3D printing of concrete is an automated manufacturing technology being developed for the construction industry. The objective is that the technology can make the construction industry more time-, labour- and material-efficient as well as increase the formability of structural designs. 3Dconcrete printing has been in development for over a decade and the technology has recently made its introduction into the built world. That being said, there are still several challenges that call for attention before the technology can be widely implemented. One of the most distinct challenges is the reinforcement of the printed element. Concrete is by nature a brittle material, which is strong in compression but relatively weak in tension. Under loads that cause tensile forces, such as bending in beams, plain concrete can easily crack or even break completely. Traditionally, concrete is therefore reinforced with embedded steel reinforcement bars that prevent the concrete from brittle failure. In the application of 3DCP the implementation of steel rebars is not trivial, and thus alternative reinforcement methods are being developed. Based on the concept of strain hardening cementitious composites (SHCC), one possible solution can be found in the development of 3Dprintable strain hardening cementitious composites (3DP-SHCC). SHCC is characterised by its ductility and its capacity to show strain hardening behaviour under uniaxial tensile loading. The combination of these two relatively new innovations imposes strict and occasionally contradicting requirements on the fresh and hardened mechanical properties. Where the 3DCP aspect of the material imposes requirements on the fresh mechanical properties, such as pumpability, extrudability and buildability, the strain hardening aspect requires very balanced and fragile criteria on the (micro)mechanical properties in the hardened state.

This PhD project ventures to incorporate SHCC into the 3-dimensional printing technology, with the underlying objective to systematically develop a printable strain - hardening cementitious composite for structural purposes.
• Optimisation of the particle size distribution formed the basis for developing a new 3DP-SHCC material. Grain size distribution analysis based on the "modified Andreasen and Andersen" model were performed on several dry-mix compositions to ensure the buildability and stability of the 3DP-SHCC in its fresh state. After this, four-point bending tests were carried out to verify the ductility in the hardened state. Two dry mixes were selected and further optimised with the addition of a viscosity modifier agent (VMA) and a super plasticiser (SP). Print speed trials and buildability tests were conducted to assess the printability of the final mix designs, followed by experimental testing of both the fresh and curedmechanical properties.
• To enable large-scale structural applications of 3DP-SHCC, various pumping strategies were evaluated, revealing bottlenecks such as the formation of fibre agglomxi xii SUMMARY erates and blockages. Individual components of the pumping system were tested and adjusted to achieve constant and adequate material flow as well as material stability throughout the printing process.
• The effect of the printing process on hardened mechanical properties underwent evaluation through two experimental studies. Firstly, an examination of the consistency of hardened mechanical properties was conducted across three nominally identical printing sessions. The second study focused on the different phases of one single printing session. It determined the influence of subsequent printing phases (mixing, pumping, extruding, and printing) on the hardened mechanical properties of the printed 3DP-SHCC elements.
• The fresh and hardened mechanical properties of a 3DP-SHCC matrixwith various fibre types were evaluated in an experimental study. High variation in workability, ductility and strain hardening capacity was encountered.
• Finally, given the importance of the fresh mechanical properties of 3DP-SHCC, research was conducted on several fresh mechanical tests. The tests were evaluated for their applicability to 3DP-SHCC based on five criteria: Reliability, precision, un-ambiguity, reproducibility and feasibility.

The study successfully developed 3Dprintable SHCCs that meet the requirements on printability and strain hardening to the achieve ductility and strain hardening capacity of printed elements.

Furthermore, the study helps to understand the interaction between the printing system and the 3DP-SHCC. It highlights the implications the material has on the pumping system and how the printing process influences the hardened mechanical properties. It discusses the key parameters of fibre types and how these influence the fresh and hardened mechanical properties, and last but not least, it gives insight into fresh mechanical testing protocols for the high-yield stress 3DP-SHCCs.

All of these give a solid base for researchers to design fibre-reinforced printable composites, with required printability and strain hardening properties. In particular, it contributes to the design and tailoring strategies for high-performance strain hardening composites.

Several studies have shown the potential of strain-hardening cementitious composites (SHCC) as a self-reinforcing printable mortar. However, papers published on the development of three-dimensional printable SHCC (3DP-SHCC) often report a discrepancy between the mechanical properties of the cast and printed specimens. This paper evaluates the effect of each successive phase of the printing process on the mechanical properties of the composite. To this end, materials were collected at three different stages in the printing process, i.e., after each of mixing, pumping, and extruding. The collected 3DP-SHCC materials were then cast in specimen moulds and their mechanical properties after curing were obtained. The resulting findings were juxtaposed with the mechanical properties of the specimens derived from a fully printed 3DP-SHCC element, and our findings indicate that while the density and the compressive strength are not significantly influenced by the printing process, the flexural and tensile strength, along with their associated deflection and strain, are strongly affected. Additionally, this research identifies the pumping phase as the primary phase influencing the mechanical properties during the printing process.

With the introduction of 3D concrete printing, research started on how to include reinforcement in 3D printed structures. Initial studies on the implementation of strain hardening cementitious composites (SHCC) as self-reinforcing printable mortars have shown promising results. The development of this new type of SHCC comes with additional challenges. Where SHCC by itself is already a complex material engineered to achieve specific micromechanical behaviour under tensile loading, its application in 3D printing techniques imposes even more requirements - the so-called ‘printability’ requirements. The question that rises for the development of this new material is how to achieve printability without losing strain hardening capacity. This paper investigates the influence of raw materials and additives, such as silica fume, limestone powder, viscosity modifying agents and water, on the fresh and hardened mechanical properties of printable SHCC, by improving on a previously developed mixture. The fresh material mixtures were subjected to slump flow tests to analyse their applicability for 3D printing. In hardened state, the mixtures were tested on their compressive strength and flexural strength to assess their potential for strain hardening capacity. Finally, two mixtures were selected for printing. The mixtures were assessed on print quality and buildability by the deployment of a buildability test. Furthermore, the printed elements were mechanically tested at 28 days, on compressive strength, flexural strength and uniaxial tensile strength and strain. It was concluded that the silica fume content and water to solid ratio are relevant variables for 3DP-SHCC optimization. The study has yielded two 3DP-SHCC mix designs that display significant strain hardening capacity and good printability properties.

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.

Since the advent of three-dimensional concrete printing (3DCP), several studies have shown the potential of strain hardening cementitious composites (SHCC) as a self-reinforcing printable mortar. However, only a few papers focus on achieving sufficient buildability when developing printable SHCC. This study investigates the role of the particle size distribution (PSD) in relation to the buildability properties of the mixture in the fresh state and strain hardening properties in the hardened state. To this end 6 mixtures were designed based on optimal particle packing with the application of the Modified Andreasen and Andersen Model. The two mix designs showed the highest displacement at maximum stress were selected for further development of their fresh state rheological properties. This was achieved by addition of a viscosity modifying agent (VMA) and a super plasticizer (SP) and through material analysis by means of ram extrusion tests. Further fresh material characterization on the final two 3DP-SHCC mix designs was attained by the deployment of uniaxial unconfined compression tests (UUCT), Vicat tests and Buildability tests. After successful printing of the two SHCC composites, the compressive strength, the 4-point bending strength and the uniaxial tensile strength and strain were determined at an age of 28 days. The research shows that optimization of the PSD in a 3DP-SHCC mix design results in an improvement of the buildability, but can introduce decreased pumpability and strain hardening capacity.

Over the past few years, several studies have shown the potential of three-dimensional concrete printing (3DCP) for applications in building and civil engineering. However, only a few studies have compared the properties of the fresh printing material and the quality of the printed elements from different printing facilities. Variations in the manufacturing conditions caused by the mixing procedures, the pumping device and the nozzle shape and/or dimensions may influence the quality of the printed elements. This study investigates the differences in the fresh and hardened properties of a printing material tested in two different printing facilities. The pump pressure and temperature experienced by the printing material during the printing session are monitored real-time. Hardened properties are measured for the printed elements, such as the bending capacity, the apparent density, and the air void content. The research shows that two different printing facilities may result in printed elements with relative differences in flexural strength and volumetric density of 49% and 7%, respectively.