Document [original]

Citation: Glotz, T.; Petryna, Y.

Experimental Characterization of

Anisotropic Mechanical Behaviour

and Failure Mechanisms of Hardened

Printed Concrete. Materials 2024,17,

3931. https://doi.org/10.3390/

ma17163931

Academic Editor: Arnaud Perrot

Received: 8 July 2024

Revised: 2 August 2024

Accepted: 3 August 2024

Published: 7 August 2024

Licensee MDPI, Basel, Switzerland.

This article is an open access article

distributed under the terms and

conditions of the Creative Commons

Attribution (CC BY) license (https://

creativecommons.org/licenses/by/

4.0/).

materials

Article

Experimental Characterization of Anisotropic Mechanical

Behaviour and Failure Mechanisms of Hardened

Printed Concrete

Theresa Glotz * and Yuri Petryna *

Chair of Structural Mechanics, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany

*Correspondence: [email protected] (T.G.); yuriy[email protected] (Y.P.)

Abstract: Extrusion-based printing of cementitious materials represents an innovative technology in

civil engineering. The additive manufacturing process significantly influences the material properties

in the hardened state, leading to anisotropic behaviour in terms of stiffness and strength compared

to conventionally cast concrete. This experimental study aims to deepen the understanding of the

mechanical behaviour of hardened printed concrete. Beam-like specimens with varying printing

patterns, loading orientations and lengths are investigated within three-point bending tests (3PBT)

and uniaxial compression tests (UCT). Homogenized material parameters such as Young’s modulus,

compressive and flexural tensile strength and density are statistically evaluated using optically

measured displacement and strain fields on the specimen surface. The qualitative and quantitative

results demonstrate a strong dependency of material properties and failure mechanisms on the

printing pattern. The interfilamental and interlayer areas with weak adhesion are identified as the

main reason for anisotropy.

Keywords: 3D printing; additive manufacturing; hardened concrete; mechanical properties; anisotropy;

failure mechanisms; experiment; DIC

1. Introduction

The adoption of 3D printing processes in the construction industry is transforming

the building sector and expanding perspectives on construction methods. The contour

crafting approach introduced by Khoshnevis [

] marked a significant milestone, paving

the way for subsequent developments in additive manufactured concrete, as summarized

in [

]. 3D concrete printing (3DCP) enables the creation of complex structures with high

design flexibility, enhanced safety standard by reducing physical labour, improved material

efficiency and cost reductions [

–

]. An overview of the different additive manufacturing

techniques can be gained in [

]. The research field for extrusion-based 3DCP is vast, with

various aspects requiring focused study. In the fresh state, materials must be pumpable,

extrudable, printable and capable of meeting buildability requirements [

]. Consequently,

significant emphasis is placed on material mix design in 3DCP research. Further research

areas include the integration of reinforcement and digital aspects such as process control

and modelling of printed concrete and related processes [9,10].

However, the properties and performance of the hardened printed components are

decisive for the practical application. Unlike conventionally cast concrete, the layer-by-layer

deposition process of 3DCP results in anisotropic material behaviour, which significantly

influences the mechanical performance and emerges as a critical mechanical property [

–

This anisotropy has been investigated in numerous studies. Ding et al. provide a critical

overview of recent research focused on interfaces in printed concrete. From a microstructural

perspective, the primary influencing factors include interface humidity, moisture transport

processes and the characteristics of the microporous structure [11].

Materials 2024,17, 3931. https://doi.org/10.3390/ma17163931 https://www.mdpi.com/journal/materials

Materials 2024,17, 3931 2 of 21

Anisotropy in printed concrete is evident at multiple levels. Wolfs et al. identify the

interlayer interval time as a significant factor influencing bond strength [

]. Other studies

also report that the mechanical performance depends on the adhesion between printed

layers, with interface properties deteriorating as interval times increase [

]. Similarly,

Sanjayan et al. found that the print-time interval affects interlayer strength and correlated

this to the surface moisture content at the interface of the layers [16].

Investigations focus not only on the presence of interlayers but also on their orientation

concerning the applied load. The mechanical properties exhibit anisotropic character

depending on the orientation of the layers and the direction of load application [

]. Ding

et al. showed the effect of orientation on tensile splitting strength through experimental

investigations with recycled sand [

]. Additionally, Kumar et al. reported that in a test

series with translational and depositional interfaces, the compressive strength of printed

concrete was lower, but the flexural strength was higher compared to cast specimens [19].

From a micro-structural perspective, several investigations have demonstrated higher

local porosity in the interlayers by analyzing the air void content in both the bulk material

and interlayer regions [

]. The presence of macro pores is a determining factor for weak

interface adhesion [

]. Liu et al. analyzed the effect of pore structure on printed concrete

with coarse aggregate, establishing a connection between pore defect geometry and cracking

damage [

]. Some studies have advanced further by applying cementitious paste at the

interface to increase bond strength, a technique successfully demonstrated by [23].

The analysis of fracture mechanisms due to the additive manufacturing process is

addressed in only a few studies [

]. Crack propagation in specimens has been shown to

occur predominantly along the interface of two layers during splitting prism tests [

]. An

examination of layer height effects on the flexural and fracture response of plain and fiber-

reinforced 3D-printed beams indicated that smaller layer heights offer benefits, despite

introducing more interfaces and requiring longer printing times [

]. Some studies have

included numerical simulations using finite element (FE) modelling strategies to predict

structural performance in the hardened state and the failure mechanisms of reinforced

concrete beams under various loading conditions, showing good agreement with experi-

mental results [

]. Pi et al. investigated the crack propagation and failure mechanisms

of specimens reinforced with 2% polyvinyl alcohol fibers with a length of 12

. Un-

der bending loads, two new crack propagation modes were identified: bending cracks

leading to the splitting of adjacent filaments and localized shear cracks that dominate

in cases of excessively low interlayer bond strength [

]. In [

], the acoustic emission

technique was used to monitor the micro-cracking mechanism in printed concrete. The

most recent study by Tang et al. distinguishes between two different crack types depending

on the propagation mode in notched three-point bending tests: trans-layer and inter-layer

fractures [24].

Despite the extensive research conducted so far, there is a remaining need for further

investigation into certain aspects of the mechanical performance of printed concrete. Most

experimental studies use specimens that have been printed and then saw-cut for testing.

While this approach facilitates comparative analysis, it is limited by the lack of unified

test methods and standards, which complicates the comparison of results from different

studies [

]. Mechtcherine et al. have noted that by cutting the sides of the outer

regions, the effects on mechanical performance can no longer be accurately assessed [

Furthermore, there is a need for a more thorough investigation of the fracture mechanisms

of the entire component.

This study aims to address these gaps by investigating a series of beam-like printed

concrete specimens with different printing patterns in three-point bending tests (3PBT) with

varying loading orientations and uniaxial compression tests (UCT) with different specimen

lengths. The specimens feature multiple filaments per layer, introducing not only interlayer

but also interfilament interfaces. UCT is conducted on whole specimens rather than cubes

to better observe failure modes. Digital image correlation (DIC) techniques are used for

optical measurements, as applied in previous studies [

]. For both tests, the

Materials 2024,17, 3931 3 of 21

results are evaluated with respect to mechanical parameters such as stiffness, strength and

density. Additionally, the failure mechanisms are analyzed in detail to gain a comprehensive

understanding of the mechanical processes in the material at a macroscopic scale, without

the influence of reinforcement. This research seeks to gain a deeper understanding of

the structural performance of printed concrete, taking into account the actual printed

cross-sectional geometry that varies due to the additive manufacturing process.

2. Materials and Methods

2.1. Printing System and Concrete Mix Design

The printer used to manufacture the specimens is a

xyz

-gantry system incorporat-

ing an active pumping nozzle for material deposition designed at TU Berlin, for further

information see [

]. The material employed for the printing process is a cement-based

mortar consisting of 900g CEM III/A 42.5 N, 600g aggregates (

∅

0.1

mm to

0.6

), 470g

powdered limestone, 485g water, 9 g defoaming agent (SIKA Control 300 PerFin) and 4g

short basalt fibers of 6

length. A detailed investigation of the material composition

referred to as G0 was carried out by Cuevas et al. in [36], and it was also applied in [37].

2.2. Test Series

In order to systematically investigate the anisotropic properties of the material in

longitudinal and transverse direction, three different printing patterns are applied with

lengthwise (L), alternately length- and crosswise (LC) and crosswise (C) printed filaments,

see Figure 1a–c. For LC specimens, this means that the filaments of the first layer are

printed in the lengthwise direction, the second layer in the crosswise direction, the third

layer again in the lengthwise direction, and so on. The printing time for a single L specimen

is approximately 20

min

, whereas the printing time for LC and C specimens is increased

by 10% and 15% respectively, due to the higher number of directional changes in the

printing path for crosswise printed filaments. In total, 30 beam-like specimens with a

desired geometry of

50 ×50 ×500mm

are printed, with ten specimens for each of the

three printing patterns. The primary objective is to test the specimens in their original

geometry. Nevertheless, some specimens need to be saw-cut to achieve a plain load

application surface.

(a)

(b)

(c)

100 mm

side view top view

Figure 1. Overview of printed specimens in side and top view with different printing patterns:

(a) lengthwise (L), (b) alternately length-/crosswise (LC) and (c) crosswise (C) printed filaments.

2.3. Experimental Setup

The experimental testing arrangement consists of two parts. The first part involves a

3PBT conducted until failure, with a support span of 400

using two different combina-

tions of layer orientation and loading direction, see Figure 2a. A 0

◦

orientation describes a

loading direction perpendicular to the printing path direction, whereas a 90

◦

orientation

indicates load application to the perpendicular edge of the beam. The specimens tested

with 90

◦

orientation are saw-cut along their long side to obtain even surfaces at the support

and load application points.

Materials 2024,17, 3931 4 of 21

As a second part, a UCT with two different specimen length is conducted in order to

investigate the influence of the specimen length on the mechanical performance (Figure 2b).

The long and short specimens are saw-cut to a length of 400

and 190

respectively,

with minor deviations due to the sawing process. As shown in Figure 2, the

-axis of the

applied coordinate system is always aligned with the longitudinal axis of the specimen

for both 3PBT and UCT. The

-plane spans the surface for the optical displacement and

strain measurements.

All test are conducted under displacement control with rates of 100

µm/min

and

250

µm/min

(3PBT) and 500

µm/min

(UCT). The applied machine force is continuously

monitored for the induced displacement at the midpoint of the specimen. Additionally, the

displacement and strain fields on the front side of the specimen are continuously measured

(see Section 2.4.2).

400 mm

90°

(a) (b)

u, F

long short

0°

400 mm

190 mm

u, F

Figure 2. Overview of experimental test setup: (a) Displacement-controlled three-point bending test

(3PBT) with 0

◦

- and 90

◦

-oriented specimens, (b) displacement-controlled uniaxial compression test

(UCT) with long and short specimens.

Tables 1and 2show an overview of the performed tests for each configuration. Some

specimens could not be tested due to defects, leading to the shown number of specimens.

Due to their poor performance in terms of interfilamental bond strength in 3PBT, C spec-

imens are excluded after the first 3PBT_C. All remaining C specimens are tested under

compressional load, resulting in a higher number of specimens for UCT_C (see Table 2).

Table 1. Overview of specimens for 3PBT.

Test Printing Pattern Orientation Number of Specimens

Three-point

bending test

(3PBT)

Lengthwise (L) 0◦4

90◦3

Length-/crosswise (LC) 0◦3

90◦3

Crosswise (C) 0◦1

90◦-

Materials 2024,17, 3931 5 of 21

Table 2. Overview of specimens for UCT.

Test Printing Pattern Specimen Length Number of

Specimens

Uniaxial

compression test

(UCT)

Lengthwise (L) Long (l) 2

Short (s) 2

Length-/crosswise (LC) l 3

s 2

Crosswise (C) l 4

s 6

2.4. Optical Measurements

2.4.1. Geometric Surveying by Structured Light Scanning

The additive manufacturing process entails deviations of the manufactured specimen

geometry from the target geometry. The actual printed geometry is consciously taken into

account in the present test series. The optical structured light scanning system COMET 5

by Steinbichler Optotechnik GmbH, Neubeuern, Germany (now Carl Zeiss Optotech-

nik GmbH), is employed to capture the specimen’s precise surface geometry prior to testing

in order to quantify deviations (Figure 3). The system projects a series of stripe patterns

onto the specimen’s surface and subsequently calculates 3D surface coordinates from the

intersections of the stripe patterns with the camera grid using the principle of triangulation.

For further information on the operating principle, see [38].

Figure 3. Structured light scanning system COMET 5 with specimen on rotary table used for geometric

all-round surveying.

2.4.2. Optical Deformation Measurements

During the test, the deformations on the specimen’s visible surface are measured

using the optical stereo camera system ARAMIS 4M manufactured by GOM GmbH, Braun-

schweig, Germany (now Carl Zeiss Industrial Quality Solutions GmbH; Figure 4a). The

system operates contactless with one sensor and two cameras and is able to capture 3D

coordinates, 3D displacements and surface strains. The measurement method of the sensor

is based on the principle of DIC. The recognition of image areas, referred to as facets, in

both the left and right camera enables the process of DIC [

]. Therefore, the specimen’s

surface is prepared with a thin sprayed random grey value pattern that doesn’t influence

the mechanical behaviour (see Figure 4b).

Materials 2024,17, 3931 6 of 21

(a) (b) 50 mm

Figure 4. (a) Three-point bending test setup with ARAMIS 4M optical stereo camera system.

(b) Sprayed stochastic grey value pattern on the specimen surface.

The measurement accuracy, which varies with the measurement volume, achieves

a precision of 0.01

for the current configuration. For 3PBT, the image generation

sampling frequency is set at

fs=2Hz to 4Hz

. In UCT, it is possible to generate images

with

fs=2Hz

while utilizing a ring memory to store the last 100 images at a higher

frequency of fs=20Hz due to a system upgrade.

3. Results

3.1. Three-Point Bending Test

3.1.1. Failure Mechanisms

As expected, the beam-like unreinforced conrete specimens fail in 3PBT due to a

discrete crack in the beam centre beneath the load application point. The brittle fracture

surfaces of 3PBT are shown in Figure 5. The filaments of L_0 specimens behave like four

individual planes (Figure 5a). The very low interfilamental bond even leads to a separation

of a filamental plane. The cracks develop in each filament row individually.

Similarly, the crack evolves gradually over the specimen height in L_90 specimens

(Figure 5b). LC specimens behave more homogeneously both for 0

◦

and 90

◦

orientation

(Figure 5c,d). A sharp edge on the fracture surface of Specimen 3PBT_LC_90_#3 can be

detected as an indication for an interfilamental gap between crosswise printed filaments

(Figure 5d). The fracture surface of the only 3PBT_C specimen in Figure 5e reveals just

a selective filamental bond at the location of directional changes in the printing path,

explaining the poor performance in 3PBT.

The evaluation of the measured strain field for 0

◦

-oriented specimens shows mainly

crack patterns with one dominant crack leading to failure, independent of the printing

pattern (Figure 6a,c,d). The crack does not always necessarily initiate in the beam middle,

as demonstrated in Figure 6d.

In contrast to one crack, 90

◦

-oriented L and LC specimens show a crack pattern

with formation of multiple cracks, see Figures 6b and 7. The gradually developing crack

observed for L_90 specimens from the fracture surface (Figure 5b) can also be seen in the

strain data in Figure 6b.

Materials 2024,17, 3931 7 of 21

(a) 3PBT_L_0_#3 (b) 3PBT_L_90_#3

(e) 3PBT_C_0_#1(c) 3PBT_LC_0_#1 (d) 3PBT_LC_90_#3

Figure 5. Fracture surfaces of 3PBT: lengthwise printed specimens with (a) 0

◦

and (b) 90

◦

orientation,

length-/crosswise printed specimens with (c) 0

◦

and (d) 90

◦

orientation and (e) crosswise printed

specimen with 0◦orientation.

[μm/m]

1500

900

600

300

−300

−600

−900

−1500

−1200

1200

(b) 3PBT_L_90_#1

(a) 3PBT_L_0_#3

Figure 6. Strain field

εx

at failure: (a) 3PBT_L_0_#3, (b) 3PBT_L_90_#1, (c) 3PBT_LC_0_#3 and

(d) 3PBT_LC_0_#2.

The optical measurement data allows a detailed analysis of the failure mechanism of

Specimen 3PBT_LC_90_#3 over time in Figure 7. The strain data

εx

at the location of each

crack obtained at the bottom edge of the specimen shows a continuous increase of strain

for Crack 1, whereas the strain development in Cracks 2 and 3 is dominated by a sudden

increase at specific time points. Crack 4 in the beam centre leads to failure (Figure 7a,b).

Materials 2024,17, 3931 8 of 21

Figure 7c shows the strain field for different time points

right after the strain in the

cracks increases. An increasing strain in Crack 4 is accompanied by decreasing strains in

the other cracks.

It can be observed that the machine force correlates with the crack appearance and

evolution. A slight increase and subsequent decrease in the load occurs simultaneously with

the increase in strain within the cracks (indicated by dashed coloured lines in Figure 7b).

After a crack opening, the stresses redistribute and the load can be increased further. The

other peaks in the force curve suggest the formation of additional cracks in other areas of

the specimen, not visible on the measured surface.

400 mm

1 24Crack 3

(b)

(a)

t 1 = 32.5 s

t 2 = 50.0 s

t 5 = 165.5 s

t 4 = 133.0 s

t 3 = 68.0 s

170 mm

[μm/m]

400

300

200

100

−100

−200

−300

−400

(c)

u, F

Figure 7. (a) Schematic illustration of displacement-controlled 3PBT for Specimen 3PBT_LC_90_#3

with order of crack appearance. (b) Evolution of strain

εx

at location of Cracks 1 to 4 at the bottom

edge of the specimen and machine force

over time

. (c) Strain field

εx

with crack evolution for

specific time points t1to t5.

3.1.2. Material Properties

The effect of anisotropy can be made measurable by applying the assumption of

homogeneous linear-elastic material behaviour to determine the material properties from

measured load, displacement and strain values. To calculate the Young’s modulus from

3PBT, the displacement at the load application point on the top edge of the beam centre is

determined from the optical measurements. Subsequently, the Young’s modulus can be

calculated using linear beam theory:

∆F·L3

∆u·48I, (1)

Materials 2024,17, 3931 9 of 21

with moment of inertia

I=b·h3

, support span

and incremental values for force

∆F

and

displacement

∆u

. The latter can be approximated using a linear fit of the force-displacement

relationship, which is linear until reaching the ultimate load due to tensile failure. The cross-

sectional information for height

and width

are measured individually for each specimen.

The brittle failure in 3PBT occurs due to a rapid growth of the central crack when the

tensile stress on the bottom side of a specimen exceeds the tensile strength. The ultimate

load at failure is not explicitly shown due to varying cross-sections, but its normalised

value correlates with the flexural tensile strength

(also referred to as flexural strength,

e.g., [18,21,24]) that is calculated using

ft=

F·L

b·h2

3·FL

2·bh2, (2)

where Fis the recorded ultimate load.

The results are shown quantitatively in Table 3and are visualized in Figure 8. For

L specimens, the orientation does not significantly affect the material parameters. In

contrast, LC specimens exhibit a 22% lower mean value for the Young’s modulus and a

29% lower mean value for the flexural tensile strength when oriented at 90

◦

. However,

the standard deviation for LC_90 specimens is higher. The material parameters for the

C_0 specimen confirm the already observed underperforming of the C pattern in 3PBT.

Table 3shows homogenized material properties for the printed concrete which are signifi-

cantly lower than those of cast concrete and exhibit differences conditioned by the applied

printing pattern.

Table 3. Young’s modulus

and flexural tensile strength

derived from 3PBT with mean value

standard deviation SD and coefficient of variation CV.

Printing Pattern Young’s Modulus EFlexural Tensile Strength ft

µ[MPa] SD [MPa] CV [%] µ[MPa] SD [MPa] CV [%]

L_0 9564 1238 12.9 4.55 0.43 9.5

L_90 9803 1243 12.7 4.36 0.87 19.9

LC_0 10,879 1386 12.7 4.48 0.65 14.5

LC_90 8450 2433 28.8 3.18 0.43 13.7

C_0 4750 0 0.0 1.84 0.00 0.0

Figure 8. Young’s modulus

and flexural tensile strength

derived from 3PBT with mean value

and standard deviation SD.

Materials 2024,17, 3931 10 of 21

3.2. Uniaxial Compression Test

3.2.1. Failure Mechanisms

The printed specimens exhibit quite different failure mechanisms in UCT and therefore

require individual investigation. The failure behaviour of L specimens under compres-

sion is characterized by the detachment of filaments. The optical measurement data for

Specimen UCT_L_l_#1, shown in Figure 9a–c, reveals increasing vertical and horizontal

displacements

and

and strains

εx

for the right filament compared to the other three fil-

aments. The strain component in

-direction (Figure 9d) further illustrates the delamination,

with increased strain values at the filamental boundaries.

(a) u

(b) u

(d) ε

[mm]

1.600

1.500

1.350

1.200

1.050

0.900

0.750

0.600

0.450

0.240

0.200

0.175

0.150

0.125

0.100

0.075

0.050

0.030

0.225

[mm]

−1000

−1500

−2000

−2500

−3000

−3500

−4000

−4500

−500

[μm/m]

2000

1500

1250

1000

750

500

250

−250

1750

[μm/m]

Figure 9. Measured displacement fields (a)

and (b)

and strain fields (c)

εx

and (d)

εy

just before

failure for Specimen UCT_L_l_#1.

An extended evaluation of the optical measurement data provides a deeper under-

standing of the mechanical behaviour. Figure 10a displays the displacement of each of

the four filaments at the bottom of Specimen UCT_L_l_#1 in the three spatial directions

over time. The displacement in

-direction parallel to the loading direction dominates.

Beginning at

t≈

440s, the displacement values for Filament 4 start to diverge from those

of the other three filaments. The

-displacement indicates that Filament 4 moves forward

while its vertical displacement increases significantly more than that of the other three

filaments. This implies that Filament 4 bears a greater load and consequently fails ear-

lier under compressive load due to lateral deflection. Not only does Filament 4 detach,

but Filaments 1 to 3 also detach in the area of load application, as shown in Figure 10b.

Figure 10c

illustrates the broken specimen with the completely detached filament and the

upper fractured part.

Materials 2024,17, 3931 11 of 21

1 432Filament

measurement points

(b) (c)(a)

Figure 10. (a) Evolution of displacement

- and

-direction measured at the bottom of each

Filament 1 to 4 over time

for Specimen UCT_L_l_#1. (b) Failure mechanism with detachment of

right filament at the moment of failure captured from optical measurements. (c) Broken specimen.

The evaluation of the strain field data for Specimen UCT_L_s_#2 enables the detection

of localized weak points that are characterised by increased compression, see Figure 11.

These areas arise due to a fluctuating extrusion rate of the material from the printing nozzle.

[μm/m]

500

−500

−1000

−1500

−2000

−2500

−3000

−3500

−4000

Figure 11. Strain field εxfor Specimen UCT_L_s_#2 just before failure.

Materials 2024,17, 3931 12 of 21

In long C specimens under compression, two types of cracks appear: slanted cracks

originating from the top, caused by the constraint of lateral expansion at the load application

plate, and vertical cracks along the layer boundary (Figure 12b). For Specimen UCT_C_l_#1,

detachment of the upper part of the front layer is observed (Figure 12a). The increased

negative strain data in Figure 13 indicate compression of the interfilamental gaps resulting

from the crosswise printing process. The significant compression in these regions leads to a

lateral expansion, causing fractures at points of weak adhesion.

(a) (b)

front

Figure 12. Failure mechanism of Specimen UCT_C_l_#1: (a) front view with detached upper part of

front layer, (b) broken specimen in side view.

The displacement-controlled test setup allows a further insight into the failure mech-

anism once the ultimate load is reached. For C specimens, the measured load gradually

decreases after exceeding the ultimate load due to the formation of new cracks (see

Figure 14

for Specimen UCT_C_l_#4). As the cracks propagate, the load-bearing cross-section dimin-

ishes, leading to a gradual reduction in load capacity with each new crack. This observation

further emphasises the anisotropic structure of the material.

Short C specimens exhibit similar crack patterns, with shear cracks originating at

the top and propagating along the filamental printing boundary (Figure 15a–d). In the

lower half, the crack propagates vertically along the layer boundary, parallel to the loading

direction. Additionally, specimens #2, #3 and #5 display multiple cracks parallel to the

loading direction (Figure 15a,b,d). The C printing pattern is characterized by a higher

number of directional changes in the printing path and therefore a longer interlayer interval

time, leading to reduced adhesion between both filaments and layers.

Materials 2024,17, 3931 13 of 21

[μm/m]ε

400

−400

−800

−1200

−1600

−2000

−2400

−2800

−3200

Figure 13. Strain field εxfor Specimen UCT_C_l_#4 just before failure.

Figure 14. Applied machine force

over time

with detailed view of post-fracture range measured

with sampling frequency fs=20Hz for Specimen UCT_C_l_#4.

For LC specimens, the boundaries between lengthwise and crosswise printed filaments

are visible in the strain field distribution (see Figure 16a). Increased strain values on the

visible surface layer with lengthwise printed filaments follow a pattern that orients on

the underlying second layer printed with crosswise filaments. Figure 16b highlights the

trajectory of the underlying crosswise printed filaments, where improved adhesion results

in lower compression. The porous space between crosswise printed filaments at directional

changes in the printing path leads to a higher compression and therefore increased strains

under compressive load.

Materials 2024,17, 3931 14 of 21

(b) UCT_C_s_#3 (c) UCT_C_s_#4 (d) UCT_C_s_#5(a) UCT_C_s_#2

Figure 15. Failure mechanism of short crosswise printed specimens: (a) UCT_C_s_#2, (b) UCT_C_s_#3,

LC specimens perform significantly better in maintaining interfilamental and inter-

layer connections. The failure mechanisms of both long and short LC specimens are

predominantly similar to those for conventionally cast concrete, as shown in Figure 16c.

[μm/m]

−2000

−2200

−2400

−2600

−2800

−3000

−3200

−3400

−1800

(c)(a) (b)

Figure 16. Specimen UCT_LC_l_#2: (a) Strain field

εx

just before failure with (b) highlighted regions.

3.2.2. Material Properties

Figure 17 schematically illustrates how the 3D scan of the specimens, conducted prior

to testing, is utilized to obtain geometric information. The specimen’s surface (Figure 17a)

undergoes structured light scanning (Figure 17c). Since UCT specimens are cut to ensure

an even load application area (Figure 17b), the CAD model is similarly trimmed to match

this length (Figure 17d). Data from six cross-sectional cuts along the specimen’s length

(Figure 17e) are processed to derive mean values for each specimen’s cross-sectional area.

Materials 2024,17, 3931 15 of 21

Variations in cross-sectional shapes are depicted in Figure 17f, presented in an overlaid

front view. Additionally, the CAD model is used to calculate the volume of the specimens.

(a) top view of printed specimen

(b) sawn specimen

(e) cross-sectional cuts (f) front view of cross-sections

Figure 17. Post-processing scheme of 3D surface scans of specimens to obtain the cross-sectional area

and volume information, exemplary shown for Specimen UCT_L_l_#1.

The variation of the cross-section along the specimen’s length is exemplary inves-

tigated on Specimen UCT_L_l_#1, see Figure 18a. Therefore, twenty-one cross-sections

are individually evaluated from the scanned geometry. The cross-sectional area varies,

although only with minor deviations from the mean value (Figure 18b). The economic

compromise involves using six cuts while still achieving satisfactory results.

(a) (b)

Figure 18. (a) Visualization of cross-sectional cuts in CAD model. (b) Discrete values of cross-

sectional area

, mean value

Aµ

and standard deviation

ASD

over specimen length

for Speci-

men UCT_L_l_#1.

Materials 2024,17, 3931 16 of 21

The coefficient of variation for the cross-sectional area

among all UCT specimens is

less than 1.6%, indicating relatively low variability in area (see Table 4and visualized in

Figure 19). However, the area deviates from the target geometry of

Atarget =50mm ·50mm =2500mm2, (3)

see Table 4. The cross-sectional area for L specimens exceeds the target value by around

20%, while for C specimens, it is 8% lower. Therefore, considering the actual geometric

dimensions has a relevant impact on the results.

Since the specimens are also weighted before testing, the density

can be determined

using the mass mand the volume Vobtained from the 3D scan:

ρ=

V. (4)

The calculated densities in Table 4represent values that include all interfilamental and inter-

layer voids resulting from the printing process. LC and C specimens achieve approximately

92% of the mean density value calculated for L specimens, indicating that L specimens are

printed in a more compact manner.

Table 4. Cross-sectional area

and density

for UCT specimens with mean value

, standard

deviation SD and coefficient of variation CV.

Printing Pattern Cross-Sectional Area ADensity ρ

µ[mm2] SD [mm2] CV [%] µhkg

m3iSD hkg

m3iCV [%]

L_l 3008.6 10.9 0.4 2012.1 1.8 0.1

L_s 2969.1 16.3 0.5 2005.9 4.5 0.2

LC_l 2657.5 13.2 0.5 1881.4 17.8 0.9

LC_s 2714.0 16.4 0.6 1847.8 6.0 0.3

C_l 2307.8 25.8 1.1 1829.0 25.6 1.4

C_s 2298.0 37.3 1.6 1834.7 16.6 0.9

Figure 19. Cross-sectional area

and density

for UCT specimens with mean value

and standard

deviation SD.

Based on the acquired cross-sectional area data, the stress in the specimen is subse-

quently calculated by

σ=

A, (5)

where

represents the recorded machine force and

the mean value of the cross-sectional

area obtained for each specimen from structured light scanning. With the surface strains de-

Materials 2024,17, 3931 17 of 21

termined by optical measurements it is possible to determine an integral Young’s modulus

Evia

ε, (6)

where

is computed using a linear fit of the stress-strain curve within the initial linear

range. Besides, the maximum measured stress

σmax

can be considered as the compressive

strength fc:

fc=σmax . (7)

Table 5and Figure 20 show the Young’s modulus and the compressive strength for

each combination of printing pattern and specimen length. L specimens have the highest

Young’s modulus, while C specimens show the lowest one with significant differences in

the magnitude. The same trend is observed in compressive strength. The Young’s modulus

for short specimens is slightly higher than for long specimens, except for C specimens.

However, short specimens C_s have a notably high coefficient of variation at 39.4%. The

difference in compressive strength between short and long specimens for each printing

pattern is relatively small.

Table 5. Young’s modulus

and compressive strength

derived from UCT with mean value

standard deviation SD and coefficient of variation CV.

Printing Pattern Young’s Modulus ECompressive Strength fc

µ[MPa] SD [MPa] CV [%] µ[MPa] SD [MPa] CV [%]

L_l 18,119 814 4.5 44.15 0.84 1.9

L_s 19,785 1503 7.6 43.03 0.49 1.1

LC_l 13,096 593 4.5 26.92 5.46 20.3

LC_s 13,816 469 3.4 28.16 1.40 5.0

C_l 9771 1787 18.3 11.18 2.82 25.3

C_s 7756 3055 39.4 12.43 2.71 21.8

Figure 20. Young’s modulus

and compressive strength

derived from UCT with mean value

and standard deviation SD.

4. Discussion

The experimental investigation reveals significant differences in failure mechanisms

and mechanical properties among the tested specimens. The variation of the printing path

in longitudinal and crosswise direction allows a systematic evaluation regarding different

combinations of printing path and loading direction.

Materials 2024,17, 3931 18 of 21

The first observation relates to the failure mechanisms. Fracture surfaces and optical

measurements from 3PBT indicate that the failure mechanism of the specimens depend on

the orientation of the printed layers. Specimens with a 0

◦

orientation tend to fail with one

dominant crack, whereas those with a 90

◦

orientation show a pattern with multiple cracks.

The interfilamental region is particularly critical. For instance, in LC_90 specimens, the

results suggest that crack formation occurs at weak connection points of crosswise printed

filaments. This affects the spatial stiffness distribution, which adapts over time as new

cracks appear.

The failure mechanisms for UCT vary significantly depending on the printing pat-

tern. For L specimens, the failure mode is primarily characterized by the detachment

of filaments, which may be influenced by uneven load distribution or imperfections in

load application or geometry. In contrast, for LC and C specimens, gaps at the location of

directional changes in the printing path in crosswise printed filaments create weak points

that become more compressed during testing. The interfilamental and interlayer bonds

are critical factors for C specimens, as evident in the fracture surfaces of both long and

short specimens. Among the three investigated printing patterns, LC specimens exhibit the

most homogeneous behaviour. The results obtained regarding crack propagation along

the interface of two layers in compression tests are consistent with observations made by

Zareiyan and Khoshnevis in [

] for some of their specimens, despite differences in the

experimental setup.

The calculated material parameters yield the following observations: Within the same

test setup, L specimens exhibit the best performance with respect to flexural tensile strength

in 3PBT and regarding compressive strength and Young’s modulus in UCT, implying

superior load-bearing capacity. Conversely, C specimens demonstrate the poorest per-

formance across all disciplines. The two effects leading to the results for C specimens

are an increased interlayer interval time, which influences the interlayer bond, as shown

in [

], and the geometrical configuration, which influences the interfilamental bond.

The experimental results underline that the latter has a greater influence in the present

test series. The different specimen lengths tested do not significantly influence the results.

The increased variation in the resulting parameters indicates that control of the printing

process is essential for generating replicable results. For 3PBT, the highest Young’s modulus

is observed in LC_0 specimens. However, the absolute values must be interpreted with

caution, as there is a significant discrepancy between the Young’s modulus results obtained

from 3PBT and UCT. This discrepancy indicates that calculating an averaged value for the

entire specimen based on linear beam theory or a homogenized compression member is

insufficient for determining accurate material parameters.

The investigation of the actual specimen geometry using 3D structured light scanned

surface data demonstrates that the actual geometry differs for the three different printing

patterns and also does not meet the targeted dimensions. Accounting for the measured

cross-sectional area significantly impacts the resulting material parameters.

5. Conclusions

The findings of this research contribute to a deeper understanding of the mechanical

performance of additively manufactured concrete. The investigation of failure mechanisms

and material properties of beam-like printed specimens with various printing patterns-

lengthwise, alternately length- and crosswise and crosswise printed filaments-reveals

distinct performance characteristics.

The study confirms the anisotropic behaviour of printed concrete. Furthermore, it

provides profound insights into failure mechanisms under bending and compression.

Specimens oriented at 90

◦

exhibit a multiple crack pattern under bending load, contrasting

with 0

◦

-oriented specimens dominated by a single crack. The interfilamental bond emerges

as a critical factor influencing failure behaviour. Weak points can lead to inducement of

cracks that are not necessarily originating at points of maximum stress. Under compression,

failure mechanisms vary significantly depending on the printing pattern. While the failure

Materials 2024,17, 3931 19 of 21

mechanism for L specimens is characterized by filament detachment, for C specimens it is

dominated by cracks propagation along interfilamental and interlayer boundaries. Only

the failure mode of LC specimens is comparable to that of cast concrete. Standardized cubic

geometries that are typically used for determining compressive strength parameters would

not allow for the observation of these structural behaviours.

The obtained material parameters, such as Young’s modulus, compressive and flexural

tensile strength and density, highlight pattern-dependent mechanical properties. This is

particularly problematic for C specimens due to a low interlayer bond observed in UCT

and a weak interfilamental bond manifested in 3PBT. This printing pattern is critical for

achieving optimal structural performance.

Understanding the actual specimen geometry using 3D structured light scanned

surface data underscores its crucial role in accurate assessment. While research methods

that saw-cut specimens from objects are important for determining material parameters,

they do not capture the true printed geometry.

For further research, such as FE modelling of printed concrete, this study offers

essential insights, emphasizing the need for a comprehensive approach to account for

direction-dependent mechanical behavior in printed concrete.

Author Contributions: Conceptualization, T.G. and Y.P.; methodology, T.G. and Y.P.; validation,

T.G. and Y.P.; formal analysis, T.G.; investigation, T.G.; data curation, T.G.; writing—original draft

preparation, T.G.; writing—review and editing, T.G. and Y.P.; visualization, T.G.; supervision, Y.P. All

authors have read and agreed to the published version of the manuscript.

Funding: The authors acknowledge support by the Open Access Publication Fund of TU Berlin.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The raw data supporting the conclusions of this article will be made

available by the authors on request.

Acknowledgments: The authors would like to thank Dietmar Stephan and his team from the Chair

of Building Materials and Construction Chemistry, TU Berlin for printing the test specimens.

Conflicts of Interest: The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:

3DCP 3D concrete printing

3PBT Three-point bending test

C Crosswise printing pattern

DIC Digital image correlation

FE Finite element

l Long specimen length

L Lengthwise printing pattern

LC Length-/crosswise printing pattern

s Short specimen length

UCT Uniaxial compression test

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