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2022 | Book

Third RILEM International Conference on Concrete and Digital Fabrication

Digital Concrete 2022

Editors: Prof. Richard Buswell, Dr. Ana Blanco, Prof. Sergio Cavalaro, Dr. Peter Kinnell

Publisher: Springer International Publishing

Book Series : RILEM Bookseries


About this book

This book gathers peer-reviewed contributions presented at the 3rd RILEM International Conference on Concrete and Digital Fabrication (Digital Concrete), held in Loughborough, UK, on June 27-29, 2022. Focusing on additive and automated manufacturing technologies for the fabrication of cementitious construction materials, such as 3D concrete printing, powder bed printing, and shotcrete 3D printing, the papers highlight the latest findings in this fast-growing field, addressing topics like mixture design, admixtures, rheology and fresh-state behavior, alternative materials, microstructure, cold joints & interfaces, mechanical performance, reinforcement, structural engineering, durability and sustainability, automation and industrialization.

Table of Contents


Alternative Processes

Zero-Waste Production of Lightweight Concrete Structures with Water-Soluble Sand Formwork

In the face of climate change and resource crisis, the construction industry can contribute to sustainable economic development by reducing its material consumption, emissions and waste. The application of lightweight construction principles in combination with circular production processes offers a comprehensive approach to this problem. The newly developed zero-waste formwork production method, based on the additive manufacturing of water-soluble sand mixture, enables to produce geometrically complex concrete structures and to reuse formwork material repeatedly in production cycles. This paper demonstrates the potential of this method for the production of the topologically optimised concrete single-span beam with spatial lattice structure (Fig. 1). First, a brief overview of the method will be given, followed then by a description of the design and production of the concrete component under the consideration of production parameters.

Daria Kovaleva, Maximilian Nistler, Alexander Verl, Lucio Blandini, Werner Sobek
An Early Trial on Milling 3D Printed Concrete Geometries: Observations and Insights of the Process

As 3D Concrete Printing (3DCP) technology develops, requirements on the form and surface quality of the final products are increasing. Layer-wise deposition results in the so-called ‘staircase effect’ which can lead to limitations on the attained precision and accuracy of geometries. Applying other shaping processes with a higher manufacturing precision can be deployed to combat this and milling is one example that has been shown to yield benefits. This paper presents an early trial of a milling process applied after printing and before the final hardened state of the material. A case study of a panel component is presented and observations are reported, which include: the critical nature of the material state, the control of debris and milling path sequence and direction. Insights are formulated into a three-tier structure to help develop signpost issues for the development of the approach.

Jie Xu, John Temitope Kolawole, John Provis, James Dobrzanski, Peter Kinnell, Sergio Cavalaro, Weiqiang Wang, Richard Buswell
Mobile Additive Manufacturing: A Case Study of Clay Formwork for Bespoke in Situ Concrete Construction

The in situ production of concrete building components with Additive Manufacturing (AM) provides new possibilities in design and function. Current deployable solutions are often stationary gantry systems, which need to increase in size with the constructed object. This research aims to address this issue by using mobile robotic systems for in situ AM instead, which can manufacture structures that exceed their static work range. However, where stationary AM systems inherently exhibit a high level of accuracy, mobile AM systems must be context-aware through onboard sensing and therefore pose a significant research challenge in their deployment and operation. A case study is performed with a mobile AM system using a print-drive-print approach for the sequential fabrication of a 1:1 scale clay formwork of a bespoke, reinforced, and lightweight-concrete column, on which this paper presents first results. A two-tiered system is applied and validated, with initial global localization through 2D SLAM, and a second refinement relative to the work piece through a 2D scanner fitted at the end-effector.

Gido Dielemans, Lukas Lachmayer, Tobias Recker, Lidia Atanasova, Christian Maximilian Hechtl, Carla Matthäus, Annika Raatz, Kathrin Dörfler
Adaptive Foam Concrete in Digital Fabrication

One of the process-related advantages of 3D concrete printing (3DCP) is the exact placement of concrete. It only seems logical, that the production method has to be developed further so that a specific concrete with specific properties can be actively placed where it needs to be. For this purpose, a suitable technology has been developed, which makes it possible to print foam concrete in different densities by extrusion method. (150–1000 kg/m3).This innovation opens up new areas of shaping and manufacturing of effective building structures. In contrast to conventional casting of concrete, construction components can now be composed of predefined zones that have different building physic, static and structural requirements due to the corresponding concrete type. (gradient concrete structures).The research at hand determines the requirements along the entire process chain through several prototypes. The results provide information about the feasibility of 3DP with adaptive foam concrete. With the possibility of grading the properties of the concrete within each building component, lighter and more sustainable buildings can be realized while conserving valuable resources.

Robert Schmid, Georg Hansemann, Michael Autischer, Joachim Juhart

Structural Design and Optimisation

Mesh Mould Prefabrication

The Mesh Mould technology combines formwork and structural reinforcement into a robotically fabricated construction system. This method allows for the industrial and full-scale realisation of complex curved, steel-reinforced concrete structures without the need for conventional formwork. The paper presents a new material and cost efficient industrial robotic prefabrication process of 3D reinforcement cages (mesh elements). A novel robotic wire application process makes it possible to now fabricate mesh elements with continuous reinforcement in two commonly orthogonal directions. Thanks to the implementation of an automated structural design approach, complex structures can be dimensioned and optimised following international standards for steel-reinforced concrete structures. Furthermore, the paper presents the development of a new, appropriate concrete mixture that is stable to fill the permeable meshes with.

Ammar Mirjan, Jaime Mata-Falcón, Carsten Rieger, Janin Herkrath, Walter Kaufmann, Fabio Gramazio, Matthias Kohler
The Production of a Topology-Optimized 3D-Printed Concrete Bridge

In the last few years, the development of 3D concrete printing (3DCP) technology has flourished exponentially both in academics and the construction industry. Many problems inherent to 3DCP are already being tackled on a material level. However, in the practical realization of large-scale components there are still a lot of questions to be answered. In this study, we discuss the production process of a topology-optimized 3D-printed concrete bridge structure. As the entire process is largely different compared to the manufacturing of traditional concrete structures, the problems, workarounds, and insights gathered from this project are valuable for future constructions using 3DCP. The geometry of the bridge was based on topology optimization results and further developed through the use of parametric modelling. After careful considerations, the bridge geometry was discretized into four segments and printed as integrated formwork. Several measures were taken during the printing process in order to produce the separate sections. The assembly process entailed the handling of the printed components, the placement of reinforcement and prestressing tendons, the production of the end blocks, and the handling and joining of the printed sections. For the latter, also the process of pouring self-compacting concrete in the printed formwork is discussed and more details about the post-tensioning procedure are provided.

Ticho Ooms, Gieljan Vantyghem, Yaxin Tao, Michiel Bekaert, Geert De Schutter, Kim Van Tittelboom, Wouter De Corte
Injection 3D Concrete Printing (I3DCP) Combined with Vector-Based 3D Graphic Statics

This paper introduces the combination of Injection 3D Concrete Printing (I3DCP) with Vector-based 3D Graphic Statics (V3DGS). I3DCP is a technique that robotically injects concrete into a non-hardening carrier liquid which acts as a supporting structure for the printed strands. The printing path of I3DCP is exactly aligned with the spatial stress trajectories, which can be simply treated as strut-and-tie networks in the truss system. V3DGS matches the core of I3DCP, which becomes an ideal system for practically operating the strut-and-tie networks at the design phase, as well as intuitively improving the structural performance. Accordingly, grounded on V3DGS, the design framework called Combinatorial Equilibrium Modelling (CEM) lends itself particularly effective to be integrated with the design process. In terms of fabrication, a modularization strategy is described for fabricating objects larger than the suspension vessel. Finally, a segmentally printed and assembled demonstrator is presented.

Yinan Xiao, Noor Khader, Aileen Vandenberg, Dirk Lowke, Harald Kloft, Norman Hack
3DCP Structures: The Roadmap to Standardization

During the last decades, 3D concrete printing (3DCP) is evolving rapidly. This newly developed technique allows the manufacturing of structures layer-by layer, based on a virtual model and without human intervention. The elimination of formwork, the reduction in labor time and cost and the increased architectural freedom are generally accepted as the major benefits compared to traditional concrete. At this moment, there are multiple examples of large-scale 3DCP projects worldwide, but the manufacturing process is still hindered from developing its full potential as developments are currently limited to trial and error related to large knowledge gaps in fundamental understanding, experimental methods and predictive models. Additionally, as concrete printing deviates on almost every aspect from mold-casted concrete, new standards with regards to the concrete mix design, the manufacturing process itself and the structural performance are required. All these categories should contain technical recommendations to ensure the structural stability, and should exclude the effect of the applied print parameters or cementitious material. To encounter the latter, researchers apply nowadays the ‘Design by Testing’ principle, with a project-specific testing program. This paper shows how structural components are designed based on the ‘Design by Testing’ principle and presents in addition an outlook for the legislation and standardization of future projects.

Jolien Van Der Putten, Maartje J. Hoogeveen, Marijn J. A. M. Bruurs, Hans L. M. Laagland

Binders and Aggregates 1: Aggregates

Mix Design for a 3D-Printable Concrete with Coarse Aggregates and Consideration of Standardisation

Driven by promising efficiency gains, 3D concrete printing is rapidly evolving. To facilitate the transfer of the findings into construction practice, it is recommended to develop concretes within the specifications of standards. It is shown, that limiting the fine particle content is a major challenge. A generalizable, numerically supported application of the particle size distribution according to andreasen and andersen is proposed as a solution. Its application in printing tests proves good extrudability and buildability of standard-compliant concrete with 16 mm largest grain size and a fine particle content of 500 kg/m3.

Markus Taubert, Viktor Mechtcherine
Fresh and Hardened Properties of 3D Printable Foam Concrete Containing Porous Aggregates

The foam concrete or aerated concrete is most widely used for construction applications where, lightweight, thermal insulation and fire resistance are crucial. While the application of foam concrete is successful in the construction industry, the emerging technology of concrete 3D printing possesses many challenges when using foam concrete. This is primarily due to the high flowability of foam concrete, since it contains a large proportion of air bubbles, resulting in poor shape retention and buildability characteristics. This study investigates the utilization of lightweight aggregate to reduce the foam content to achieve a lightweight 3D printable mix. The expanded perlite (EP) aggregate was used as a volumetric replacement to fine sand that substantially reduced the foam content in the mix. The effect of EP on the physical and mechanical properties was studied and compared with the control 3D printable foam concrete containing sand. The results showed that the replacement of sand with EP aggregate reduces the flow properties that is suitable for 3D printing. Besides, the compressive strength of 3D printed samples was also enhanced with the addition of EP aggregates. The compressive strength of EP based 3D printed specimens at 28 days was determined as 12.95 MPa, 15.5 MPa and 10.6 MPa in the perpendicular, longitudinal, and lateral directions respectively. On the other hand, control 3D printed samples displayed the compressive strength of 5.5 MPa, 8.4 MPa and 4.2 MPa at a slightly lower density range.

Kirubajiny Pasupathy, Sayanthan Ramakrishnan, Jay Sanjayan
Sustainable 3D Concrete Printing with Large Aggregates

The number of large-scale projects featuring the use of 3D Concrete Printing (3DCP) had a steep increase in the past years. The most remarkable applications include apartment and residential buildings and promise to deliver a robust, cost-effective and sustainable solution compared to conventional construction. One of the paths to boost 3DCP’s sustainability lies in the material upscaling from mortars to concrete; thus, our work focuses on the concrete mixes for 3DCP. For that, we use a calcined-clay limestone-based cement (FutureCEM®) and large aggregates (up to 8.0 mm) to produce printable mixes. In this work, we present a comparative analysis of mixes produced with CEM I and FutureCEM. Specifically, mixes with strength classes of C25 and C45 produced with these cements are compared to a mortar produced with White Cement; a mortar produced with FutureCEM®; and a concrete produced with CEM I. Also, the CO2 footprint of the mixes is compared to that of Ready Mix Concrete produced in Denmark; such analysis validates the environmental benefits of material upscaling and points out that 3DCP mixes can be produced with a similar CO2 footprint to that of conventional concrete.

Wilson Ricardo Leal da Silva, Martin Kaasgaard, Thomas J. Andersen
Design and Fabrication of Spatially Graded Concrete Elements with Ice Aggregate Method

An important research perspective for optimization of concrete structures today can be recognized in the concept of controlled spatial grading of concrete elements. Spatial grading aims to control the volumetric distribution of concrete within an element based on the internal stresses, which enables a more efficient structural performance with less material without changing the overall geometry of a structural part.Due to high geometric complexity, the fabrication of spatially graded structures are typically based on additive manufacturing techniques such as extrusion or voxel-based 3D printing. However, for large-scale production and fabrication, 3D printing is ultimately constrained by speed and cost. This article presents the potential of regular ice as aggregate for casting spatially graded concrete components. Prefabricated ice aggregates of varying size and geometry are carefully placed into a mould to form regions with lower and higher relative densities, into which self-compacting concrete is poured. We investigate various parameters of customized ice aggregates including geometry, scale and relative densities. The presented experiments demonstrate how ice aggregate can be used to efficiently fabricate spatially graded concrete elements with high geometric complexity, while improving the structural performance by concentrating concrete only where it is needed. In addition, the intricate patterns of voids created by ice aggregate result in unique visual and lighting qualities that would be difficult to achieve with other fabrication methods. This article addresses the fundamental research questions and provides a ground for further research in digitization and automation of the ice aggregate production method.

Vasily Sitnikov, Lena Kitani, Artemis Maneka, Ena Lloret-Fritsch, Juney Lee, Benjamin Dillenburger

Binders and Aggregates 2: Alternative Binders

Accelerating Early Age Properties of Ultra-Low Clinker Cements for Extrusion-Based 3D Printing

In this study, we investigated the influence of commercial sodium nitrate/thiocyanate accelerator compared to calcium sulfoaluminate cement addition on setting time, rheology and reaction kinetics of ultra-low clinker composite cement for extrusion-based 3D printing application. CEM I 52.5 N and a ternary composite cement with 70% clinker replaced by slag and limestone were evaluated. Results indicate that final setting time of 30 min and buildable yield stresses can be attained with less than 5% addition of calcium sulfoaluminate, with ettringite and C-(A)-S-H as main reaction products. This demonstrates the synergy between slag and calcium sulfoaluminate cements can be harnessed to control rheology and hardening. This is significance for evidencing suitability of ultra-low clinker composite cements for extrusion-based 3DCP, thus helping to fulfil its wider potential as a low-carbon concrete technology.

Rutendo Rusike, Michael Sataya, Alastair T. M. Marsh, Sergio Cavalaro, Chris Goodier, Susan A. Bernal, Samuel Adu-Amankwah
Developing Printable Fly Ash–Slag Geopolymer Binders with Rheology Modification

The rheology of mixtures of fly ash-slag geopolymers, optimized for strength is not favorable for printing. Rheology modification is required using additives, which provide specific improvements in yield stress and thixotropy. These binders typically exhibit a pseudo-yield type behavior with a continuously deformable response under applied stress. A printable (both extrudable and buildable) material requires a yield-type behavior and adequate thixotropy, which can be brought out by addition of clay and Carboxymethyl Cellulose (CMC). The modification in rheology is attempted using commonly available Kaolinite clay. Specific changes in rheology caused due to the rheology modifiers are evaluated and are related with the performance in printing. Addition of clay contributes to an increase in the stiffness of the paste and improves buildability of the mix. A synergy between clay and CMC is established for proper printability. Clay in combination with CMC increases the storage modulus and produces a yield type behavior. CMC improves flocculation of clay but delays buildup due to its negative influence on reaction kinetics. Excess CMC increases the resistance to flow and produces a continuously deformable Maxwell response, which is not suitable for buildability.

Tippabhotla A. Kamakshi, Kolluru V. L. Subramaniam
Formulation and Characterization of a Low Carbon Impact Cementitious Ink for 3D Printing

3D printing is considered to be the most innovative construction technique. A challenge encountered with this technique is the ecological impact of the used mortar (ink). Indeed, due to the small particle size of the used sand, inks contain large amounts of Portland cement, which increases the carbon footprint of the material. This study is carried out in a partnership with Constructions 3D company, it concerns the design and characterization of a cementitious ink with a high cement substitution rate in order to reduce its environmental impact. This mortar is made from a reference ink composed of cement. We chose to work with constant workability based on the slump flow test and an equivalent Water/Binder ratio for the two mortars by adapting the admixture. A fresh state characterization allows evaluating the influence of the materials on the buildability of the mortars (fall cone test) and a hardened state characterization allows evaluating their influence on the mechanical aspect (compressive strength at 2, 7, 28 and 90 days, and shrinkage). Finally, full-scale prints are made in order to confirm the printability of this low carbon impact ink. Although 70% of the cement is replaced by metakaolin and ground granulated blast-furnace slag, the mortar produced is not less effective than the reference mixture and fully meets the specifications imposed by the 3D printing technique. The ternary ink formulated in this way will be tested on site to assess its properties in real conditions.

Estelle Hynek, David Bulteel, Antoine Urquizar, Sébastien Remond
Strategies for Reducing the Environmental Footprint of Additive Manufacturing via Sprayed Concrete

Lately, the construction industry has been trying to reduce its environmental footprint with the help of an additive manufacturing. This emerging technique allows for an automated construction of free-of-form structural elements. This, in turn, enables optimization of geometrical dimensions during design. However, the 3D concrete printing (3DCP) based on extrusion of material requires high amount of Portland cement, additives, and consumes an already scarce fine sand. On the other hand, the additive manufacturing using spraying (S-3DCP) offers larger flexibility in material composition, allowing for further reduction of the carbon footprint. It also facilitates implementation of larger aggregates, which improve material mechanical stability during manufacturing process and ameliorate its flexural strength in the hardened state. In this work, strategies related to the circular economy and the reduction of CO2 emissions through the concrete recipe optimization are assessed and demonstrated. The first strategy is based on the use of low-CO2 cement, which can reduce CO2 emissions by about 40% compared to using CEM II. The second strategy is based on the design of elements, which is to reduce the thickness while satisfying all mechanical constraints, that is, to reduce the concrete by approximately 25%. Finally, this paper will show the possibility of using demolition waste to replace 45% of aggregate (i.e., 35% of 0–4 mm sand and 10% of 48 mm aggregate) and the potential circular economy of S-3DCP. Due to the use of high-speed spraying (shotcrete) as the material placement method, continuous and isotropic elements with a low environmental footprint are produced.

Aurélie Favier, Agnès Petit
Mechanical Performance of 3-D Printed Concrete Containing Fly Ash, Metakaolin and Nanoclay

Similar to conventional concrete, the use of supplementary cementitious materials (SCMs) in 3-D printing concrete (3DPC) can be technically and environmentally beneficial. 3DPC consumes larger amount of Portland cement compared to conventional concrete so replacing the cement with SCMs will reduce the carbon footprint of 3DPC. However, it is important that the level of replacement does not lead to a significance loss of the mechanical performance of 3DCP. Therefore, this study investigates the mechanical strengths of 3DPC with various mix compositions. Mixes containing fly ash, metakaolin, nanoclay, and combination of them were studied. The test program includes measuring the compressive strength and flexural strength on standard cubes/prisms as well as 3D printed cubes/prisms at 7 and 28 days. The findings showed that fly ash decreased the early age strengths but led to an increase in the late age strengths. On the other hand, metakaolin and nanoclay did not remarkedly affect the strengths. It was found that mixes containing both fly ash and metakaolin showed better performance. Finally, it was observed that samples from 3D printed concrete had lower strengths compared to the same mixes poured in standard cubes and prisms. Optimisation of the mix compositions using these SCMs is required to achieve acceptable mechanical performance.

Ahmed Abdalqader, Mohammed Sonebi, Marie Dedenis, Sofiane Amziane, Arnaud Perrot

Binders and Aggregates 3: Strain Hardening Materials

Incorporation and Characterization of Multi-walled Carbon Nanotube Concrete Composites for 3D Printing Applications

Being 3D concrete printing (3DCP) a relatively new technology, there is a constant need for development, not only in hardware but also in the materials we currently use. The use of nanomaterials, notably carbon nanotubes, in concrete and cement mixes is not new, but it is a novelty in 3DCP.This paper investigates the effect on the fresh and hardened state properties of adding multi-walled carbon nanotubes (MWCNTs) to a printable concrete mix. Cement composites reinforced with MWCNTs may exhibit an irreversible change in resistivity when subject to damage or microstructural changes caused by strain or stress. Research also points to an improvement in flexural and compressive strength and lower shrinkage for CNT-cement mortars. The combined electrical and mechanical properties of these mixtures are of interest in crack self-detection for structural health monitoring. In this paper, two commercial solutions of MWCNTs dispersed in water were used to determine their effect on a cement mortar suitable for 3D printing. Three different mixes were prepared: a reference mix, a mix with 0.05% of MWCNTs per binder content and a mix with 0.1% of MWCNTs per binder content. Higher percentages of carbon nanotubes resulted in a decrease in flowability and mechanical properties due to the difficulty in dispersing the nanotubes. The results show that one of the two batches of MWCNTs used performed better overall than the other and the mix with 0.05% of MWCNTs per binder content revealed a better performance both in the fresh and hardened state than the mix with 0.1% of MWCNTs.

Albanela Dulaj, Monica P. M. Suijs, Theo A. M. Salet, Sandra S. Lucas
Properties of 3D-Printable Ductile Fiber-Reinforced Geopolymer Composite

This paper presents the performances of a 3D printable ductile fiber-reinforced geopolymer composite (3DP-DFRGC). An ambient temperature cured one-part geopolymer was utilized as the binder for manufacture of the developed 3DP-DFRGC, which eliminates the necessity for curing at elevated temperature and handling of user-hostile alkaline solutions. Herewith, it considerably enhances the possibility of in-situ applications and commercial viability of the 3DP-DFRGC. The rheological behavior and mechanical properties of the 3DP-DFRGC were experimentally characterized. The mold-cast DFRGC was also prepared and tested for comparison. The 3DP-DFRGC exhibited pronounced deflection-hardening behavior under bending. The modulus of rupture and the corresponding deflection of the 3DP-DFRGC were 18% and 28% higher, respectively than those of the mold-cast DFRGC. This can be due to the preferential orientation of fibers in the 3D-printed specimens.

Shin Hau Bong, Behzad Nematollahi, Venkatesh Naidu Nerella, Viktor Mechtcherine
Feasibility of Using Ultra-High Ductile Concrete to Print Self-reinforced Hollow Structures

3D concrete printing (3DCP) is facing the challenge of introducing steel reinforcement. Using ultra-high ductile concrete (UHDC) to print self-reinforced structures is a potential method to address the challenge. However, few researches have been conducted in this area. This work aims to demonstrate the feasibility of using UHDC to print self-reinforced structures with hollow sections. 6 different hollow beams without steel reinforcement were printed and tested under four-point loading. The results indicate that the printed beams with hollow sections have multiple cracking, flexural hardening, and ductile failure mode despite the absence of steel reinforcement. The flexural strength of hollow beams ranged from 6.06 MPa to 7.86 MPa. The deflection to span ratio of all printed beams was higher than 1/50 at the ultimate state. The combination of UHDC and hollow structures provides a new concept for 3DCP without steel reinforcement.

Junhong Ye, Yiwei Weng, Hongjian Du, Mingyang Li, Jiangtao Yu, Md Nasir Uddin
Development of Cementitious Metamaterial with Compressive Strain Hardening Characteristics

The aim of this study is to fabricate a cementitious metamaterial with compressive strain-hardening characteristics by directly 3D printing fiber reinforced cement mortar. The internal structure of the metamaterial was designed such that vertical and horizontal elliptical holes were hollowed out alternately from a rectangular prism. This structure was intended to “squeeze” inward when compressed even after cracking. To validate the conceptual design, specimens were prepared by using two methods: casting fiber reinforced cement mortar in 3D printed plastic molds and directly printing fiber reinforced mortar by an extrusion-based 3D mortar printer. The hollowed specimens, as well as a control cast specimen without holes, were subjected to compression testing, and the surface strain was measured by using Digital Image Correlation. The test result showed that the load first increased almost linearly with the increase of deformation, followed by 76% load drop at the onset of the first crack. After the load drop, the load increased again, instead of showing brittle failure, and could recover up to 76% of the peak. When compared with the filled specimen, the hollowed cast specimen showed significantly larger deformation capacity with 14 times larger energy absorption due to the post-peak strain hardening behavior. On the other hand, the hollowed 3D-printed specimen showed the strain-softening behavior after the peak load, which could be attributed to the lack of accuracy in printing the elliptical holes. Nevertheless, significantly high energy absorption could be achieved in the printed specimen as well.

Keisuke Nishijo, Motohiro Ohno, Tetsuya Ishida
Consistency of Mechanical Properties of 3D Printed Strain Hardening Cementitious Composites Within One Printing System

Previous research has shown that the material properties of a three-dimensional printed strain hardening cementitious composite (3DP-SHCC) can significantly vary, depending on the printing system with which it is produced. However, limited research has been performed on the reproducibility of hardened mechanical properties under identical printing conditions. In this study, the consistency of hardened properties, including compressive strength, flexural strength and deflection, and tensile strength and strain, was tested from materials printed during three separate but identical printing sessions. The research shows that with 3DP-SHCC, significant variations in mechanical properties between printing sessions can be expected.

Karsten Nefs, A. L. van Overmeir, Theo A. M. Salet, A. S. J. Suiker, B. Šavija, E. Schlangen, Freek Bos

Design and Digital Workflow

Uncertainty Quantification of Concrete Properties at Fresh State and Stability Analysis of the 3D Printing Process by Stochastic Approach

This study aims at developing an efficient method that allows quantifying the uncertainties of concrete properties and their effect on the stability of structure during the 3D printing process. Following that, the well-known Bayesian inference will be chosen to characterize the uncertainties of the elastic and plastic properties of the cementitious material at fresh state using the results of experiments available in the literature. These characterized mechanical properties and their associated uncertainty will be then taken as input parameters for the stochastic analysis through which the probability of instability of the printed structure due to plastic and buckling collapses can be estimated. Our numerical results highlight the significant effect of uncertainty on the stability during printing of concrete structure, that has been ignored in the literature.

Zeinab Diab, Duc Phi Do, Sébastien Rémond, Dashnor Hoxha
Simulation of 3D Concrete Printing Using Discrete Element Method

The article at hand presents an approach for analyzing the 3D Concrete Printing (3DCP) by means of the Discrete Element Method (DEM). An advanced user-defined simulation material model for fresh printable concrete has been developed to simulate extrusion, discharge and deposition. In addition, a calibration procedure is shown to find a fitting parameter set for the material model parameters based on experimental data. The calibration of the latter is an iterative adaption process, leading to a realistic representation of real printable concrete. Finally, an extrusion-based 3DCP process is exemplary simulated to show the potential of the simulation method for process analyses and to verify the applicability of the model. The developed simulation tool enables a better understanding of the extrusion process during 3DCP and a profound analysis of the material flow within the extruder. Based on this information, improvements in the machine layout and the process parameter settings can be identified, allowing for further printing process optimizations.

Knut Krenzer, Ulrich Palzer, Steffen Müller, Viktor Mechtcherine
Influence of Infill Pattern on Reactive MgO Printed Structures

The construction industry’s increasing interest in additive manufacturing has generated a parallel interest in alternative and supplementary binders. Reactive magnesia (MgO) binders are an attractive and sustainable alternative to Portland cement; they are produced at lower temperatures and can potentially absorb the equivalent amount of CO2 emitted during production within their service life via carbonation curing. While excessive evaporation in 3D printed Portland cement is usually sought to be prevented, the resulting increase in porosity from the higher surface exposure is a desirable feature for 3D printed MgO that can lead to higher carbon intake. In this work, we utilize nanoclays in combination with methylcellulose as rheological modifiers to produce 25.4 mm MgO paste cylinders with different infill patterns: two open with continuous hollow channels and one solid/closed to mimic a conventionally cast one. Two additional configurations were introduced where the top and bottom layers of the open infills were replaced by a closed layer or a “lid” to conceal the infills’ internal structure. The results show that 3D printing of MgO improves compressive strength over cast ones by up to 380% at 28 days of carbonation curing, reaching 40–48 MPa at 1.1 w/b. The results also suggest that infill patterns play a more critical role in stress transfer than carbon intake as finite element analysis confirmed the introduction of localized stresses at the lid layers.

AlaEddin Douba, Palash Badjatya, Shiho Kawashima


Evaluation of Durability of 3D-Printed Cementitious Materials for Potential Applications in Structures Exposed to Marine Environments

The rising interest in 3D-printing of concrete structures for use in marine environments requires development of concrete mixtures with adequate mechanical and durability characteristics. The incorporation of alternative cementitious materials, combined with careful selection of printing parameters has emerged as an effective way of controlling not only the fresh properties and printability of mixtures, but also their mechanical and durability properties. This paper presents the results of various durability related tests performed on 3D-printed mortars, including density, porosity, rate of water absorption and resistance to chloride penetration. Results of these tests indicate that the performance of mortar elements 3D-printed using controlled overlap process was similar to the performance of conventionally cast mortar elements with the same composition. Moreover, the results of the chloride transport related tests obtained from all specimens evaluated during the course of the study indicate low chloride ion penetrability, thus re-affirming that combination of the proposed material and 3D-printing method of fabrication have a potential for producing structural elements for applications in marine environments.

Fabian B. Rodriguez, Cristian Garzon Lopez, Yu Wang, Jan Olek, Pablo D. Zavattieri, Jeffrey P. Youngblood, Gabriel Falzone, Jason Cotrell
Two Year Exposure of 3D Printed Cementitious Columns in a High Alpine Environment

While digital fabrication technologies with concrete have focused primarily on process development and compressive strength in terms of performance, more recently studies evaluating the durability performance of 3D printed cementitious materials have started to appear. To date, however, no field performance assessments have been published regarding 3D printed cementitious materials. In this study, we present the condition of bespoke 3D printed columns that have been exposed to the high alpine environment of Riom-Parsonz, Switzerland, for a period of two years. Damage levels are variable from column to column, often appearing as vertically oriented cracks that can also track along weak layer interfaces. The damage is conjectured to be linked primarily to freeze-thaw damage and/or thermal or differential shrinkage strains. The link between column design and damage is discussed in depth, in particular as it relates to the current predominant use case of 3D printed cementitious materials as a lost formwork.

Timothy Wangler, Asel Maria Aguilar Sanchez, Ana Anton, Benjamin Dillenburger, Robert J. Flatt
Salt Scaling Resistance of 3D Printed Concrete

Extrusion-based 3D concrete printing is an emerging technology in the construction field due to the many advantages associated with it as compared to conventional mould casting technology. However, many aspects like durability and long-term service performance are yet to be investigated in detail. The present study focuses on understanding the salt scaling resistance of 3D printed concrete samples. 3D printed concrete samples were prepared with a Portland cement mixture on the one hand and a mixture containing a blend of Portland cement and blast furnace slag on the other hand. The printed samples were subjected to freeze and thaw cycles with a 3% saltwater concentration. It was observed that the 3D printed samples exhibited better resistance against salt scaling compared to the mould cast samples made with the same mixture. The pore structure of the 3D printed samples was characterized by mercury intrusion porosimetry. It was observed that the presence of a higher amount of interconnected and coarser pores at the interlayer region of the 3D printed samples, acting like pockets of air voids, facilitates the release of ice crystallization pressure during the freezing phase. The study gives insights into the durability characteristics and feasibility of using 3D printed concrete elements exposed to aggressive environmental conditions.

Manu K. Mohan, A. V. Rahul, Geert De Schutter, Kim Van Tittelboom
Influence of the Print Process on the Durability of Printed Cementitious Materials

3D concrete printing (3DCP) is currently being explored both in academia and practice and ensures a fast, economic, safe and formwork-free construction process. Although the elimination of formwork is one of the biggest advantages, it also removes the protection between the curing concrete and the surrounding environment and consequently, cracking resulting from shrinkage can be more pronounced. Additionally, the effect of the layered fabrication process and the absence of compaction could increase the porosity and the occurrence of weakly bonded interfaces. The combination of these three phenomena might affect the durability of 3DCP elements, as these interfaces form ideal ingress paths for chemical substances. In order to improve the long-term behavior, this study aimed to comprehend the correlation between different print process parameters and the resistance against carbonation. Therefore, multi-layered elements were fabricated with two different print techniques (2D and 3D) and two interlayer time gaps (0 and 30 min). To enable a complete comparison between both fabrication processes also conventional cast elements were considered. In general, a more pronounced CO2 penetration could be observed for printed elements, related to the higher porosity. Additionally, enlarged time gaps tend to be detrimental for the durability, however, this effect could be counteracted the 3D-print technique. The higher pump pressure improves the bonding between subsequent layers and the general long-term behavior.

Jolien Van Der Putten, M. De Smet, P. Van den Heede, Geert De Schutter, Kim Van Tittelboom
Freeze-Thaw Performance of 3D Printed Concrete: Influence of Interfaces

The long-term performance of 3D printed concrete structures is essential and among the various durability issues, frost damage is one of key importance, especially in cold locations such as Switzerland. For 3D printed materials, the presence of layer interfaces and cold joints is a potential issue in terms of frost resistance. Therefore, after extrusion, both cast and printed samples were prepared, and they were subjected to 300 cycles of freeze-thaw in accordance with ASTM C666. It was found that printed samples have lower resistance to freeze-thaw conditions compared to their cast counterparts. The lower resistance of the printed samples could be attributed to the heterogeneity in the microstructure, in particular to the higher capillary porosity in the interface region compared to that in the bulk. The higher capillary porosity could be confirmed based on the sorptivity test results.

Arnesh Das, Asel Maria Aguilar Sanchez, Timothy Wangler, Robert J. Flatt

Heterogeneities and Defects

Mechanical Properties and Failure Pattern of 3D Printed Hollow Cylinders and Wall Segments Under Uniaxial Loading

Extrusion-based 3D printed structures are heterogeneous with a combination of solid layers and weak bonds. The weak bonds can be considered to be an amalgamation of hydrated products and air voids. The hardened state properties of 3D printed structures depend on several factors such as layer strength, bond strength, geometrical imperfection, anisotropy, and printing parameters. The geometrical imperfections may be due to compression of individual layers or localized buckling during printing. This research aims to study the mechanical properties of 3D printed hollow cylinders and wall segments. The hollow cylinders correspond to hollow printed columns, whereas wall segments are cut from printed walls. The wall segment used in this study had a fixed design. The study is divided into three phases: hollow cylinders were printed with different aspect ratios (L/D), and compressive strength was measured at different ages in the first phase. The second phase included displacement control tests on 150 mm diameter and 300 mm height hollow printed cylinders. The post-peak behaviour was evaluated. The cylinder fails much later than the initiation of the first crack, but the crack propagates diagonally through the bonds and layers at ultimate failure. The effect of curing (water and air curing) on the compressive strength of hollow cylinders was further evaluated. In the last phase, 1 m by 1 m walls were printed, and segments were cut. The compressive strength was evaluated on the cut segments. This study shows that the initiation of crack is majorly influenced by geometrical irregularity and bond strength between the layers for the hollow cylinders. Whereas, for the wall segment, cracks initiated and propagated through the wall leaves and ribs connection.

Shantanu Bhattacherjee, Smrati Jain, Manu Santhanam, G. Thiruvenkatamani
Impact of Drying of 3D Printed Cementitious Pastes on Their Degree of Hydration

The 3D printing of cementitious materials has been proposed as a method to reduce costs and waste associated with formwork. However, the 3D printing of concrete faces some challenges. One specific obstacle is related to the curing and shrinkage of the printed cementitious materials. These elements often contain a high paste content and poor curing conditions. Information is needed about the drying behavior of cementitious elements with very high surface to volume ratio that are exposed to drying nearly immediately upon placement. This study uses neutron radiography to evaluate the impact of the drying of printed cement paste samples on their degree of hydration. Exposing 3D printed samples to the atmosphere, (i.e., drying) severely limited the hydration. The impact of drying was highly dependent on the surface to volume ratio of the element. The degree of hydration of a material is linearly correlated to the square root of its volume to surface ratio.

Rita M. Ghantous, Yvette Valadez-Carranza, Steven R. Reese, W. Jason Weiss
The Environment’s Effect on the Interlayer Bond Strength of 3D Printed Concrete

Estimating the bond strength between extruded concrete filaments is of paramount importance for the structural design of 3D printed concrete elements. This study investigates the effect of the environmental printing conditions at various pass times on the interlayer bond strength (IBS), elaborates on the underlying mechanism and investigates a possible mitigation measure. Elements are printed with seven different pass times ranging from 1 min to 30 min while exposed to a benign (indoor-like) and severe (site-like) evaporation rates. The IBS is quantified through flexure tests, with the interlayer aligned vertically, at a concrete age of 28 days. The experimental results show that increasing the pass time from 1 min to 30 min decreases the IBS by 107% for the benign condition. When the specimens were exposed to the severe evaporation rate for 30 min the IBS decreased an additional 35%. Applying a curing compound to the exposed filament surface, to reduce the moisture evaporation, weakened the bond strength.

Gerrit M. Moelich, J. J. Janse van Rensburg, Jacques Kruger, Riaan Combrinck
Evaluation of the Bond Strength Between 3D Printed and Self-compacting Concrete

Additive manufacturing is a perfect tool to manufacture topologically optimized elements. This is mostly done by 3D printing the contour of the desired element. As these printed constructions are hollow, their loadbearing capacity is limited. The use of a proper infill material can help to get rid of this shortcoming by properly integrating a conventional reinforcement system. Self-compacting concrete (SCC) seems an excellent material choice as it can flow into the irregular printed spaces. Not only the high flow properties are important, but the adhesion between the two types of concrete is of interest too. A good bond is needed to prevent delamination and assure a proper force transfer between the two materials. In this study, the bond strength between 3D printed and self-compacting concrete was tested. Linear 3D printed elements were cured at different conditions (dry and submerged in water) for different time periods (2, 6, 13 and 28 days) before casting SCC adjacent to it. After 28 days of curing of the SCC, cores were drilled out of the composite specimens and a direct tensile test was performed. Results showed that the bond strength between the two materials is neither dependent on the curing time of the printed material nor on the curing condition. However, the fracture plane does seem to be influenced by these factors. Specimens cured under water or specimens cured for a limited period tend to fail more at the interface between the two materials, while the ones cured in a dry environment or for a longer period do fail in the 3D printed material.

Michiel Bekaert, Kim Van Tittelboom, Geert De Schutter
Interlocking 3D Printed Concrete Filaments Through Surface Topology Modifications for Improved Tensile Bond Strength

Numerous organizations in the additive concrete manufacturing industry generally observe weak bonding between deposited filaments in 3D concrete printing. The consequences thereof are far reaching, including significant anisotropic mechanical behaviour as well as impaired durability. Several attempts have been made to address this issue, for example via chemical modification of the interlayer and repair mortar inclusion. However, limited literature exists on physical alteration of the interlayer toward enhanced filament bonding. This paper attempts to employ a biomimicry principle, namely topological interlocking, in order to facilitate filament bonding. This is achieved by exploiting the enhanced mechanical resistance provided in shear compared to tension loading only. Three simple geometrical patterns, namely square, sinusoidal and zigzag are selected to incorporate a combination of tension and shear resistance at the interlayer region. The direct tensile test (DTT) is used to experimentally evaluate the bond strength improvement capability of each pattern while maintaining a constant groove frequency, amplitude and width throughout the experiment. All three patterns indicate a noticeable improvement over the conventional horizontal interlayer with the square pattern achieving the highest followed by the sinusoidal pattern and lastly the zigzag pattern. Practical implications relating to the implementation of topological interlocking are discussed, and recommendations provided for further research into this novel application.

Jean-Pierre Mostert, Jacques Kruger
Digitally Fabricated Keyed Concrete Connections

Most current technologies in digital fabrication with concrete (DFC) rely on controlled environmental conditions and, thus, have been used in prefabricated construction. Prefabricated reinforced concrete elements produced in factories need assembly and connection on-site. Using DFC for producing tailor-made geometries and applying surface roughness generates new possibilities for the design of connections. DFC enables (i) fabricating dry connections, for example, by using exact formworking or milling processes, and (ii) the relatively straightforward preparation of rough construction joints, for example, by using extrusion processes. In a recent study, a series of different specimens incorporating connections were tested using deformation-controlled push-off tests. This contribution presents the experimental campaign including design, preparation and test results employing keyed connections produced with the Eggshell technology, a fabrication process using 3D printed thin plastic formwork.

Patrick Bischof, Jaime Mata-Falcón, Joris Burger, Walter Kaufmann

Material Jetting

A 3D Printing Platform for Reinforced Printed-Sprayed Concrete Composites

The issue of reinforcement remains a difficult one for digital fabrication in concrete construction, with many solutions offered on how to best incorporate it with the technology in a practical manner. While certain technologies, such as digital casting systems, allow more seamless introduction of traditional steel reinforcement bars, the dominant technology of extrusion-based 3D printing still shows great limitations, with the most typical application being that of a lost formwork for traditionally cast concrete, or as unreinforced masonry.The use of both conventional and unconventional reinforcement materials, and their incorporation into processes, is a large and active area of research with the digital concrete community. In this work, we present a 3D printing platform and manufacturing concept in which a 3D printed form serves as a vertical substrate for a reinforcement, either textile or conventional steel. The structural concrete is then either applied by hand or sprayed around the reinforcement.

Lex Reiter, Ana Anton, Timothy Wangler, Benjamin Dillenburger, Robert J. Flatt
Influence of Material and Process Parameters on Hardened State Properties of Shotcrete 3D-Printed Elements

The layer-by-layer nature of additive manufacturing processes creates an interface between the individual strands and can therefore affect the printed element in the hardened state. In this paper, the Shotcrete 3D Printing (SC3DP) technique is investigated. Here, the effect of process and material parameters during production, namely concrete volume flow (0.4; 0.6; 0.8 m3/h), air volume flow (30; 40; 50 m3/h), and accelerator dosage (corresponding with 0; 3; 6% bwoc) on layer geometry (width/height), interface tortuosity and flexural strength are evaluated.The results presented in this paper show that the strand geometry is essentially determined by the accelerator dosage and concrete volume flow. The interface tortuosity is influenced by an interaction of all three parameters, but the air volume flow has the greatest influence. The flexural strength is dominated by the accelerator dosage and the air volume flow. In addition, a correlation between interface tortuosity and flexural strength is demonstrated. Finally, the consequences of how to use the findings of the effect of process parameters, accelerator dosage, and the layer-by-layer nature during printing in practical application are discussed.

David Böhler, Inka Mai, Niklas Freund, Lukas Lachmayer, Annika Raatz, Dirk Lowke
Shotcrete 3DCP Projection Angle and Speed Optimization: Experimental Approaches and Theoretical Modelling

Shotcrete has been lately used for additive manufacturing because of the high versatility of this method and possibility to integrate reinforcement. However, it is known that shotcrete generates waste due to rebound. To minimize the waste, the spraying process needs to be optimized. This work shows the optimization of wet-mix shotcrete from both experimental and theoretical sides for the use in shotcrete 3D concrete printing (S-3DCP). A better control must be reached to reduce the waste from rebounds and for improving the concrete mechanical properties.A strategy to reduce the waste coming from rebounds is to narrow the concrete jet angle, accelerate all particles at the same speed and ensure a homogeneous sprayed material across the jet. Several types of nozzles have been developed and assessed to ensure high mechanical properties, minimal waste and homogeneous spraying. The classic value of the cone angle is about 15°. The nozzle developed and assessed in experiments managed to reduce it to 8° by optimizing the air inlet and the internal shape of the nozzle, and it was reduced to 2.5° by using an additional air intake.A model of S-3DCP using aerodynamics (drag force), considering both paste and aggregate phases and the rheological properties of concrete has been developed to evaluate the influence of different parameters. The best configuration between the nozzle shape, air settings (pressure and flowrate) and concrete settings (aggregate distribution, paste composition and mass flowrate) allowed to find an optimum for the concrete jet speed. The speed of the spray has a major influence on the cohesion between the concrete layers. Using optimized nozzle geometry and particle velocity, the improvement in strength performance has been measured from 20–25%. The model has been verified by reproducing experimental conditions and comparing speed results.

Benjamin Galé, Thierry Ursenbacher, Agnès Petit, Vincent Bourquin
ARCS: Automated Robotic Concrete Spraying for the Fabrication of Variable Thickness Doubly Curved Shells

The fabrication of doubly curved concrete shells traditionally poses challenges in terms of material deposition and formwork manufacturing. Recent advances in digital fabrication techniques including robotics in construction have created tools to better tackle these challenges. We present a digital fabrication system to accurately deposit cementitious materials onto doubly curved surfaces using robotic concrete spraying in a process for prefabrication of structural shell components we call ARCS (Automated Robotic Concrete Spraying). Using inputs of a generic variable thickness shell volume, a robotic path is generated to deposit material onto the formwork surface where required. Paths are generated on the surface using geodesic lines as guides in order to create iso-curves for evenly spaced spray paths. Additionally, layers are incrementally deposited to allow for variable thicknesses across the surface. This approach is flexible enough such that additional features can be embedded onto the shell such as ribs. We demonstrate this automated fabrication process on a 1.8 m by 1.8 m scale shell specimen.

Mishael Nuh, Robin Oval, John Orr

Particle Bed Binding

Particle Bed Technique for Hempcrete

This study focuses on the particle bed additive manufacturing of hempcrete. The micronized hemp shives play an important role on the imbibition phenomenon during the 3D printing process. It appears imperative to control the key parameters involved in the imbibition process of the powder-bed. The measurements and analysis of the depth and homogeneity of the imbibition front are carried out in this study.We highlight that diffusion coefficient and the imbibition kinetic are clearly influenced by the amount of hemp in the powder bed. This study highlights that micronized hemp shives reduce the imbibition kinetics due to its swelling.

V. Danché, A. Pierre, K. Ndiaye, T. T. Ngo
Effect of Curing in Selective Cement Activation

Particle Bed 3D Printing by Selective Cement Activation (SCA) has the capability to produce high resolution and freeform elements, which offer new possibilities in terms of resource-efficient and architectural design. In this paper we present possibilities to enhance the mechanical properties of SCA-printed specimens by curing. Therefore, the focus of this study is to systematically investigate the effect of various curing conditions for two specimen series varying in water cement ratio (0.3 and 0.5):i) under water storage for 1, 3, 7, 21, 28 days and subsequent storage at 20 ℃ and 65% relative humidity andii) storage at varying relative humidity (65% and 100% at 20 ℃).The specimens are evaluated in terms of mechanical performance (28 d compressive strength), geometrical precision, water distribution over the cross section and calorimetry measurements in order to characterize the remaining hydration potential of the activated cement before and after curing. It is shown that suitable curing can lead to a significant better mechanical performance of the specimens. This is especially interesting for specimens with a low w/c-ratio, where the geometric precision is high. The effect of improved mechanical performance due to curing is attributed to a homogenous distribution of the curing medium water in the specimen. This enables a higher degree of hydration of the cement in the cured specimens, which is shown with the calorimetry measurements. Finally, the results are discussed regarding their practical applicability.

Friedrich Herding, Inka Mai, Dirk Lowke
Evaluating the Effect of Methyl Cellulose on Hardened State Properties in Selective Cement Activation

Compared with other 3D printing techniques used in construction, particle bed 3D printing by Selective Cement Activation (SCA) offers unique possibilities regarding the fabrication of high-resolution and freeform components. The printed geometry is determined by the fluid distribution in the particle bed, consisting of a dry mixture of fine aggregates and cement. The fluid intrusion behaviour and thus the shape accuracy of the printed object can be actively controlled by adding additives such as methyl cellulose (MC) to the particle bed. In this research the goal is to fundamentally understand the effect of MC varying in degree of polymerization (DOP), size and dosage on hardened state properties of SCA printed objects. Therefore, dissolution experiments of MC in the rheometer, capillary induced fluid intrusion into the particle bed as well as printing experiments are conducted. As hardened state properties of printed components, geometric precision, density as well as compressive strength are assessed.It was found that MC granules in the particle bed increases the geometric precision and the mechanical performance of SCA objects to a certain extent. This is explained by a modified fluid intrusion behaviour in the particle bed. Here, the effect of the DOP is low compared to the effect of the particle size of the dry granules and the amount of MC. When using too much MC, the geometric precision is not further increased while the mechanical performance even decreases, which is attributed to an increase in porosity in the printed specimen.

Inka Mai, Friedrich Herding, Dirk Lowke
Selective Paste Intrusion: Stability of Cement Paste Mixtures Towards Changing Ambient Temperature

The Selective Paste Intrusion (SPI) is an additive manufacturing method in which aggregates in a particle bed are selectively bonded layer-by-layer with cement paste to build complex, free-formed concrete elements. To ensure both sufficient layer bonding and shape accuracy, the adjusted paste yield stress ( $${\tau }_{0}$$ τ 0 ) needs to be almost constant during the entire printing period. The $${\tau }_{0}$$ τ 0 depends beside others on the ambient air temperature. Temperature changes, as common in the precast plants, are transferred to the raw materials, mixing tools and printer peripherals. Thus, the fresh cement paste will vary in temperature. A change especially during the production process can either lead to insufficient layer bonding and thus strength of the built component or excessive spreading of the cement paste in the particle bed and thus poor shape accuracy. We therefore investigate the stability of the paste $${\tau }_{0}$$ τ 0 at temperatures between 16 ℃ and 26 ℃ in steps of 2 ℃ each, starting at 20 ℃, which is the reference temperature. We found a continuously increasing $${\tau }_{0}$$ τ 0 for increasing temperatures. This worsens the printing process i.e. the penetration depth of the paste, shown in simulations based on Darcy’s law. To ensure consistent component quality at varying ambient temperatures, the worsened penetration depth needs to be compensated by adjustments in either the mixture composition or the process parameters, e.g. velocity of the print head.

A. Straßer, Carla Matthäus, D.Weger, T. Kränkel, C. Gehlen

Printability and Set Control

Set-On Demand Concrete by Activating Encapsulated Accelerator for 3D Printing

In-line activation of cementitious materials by introducing the accelerators at the print head is a promising solution to attain the conflicting rheological requirements in 3D concrete printing. However, there are many challenges associated with this technique including, difficulty in achieving good mixing homogeneity, formation of dead zones during in-line mixing etc. This study investigates a method to attain in-line activation of cementitious materials by encapsulating the accelerator and introducing it in the initial mixing. This is followed by thermal activation at the printing head to destroy the encapsulation and subsequently mix the accelerator with the cementitious materials. A gelatine-based capsule was used as the shell material for the encapsulation, which dissolves when the temperature is exceeded above 30 ℃. The encapsulated accelerator was used at different dosages and the fresh properties of 3D printable mixes were assessed. The results showed that the in-line activation of encapsulated accelerator at 2.5 wt% dosage transformed the highly pumpable fresh concrete to a stiffened concrete with high yield strength development. For instance, the yield strength of concrete after activation was increased by fivefold, compared to control printable mix (CPM). Meanwhile, before activation, the fresh concrete mixes with encapsulated accelerator showed excellent flowability for long durations.

Sasitharan Kanagasuntharam, Sayanthan Ramakrishnan, Jay Sanjayan
Using Limestone Powder as a Carrier for the Accelerator in Extrusion-Based 3D Concrete Printing

Aiming at obtaining good shape stability after extrusion, accelerators are used in extrusion-based 3D concrete printing. Instead of injecting a liquid accelerator into fresh concrete, limestone slurry is proposed as a carrier for the dry accelerator in this study. As such, two pumpable mixtures (a Portland cement-based mixture and a limestone powder-based mixture) and a combination of these two mixtures (a combined mixture) were prepared. The flow behaviors and structural build-up were investigated, respectively.

Yaxin Tao, Karel Lesage, Kim Van Tittelboom, Yong Yuan, Geert De Schutter
Printability Assessment of Cement-Based Materials Using Uniaxial Compression Test

Concrete 3D printing, a cutting-edge manufacturing technology, is grasping increasing interest in the construction industry. Striking a balance between extrudability and buildability of printable cement-based materials is crucial to ensure successful printing. Although rheology is widely used to assess flow properties, it can bring several challenges when assessing flow properties of stiff materials used in 3D printing, including wall slippage, liquid phase migration, jamming, shear localization, and aging. Moreover, in 3D printing applications, cement-based materials are usually subjected to compressive loads due to the weight of the printed layers. The uniaxial compression test is used to assess the printability of cement-based materials investigated in this study. Both the extrudability and buildability aspects have been discussed. Various mortar mixtures were proportioned using compatible chemical-admixture systems and different sand-to-cement ratios. The effect of the latter designing parameters on the printability of cement-based materials was evaluated. On the other hand, special attention was given to the stress-strain behavior of the investigated mixtures.

Ilhame Harbouz, Ammar Yahia, Emmanuel Rozière, Ahmed Loukili
Monitoring Strain Using Digital Image Correlation During Compressive and Tensile Loading: Assessment of Critical Strain of Cement-Based Materials Containing VMA

3D concrete printing sparks over the past few years the interest of researchers invested in concrete Digital Fabrication. While many processing issues have already been overcome to a large extent, in-depth understanding of the materials behaviour remains a challenge, especially for high yield stress materials deposited using the so-called infinite brick strategy. The highly non-linear behaviour of this kind of material enforces the need to describe the mechanical behaviour for large deformation analysis, especially because this ability to be deformed is often overcome during the printing process leading to cracks initiation and growth. This study proposes to focus on the assessment of critical strain of fresh material undergoing compression and tension. It is crucial to keep compressive and tensile strain under critical values in order to avoid possible buildability and filament tearing issues (low radius of curvature along the printing path, bending induced by deposition, control of printing speed…). Considering the inability of most of firm printable cement-based mix designs to be plastically deformed without cracking, this work proposes to monitor material deformation for large deformation using Digital Image Correlation (DIC) during mechanical testing. Such investigation is required to fully and accurately describe the elasto-visco-plastic behaviour of the material. A comparison of strain, engineering strain and DIC computed strain is proposed and the limit of validity of common assumptions used for tests data analysis will be discussed. Finally, effects of Viscosity Modifying Admixtures (VMA) on critical shear strain will be investigated to assess to what extent this one could be increased by VMA addition: the improvement of the material ability to be deformed before breaking will allow to compute influence of viscous properties of high yield stress materials more accurately.

Yohan Jacquet, Arnaud Perrot, Vincent Picandet
Temperature Impact on the Structural Build-Up of Cementitious Materials – Experimental and Modelling Study

With increasing focus on industrialized processing, investigating, understanding, and modelling the structural build-up of cementitious materials becomes more important. The structural build-up governs the key property of fresh printable materials – buildability – and it influences the mechanical properties after the deposition. The structural build-up rate can be adjusted by optimization of the mixture composition and the use of concrete admixtures. Additionally, it is known, that the environmental conditions, i.e. humidity and temperature have a significant impact on the kinetic of cement hydration and the resulting hardened properties, such as shrinkage, cracking resistance etc. In this study, small amplitude oscillatory shear (SAOS) tests are applied to examine the structural build-up rate of cement paste subject to different temperatures under controlled humidity. The results indicate significant influences of the ambient temperature on the intensity of the re-flocculation (Rthix) rate, while the structuration rate (Athix) is almost not affected. A bi-linear thixotropy model extended by temperature dependent parameters coupled with a linear viscoelastic material model is proposed to simulate the mechanical behaviour considering the structural build-up during the SAOS test.

Alexander Mezhov, Annika Robens-Radermacher, Kun Zhang, Hans-Carsten Kühne, Jörg F. Unger, Wolfram Schmidt
Early Age Shear and Tensile Fracture Properties of 3D Printable Cementitious Mortar to Assess Printability Window

Digital fabrication with concrete is a giant leap forward for the concrete construction industry in achieving its sustainability goals. Concrete 3D printing, despite being the forerunner of the digital fabrication and additive manufacturing techniques, requires a significant amount of attention to be standardised. The nebulously defined parameters of pumpability, extrudability and buildability which collectively define the printability of a mix, need to be quantified. This requires the development of testing procedures to study the early age properties of concrete mixes that are presently unavailable. This study deals with the evaluation of early age tensile and shear strength of 3D printable cementitious mortars through a novel “fracture-based” methodology and tailored set-up. Direct shear and direct tensile tests have been performed on a 3D printable mix, incorporating also high proportions of Calcium Sulpho-Aluminate cement and non-structural basalt fibres, at “very early ages” namely, 30 min, 45 min, 60 min, 75 min, 90 min and 120 min from the contact between binder and water. The phase transition between fluid state to solid state of a printable mix has been observed and the energy released during the tensile and shear tests analysed with the aim of identifying the printability window from the time-evolution trend of the aforesaid parameters. The methodology has been carefully calibrated also with reference to experimental artefact, including the effect of friction among the movable mould parts, which may significantly affect the results in the case of very small values of mechanical material parameters, as it happens for concrete in very early ages.

Andrea Marcucci, Sriram K. Kompella, Francesco Lo Monte, Marinella Levi, Liberato Ferrara
A Strain-Based Constitutive Model Ensuring Aesthetic 3D Printed Concrete Structures: Limiting Differential Settlement of Filaments

Additive technologies in construction, such as 3D printing of concrete, have brought about renewed enthusiasm in the construction sector, mainly due to the principal advantages it presents including reduced cost, time, waste, formwork free manufacturing and free-form parametric design possibilities. Although holistically impressive, the aesthetic appeal brought by distinct fissility of printed structures remains highly contended among the public. A particular aspect contributing thereto, is the inconsistent filament layer thicknesses observed over the height of a printed object. This is not atypical since each filament layer must support the weight of successive depositions while still in the plastic concrete state. Consequently, differential settlement of filament layers is observed, increasing in magnitude toward the bottom critical filament layer. This paper derives a simple strain-based constitutive analytical model that determines the number of printable layers whereby the user-defined critical strain value for an individual filament layer is not exceeded. A novel material stiffness characterisation test, taking form as a modified version of the conventional unconfined uniaxial compression test, is performed on 3D printed concrete samples extracted directly from a filament layer, whereafter the model is compared against experimentally measured displacements. Digital image correlation technology is adopted for the accurate determination of displacements during printing. Experimental results indicate that the model initially underestimates displacement, followed by overestimation closer to the material yield point. Recommendations are provided toward further model improvements. This model is particularly apt in situations where the aesthetic appeal of a printed element places stricter demands than mechanical or durability requirements.

Jacques Kruger, Jean-Pierre Mostert, Gideon van Zijl

Process Control, Toolpath and Inspection

Process Control for Additive Manufacturing of Concrete Components

Additive manufacturing (AM) processes offer new possibilities in the design of concrete components. The process chain for AM processes generally consists of component design, print path generation, and manufacturing. Within the step of print path generation, the component is commonly divided into layers and filled with waypoints based on the assumption of a constant cross-section of the applied material strands. In contrast to metal or plastic, however, the material properties of fresh concrete are more sensitive to environmental influences such as temperature and humidity. This leads to cross-section variations during the process. Therefore, exclusively relying on an apriori print path planning for large-scale components leads to significant deviations between as-planed and as-printed geometries. The presented research aims to increase the manufacturing accuracy of concrete components by compensating layer inconsistencies through a controlled material application. For this purpose, varying the printing speed and nozzle distance allows for correction of the deviations of subjacent layers. Deviation detection is performed by a 2D laser sensor mounted on the printing nozzle to generate information about the underlying cross-section. Comparing the measured values to precalculated setpoints generates the error values. The control algorithm maps the error data into an adaption of the printing speed and nozzle distance to fulfill the pre-planned geometry. Applying the controller to a medium-sized component and comparing the result to the uncontrolled process shows a considerable accuracy improvement.

Lukas Lachmayer, Robin Dörrie, Harald Kloft, Annika Raatz
Generative Structural Design: A Cross-Platform Design and Optimization Workflow for Additive Manufacturing

This paper explores a generative design, simulation, and optimization workflow for full-scale 3D printed building components using an array of different mixtures. Large-scale additive manufacturing in conjunction with algorithmic CAD design tools enables a vast amount of control when creating geometry. This can also be advantageous regarding the increasing demands to use more resourceful and sustainable construction methods and materials. The presented methodology aims to highlight these new technological advancements and offers a multimodal and integrative design solution with the potential for an immediate application in the AEC-Industry. The presented work will investigate a case study based on a structurally optimized waffle or two-way joist slab. The goal is to create permanent formwork that serves as molds during the pouring and fabrication process on-site and validates the structural integrity of those 3d concrete printed molds. For this, a hybrid validation strategy, using a digital and cross-platform simulation environment that is audited with physical prototyping is investigated. Various geometric attributes are parameterized and can be imported to a finite element method (FEM) software via a custom workflow. Here the structural behaviors and failure patterns of the molds during the pouring process are examined and incrementally optimized to satisfy minimum structural performance requirements to ultimately withstand the pressure of the cast-in-situ concrete. Subsequently, alternative and hybrid mixtures and materials, e.g., foam concrete and eco-friendly composites are numerically evaluated and compared. The results are fed back to the parametric design model to further optimize the generative workflow.

Saqib Aziz, Ji-Su Kim, Dietmar Stephan, Christoph Gengnagel
A Closed-Loop Workflow for Quality Inspection and Integrated Post-processing of 3D-Printed Concrete Elements

Additive manufacturing with concrete opens up new possibilities in the mass production of individualized and highly material-efficient building components. However, the absence of the commonly used formwork places high demands on the material during production, often leading to inaccuracies in the printed object. In order to guarantee dimensional accuracy and component quality, this paper presents an integrated and fully automatic quality inspection method as well as a subsequent fully automatic local postprocessing process.

Norman Hack, Carsten Jantzen, Leon Brohmann, Markus Gerke, Karam Mawas, Mehdi Maboudi
Force Flow Compliant Robotic Path Planning Approach for Reinforced Concrete Elements Using SC3DP

This paper presents a robotic path planning approach for additively manufactured reinforced concrete components using Shotcrete-3D-Printing Technology (SC3DP). The aim is to optimally match the concrete paths and reinforcement arrangement to the compression and tension trajectories. Typically, 3D printers can control three axes, which means that the material is built up in horizontal layers. In 3D concrete printing, this restricts the integration of reinforcement, as well as the integration of other component functions. The novel approach uses the spatial freedom of the robot-assisted SC3DP technology. The planning of the concrete paths is adjusted to the integration of the reinforcement during the printing process (concrete supports reinforcement). By curving the concrete pathways threedimensionally, the reinforcement can be optimally arranged along the tensile stress trajectories, without additional support reinforcement for positioning. In this paper, the novel approach is demonstrated on a single span beam under uniform loading. Cut carbon fibre fabrics are used as reinforcement. The paper gives an overview of the used machine setup, technical parameters and abilities of the combined technologies.

Robin Dörrie, Harald Kloft


Integrating Reinforcement with 3D Concrete Printing: Experiments and Numerical Modelling

3D Concrete Printing (3DCP) is a technology that recently has attracted the attention of both academia and industry. The technology offers an increased design flexibility and has been used at various scales, e.g. from furniture to bridges and houses. One of the current challenges in 3DCP is to produce load bearing structures in a single process, i.e. reinforced elements as part of 3DCP process. This is because the integration of vertical reinforcement during the printing process is not trivial. Although few reinforcement methods have been studied, a robust and efficient 3DCP reinforcement solution is yet to be coined. To support these studies in finding a reinforcement solution fit for 3DCP, while limiting experimental efforts, we offer a computational fluid dynamics (CFD) model that simulate concrete flow around rebars. The numerical model applies 1) an elasto-visco-plastic constitutive law to mimic the flow behavior of the concrete and 2) the volume of fluid method to track the free surface of the concrete. To validate the proposed model, 3DCP experiments are carried out by printing around horizontal and vertical rebars. The rheological behavior of the concrete is characterized on a rheometer using a vane-in-cup measuring system, and such data is included in the CFD model. The experimental and numerical results agree relatively well; providing a new venue for identifying printing strategies that ensures a good bonding between concrete and reinforcement.

Jon Spangenberg, Wilson Ricardo Leal da Silva, Md Tusher Mollah, Raphaël Comminal, Thomas Juul Andersen, Henrik Stang
Flow-Based Pultrusion of Anisotropic Concrete: Mechanical Properties at Hardened State

The issue of reinforcement for 3D concrete printing has received considerable attention, as constructions have to comply with reliability principles and building standards. Here a specific process called Flow-Based Pultrusion for additive manufacturing (FBP), inspired by pultruded composite manufacturing and built on existing extrusion-based 3D printing technology, permits to impregnate numerous continuous fiber rovings pulled by the extrusion flow of a fine mortar [4]. The resulting extruded material, an Anisotropic Concrete, is isotropic transverse like unidirectional long fibers composite. The mechanical properties are greatly influenced by the quantity of fibres (reinforcement ratio) and their impregnation quality [2]. These factors are related to the process parameters [3], the mortar rheology for impregnation and the fibre dosage (roving thickness, roving count) for reinforcement ratio. Full-scale experiments are presented, using fibre rovings to increase the reinforcement ratio up to 3%, which is comparable to the steel/concrete ratios in traditional rebar-reinforced concrete elements. The influence of reinforcement ratio on the tensile strength and ductility of the hardened material is presented.

Léo Demont, Malo Charrier, Pierre Margerit, Nicolas Ducoulombier, Romain Mesnil, Jean-François Caron
Core Winding: Force-Flow Oriented Fibre Reinforcement in Additive Manufacturing with Concrete

In this paper we present a novel reinforcement method for concrete 3D printed elements using fibre winding. This technique, termed Core Winding Reinforcement (CWR), allows for force-flow oriented alignment of fibre reinforcement along the part’s faces. It has been tested in combination with Shotcrete 3D Printing (SC3DP), but is suitable for both, material extrusion and material jetting. The paper describes the fully automated design-to-fabrication workflow for a 1:1 demonstrator using SC3DP: First, the stress distribution of a given wall geometry is analysed digitally, second, the core is printed, third, the reinforcement fibre is wound and fixed onto the core, fourth, it is embedded by applying a cover layer of shotcrete, and finally, the surface is trowelled in an automated manner.

Stefan Gantner, Philipp Rennen, Tom Rothe, Christian Hühne, Norman Hack
Integration of Mineral Impregnated Carbon Fibre (MCF) into Fine 3D-Printed Concrete Filaments

Additive manufacturing with concrete, also often called 3D concrete printing (3DCP), has been gaining land speedily in recent years. This technology not only opens up higher productivity and relieves labor of heavy work, but also offers completely new design possibilities. Here, novel architectural form languages and design approaches can be developed, which are based, for example, on growth principles learned from biology. The structures created in this way are characterized by a high degree of optimization with regard to material use. However, introduction of such approaches in the concrete construction requires an adequate reinforcement material and flexible technology for its integration into the 3DCP process. The article at hand presents a new technology developed for integrating such reinforcement into 3DCP. Mineral-impregnated carbon fibre (MCF) rowings are automatically introduced into the printing nozzle and subsequently centrally integrated into fine concrete filaments. In addition to the description of the fabrication process, the mechanical characteristics of the printed composite material are provided. For this purpose, both the stress-strain behavior of the composite and the bond properties between reinforcement and concrete matrix are investigated. Furthermore, a comparison is made between the performance of the printed material with reinforcement integrated in the concrete filament and a conventionally casted concrete. The evaluation of the results is accompanied by image-based examinations using computer tomography and scanning electron microscope to explain the effects of parameter under investigation on the bond quality and other properties.

Tobias Neef, Steffen Müller, Viktor Mechtcherine
Flexural Behaviour of Steel-Reinforced Topology-Optimised Beams Fabricated by 3D Concrete Printing

In this study, steel-reinforced topology optimised beams were 3D printed through the layered extrusion technique and in combination with cementitious printable mortar. Cementitious material properties were used as input data of the topology optimisation algorithm to identify the optimised beam shape reducing the material used. The steel reinforcement was designed using small-diameter deformed bars (i.e., ϕ8) placed into the inter-layer plane during printing. The main structural design features are discussed, along with the results of flexural tests carried out on a 2 m long beam with/without reinforcement tested in a 3-point bending configuration. First, the experimental results enabled the validation of the topology optimisation algorithm by comparing the experimental failure loads with the design ones. Secondly, the experimental outcomes allowed the comparison between the mechanical response of 3D printed beams with and without reinforcement, highlighting the significant role of the joints of the printing path.

Costantino Menna, Laura Esposito
Fundamental Study on Automated Interlayer Reinforcing System with Metal Fiber Insertion for 3D Concrete Printer

In recent years, 3D concrete printing (3DCP) systems have been extensively developed in the construction field as a promising new technology. In particular, additive manufacturing (AM) systems using mortar are among the most promising methods. 3DCP has great advantages such as labor work saving at labor-intensive construction sites and creating complex-shaped members with high accuracy that are difficult to create using conventional construction methods. However, printed members using AM-based 3DCP systems inevitably include discontinuous interlayers, such as cold joints of concrete structures. The presence of such defects causes strength degradation in printed structures. In addition, it is challenging to introduce reinforcing steel bars as in general RC structures for AM-based 3DCP systems. Therefore, it is necessary to overcome the defects in the interlayers using new reinforcement methods. In this study, a fundamental investigation was conducted to develop an automatic reinforcing system suitable for AM-based laminate structures by metallic fiber insertion at the interlayers. A prototype automatic metallic fiber insertion system was established. Specimens were obtained and subjected to mechanical tests to confirm the reinforcing effect of metallic fibers. In addition, the interior of the specimen was measured by X-ray CT. It was confirmed that the reinforcing effect of metallic fibers enhanced the mechanical properties of the printed specimens.

Tomoya Asakawa, Tomoya Nishiwaki, Kazunori Ohno, Shigeru Yokoyama, Yoshito Okada, Shotaro Kojima, Youichi Satake, Yoshihiro Miyata, Yuki Miyazawa, Youhei Ito, Hideyuki Kajita
Robotically Placed Reinforcement Using the Automated Screwing Device – An Application Perspective for 3D Concrete Printing

In structural applications reinforcement is required to achieve sufficient tensile strength and ductility. While several concepts are being developed to achieve this in 3D concrete printing, no generic method has been established yet and as a result all built objects are solved on a case-by-case basis. In this study one of the potential solutions, inserting helical reinforcement by a screwing motion, is further explored by presenting the Automated Screwing Device and its potential application domains in 3DCP.

Lauri Hass, Freek Bos
Proof-of-Concept: Sprayable SHCC Overlay Reinforcement Regime for Unreinforced 3D Printed Concrete Structure

Rebar penetration is the most common vertical reinforcement strategy for unreinforced 3D printed concrete (U3DPC) structures to improve the axial, flexural and shear capacities, and interlayer bond behaviour. Despite the reasonable simplicity of the application either by a manual or automated system, the method may disturb the fresh material during insertion which may cause plastic yield or elastic buckling failure, in particular with ribbed rebars, and generally has a deficiency in bond behaviour with non-ribbed rebar.Sprayable strain-hardening cementitious composite (S-SHCC) overlay retrofit- ting has been extensively studied for shear and ductility enhancement of unreinforced masonry (URM) walls by several researchers. Depending on the overlay thickness and failure mode, the shear capacity can be increased by up to 465%. Due to the layer-by-layer construction nature of 3DPC, the similarity in structural behaviour of the U3DPC and URM is evident despite the apparent difference in interlayer bond strengths of the construction methodologies. Thus, the S-SHCC overlay retrofitting reinforcement regime may be applied to U3DPC structures to increase the mechanical resistance to applied loads.This paper introduces the reinforcement method, and presents experimental results of enhanced in-plane and out-of-plane flexural behaviour of 3DPC elements reinforced with thin, bonded SHCC layers.

Seung Cho, Marchant van den Heever, Jacques Kruger, Gideon van Zijl
Pre-installed Reinforcement for 3D Concrete Printing

Providing reinforcement is essential for the structural integrity of concrete elements and for safely handling, transporting, and assembling prefabricated concrete parts. However, the integration of reinforcement is a persisting challenge in 3D concrete printing with extruded concrete. This paper presents a production process consisting of 3D printing around pre-installed reinforcement. The reinforcement is composed of conventional reinforcing steel bars, which can be pre-assembled in cages independently of casting, boosting the specialisation and efficiency in production. This approach was used to produce a 3.4 m span T-beam with optimised topology, consisting of three segments connected with matching surfaces. The beam segments were printed upside down, with an open web on top of the flange. Each segment featured reinforcing steel installed in the flange and web. After printing and assembling the segments, a conventional reinforcing bar was inserted in the web as bending reinforcement and grouted subsequently. The structural performance was assessed in a six-point bending test. The fabrication and structural testing of this case study beam showed that pre-installed reinforcement imposes several challenges to the extruder precision, the precision of the bent reinforcement, and – if applied – the casting after printing.

Lukas Gebhard, Patrick Bischof, Ana Anton, Jaime Mata-Falcón, Benjamin Dillenburger, Walter Kaufmann

Wet Material Property Control

Material Design and Rheological Behavior of Sustainable Cement-Based Materials in the Context of 3D Printing

In recent years, the development of digital construction methods such as 3D concrete printing (3DCP) has attracted increasing interest by the construction industry.  3DCP has a potential to solving many problems currently encountered in this business sector. At the same time, the urgent need in introducing sustainable binders such as limestone calcined clay cement into the practice of construction has been increasingly understood. It seems logical and very promising to merge both concepts in order to achieve a considerable progress in the field. From a technical perspective, however, the implementation of such sustainable approaches can only succeed if fresh 3D printable materials yield rheological properties in accordance with the high requirements posed by the complex process chain of 3DCP. This article presents a methodological approach for the development of 3D printable concretes (3DPC). Rheometer tests and penetration tests were conducted to systematically investigate the time-dependent increase in the static yield stress. The extrudability and buildability of the mixtures were verified by printing wall elements. Due to the low cement content of the mixtures (<270 kg/m3), strength tests were performed as well. Despite the low clinker content, compressive strength values of approximately 30 MPa could be reached. The results attained in this work show that the use of supplementary cementitious materials enables establishing low-clinker concretes in the context of 3D concrete printing.

Silvia Reißig, Venkatesh Naidu Nerella, Viktor Mechtcherine
Measuring Plastic Shrinkage and Related Cracking of 3D Printed Concretes

In the first hours after extrusion 3D-printed concrete elements are exposed to the increased risk of cracking due to plastic shrinkage. Ability to control and mitigate plastic shrinkage and related cracking is significant challenge along the way to wide application of the 3D-printing technology in the construction praxis. While plastic shrinkage can compromise economic viability and structural stability of the printed elements, until now only few studies were dedicated to this issue. The article at hand analyses applicability of different setups for quantification of plastic shrinkage and related cracking. Suggested approach enables to determine shrinkage induced deformations along with evolution of the material properties.

Slava Markin, Viktor Mechtcherine
Automated Visual Inspection of Near Nozzle Droplet Formation for Quality Control of Additive Manufacturing

The formation of droplets near the nozzle exit gives information about the production process and extruded material properties, which can be used for quality control, and process and material optimization. Mass and length of these droplets, or “slugs”, have been measured in previous studies [1, 2]. In this study, the experiment is expanded by including a fully automated visual inspection procedure to calculate the volume of the droplets for all axisymmetric geometry. The method extends the application domain of the experiment since the material can be analyzed when the droplet frequency is high or even without the formation of separate droplets. By measuring the droplet mass in parallel to the volume estimation, the density of the material can be determined when an infrequent slug-by-slug flow exists.

Derk Bos, Rob Wolfs
Selected Test Methods for Assessing Fresh and Plastic-State 3D Concrete Printing Materials

Materials’ requirements for 3D concrete printing centre around printability and buildability. The concrete must be pumpable, extrudable, yet retain its shape after extrusion (fresh state) and stack over each other without yielding and buckling failure (plastic state). The nature of 3D printed concrete materials has driven research into adapting various test methods to assess these requirements. This study reports selected test methods evaluated for 3DCP in its fresh and plastic state – rheometry, unconfined compression tests, gravity-driven (slug) tests and slow penetration tests. The study further highlights their correlations, applications, and practicality for the two main types of printable materials – enhanced-thixotropic and accelerated-hydration concrete.

John Temitope Kolawole, Danny De-Becker, Jie Xu, James Dobrzanski, Sergio Cavalaro, Simon Austin, Nicolas Roussel, Richard Buswell
Third RILEM International Conference on Concrete and Digital Fabrication
Prof. Richard Buswell
Dr. Ana Blanco
Prof. Sergio Cavalaro
Dr. Peter Kinnell
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