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\begin{document}

\title{Anisotropic properties of additively manufactured lightweight concrete
elements}



\author[1]{Marco Lindner}%
\author[1]{Ralf Gliniorz}%
\author[1]{Henrik Funke}%
\author[1]{Sandra Gelbrich}%
\affil[1]{no affiliation}%


\vspace{-1em}



  
  \date{May 16, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
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\selectlanguage{english}
\begin{abstract}
Robotic concrete extrusion is a novel additive manufacturing process (3D
concrete printing) and is part of a continuously digitally controlled
value chain. According to the state of the art, concrete is considered
to be an isotropic material due to the manufacturing process. However,
for the additive manufacturing process, the isotropic approach hast to
be reconsidered due to the layered structure. It can be assumed that due
to the layered structure, the material properties vary depending on the
deposition direction and the geometry of the layers. The aim of the work
was to record the material-technical characteristics of extruded
elements manufactured according to standards in comparison with concrete
recipes. Process-related influences on the mechanical parameters of
additively manufactured concrete elements were examined and evaluated in
more detail. Based on the findings obtained, the dimensioning, design
and measurement of components can be carried out and thus guidelines for
components can be derived. With these derived guidelines, the material
utilization and economic efficiency can be improved.%
\end{abstract}%



\sloppy


Anisotropic properties of additively manufactured lightweight concrete
elements

Marco Lindner*, Ralf Gliniorz, Henrik Funke, Sandra Gelbrich

E-mail: marco.lindner@mb.tu-chemnitz.de

Keywords: Robotic 3D printing, additive manufacturing, extrusion with
concrete, anisotropic properties,

Robotic concrete extrusion is a novel additive manufacturing process (3D
concrete printing) and is part of a continuously digitally controlled
value chain. According to the state of the art, concrete is considered
to be an isotropic material due to the manufacturing process. However,
for the additive manufacturing process, the isotropic approach hast to
be reconsidered due to the layered structure. It can be assumed that due
to the layered structure, the material properties vary depending on the
deposition direction and the geometry of the layers. The aim of the work
was to record the material-technical characteristics of extruded
elements manufactured according to standards in comparison with concrete
recipes. Process-related influences on the mechanical parameters of
additively manufactured concrete elements were examined and evaluated in
more detail. Based on the findings obtained, the dimensioning, design
and measurement of components can be carried out and thus guidelines for
components can be derived. With these derived guidelines, the material
utilization and economic efficiency can be improved.

1. Introduction

Rapidity, precision, economics and resource saving are the most
important requirements of the construction industry of the future.
Modern lightweight construction technologies can implement these
requirements and add the aspects of design freedom, efficiency, and
sustainability. Therefore, the development of new manufacturing
technologies for highly stressable, durable and free-forming
prefabricated concrete components with low material and energy input is
the focus of the research group ``Lightweight Constructions in Civil
Engineering'' (LBW) at the Chemnitz University of Technology. In
general, the well-known building material concrete is considered
isotropic due to the material properties and classical processing
methods (casting, laminating e.g.). In contrast, the application of
additive manufacturing processes result in anisotropic material
properties because of the layered structure of the concrete elements by
means of extrusion.

The aim of the work was to determine and compare the material properties
of test specimens produced by casting (in accordance with standards DIN
EN 12390) and by means of extrusion technology to be able to draw
conclusions about anisotropic material behavior because of the extrusion
process.

In addition, numerical investigations were conducted to reproduce the
results of the mechanical tests. As a part of the investigation,
process-related influences on the mechanical parameters of additively
manufactured concrete elements were examined and evaluated in more
detail. Based on the findings obtained, design and dimensioning of
components can be conducted and thus guidelines for components can be
achieved. With these derived guidelines, material utilization and
economic efficiency can be improved and the lightweight construction
principles can be implemented.

2. Material and methods

2.1. Experimental procedure

Additive manufacturing with mineral materials, in particular
robot-assisted concrete extrusion, is a new manufacturing technology in
construction and offers architects and civil engineers a variety of
options for implementing lightweight construction concepts. At the same
time, it requires great care in selecting the correct manufacturing
parameters and material composition. The development was executed at the
research facility ``LBW RoboArena'' (\textbf{Table 1} .) at Chemnitz
University of Technology {[}1, 2, 3, 4, 5, and 6{]}.

\textbf{Table 1.} Research facility ``LBW RoboArena'' at research group
LBW\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
& research facility & research facility\tabularnewline
\midrule
\endhead
& robot system & KUKA KR 90 R2700 pro\tabularnewline
& conveyor system & PFT BOLERO FC-230V\tabularnewline
& max. rate of delivery Q\textsubscript{max} & 2.5 l/min\tabularnewline
& control technology & KUKA KR C4\tabularnewline
& control technology & SPS Siemens S7\tabularnewline
& hose type & RSI 40 BD\tabularnewline
\bottomrule
\end{longtable}

The extrusion technology, consisting of a conveyor system (screw
conveyor on the mixer) and a robot-guided tool head, generate the
concrete layers. A spray-extrusion multi-tool nozzle (SEMT) was
developed and implemented for the production of the concrete modules in
the robot-supported flow production process in order to avoid a changing
of tools in the production process while changing from the spray to the
extrusion technology. This allows a thin additive application of
material by spraying concrete and then the application of an adapted
concrete extrudate. For example, facade modules consist of a
fibre-reinforced shotcrete surface with a high surface quality and a
support structure made of extruded thin concrete, which is applied
directly to the shotcrete in the green state. On the one hand, the new
type of nozzle allows the concentric, fibre-reinforced extrusion of a
concrete layer and, on the other hand, the generation of a spray mist
through the controlled activation of compressed air and thus the shaping
by means of concrete spraying. The SEMT is attached to the robot arm,
using a quick coupling. The following table 2 summarizes the tool
parameters for testing.

\textbf{Table 2.} Tool parameters\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& technical parameters of the experiment & technical parameters of the
experiment & technical parameters of the experiment\tabularnewline
\midrule
\endhead
& nozzle diameter spraying & {[}mm{]} & 10\tabularnewline
& air pressure spraying & {[}bar{]} & 5\tabularnewline
& nozzle diameter extrusion & {[}mm{]} & 20\tabularnewline
& rate of delivery & {[}l/min{]} & 1.3\tabularnewline
& feed nozzle speed & {[}m/s{]} & 0.06\tabularnewline
& endless fibre & {[}-{]} & non\tabularnewline
\bottomrule
\end{longtable}

As a result, several process steps (spraying, extrusion, endless fibre
reinforcement) can be implemented in an automated process with the
spray-extrusion multi-tool nozzle without changing the tool or position
of the component. In this publication, reinforcement with endless fibres
for better comparability was not used.

2.2. Materials

The material composition bases on a complex of technological
restrictions and requirements of the precast concrete element. For
example, a high-strength and durable fine-grained concrete with
short-fibre reinforcement was developed, so these could be processed
using spraying and extrusion. When characterizing the material, the
green strength as well as the material and component-related strength
and failure behaviour under compressive and flexural stress were of
particular interest. In addition to high strength in the use state, the
concrete for this process combination of spraying and extrusion also
requires application-specific properties in the uncured state, such as
high green strength, thixotropy, low shrinkage, adjusted processing time
and good bonding behaviour of the individual layers as well as the
contact zone between the sprayed and extruded layers.

To evaluate the durability, crack and failure behaviour of the reference
components, climate change tests and angle-dependent insert pull-out
tests were also carried out. Furthermore, the influence of the recipe
and the process parameters on the material characteristics in the fresh
and hardened state was examined and described.

Another focus was on the exposed concrete properties of the sprayed
concrete shell for facade elements. For this purpose, the air void
content in the fresh concrete, the exposed concrete surface according to
the DBV/VDZ data sheet in porosity class and deviations from evenness
and colour requirements according to DIN 18202 were determined and
evaluated {[}7, 8{]}.

The concrete recipe was determined empirically with constant testing of
the consistency and is summarized in table 3. The mixing process was
using an Eirich R05T.

\textbf{Table 3.} Concrete mixture\selectlanguage{english}
\begin{longtable}[]{@{}lll@{}}
\toprule
& concrete mixture kg/m\selectlanguage{ngerman}³ & concrete mixture kg/m\selectlanguage{ngerman}³\tabularnewline
\midrule
\endhead
& cement & 620\tabularnewline
& silica fume & 120\tabularnewline
& Dolomite filler & 300\tabularnewline
& dispersible AR--glass fibre 12 mm & 0.75\tabularnewline
& PCE superplasticizer & 8.6\tabularnewline
& stabilizer & 1.5\tabularnewline
& retarder & 6,2\tabularnewline
& W/B & 0.34\tabularnewline
\bottomrule
\end{longtable}

2.3. Proof of usability

Concrete is regarded as a homogeneous, joint-free material in the
construction industry. However, this cannot be assumed for additive
manufacturing, since several layers of concrete are build up. Due to the
production-related anisotropy, the compressive strength was determined
in three load directions. Figure 1 serves to clarify the load directions
and the designation. Here, the placement direction is in the
X-direction. The installation height in the Z-direction is based on the
number and dimensions of the layers. The width of the component in the
Y-direction is depending on the width and number of the extrudate.
Furthermore, samples were produced in two ways: the concrete was
injected directly and extruded into the moulds.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 1.} Extrusion test specimen: CAD (left) and physical
while production (right)

To determine the compressive strength (f\textsubscript{ck}, cube, 28 d),
cube-shaped specimens with an edge length of 150 mm were produced and
stored under water for 28 days (according to DIN EN 12390-2 {[}11{]}).
The compressive strength was determined in accordance with DIN EN
12390-3 {[}12{]}. Table 4 summarizes the test parameters.

\textbf{Table 4.} Test parameters\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& test parameters on the basis DIN EN 12390-3 (n=3) {[}12{]} & test
parameters on the basis DIN EN 12390-3 (n=3) {[}12{]} & test parameters
on the basis DIN EN 12390-3 (n=3) {[}12{]}\tabularnewline
\midrule
\endhead
& testing machine & & ToniTechnik\tabularnewline
& sample dimension & {[}mm{]} & 150 x 150 x 150\tabularnewline
& testing age & {[}d{]} & 28\tabularnewline
& speed & {[}MPa{]} & 0.5\tabularnewline
& test temperature & {[}\selectlanguage{ngerman}°C{]} & 20\tabularnewline
\bottomrule
\end{longtable}

Due to the spatial orientation of additively manufactured specimens, the
flexural strength is also dependent on the direction of loading (Figure
2).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 2.} Extrusion

A four-point bending tensile test was chosen to determine the load
properties, because in contrast to the three-point flexural test, there
is a constant bending moment between the upper force application points.
In this way, the material parameters are determined more realistically,
especially in the case of plate-shaped samples, and these are less
influenced by possible inhomogeneities in the sample. For the
determination of the four-point bending tensile strength, based on DIN
EN 12390-5 {[}13{]}, were prismatic specimens (extrudate) manufactured
and stored under water. The Test parameters are summarized in table 5.

\textbf{Table 5.} Test parameters 4-point bending tensile test\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& test parameters on the basis DIN EN 12390-5 (n=6) {[}13{]} & test
parameters on the basis DIN EN 12390-5 (n=6) {[}13{]} & test parameters
on the basis DIN EN 12390-5 (n=6) {[}13{]}\tabularnewline
\midrule
\endhead
& testing machine & & ToniTechnik\tabularnewline
& sample dimension & {[}mm{]} & 40 x 40 x
\textgreater{}160\tabularnewline
& testing age & {[}d{]} & 28\tabularnewline
& speed & {[}N/mm\selectlanguage{ngerman}²s{]} & 0.05\tabularnewline
& test temperature & {[}\selectlanguage{ngerman}°C{]} & 20\tabularnewline
\bottomrule
\end{longtable}

In addition to the classic parameters, there is a focus on the bond
between the sprayed and extruded layer (Figure 3). For assessment, a 15
mm thin layer was sprayed and a 40 mm thick layer of extrudate was built
up on top of this.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 3.} Spraying (left) and extrusion (right)

After the 28 days of hardening, the samples were prepared in such a way
that this contact zone could be subjected to tensile loads. Here drill
cores were created with a depth of 5 mm protruding into the sprayed
layer (Figure 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 4.} Set-up test specimen

A defined test specimen (round) is glued to the surface of the drill
core with epoxy resin adhesive. The tensile force is now initiated into
the construction via the test plate until a fracture occurs. The test
parameters are summarized in table 6.

\textbf{Table 6.} Compilation of test parameters\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& test parameters on the basis DIN 18555-6 {[}14{]} & test parameters on
the basis DIN 18555-6 {[}14{]} & test parameters on the basis DIN
18555-6 {[}14{]}\tabularnewline
\midrule
\endhead
& testing machine & & Form+Test ComTest\tabularnewline
& sample dimension & {[}mm{]} & \selectlanguage{ngerman}Ø = 50\tabularnewline
& testing age & {[}d{]} & 28\tabularnewline
& speed & {[}N/mm\selectlanguage{ngerman}²s{]} & 0.05\tabularnewline
& test temperature & {[}\selectlanguage{ngerman}°C{]} & 20\tabularnewline
& adhesive & & Silikal RI/21\tabularnewline
\bottomrule
\end{longtable}

3. Results

The practical proof of the suitability of the newly developed nozzle
tool for the production of reinforced and non-reinforced extrudates and
sprayed layers is based on pressure, flexural and tensile adhesion
tests. The samples with the designation ''M'' are characterized by the
fact, that the concrete was taken directly from the concrete mixer. The
samples with ''S'' and ''E'' were \emph{s} prayed or \emph{e} xtruded
directly into the corresponding mould. The samples marked ``X'', ``Y'',
``Z'' depict the direction of loading in the coordinate space. The
following figure 5 summarizes the compressive strength results.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 5.} Summary of compressive strength results

In comparison to the compressive strengths, the sprayed ''S'' and the
extruded ''E'' samples characterize significantly high values. It is
assumed that the particle acceleration minimizes the defects. The
samples with ''X'' are loaded in the direction of feed, which means that
the front sides of the layers are subjected to pressure. It can be
assumed that the layer geometry has less influence since the layers
support each other against buckling. It is assumed that the layer
geometry has an influence on the two sample batches ''Z'' and ''Y''. The
cross-section of the extrudate is not rectangular with sharp edges, but
rounded, so that defects accumulate.

A comparison of the bending tensile strengths of the individual batches
shows figure 6.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure 6.} Summary of the flexural strength results

Because of the compilation, it can be stated that all sample batches
produced with the new tool can generate high flexural strengths with
little deviation. Similar to the pressure tests, the sprayed samples
also showed the highest strength.

To assess the bond between the sprayed layer and the extruded layer, the
area was subjected to a tensile test. The mean value is 2 MPa (Min. 1.9
MPa Max. 2.2 MPa). The fracture took place in the extrudate. According
to DIN 18555-6 it is a cohesive fracture. This means that the contact
zone is more resilient than the joining partners are.

Overall, it can be stated that the extrudates produced with the new tool
achieve a very high level in terms of strength and deviations.

4. Conclusions

The aim of the work was to determine and compare the material properties
of test specimens produced by casting (in accordance with standards) and
by means of extrusion technology to be able to draw conclusions about
anisotropic material behaviour caused by the extrusion process. As a
part of the investigation, process-related influences on the mechanical
parameters of additively manufactured concrete elements were examined
and evaluated in more details. Based on the findings obtained, design
and dimensioning of components can be conducted and thus guidelines for
components can be achieved. With the guidelines derived from this,
material utilization and economic efficiency can be improved and the
principles of lightweight construction can be implemented.

Acknowledgements

The European Regional Development Fund (ERDF) via the Saxon Development
Bank (SAB) funded the extrusion system used for this article. A priority
program SPP 2187 of the DFG funded the development of the tool and the
research performance. The authors would like to thank the donor of
subsidies for the financial support.

References

{[}1{]} M. Lindner, E. Rudolph R. Gliniorz, H. Funke, S. Gelbrich, L.
Kroll. \textbf{2021} , \selectlanguage{ngerman}\emph{Kontinuierliche Fertigung von dünnwandigen
faserverstärkten Präzisions-Betonelementen für freigeformte
Modulbauweisen} , ICCX Western Europe; Poster.

{[}2{]} M. Lindner, R. Scharf-Wildenhain, R. Gliniorz, H. Funke, S.
Gelbrich, L. Kroll. \textbf{2021} , \emph{Kalibrierverfahren für
konstante Materialstränge bei robotergestützer Betonextrusion} , Beton-
und Stahl-betonbau 116, Sonderheft Schneller bauen S2,
https://doi.org/10.1002/best.202100051, S. 42-47.

{[}3{]} M. Lindner, K. Mandel, R. Gliniorz, H. Funke, S. Gelbrich, L.
Kroll. \textbf{2021} , \emph{Kontinuierliche Fertigung von dünnwandigen
faserverstärkten Präzisions-Betonelementen für freigeformte
Modulbauweisen (KoBeMo)} , BetonWerk International Nr. 1, ISSN
1437-9023, S. 28-29.

{[}4{]} M. Lindner, K. Vanselow, R. Gliniorz, S. Gelbrich.\textbf{2020}
, \emph{Additiv gefertigte Leichtbaustrukturen mit Beton und
Faserbewehrung} , Fachtagung Werkstoffe und Additive Fertigung,
https://additive-fertigung-2020.dgm.de/home.

{[}5{]} S. Gelbrich, M. Lindner, R. Wagler, R. Gliniorz, K.
Vanselow.\textbf{2020} , \emph{Verfahren und Vorrichtung zur Herstellung
eines faser- und/oder textilbewehrten mineralischen Bauteils} ,
DE102019133755A1.

{[}6{]} K. Vanselow, M. Lindner, R. Gliniorz, S. Gelbrich (2021),
Innovative technology of additive manufacturing of textile concrete for
lightweight structures, Aachen-Dresden-Denkendorf In-ternational Textile
Conference 2021.

{[}7{]} DBV/VDZ DBV/VDZ Merkblatt Sichtbeton;
https://www.baunetzwissen.de/beton/tipps/planungshilfen/dbv-vdz-merkblatt-sichtbeton-4581763,
12.04.2022.

{[}8{]} DIN 18202: Toleranzen im Hochbau - Bauwerke. Ausg. 2019-07.

{[}9{]} DIN EN - 206-1: Beton - Teil 1: Festlegung, Eigenschaften,
Herstellung und Konformität. Ausg. 2001-07.

{[}10{]} DIN 1045-2: \emph{Tragwerke aus Beton, Stahlbeton und
Spannbeton - Teil 2: Beton - Festlegung, Eigenschaften, Herstellung und
Konformität - Anwendungsregeln zu DIN EN 206-1} . \textbf{2008} .

{[}11{]} DIN EN 12390-2: \emph{Prüfung von Festbeton - Teil 2:
Herstellung und Lagerung von Probekörpern für Festigkeitsprüfungen}
.\textbf{2009} .

{[}12{]} DIN EN 12390-3: Prüfung von Festbeton - Teil 3: Druckfestigkeit
von Probekörpern. \textbf{2009} .

{[}13{]} DIN EN 12390-5: \emph{Prüfung von Festbeton - Teil 5:
Biegezugfestigkeit von Probekörpern} . \textbf{2009} .

{[}14{]} DIN 18555-6: \emph{Prüfung von Mörteln mit mineralischen
Bindemitteln; Festmörtel; Bestimmung der Haftzugfestigkeit}
.\textbf{1987} .

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\selectlanguage{english}
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