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2019 | Book

First RILEM International Conference on Concrete and Digital Fabrication – Digital Concrete 2018


About this book

Digital fabrication has been termed the “third industrial revolution”, and is promising to revolutionize many disciplines, including most recently the construction sector. Both academia and industry see immense promise in cementitious materials, which lend themselves well to additive manufacturing techniques for digital fabrication in construction. With this recent trend and high interest in this new research field, the 1st RILEM International Conference on Concrete and Digital Fabrication (Digital Concrete 2018) was organized.

Since 2014, ETH Zurich has been host for the Swiss National Centre for Competence in Research (NCCR) for Digital Fabrication in Architecture, which is highly interdisciplinary and unique worldwide. In 2018, this NCCR opened the “DFAB House”, which incorporates many digital fabrication principles for architecture. It is also responsible for the 600 m2 Robotic Fabrication Lab and the first robotically built roof in the world. Held in tandem with Rob|Arch 2018, the leading conference for robotics in architecture, RILEM deemed it the right time to combine forces at this new conference, which will be the first large conference to feature the work of the recently created RILEM Technical Committee on Digital Fabrication with Cement-based Materials, among other leaders in this new field worldwide.

This conference proceedings brings together papers that take into account the findings in this new area. Papers reflect the varying themes of the conference, including Materials, Processing, Structure, and Applications.

Table of Contents

Correction to: First RILEM International Conference on Concrete and Digital Fabrication – Digital Concrete 2018

In Chapter “Challenges of Real-Scale Production with Smart Dynamic Casting”, low-resolution Figure 4 is replaced with high resolution, Figure 5 is replaced with new figure and Figure 6 and the graph near are positioned as per the standard.

Timothy Wangler, Robert J. Flatt

Materials and Processing

Fresh and Hardened Properties of 3D Printable Geopolymer Cured in Ambient Temperature

This paper reports the fresh and hardened properties of an ambient temperature cured 3D printable geopolymer suitable for extrusion-based 3D concrete printing process. Effects of several key geopolymer synthesis parameters including type of alkaline activator (sodium (Na)-based versus potassium (K)-based), mass ratio of silicate to hydroxide solutions, viscosity and SiO2/M2O ratio (where M = Na or K) of silicate solution on extrudability, open time, shape retention ability and compressive strength of the 3D printable geopolymers were investigated. The results revealed that the type of alkaline activator solution and SiO2/Na2O ratio of the silicate solution had a significant influence on the open time and shape retention ability of the mixtures. The parameters investigated in this study did not have significant effect on the extrudability of the mixtures. The Na-based activators resulted in higher compressive strength of 3D printed geopolymer than the K-based activators. The 3D printable geopolymer mixture made by 8.0 M NaOH solution (25% w/w) and Na2SiO3 solution (75% w/w) with a SiO2/Na2O ratio = 2.0 exhibited the highest compressive strength of 16.6 MPa when cured for only 3 days in the ambient temperature.

Shin Hau Bong, Behzad Nematollahi, Ali Nazari, Ming Xia, Jay G. Sanjayan
Evolution of Concrete/Formwork Interface in Slipforming Process

In a slipforming process, concrete is continuously poured and the formwork simultaneously raised so that the older concrete at the bottom supports the fresher at the top after a few hours of hydration. Such a complex process has to be industrially optimized to ensure the quality surface of concrete. The objective of the present work is to study the evolution of the concrete/formwork interface at very early age.An experimental device has been designed in order to measure the evolution of the friction at this interface during the first hydration period. With cement hydration and thus water consumption, pore water pressure decreases and creates suction which could increase granular stress on the formwork. The role of pore water pressure on formwork friction is investigated. The first hydrates may also bond to the surface and create a strong adhesion at the same time.

T. Craipeau, T. Lecompte, F. Toussaint, A. Perrot
Experience in Online Modification of Rheology and Strength Acquisition of 3D Printable Mortars

This study focus on the early age properties of two mortar formulations designed for a 3D printing extrusion process. They follow a new design and process strategy, which consists in formulating a mortar to be self-levelling, to optimize pumpability, and then incorporating an additive in the extrusion nozzle to modify rheology properties and setting properties to adapt it to the requirement of the printing process (self-sustaining as soon as the material exits the nozzle, and fast strength acquisition). Two types of additives are considered: an alkali-free shotcrete accelerator and a starch ether based VMA. Compression and shear strength measurements from 2 min to 4 h after the incorporation of the additive demonstrate the capacity of the method to create mortars with strength acquisition vastly superior to results from the literature. Lab-scale extrusion and operational feedback from 3D printing customers demonstrate the feasibility at operational scale. The variety of properties obtainable by playing with different types of additives is also discussed.

V. Esnault, A. Labyad, M. Chantin, F. Toussaint
A Framework for Performance-Based Testing of Fresh Mixtures for Construction-Scale 3D Printing

A step-by-step procedure for performance-based testing of mixtures for construction-scale 3D printing is proposed. Workability of a fresh “printing mixture” is described in terms of print quality, shape stability, robustness, and printability window. To demonstrate the proposed procedure and test methods, an experimental program is carried out using four different mixtures. The experimental results are used as the basis for discussion and comparison of performance of developed mixtures for use in construction-scale 3D printing. Finally, perspectives on the future research areas as critical steps for advancement of construction-scale 3D printing are provided.

Ali Kazemian, Xiao Yuan, Ryan Meier, Behrokh Khoshnevis
Characterization of 3D Printing Mortars Made with OPC/CSA Mixes

Printable mortars used in 3D printing of cementitious materials must have rheological behaviour and setting rigorously controlled. In this research, mixes made of two types of cement, ordinary Portland cement (OPC) and Calcium Sulfoaluminate cement (CSA) are adopted to control the printability of a mortar. Extrudability, buildability and comparable compressive strength to that of a traditional mortar are the specifications required by the printable mortar. Different mixes of OPC/CSA cement pastes (ranging between 0 and 10% of CSA) and 2 mixes of OPC/CSA mortars (0 and 7% of CSA) are studied. The cement pastes are studied by isothermal calorimetry and rheometer. Heat of hydration and cement pastes' yield stress increase with the dosage of CSA in the mix. A mortar made out of 7% CSA in previous article [1] is then tested in laboratory and their behaviour is compared to the results of the cement pastes.

Noura Khalil, Sébastien Rémond, Bilal Baz, Georges Aouad
Rheological and Water Transport Properties of Cement Pastes Modified with Diutan Gum and Attapulgite/Palygorskite Nanoclays for 3D Concrete Printing

This paper identifies and addresses two challenges in extrusion-based 3D concrete printing from a materials perspective. The first is the effect of self-weight and the weight of subsequent layers on structural build-up. And the second is the excessive water loss of printed materials due to the absence of formwork. Viscosity modifying admixtures (VMAs) are extensively used in cement-based 3D printing projects to achieve sufficient print quality, shape stability, and printability window. This study aims to evaluate VMAs’ effects on the two aforementioned challenges through investigating the evolution of static yield stress under sustained stress at rest and water retention capacity of cement pastes modified with nanoclay and diutan gum.

Siwei Ma, Shiho Kawashima
Rheological Control of 3D Printable Cement Paste and Mortars

Recent advances in concrete construction such as three-dimensional concrete printing (3DCP) have given rise to new requirements on the control of both the hydration and rheology of cementitious materials. To meet these new demands, and to move toward adoption of 3DCP on a commercial scale, in-operando control of hydration and rheology will be required. In this study, two cement paste mixtures containing limestone powder of two different median particle sizes are used to create 3D printed structures with a cement paste printer. Hydration control in the form of acceleration is achieved with the addition of the limestone powder to the cement and rheology control is achieved by using limestone with different median particle sizes. Rheology measurements conducted concurrently with printed structures indicate that yield stress and a measure of thixotropy of the cement paste provide an indicator as to whether a material will produce a multi-filament free-standing structure for a given 3DCP system. Simulations of particles flowing in a pipe are used to study the rheological behavior of paste and mortar. For the case of a mortar, the flow rate of suspended particles (sand) follows the same functional form with driving force as the matrix fluid (cement paste). Shear-induced particle migration increases the density of particles toward the center of the pipe, a result that implies that the aggregates may not be uniformly distributed.

Scott Z. Jones, Dale P. Bentz, Nicos S. Martys, William L. George, Austin Thomas
Adapting Smart Dynamic Casting to Thin Folded Geometries

The first thin folded concrete prototypes produced with Smart Dynamic Casting (SDC) exposed numerous challenges concerning concrete. The SDC process is modelled to explain the increased difficulty to fabricate thin folded members compared to columns. Due to the smaller volume to surface ratio in formworks for thin folded structures the effect of friction is amplified and the process window narrows down. In order to compensate for this, retarded self-compacting mortar mix designs and acceleration strategies are investigated.Material testing results provide guidelines of how to achieve a uniform hardening rate over the course of an experiment, while preserving sufficient fluidity and dealing with variations in raw materials. For this, offline penetrometer tests are performed to evaluate material properties and online measurements are recorded to follow the strength evolution of the same mix processed with the experimental setup. In addition, the slipping criterion and the deformability of the concrete are tested in a 1:1 scale robotic experiment to evaluate the fabrication feasibility with the adapted mix. This unveils the potential to produce thin folded members for architectural applications.

Anna Szabo, Lex Reiter, Ena Lloret-Fritschi, Fabio Gramazio, Matthias Kohler, Robert J. Flatt
Enhancing Printable Concrete Thixotropy by High Shear Mixing

Our results show that the storage elastic modulus as a function of time increases at a higher rate for cement paste mixed at higher vesus lower mixing intensity. Hence, higher mixing appears to be enhancing thixotropy. Using calorimetry analysis we find that higher mixing decreases the setting time and enhances the peak of the heat flow. By analyzing the nanoparticles present in the suspending fluid of the cement paste, we show, in accordance with literature, that an appropriate combination of mixing energy and super-plasticizer dosage promotes hydration by scratching hydrates from the surface of cement particles, stabilizing them in the suspending fluid and hence generating additional nucleation surfaces. These results open the door for the design of printing heads including high-shear micro mixers allowing for a faster liquid-to-solid transition of the printable material.

Aileen Vandenberg, Hela Bessaies-Bey, Kay Wille, Nicolas Roussel
Discrete Element Simulations of Rheological Response of Cementitious Binders as Applied to 3D Printing

This paper aims to model the extrusion-based 3D printing process of a plain ordinary Portland cement (OPC) paste using the discrete element method (DEM), and outlines the methodology adopted to evaluate the linkage between particle scale processes and extrusion process. A mini slump test is used to define the rheological model to be used in DEM, and extract the relevant parameters. They are then implemented in a scaled-down extrusion printing model to determine the influence of particle-scale effects on extrusion force. The DEM model is able to capture the differences in extrusion load-displacement responses similar to the experiments. Refinements to the model based on extracted parameters are also discussed.

Pu Yang, Sooraj Kumar A. O. Nair, Narayanan Neithalath

Mechanics and Structure

Three-Dimensional Printing Multifunctional Engineered Cementitious Composites (ECC) for Structural Elements

Three-dimensional printing (3DP) has great potential to facilitate fabrication of structures with smart functions. This research aims to develop an effective and efficient method to fabricate multifunctional structural elements using Engineered Cementitious Composites (ECC) through 3DP. To this end, ECC slabs measuring 304.8 mm by 76.2 mm by 12.7 mm (length by width by thickness) are prepared for experimental testing. Titanium dioxide nanoparticles are incorporated in the slabs to deliver photocatalytic functionality for chemical reduction of gaseous air pollutants. Two schemes for incorporating titanium dioxide nanoparticles into the ECC slabs are investigated and compared. 3DP is employed to fabricate the slabs and compared with the conventional cast-in-mold fabrication method. The photocatalytic functionality of different slabs is evaluated through nitrogen oxides abatement testing under ultraviolet light. The concentration of nitrogen oxides is measured in real time. After the nitrogen oxides abatement testing, all slabs are tested to failure under four-point bending to evaluate their flexural properties. The results show that 3DP is promising to fabricate multifunctional ECC structural elements with improved efficiency.

Yi Bao, Mingfeng Xu, Daniel Soltan, Tian Xia, Albert Shih, Herek L. Clack, Victor C. Li
Large Scale Testing of Digitally Fabricated Concrete (DFC) Elements

Case study projects based on Digitally Fabricated Concrete (DFC) are presented in an increasing pace around the globe. Generally, though, it is not reported what structural requirements (if any) these structures meet and how compliance to these requirements was established. Published material research is often not connected to the presented case studies, and even when it is, it is not necessarily obvious their small scale results can be applied to full scale structures as some scale effects should be anticipated. Caution is required as DFC related material tests are still under development and scale effects in DFC have hardly been studied. Therefore, it is recommendable to perform large scale testing, in the range of 1:5 to 1:1, if DFC is applied to actual use structures. This paper presents such testing for two projects, a pavilion in Denmark (not realized) and a bridge in the Netherlands (realized). In both cases, elements printed with the 3D Concrete Printing facility of the Eindhoven University of Technology were intended for actual load bearing performance. The conservative designs past the test requirements, but nevertheless some important findings with regard to element manufacturing and structural behaviour were experienced. It is concluded that large scale testing remains advisable for DFC structures as long as not all relevant aspects of the technology are quantitatively understood, at least when new concepts are being applied.

Freek Bos, Rob Wolfs, Zeeshan Ahmed, Theo Salet
Method of Enhancing Interlayer Bond Strength in 3D Concrete Printing

Additive manufacturing is predicted to revolutionize the way in which we construct our cities and structures. These technologies can create a big potential for freeform design whilst also providing reductions in cost, materials wastage and workplace injuries. 3D concrete printing (3DCP) is one technique that is being investigated. Although many benefits are evident, there are many technological issues that have yet to be explored, particularly that of the bonding strength in extrusion based 3DCP. Extrusion based 3DCP works on a layer by layer deposition of a stiff cementitious mix, forming a material interface. This interface essentially becomes a position of weakness, forming a weak bond.Currently the interlayer bond is assumed to be related to either mechanical anchorage effects or chemical hydration effects. This paper hypothesis that the mechanical effects are predominant and to prove the hypothesis presents a series of experiments that were carried out to analyse and enhance the bond at this interface. The methodology employed in this study will focus on applying a cement paste to the top of an extruded substrate layer before the secondary layer is deposited.We demonstrated that the application of a paste at the interlayer does show an increase in bond strength. The greatest bond strength was found in pastes mixed with additives to sustain flow characteristics, over a time gap. The increase in contact area on both layers is now verified to be a crucial factor in bond strength development.

Taylor Marchment, Jay Sanjayan
Exploiting the Potential of Digital Fabrication for Sustainable and Economic Concrete Structures

Digital technologies overcome typical constraints of traditional concrete construction processes caused by the high impact of labour costs and bring about many new possibilities to the conceptual design, dimensioning, detailing, and production of concrete structures. While the potential of geometric flexibility is being extensively explored, most digital technologies encounter difficulties in penetrating the market due to lacking compliance with structural integrity requirements. To maximise their impact, it is essential that digital concrete processes (i) integrate reinforcement resisting tensile forces and (ii) address conventional structures with geometric simplicity. This paper discusses the potential of digital concrete fabrication processes to reduce the quantity of reinforcement required in concrete structures. For example, “minimum reinforcement” can be tremendously reduced by (i) tailoring the concrete grade locally to the actual needs and (ii) ensuring small crack spacings and correspondingly reduced crack widths by means of crack initiators. An experimental study shows that the strength reduction in the interfaces between layers from extrusion processes can be quantified with reasonable accuracy, which allows using these weak interfaces as crack initiators. A mechanical model to quantify the corresponding potential for saving “minimum reinforcement” when using 3D printing is presented. It is found that weak interfaces in layer joints with 33% of the concrete tensile strength inside the layer allow reducing up to 80% the minimum reinforcement for a given maximum crack width requirement under imposed deformations.

J. Mata-Falcón, P. Bischof, W. Kaufmann
Alternative Reinforcements for Digital Concrete Construction

Applications of structural concrete require use of reinforcement in one form or another. The known reinforcement concepts in additive concrete construction typically rely on conventional reinforcement approaches, which provide a solid basis for structural design, since existing guidelines and codes can be used. However, the use of conventional steel reinforcement poses serious limitations to the digitalization and automation of fabrication techniques. The article at hand presents two alternative approaches of reinforcing 3D-printed concrete structures: (1) additive manufacturing of steel reinforcement elements, (2) use of high-performance microfiber to achieve strain-hardening behavior of printed concrete. For both approaches materials and manufacturing techniques are briefly described followed by the results of mechanical testing and complimentary microscopic investigations. The printed steel bars showed similar mechanical performance in comparison to ordinary steel bars of the same cross-section area and comparable bond behavior to concrete too, as observed in pull-out experiments. The addition of 1% to 1.5% high-density polyethylene microfiber to fine-grained matrix enabled for printable strain-hardening cement-based composites (SHCC) with a tensile strain capacity of up to 3.2%.

Viktor Mechtcherine, Venkatesh Naidu Nerella, Hiroki Ogura, Jasmin Grafe, Erik Spaniol, Martin Hertel, Uwe Füssel
Additive Manufacturing and Characterization of Architectured Cement-Based Materials via X-ray Micro-computed Tomography

There is an increasing interest in the fabrication of cement-based materials via additive manufacturing (AM) techniques. However, the processing-induced heterogeneities and interfaces represent a major challenge. The role of processing in creating interfaces and their characteristics requires understanding of the microstructure of 3D-printed hardened cement paste (hcp). This work investigates the microstructural features of architectured cement-based materials, including processing-induced heterogeneous patterns, interfacial regions (IRs), and pore network distributions with respect to the architectural patterns. A 3D printer was modified and merged with an extrusion system and specimens were 3D-printed using a layer-wise direct ink writing (DIW) process capable of fabrication of ‘lamellar’ architectures of materials. A lab-based X-ray microscope (XRM) was used to perform X-ray micro-computed tomography (micro-CT) evaluations to explore the microstructural characteristics of 3-day old intact (i.e. not tested) 3D printed and cast specimens at two levels of magnification: 0.4X and 4X. CT scans of printed specimen revealed a patterned pore network and several microstructural features, including: (a) macropores (visible during printing), (b) micropores at interfacial regions (IRs), (c) accumulation of anhydrous cement particles near macropores, and (d) rearrangement of filaments away from their designed toolpath. In comparison, microstructural investigation of cast specimen at 4X scan revealed randomly distributed pores with no connectivity throughout the specimen. The aptitude of micro-CT as a non-destructive technique for microstructural characterization of architectured cement-based materials is discussed. The role of processing to induce and to pattern heterogeneities such as IRs in materials is demonstrated and the role of architecture in controlling such heterogeneities and their directionality through the interface is discussed.

Mohamadreza Moini, Jan Olek, Bryan Magee, Pablo Zavattieri, Jeffrey Youngblood
Hardened Properties of 3D Printable ‘One-Part’ Geopolymer for Construction Applications

This paper reports the hardened properties of an extrusion-based 3D printable ‘one-part’ geopolymer for construction applications. To date, all of the 3D printable geopolymers reported in the literature had ‘two-part’ mix formulations, made by using liquid activators. In contrast, the 3D printable geopolymer developed in this study has a ‘one-part’ (just-add-water) mix formulation, made by using a small amount of solid activator instead of the commonly used liquid activators. Handling a small amount of solid activators instead of large quantities of user-hostile liquid activators significantly enhances commercial viability and large-scale application of the 3D printable geopolymers in construction industry. Effects of print-time interval on the inter-layer strength, along with compressive and flexural strengths of the developed 3D printed ‘one-part’ geopolymer in different directions were investigated. Specimens were printed with 2 and 15 min delay times (print-time intervals). Compressive, flexural and inter-layer strengths of the 3D printed ‘one-part’ geopolymer were measured. The results showed that the print-time interval had a significant effect on the inter-layer strength of the 3D printed ‘one-part’ geopolymer. However, the effect of the print-time interval on the compressive and flexural strengths of the 3D printed ‘one-part’ geopolymer was negligible. The results also showed that the compressive and flexural strengths of the 3D printed ‘one-part’ geopolymer depended on the loading direction.

Behzad Nematollahi, Ming Xia, Shin Hau Bong, Jay Sanjayan
Bond Strength in 3D Printed Geopolymer Mortar

3D printing is getting significant attention in the construction industry. This technology, which has been talked for years, is now delivering tangible results, however it is not yet ready for mass production in mainstream construction. Thixotropy material, 3D printer together with 3-dimensional computer model are the key elements required for robust concrete printing. For a freshly printed material, the increase in structural buildup at rest, prior to the placement of a successive layer can result poor interlayer bond strength and therefore, in this paper, we aimed to investigate the structuration effect of a nano-clay modified geopolymer, used for 3D concrete printing. Different structuration rate was achieved by changing molar ratio of activator and experimental results conclude that, there exists an optimum printing zone for different molar ratios, beyond which interlayer bond strength will be very weak.

Biranchi Panda, Nisar Ahamed Noor Mohamed, Yi Wei Daniel Tay, Ming Jen Tan
Potentials of Steel Fibres for Mesh Mould Elements

Mesh Mould is a digital fabrication technique developed at ETH Zurich in which the reinforcement and formwork production are unified in a robotically controlled system. An industrial robot fabricates a dense, three-dimensional, double-sided, welded reinforcement mesh that is infilled with a special concrete mix that achieves sufficient compaction without flowing out the mesh, which acts as porous formwork. Since the project started in 2012, the actual generation of robot end-effector is capable of bending and welding conventional steel reinforcement of 6 and 4.5 mm in diameter. Due to the process, the load-bearing capacity of these Mesh Mould elements is not equal in both directions due to geometrical restrictions in the end-effector. This study aims to increase the load-bearing capacity in the weaker direction by using steel fibre reinforced concrete (SFRC), which orients the fibres during flowing in this direction and in addition prevents the leakage of the concrete by enhancing jamming. A total of 10 specimens with 540 × 210 × 80 mm dimensions were tested in a displacement controlled symmetric four-point bending test. By combining SFRC with a mesh, the bending strength increased significantly with respect to the samples without fibres. The capacity is higher than the capacity of the individual parts, which are evaluated in separate material tests. Nonetheless, the bending strength in this study was limited by the weld strength, which was considerably lower than the one achieved by the robot. Higher weld strength would lead to better performance than in this first study, which is a part of an ongoing research effort.

P. Pfändler, T. Wangler, J. Mata-Falcón, R. J. Flatt, W. Kaufmann
Capillary Water Intake by 3D-Printed Concrete Visualised and Quantified by Neutron Radiography

Water uptake into two formulations of 3D-printed concrete via capillary suction was assessed by neutron radiography. The samples varied in their layer-to-layer deposition time intervals (TI) and the use of different binders. TI of two and 13 min were short enough to avoid preferential capillary suction at interlayer bonding areas in the fine-grained printable concretes containing supplementary cementitious materials. An increase in the time interval to 24 h gave rise to quick capillary suction through the layer-to-layer interfaces. However, moisture did not redistribute into the matrix regions from the interfaces. For mixture with Portland cement as sole binder and addition of a superabsorbent polymer (SAP), the short layer-to-layer deposition interval of two minutes resulted in tight interlayer bonds with quasi-null capillary suction. Intervals of 13 and 36 min, however, resulted in partially quick and intense absorption of water and immediate absorption by adjacent SAP particles.

Christof Schröfl, Venkatesh Naidu Nerella, Viktor Mechtcherine
Corrosion Challenges and Opportunities in Digital Fabrication of Reinforced Concrete

This contribution addresses corrosion of steel in digitally fabricated concrete. In recent times the concrete processing for digital fabrication applications has been greatly advancing, rising the interest of research institutions, industrial partners, governments and public media. Nevertheless, for a broad large scale application, not just the technological feasibility, but also the long term durability needs to be ensured. This contribution presents a general overview of recently developed digital fabrication technologies and assesses them from the point of view of reinforcement corrosion risks. Experimental results are presented and a number of potential durability issues specific to digital fabrication are raised. On the other hand, we highlight opportunities for making more corrosion-resistant concrete structures by taking advantage of digital fabrication technology.

M. Stefanoni, U. Angst, B. Elsener
The Effect of Print Parameters on the (Micro)structure of 3D Printed Cementitious Materials

The extrusion-based 3D printing method is one of the main additive manufacturing techniques worldwide in construction industry. This method is capable to produce large scale components with complex geometries without the use of an expensive formwork. The main advantages of this technique are encountered by the fact that the end result is a layered structure. Within these elements, voids can form between the filaments and also the time gap between the different layers will be of great importance. These factors will not only affect the mechanical performance but will also have an influence on the durability of the components. In this research, a custom-made 3D printing apparatus was used to simulate the printing process. Layered specimens with 0, 10 and 60 min delay time (i.e. the time between printing of subsequent layers) have been printed with two different printing speeds (1.7 cm/s and 3 cm/s). Mechanical properties including compressive and inter-layer bonding strength have been measured and the effect on the pore size and pore size distribution was taken into account by performing Mercury Intrusion Porosimetry (MIP) tests. First results showed that the mechanical performance of high speed printed specimens is lower for every time gap due to a decreased surface roughness and the formation of bigger voids. The porosity of the elements shows an increasing trend when enlarging the time gap and a higher printing speed will create bigger voids and pores inside the printed material.

J. Van Der Putten, G. De Schutter, K. Van Tittelboom
Compressive Strength and Dimensional Accuracy of Portland Cement Mortar Made Using Powder-Based 3D Printing for Construction Applications

An innovative methodology has recently been developed by the authors of this study for geopolymer formulations for the requirements and demands of commercially available powder-based 3D printers. In this study, the formulation is extended to conventional Portland cement to expand the scope of printable materials that can be used in the commercially available powder-based 3D printers for construction applications. A Portland cement-based powder composed of Portland cement, amorphous calcium aluminate and fine silica sand was developed for powder-based 3D printing process. Effects of different printing parameters including binder saturation level (100%, 135% and 170%) and shell to core ratio (1:1 and 1:2) on dimensional accuracy and compressive strength of the green specimens have been investigated. A compressive strength of up to 8.4 MPa was achieved for the ‘green’ 3D printed samples before any post-processing process. The results indicated that the increase in the binder saturation level and the change in the Shell/Core ratio from 1:1 to 1:2 significantly increased the compressive strength, but considerably reduced the linear dimensional accuracy of the green samples. The compressive strength and linear dimensional accuracy of the green samples exhibited an anisotropic behavior, depending on the testing direction.

Ming Xia, Behzad Nematollahi, Jay Sanjayan
Impact of 3D Printing Direction on Mechanical Performance of Strain-Hardening Cementitious Composite (SHCC)

Automatically adding or even printing steel reinforcements into a 3D-printed concrete structure is antithetical to the design freedom as well as construction ease and efficiency. Strain-Hardening Cementitious Composite (SHCC) is a kind of short random fibre reinforced cementitious composites exhibiting robust tensile strain-hardening and multiple cracking, which has potentials to reduce or even eliminate the need for steel reinforcements in printed concrete structures. Since one of the main disadvantages of 3D-printed structures is the anisotropy, this study aims to evaluate the impact of 3D printing directions on the tensile and compressive performance of self-reinforced SHCC materials. Four series of SHCC specimens with the same mix proportion but different printing patterns (including Parallel, Perpendicular, Cross and Normal Casting) were prepared and tested under uniaxial tension and compression, and the single-crack fibre-bridging constitutive relations were micromechanically modelled to physically support the experimental results. The findings of this study can support the future design and manufacturing of 3D-printed concrete structures using fibre-reinforced materials.

Jing Yu, Christopher K. Y. Leung

Applications and More

Feasibility of Using Low CO2 Concrete Alternatives in Extrusion-Based 3D Concrete Printing

In conventional concrete, replacing high-volume (more than 45%) of ordinary Portland cement (OPC) by supplementary cementitious materials (SCMs) is not a novel CO2 reduction method, whereas rarely in 3D printable concrete. This study attempts to explore the feasibility of using SCMs in 3D printable concrete. Initially, the existing binder mixes, required fresh properties and a research method of 3D printable concrete are investigated by reviewing the relevant papers. Additionally, the constraints and opportunities of using SCMs in 3D printable concrete are illustrated and summarized. Finally, it has been found that up to 45% of cement can be replaced by a blend of fly ash and silica fume. The essential fresh properties of 3D printable concrete include extrudability, workability, open time, buildability and structural build-up, which are influenced by the binder mix, particle size distribution, water to binder ratio, binder to aggregate ratio, admixture addition, the dosage of reinforced-fibers, etc. On the other hand, there are many limitations to develop SCMs-based 3D printable concrete, such as few relevant studies, a lack of the certificated standard, massive related-parameters and the shortage of common SCMs. For the first three problems, it can be solved with the development of 3D printable concrete. For the last one, calcined clay is one potential alternative for developing sustainable 3D printable concrete in the areas where are in short supply of fly ash and silica fume.

Yu Chen, Fred Veer, Oguzhan Copuroglu, Erik Schlangen
Experimental Investigation on the Mechanical Strength and Thermal Conductivity of Extrudable Foamed Concrete and Preliminary Views on Its Potential Application in 3D Printed Multilayer Insulating Panels

This contribution is focused on the properties of a particular type of foamed concrete, the extrudable foamed concrete, which is characterized by the dimensional stability in the green state, that is the ability to maintain its shape in the fresh state (green strength). In particular, after an overview of both the compressive and the indirect tensile strength, the effect of density on thermal conductivity values is presented. Interestingly, the thermal conductivity of this particular kind of lightweight cementitious material is lower compared to both classical foamed concrete and aerated autoclaved concrete (AAC) at comparable density. Moreover, the remarkable inherent green strength makes this material potentially suitable for in situ 3D printing applications in co-extruded elements with both thermal insulation and structural purposes.

Devid Falliano, Ernesto Gugliandolo, Dario De Domenico, Giuseppe Ricciardi
Development of a Shotcrete 3D-Printing (SC3DP) Technology for Additive Manufacturing of Reinforced Freeform Concrete Structures

In this paper, a novel Additive Manufacturing (AM) technique for robot-based fabrication of large-scale freeform reinforced concrete elements is presented. The AM technology, called Shotcrete 3D Printing (SC3DP), is based on an automated shotcreteing process and offers the ability to integrate structural reinforcement in both principal directions and enables printing of horizontal cantilevers onto vertical surfaces. Moreover, the SC3DP technique effectively addresses the problem of cold joints that is inherent to other 3D printing techniques. However, as controlling the process parameters of the SC3DP technique is significantly more complex than it is for conventional 3D concrete printing processes, several closed-loop online control routines were developed and integrated. The resulting gain of control for this adaptive fabrication process is demonstrated through a case study for the production of a complexes reinforced concrete component. Moreover, its conceptual implications are discussed and an outlook for future work is given.

H. Lindemann, R. Gerbers, S. Ibrahim, F. Dietrich, E. Herrmann, K. Dröder, A. Raatz, H. Kloft
Challenges of Real-Scale Production with Smart Dynamic Casting

Digital fabrication with concrete has for more than a decade been of high interest in both research institutions and industries. A particular interest has been set on Contour Crafting, a type of layered extrusion with concrete, which in recent years has been used for the fabrication of single and multi-story buildings. However, these have been done with little proof of systematic integration of reinforcement, which until now still requires tedious post processing to obtain the structural capabilities required.Smart dynamic casting, a robotic fabrication process for standard and non-standard vertical structures, has recently proven a systematic integration of reinforcement and is thereby the first digital fabrication process world-wide which has unified reinforcement and concreting in a single robotic fabrication process. This paper presents the latest developments and challenges of SDC and introduces the first architectural application in the form of structurally optimised façade mullions that are to be installed in the dfab House at the EMPA premises in Dübendorf, Switzerland.

E. Lloret-Fritschi, F. Scotto, F. Gramazio, M. Kohler, K. Graser, T. Wangler, L. Reiter, R. J. Flatt, J. Mata-Falcón
The Tectonics of Digitally Fabricated Concrete. A Case for Robotic Hot Wire Cutting

In the last decades, digital fabrication technologies have stimulated the materialization of complex and customized solutions in several materials. Recently, the integration of these technologies with such a variable and rich material as concrete has prompted an explosion of possible processes and outcomes for digitally fabricated concrete structures. In this context, this paper examines current digital fabrication strategies for concrete, focusing on their applications and in identifying critical issues for their adoption. From this point, through the presentation of two case studies, we propose and discuss Robotic Hot Wire Cutting as a technically and tectonically relevant digital fabrication technology for customized concrete architecture.

Pedro Filipe Martins, Paulo Fonseca de Campos, Sandra Nunes, José Pedro Sousa
Compliance, Stress-Based and Multi-physics Topology Optimization for 3D-Printed Concrete Structures

Recent advancements in Additive Manufacturing (AM) technologies have pushed the limits of manufacturability and have encouraged the design of products with increased complexity. Topology Optimization (TO) algorithms, on the other hand, have provided engineers with a tool for intelligently exploiting this design freedom by efficiently optimizing the shape of engineering structures. In this paper, three important developments of TO that might influence the manufacturing process and design of 3D-printed concrete structures are discussed. The first example shows how general structural TO problems, such as the well-known minimum compliance problem, can help to determine the optimal printing path and can discover the ideal location of the steel reinforcements. Secondly, it is considered how stress-based TO can enhance the shape of fiber-reinforced concrete components where the lack of steel reinforcements introduces a non-negligible strength asymmetry. In a third and last example, traditional structural TO techniques are extended to allow for multi-physics optimization. The thermal transmittance through a construction component is minimized, while the overall material usage is restricted. Results show the generation of very efficient (multi-material) structures that are aesthetically pleasing at the same time. The presented techniques aid in the search for more efficient structural design and might help overcome some of the technological challenges related to large-scale concrete 3D-printing.

Gieljan Vantyghem, Veerle Boel, Wouter De Corte, Marijke Steeman
First RILEM International Conference on Concrete and Digital Fabrication – Digital Concrete 2018
Dr. Timothy Wangler
Dr. Robert J. Flatt
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