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. @en

Currently concrete is one of the most used building materials in the world. Despite its ability to withstand compressive loads, concrete has a low tensile strength, making it prone to cracking when pulled apart. Therefore, reinforcement is required to withstand the tensile stresses. This is generally achieved by applying steel rebars into the tension zone of the concrete structure. In recent years, there has been a growing trend of incorporating fibers into concrete mixtures to develop cement-based materials with properties resembling steel, such as Engineered Cementitious Composites (ECC).
This thesis explores innovative approaches to improve the mechanical properties of ECC by employing 3D-printed auxetic and non-auxetic reinforcements. Auxetic materials display a distinctive characteristic where, upon vertical stretching, they exhibit lateral expansion, and upon vertical compression, they undergo lateral contraction. In other words, when subjected to tensile loading, auxetic materials expand horizontally, and when subjected to compressive loading, they contract horizontally.
The main research question revolves around the possibility of enhancing the deformation capacity of ECC through these innovative reinforcements. To address this question, numerical simulations, experimental tests, and comprehensive analyses were conducted.
The study begin with the creation of ECC samples reinforced with 3D printed polymeric meshes, exploring different angles, volumes, and sizes of reinforcements using two distinct 3D printing materials, namely Acrylonitrile Butadiene Styrene (ABS) and Thermoplastic Poly-urethane (TPU). The mechanical characteristics of the composite materials were assessed by uniaxial tensile testing, and their response to stress was thoroughly examined.
The results conclusively demonstrate that the incorporation of 3D printed auxetic and non-auxetic reinforcements significantly increases the deformation capacity of ECC. The auxetic designs have improved deformation and flexibility, which makes them perfect for applications that value ductility and strain capacity. In contrast, non-auxetic designs, in particular honeycomb structures, exhibit higher stiffness and load-bearing capacities, making them appropriate for situations that demand structural rigidity and resistance to deformation.
Moreover, the study highlights the crucial role played by the choice of 3D printing material in influencing the strength and strain capacity of the reinforcement. ABS exhibits superior load-bearing capacity due to its high stiffness, while TPU showcases exceptional strain capacity, owing to its elastic and flexible properties. The investigation of many factors, including angles, volumes, and sizes, highlights their substantial influence on the mechanical characteristics of the ECC reinforcement. These characteristics can be changed to allow for alternatives between stiffness, load-bearing capacity, and strain capacity, which can be used to optimize the design of reinforcement for a variety of applications.
In conclusion, this thesis makes a contribution to the developing topic of "designer construction materials," where the properties of cementitious composites can be tailored and optimized through innovative reinforcement strategies. Future constructions that are durable, flexible, and sustainable will be made possible by the combination of 3D printing technology with ECC. This thesis encourages researchers to go further, where imagination and creativity meet concrete, creating a world where materials work with us to create a physical environment that is more resilient.

Conventional construction techniques are unsustainable and expensive due to longer construction time, labour-intensive work, and the use of formwork. 3D concrete printing is seen as a potential substitute to overcome these issues. However, extrusion-based 3D printed structures exhibit a weak interlayer bond, and researchers have tried to address this problem through techniques such as using cement mortar layers or vertical steel reinforcement between the layers. This study aims to address the problem of the weak interlayer bond by implementing the phenomenon of 'topological interlocking' to increase the mechanical interlocking between the layers. The study performed scale model printing in the form of mould-casting and caulk-gun extrusion to produce specimens to analyze the effect of grooving on interlayer bond strength. The results were counterintuitive to the previously observed research, and the study recommends avoiding issues that may interfere in further similar studies.

Nowadays, with the rapid development of 3D printing technology, more and more applications of this technology can be seen in daily life, and architecture is one of the important directions. There are already a lot of creative buildings completed by 3D printing in the world, and many of them have already been put into use. One of the big challenges of this technology is that since the properties of freshly extruded concrete are not high enough, it becomes necessary to use reinforcement strategies to make the concrete meet the requirements of 3D printing. One of the common reinforcement strategies is achieved by adding fibers to the concrete.

In this study, through finite element modeling and analysis of a 3D concrete printed farmhouse located in Wujiazhuang, China, the whole process of 3D printing of the farmhouse, as well as the internal forces of the structure when it enters into use after the printing is completed was analyzed, and the effect of the application of fiber reinforced concrete in the whole process of 3D printing, as well as the optimization strategy was shown in detail.

The literature study summarizes the current state of development of additive manufacturing and 3D printing, as well as the different types classified through the process; introduces FEM and describes its application in 3D printing; describes the process of making fiber reinforced concrete and its advantages over the material properties of normal concrete; reveals the changes in material properties of materials used for 3D printing in the early stages; and describes the field situation of the Wujiazhuang farmhouse and the printing process.

The deformation and stress distribution of the whole structure under the use state had been analyzed by linear static analysis in finite element software, and the material distribution of the structure had been preliminarily determined. After that, the whole printing process of the structure was analyzed by stage and structural nonlinear analysis, and the key issues such as how to simulate the truss structure wall and what parameters will affect the number of printable layers of the structure were analyzed by building multiple models. Finally, the material distribution and printing solution of the structure were given.

The results given were further modified by continuing to reduce the deformation of the structure to meet the code requirements for the displacement of the formwork structure. Several methods were given to minimize the deformation to meet the code requirements from two different perspectives: the geometry of the structure and the material properties.

The reliability of the modeling in this study and the accuracy of the results were verified by using finite element software modeling to analyze a 3D printing model from a paper and comparing the results. The conclusions of this study and the possible applications of the conclusions in 3D printing were finally given, as well as possible directions for future follow-up research.

This report describes the effect of microfibre reinforcement on the printability and strain hardening properties of Strain-Hardening Cement-based Composites (SHCC). To determine this effect, a study was conducted with three different fibre types and two different concrete matrices. In total, this has led to 6 different concrete mixtures. The three different fibre types are PVA(Poly-Vinyl-Alcohol) 6mm length, PVA 8mm length and HDPE(High Density Poly Ethylene) 6mm length. The difference between the two matrices is mainly in the water-cement ratio, which leads to a stronger and weaker matrix. The strong matrix is the C mix and the weaker one is the D mix.
The printability of the different mixtures is determined by means of a slump flow test. Printability can be divided into buildability and pumpability. Buildability describes how well the printed concrete layers hold together and do not collapse. Pumpability of the mixture says how well it is able to be pumped. If the slump flow test shows that a mixture behaves stiff, than it will have a higher buildability and lower pumpability. If the mixture behaves very fluidly in the slump flow test, it will have a lower buildability and higher pumpability.
The strain hardening properties of the SHCC are determined by doing compressive tests, four-point bending tests and tensile tests. These tests determine the compressive strength, flexural strength and tensile strength of each mixture. It was also found out to what extent each mixture is able to stretch, bend and then exhibits strain hardening. After the tests, the samples were also visually examined to see how the bending and cracking took place.
The weak D mix seems to show the most strain hardening. The fibres in this matrix can transfer the tensile stresses better, because in this matrix they are more able to stretch.
The research has shown that the mixtures with PVA 6 mm in the fresh state are very stiff and are therefore the least suitable for 3D printing. In addition, the mixtures with the PVA 6 mm microfibre reinforcement show the least strain hardening. In the four-point bending tests it was already noticeable that the samples failed at very low deflection and there was little cracking. It was therefore decided not to include these mixtures in the tensile tests in order to save time.
HDPE microfibre has shown to have good printability and significantly outperforms PVA 8 mm in strain hardening. The HDPE fibres provide a very flexible and stretchable material. Compared to PVA, HDPE shows a much finer cracking pattern, which has a positive effect on the durability of the concrete. All in all, it can be concluded that HDPE performs best and would be the best choice for application in 3D printing.

In the near future, pressure on sustainability requirements in building structures will increase further as governments around the world are forced to tackle climate change issues. The construction industry is one of the most resource intensive and wasteful sectors in the world and is estimated to keep growing at a fast pace due to growing populations. Earth building materials and construction systems are perhaps one of the most attractive options available. The advantages of raw earth are that it is considered to be widely available, recyclable, can be sourced at low costs, can exhibit high strength and can provide an excellent indoor climate for occupants. The interest in building with earth is experiencing a renaissance but advancements in Earth Building Technology is currently at its infancy. Most improvement in earth buildings has been related to fabrication methods, where current developments and opportunities in additive manufacturing has potential to create a new form of earth architecture. From a mechanical perspective, earth materials are strong in compression and weak tension, making compression-only structures like arches, vaults and domes especially interesting.
The purpose of this thesis is to determine how feasible it is to combine earth material with a robotic sprayed earth fabrication method to build wall and arch structures. Emphasis has been put in understanding the material for robotic fabrication. Therefore, an in-depth literature study of earth materials and their properties was conducted to understand how they gain strength. In AM, modelling the structural-up of a printed structure requires time-dependent material properties where early-age behaviour is especially critical. As a result, time-dependent compressive strength and Young’s modulus of potential shotearth with and without cement stabilisation was predicted. Equations from the material predictions were input into the design tool developed by Witteveen+Bos to explore geometrical shapes, assess printing time and dimensions that can be achieved.
The results of this work show that earth building materials is essentially impossible to predict without testing as soil composition has a big influence on mechanical strength. Prediction of raw earth properties was based on understanding of soil consistencies and water content. Prediction of cement stabilised earth properties was based on existing data from 3D extruded and sprayed concrete experiments. 3D printed structures, extruded and sprayed, can be compared to ancient masonry structures in their structural design and inspiration from these structures sees different layering techniques adopted to minimise formwork and maximise structural efficiency and strength. Through a geometrical non-linear FE analysis with the time-dependent properties defined and varying robotic printing speeds between 1000 -6000 mm/min, a free standing wall structure of one meter width can reach heights between 0.1 and 0.62 m. For the overhanging structures, a barrel vault and a half dome, considering no formwork, an 8.5 -10.5 degrees arch or tilt can be achieved, compared to the vertical free wall. It is found that for lower printing speeds, more structure can be printed before failure. For the half dome almost the full geometry can be achieved. The unstabilised and cement stabilised upper bound material models resulted in comparable geometric dimensions for all structures analysed. Results of this study also compared the barrel vault and half dome using the roman (radial) and nubian (inclined) layering techniques.

Concrete compressive strength is the most frequently used and most important mechanical propertyof concrete. National and international building codes (such as the Eurocode) frequently use the com­pressive strength for design with concrete.

In some cases, instead of testing the concrete specimens under compressive loading in laboratories, predicting the compressive strength using machine learning predictions could be a good alternative. The ingredients making up the concrete mix and the curing age can be used as predictors for the com­pressive strenght.

Artificial Neural Networks (ANNs) are machine learning algorithms that have been used since the nine­teen sixties and were inspired by the way neurons work in the brain. Previous researches suggest that ANNs have great potential to predict concrete compressive strength.

In this research, an Artificial Neural Network was set up using the Keras framework and was trained with concrete composition data consisting of examples of concrete recipes and their respective con­crete compressive strenghts. Then, three hyperparameter optimization methods (for loop, grid search and Bayesian optimization) were implemented in several runs. The resulting hyperparameters were used to create ANNs. Afterwards, the three methods were compared with respect to several met­ rics (R­squared score, root mean square error and running time) to see which one is the relatively best method to provide hyperparameters for the predefined ANN that learns from concrete composition data.

The best runs of the three hyperparameter optimization methods show similar goodness­of­fit (with negligible difference). Among these best runs, grid search has the shortest running time. Bayesian optimization provides the highest R­squared score, and has root mean square error of 4.45 [MPa] and a reasonable running time of 73 minutes. Therefore, Bayesian optimization is considered the preferable hyperparameter optimization algorithm for the concrete compressive data used as goodness­of­fit is in most cases to be considered the decisive metric for this application. Further research trying runs with larger parameter grids and spaces, or using other tuning methods could result in even better goodness­ of­fit results.

This research focuses on developing architected lightweight cementitious cellular composites (LCCCs) for potential thermal insulation purposes. Specifically, Voronoi cellular structures with different randomness are adopted and used as cellular structure of the LCCCs. the mechanical properties of the developed LCCCs are investigated by experimental and numerical methods. With the help of 3D printing technique, LCCCs are prepared with two mixtures: a reference mortar, denoted as REF and mortar incorporating microencapsulated phase change materials (mPCMs), denoted as PCM14. Mechanical properties of developed structures are first investigated. The compressive behaviour of the LCCCs is investigated at the age of 28 days. In general, the LCCCs show anisotropic compressive behaviour in two loading directions: the compressive strength and stiffness of the LCCCs in the out-of-plane direction is substantially higher than that of the in-plane direction. Comparing to conventional foam concrete with the same density, the out-of-plane compressive strength of the LCCCs is higher than commonly reported cases in literature. Due to the high porosity of the cellular structures, the compressive strength of the LCCCs is significantly reduced comparing to the corresponding constituent materials, as expected. In addition, this strength reduction of the LCCCs made with PCM14 is substantially lower than the that of the REF samples. Interestingly, the relative stiffness (measured stiffness of the LCCCs normalized by their porosity) of the mPCMs incorporated LCCCs is even higher than the constituent material E-modulus. Furthermore, the critical role of air voids on the compressive behaviour of the LCCCs is identified. The thermal conductivity of the LCCCs is investigated by numerical models using commercial software Abaqus and ANSYS. It is found that the thermal conductivity of the LCCCs is comparable with the conventional foam concrete of the same density. The geometry heterogeneity of the LCCCs has a minor effect on determining the thermal conductivity of the LCCCs. Moreover, increasing porosity or reducing thermal bridge on the heat transfer direction can dramatically improve the thermal insulation performance of the LCCCs. It is found that the natural convection of the air within the cellular pores and the radiation has less notable effect on the thermal conductivity of specimen. Comparing to the REF, PCM14 has enhanced thermal insulation performance. In the end, using extrusion-based 3D concrete printing technique, the LCCCs with mPCMs has been successfully printed as an implement of manufacturing LCCCs as a construction-scale brick. In general, the developed LCCCs have high porosity gives good insulation properties as well as good compressive strength. This gives them great potential to be used as novel lightweight thermal insulation materials for construction.

3D concrete printing is an additive manufacturing method that uses a printing robot to build 3D concrete elements. In the last decade, several 3D concrete printing techniques have been developed and extensive research has been conducted on suitable concrete mixes, the printing process and the material and mechanical properties of the finished product. However, certain challenges have remained unresolved. Two of these challenges are the low tensile strength and the limited ductility of the printed concrete. As concrete is a brittle material that is weak in tension, traditional structural concrete elements make use of steel reinforcement to ensure tensile strength. The incorporated steel rebars are designed to take over the tensile stress when the concrete cracks. With the linear elastic and eventually plastic behaviour of steel in tension, the steel provides a ductile behaviour to the rein- forced concrete. This ductile behaviour is required to comply with modern building regulations, as it ensures structural integrity and safety by timely warning in case of overload. However, for printed concrete the incorporation of steel rebar reinforcement is not evident.
The use of Strain-Hardening Cementitious Composites (SHCC) in the field of digital concrete printing is one of the many possible ways to incorporate reinforcement into printed structural concrete elements. However, just like regular printable concrete, the bond between subsequently printed layers has shown to be a point of concern. The research presented in this master thesis focusses on the interlayer bond of these printable SHCC’s and more specifically on developing improvement methods for this interlayer bond. To better comprehend the origins of problem a thorough literature study was performed on the following topics: printing facilities, 3d printing compatible mix designs, (printable) SHCC, anisotropy and research on related interlayer bond improvement methods. On the basis hereof an experimental research was designed and conducted on various methods unrelated to the SHCC mixture and their effect on the interlayer bond. The methods can be divided in three groups: nozzle types, surface modification by brushes and incorporation of vertical steel reinforcement. In total 7 parameters were designed and tested: three nozzles, two brushes and two steel reinforcements. Tensile interlayer bond tests were performed to assess the influence of the parameters on the interlayer bond strength. To make sure that the overall strain hardening capacities of the material were not negatively influenced by the interlayer bond improvement methods the research also included compressive tests in two directions and uni-axial tensile tests parallel to the printing direction. Finally, the research was extended with a concise CT scan analysis on porosity and reinforcement bonding. Results show that the type of nozzle has significant influence on all mechanical properties. Sur- face modification by brushes tend to result in SHCC elements that have higher interlayer bond, but a reduced strain-hardening capacity. Specimens with interlayer steel reinforcement completely lose strain-hardening capacity in printing direction. In general, it is concluded that there is potential in designing methods to improve the interlayer bond of printed SHCC.

In this study, a three-scale characterization of eight 20-years-old reinforced concrete specimens naturally deteriorated due to chloride-induced corrosion was carried out. The main aim of this research was to investigate some relevant parameters to accurately model corrosion of reinforced concrete structures, such as which are the locations where corrosion pits are more likely to initiate as well as if the Elastic modulus of corrosion product depends on the concrete mix that the reinforcement was embedded in. Each scale of characterization had a specific goal: the macro-scale characterization aimed to analyze the state of the specimens after 20 years of outside (unsheltered) exposure through Non-destructive techniques (i.e. potential measurements, corrosion rate, resistivity of concrete), visual inspections and analysis of historical records. Also, since specimens were mis-labelled during the years, the cement type that each specimen was cast with was identified through polished sections of concrete portions analyzed under the Scanning Electron Microscope (SEM). The meso-scale characterization of the reinforcement aimed to quantify the volume loss of the steel as well as to investigate which were the locations where corrosion pits occurred the most. Out of each reinforced concrete specimen, a core of 20 mm diameter and 100 mm height has been drilled, embedding a steel bar of 8 mm diameter. Each core has been then analyzed through X-ray Computed Tomography (CT-scan) and image analysis was performed. Apart from revealing different volume loss for bars embedded in different mixes (i.e. CEM I was the most corroded, CEM III/B the least), this characterization suggested that the most influencing factor for corrosion pits to form is the presence of defects (i.e. air voids) at the steel/concrete interface. Finally, the micro-scale characterization of corrosion product aimed to investigate the relation between Elastic modulus of corrosion product (퐸cp), its micro-structure and its chemical composition, with regard to the different concrete mix that steel was embedded into. 퐸cp was measured through Nano-indentation, while chemical composition was measured through SEM/EDS (Energy Dispersive Spectrometry) elemental mapping and spot analysis. This phase of the research revealed that differences between Elastic modulus of corrosion product generated in different mixes are not significant, and that the relation between 퐸cp and Fe/O seems to be (linearly) proportional. In the last chapter, the findings of the research are summarized and some recommendations are made regarding future endeavors.

In this study the cracking and debonding behaviour of cementitious screeds bonded to concrete bearing floors is
analysed. In order to obtain insight in this behaviour research is performed to the surface tensile strength of concrete,
the bond strength of screeds on concrete bearing floors, the behaviour of screeds during shrinkage and the methods to
determine the bond strength. This research is carried out by a theoretical study in which the screed shrinkage
behaviour as well as the screed / concrete behaviour during testing of the bond strength is analysed by means of
analytical and FEM studies. Besides this practical experiments are performed to obtain insight in the influence of
different parameters on the surface tensile strength of concrete / the bond strength of screeds on concrete bearing
floors. Furthermore practical experiments are executed to study the influence of different parameters on the outcome
of a preferred bond strength testing method.