region with decreasing layers because of the winding
process, as shown in Figure 8. This part is thinner than
the rest of the formwork, when assembling the segments
in the structure, it will be further reinforced by manual
GFRP wrapping, which also connects adjacent segments.
Evaluation
To evaluate the fabrication quality, a circular tube
with 200 mm diameter was fabricated using the proposed
FIG. 10. Local bump within the inner layers. Left: scan data; right: photograph.
FIG. 9. Fabrication accuracy from scan data. (A) Physical prototype; (B) scan results; (C) overlap of the scanned and
designed shape; (D) data comparison segment (150 mm high); and (E) deviation of the scanned and designed data. Color
images are available online.
8 OU ET AL.
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fabrication method. The tube was scanned to check the
fabrication accuracy compared with the designed shape,
and then split tests were carried out to check the tensile
strength of the tubes in hoop direction. The scan results are
as shown in Figure 9; a 150 mm tall part in the middle of
the tube was taken to check the geometry accuracy. It
shows that the average deviation of the tube was 3.57 mm,
while the overall shape accuracy was 98.4%. Split tests
were carried out according to ASTM D2290,
49
5 standard
rings (with 15 layers of fiber) with 29.92 mm average height
and 4.32 mm average thickness were cut from the tube for
the tests. The average tensile strength of the rings was
52.32 MPa.
Local bump (delamination between layers) was observed
in the tube, especially within the layers beside the inner
surface, as shown in Figure 10. As discussed in the previous
part, exclusive adhesive was avoided to reduce the undesired
deformation caused by the adhesive force between the sup-
porting roller and the fabric. This also makes most part of the
inner fabric layers clear from resin (Fig. 8), so with low
interlayer bonding in this part. Besides, when we look close
into the fabricated surface, inclined pattern can be observed
(Fig. 6), this was caused by the traction force of the fabric
while winding.
Prototypes B and C: tubes with nonstandard sections
and varying direction
After achieving the first successful prototype using the
proposed fabrication method, two more prototypes as shown
FIG. 12. Structural column topological optimization process: (A) initial geometry, (B) initial FEA model setting, (C) and
(D) optimization process, and (E) optimized smoothed model. FEA, finite element analysis.
FIG. 11. Left: Prototype B, a tube with nonuniform section; right: Prototype C, a tube with changing direction.
ADDITIVE FABRICATION OF FORMWORK BY FRP WINDING 9
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in Figure 11 were fabricated following the same process.
Prototype B is a 500 mm high tube with a circular base with
200 mm diameter and varies to be a shamrock-shaped section
at the top, while Prototype C is a circular hollow section
changing its direction gradually during the fabrication pro-
cess. These two prototypes further showcased the feasibility
of using this fabrication method on GFRP hollow sections as
nonstandard concrete formwork.
Discussion
The average fabrication speed of the current prototypes
was 100 mm/h (height increase). There are several possible
ways to speed up the winding process. For example, fabric
with higher stiffness can be used to reduce the required
overlapping layers for each section and larger height in-
crease with each winding; and the general-purpose epoxy
resin can be replaced by ultraviolet-curing resin to accelerate
the curing process. These aspects will be studied in future
research.
The concrete confinement in the CFFTs is mostly pro-
vided by the FRP strength in the hoop direction.
50
In this
study, the split test results showed the high strength of the
fabricated tube in hoop direction, which verifies the poten-
tial of applying it in the CFFTs. In addition, the impregna-
tion of the resin at the outer surface layers helps holding
the concrete in the tube. Traditionally, in the FRP mat-
reinforced concrete structures, the mat is wrapped on the
concrete surface by wet layup method, while in this appli-
cation the fabric is winded in a loose aggregation. Whether
the confinement provided by the FRP formwork in this study
is comparable with the traditional FRP confinement is yet
to be evaluated.
Outlook of this Application
This article explores the integration of emerging technol-
ogies in digital structural design, AM technique, and estab-
lished structure systems, including topological optimization,
robotic fabrication, and FRP reinforcements. In this research,
three prototypes have been developed with a novel fabrica-
tion method based on a synthesis of the technologies to
fabricate the highly optimized nonstandard structures in a
cost-, time-, and material-efficient way.
This fabrication method was inspired by the CFFT or
DSTA structures in which the FRP tube works as the form-
work and confinement for the concrete inside. Based on the
established construction sequence of CFFT or DSTA struc-
ture, which includes prefabrication of circular steel core
and FRP skin/formwork, FRP formwork trimming, installa-
tion, and segment jointing with wrapping,
13
an application
outlook of the proposed method is presented in this part on
FIG. 13. Processing and fabrication process of a segment
of the column:(A) original geometry, (B) segmentation of
geometry, (C) geometry preparation, (D) FRP formwork
winding, and (E) formwork trim. Color images are available
online.
FIG. 14. Assembly sequence: (A) inner structure, (B) winding of the base, (C) assembling the branches, and (D) rein-
forcement with manual wrapping at the joint and base.
10 OU ET AL.
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the in situ construction of a topologically optimized and
customized large-scale structure with FRP with similar con-
struction sequence.
Applying bidirectional evolutionary structural optimiza-
tion (BESO)
51
algorithm on the loading condition and design
domains in Figure 12, an optimized tree-branch-like structure
is generated. To fabricate the BESO-generated structure, the
generated geometry is first preprocessed and refined to be
compatible with the proposed fabrication system. The
workflow is composed of the following steps:
(a) Segmentation of geometry. The structure is divided
into segments to fit the operation range of the robot and
the compatibility of the fabrication process. The geometry
is usually separated at the branching root since the current
system does not accommodate the fabrication of branch-
ing geometry (Fig. 13A, B). Meanwhile, a steel core is
extracted from the geometry as the skeleton.
(b) Geometry preparation. The geometry of each segment
is processed by computer to extend the end-surface to be
in a full loop of the tubular geometry (Fig. 13C). The
geometry is also adjusted to ensure the clearance and
gradient are compatible with the end-effecter.
(c) Prefabrication of steel core. The prefabricated steel
core is made of standard steel profiles such as channel,
angle, or steel rod; the skeleton is prefabricated as stan-
dard segments and will be assembled with the FRP
formwork fabricated on-site.
(d) FRP formwork winding. The outer surface of each
segment of the optimized column is fabricated on-site
with the system developed in this research using FRP
winding, starting from a short 3D printed base with the
same geometry of the end or branching root region of the
optimized geometry (Fig. 13D). After the formwork is
finished, it will be removed from the base, and the extra
part at the ends will be trimmed according to the geom-
etry of the original segment (Fig. 13E).
(e) Steel core and formwork installation. The steel core
will be placed at the base (Fig. 14A), followed by the
‘‘trunk’’ part of the column, which will be fabricated
outside of the steel core using the proposed winding
method (Fig. 14B). Then, the fabricated ‘‘branch’’ form-
work will be placed at the designed location of the skeleton
and be joined with other segments by a manual FRP wrap
around the joints (Fig. 14C, D). After the FRP parts are
finished, the concrete can be cast in the tube.
This study is a preliminary research on the feasibility of
this fabrication method, while more work is needed to up-
scale this method in the upcoming research. For example,
more systematic studies will be done to check the relation-
ship between the overlapping height, fabrication speed,
and overall strength for different section sizes and shapes;
concrete will be filled inside the tube, and the compres-
sive strength of the confined concrete will be compared with
the plain concrete; epoxy resin dispenser and the dispense
rate will also be studied for a better automatic and accurate
control.
Conclusion
As demonstrated and verified with the fabricated proto-
type, this article presents a new fabrication method for the
customizable FRP tubular structure, which can be used as
both the concrete formwork and the structural reinforce-
ment for large-scale nonstandard sections. The nonuniform
structural formwork is manufactured by robotic winding of
GFRP fabric; fast cure adhesive resin is applied during the
winding process so the fabricated tube can be self-supported.
Multiple layers of fabric are winded in a spiral way to extend
the segment with changing section and direction using the
path planning of industrial robots. The proposed system
makes it possible to fabricate large-scale customized tubu-
lar formwork on-site, where the robots are accessible, and
then concrete can be cast. The unparalleled flexibility in the
system made the fabrication of highly optimized structure
system possible without a significant increase in cost and
construction waste. Although the proposed method has only
been applied on indoor condition, it has high potential to be
applied on-site, as the fabrication process is highly robotic
controlled, which guarantees the fabrication quality in dif-
ferent environments.
The method and prototypes developed in this research
provide an alternative solution to fabricating customizable
large-scale structural members for on-site construction. The
system is derived from a well-developed construction pro-
cess, allowing it to be compatible with existing building
process in the construction industry. With the rapid fabrica-
tion of customizable FRP formwork, the novel method can
significantly reduce the cost of transportation and possible
damage of the tubes, while increasing structural flexibility
and efficiency.
Author Disclosure Statement
On behalf of all authors, the corresponding author states
that there is no conflict of interest.
Funding Information
The authors received financial support provided by Uni-
versity of Queensland and the Science and Technology In-
novation Program of Hunan Province (Grant No. 2020RC4049).
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