Extrusion based additive manufacturing of cementitious materials has demonstrated strong potential to become widely used in the construction industry. However, the use of this technique in practice is conditioned by a feasible solution to implement reinforcement in such automated process. One of the most successful ductile materials in civil engineering, strain hardening cementitious composites (SHCC) have a high potential to be employed for three-dimensional printing. The match between the tailored brittle matrix and ductility of the fibres enables these composites to develop multiple cracks when loaded under tension. Using previously developed mixtures, this study investigates the physical and mechanical performance of printed SHCC. The anisotropic behavior of the materials is explored by means of mechanical tests in several directions and micro computed tomography tests. The results demonstrated a composite showing strain hardening behavior in two directions explained by the fibre orientation found in the printed elements. Moreover, the printing technique used also has guaranteed an enhanced bond in between the printed layers.

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In 3D concrete printing (3DCP), it is necessary to meet contradicting rheological requirements: high fluidity during pumping and extrusion, and high stability and viscosity at rest to build the layered structure. In this paper, the impact of the hydroxypropyl methylcellulose (HPMC)-based viscosity-modifying admixture (VMA) on the 3D printability and mechanical performance of a limestone and calcined clay based cementitious material is investigated. A combination of VMA and superplasticizer was used for that purpose. In this case, controlling the competitive effects between VMA and superplasticizer becomes critical. The main strategy for 3D printing in this study was to add an optimal dosage of VMA in the solid suspension that was already mixed with water and superplasticizer. A lab-scale 3DCP setup was developed and demonstrated as well. A series of tests was performed to characterize the effects of VMA on flowability, extrudability, open time, buildability, green strength, hydration, compressive strength, and air void content and distribution. Experiments performed in this study showed that the mixture containing 0.24% (of the binder mass) of VMA exhibited satisfactory 3D printability and optimal mechanical performance. Finally, the results, limitations, and perspectives of the current research were discussed.@en

The goal of this study is to investigate the effects of different grades of calcined clay on the extrudability and early-age strength development under ambient conditions. Four mix designs were proposed. Three of them contained high, medium, and low grades of calcined clay, respectively, and one was the reference without calcined clay. In terms of extrudability, an extrusion test method based on the ram extruder was introduced to observe the quality of extruded material filaments, and to determine the extrusion pressure of tested materials at different ages. For evaluating the very early-age strength development, the penetration resistance test, the green strength test, and the ultrasonic pulse velocity test were applied. Furthermore, the mechanical properties of the developed mix designs were determined by the compressive strength test at 1, 7 and 28 days. Finally, the main finding of this study was that increasing the metakaolin content in calcined clay could significantly increase the extrusion pressures and green strength, shorten the initial setting time and enhance the compressive strength at 1, 7, and 28 days.@en

New additive manufacturing methods for cementitious materials hold a high potential to increase automation in the construction industry. However, these methods require new materials to be developed that meet performance requirements related to specific characteristics of the manufacturing process. The appropriate characterization methods of these materials are still a matter of debate. This study proposes a rheology investigation to systematically develop a printable strain hardening cementitious composite mix design. Two known mixtures were employed and the influence of several parameters, such as the water-to-solid ratio, fibre volume percentage and employment of chemical admixtures, were investigated using a ram extruder and Benbow-Bridgwater equation. Through printing trials, rheology parameters as the initial bulk and shear yield stress were correlated with variables commonly employed to assess printing quality of cementitious materials. The rheology properties measured were used to predict the number of layers a developed mixture could support. Selected mixtures had their mechanical performance assessed through four-point bending, uni-axial tensile and compressive strength tests, to confirm that strain hardening behaviour was obtained. It was concluded that the presented experimental and theoretical framework are promising tools, as the bulk yield stress seems to predict buildability, while shear yield stress may indicate a threshold for pumpability.

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Recent years have seen a rapid growth of additive manufacturing methods for concrete construction. Potential advantages include reduced material use and cost, reduced labor, mass customization and CO2 footprint reduction. None of these methods, however, has yet been able to produce additively manufactured concrete with material properties suitable for structural applications, i.e. ductility and (flexural) tensile strength. In order to make additive manufacturing viable as a production method for structural concrete, a quality leap had to be made. In the project ‘3D Concrete Printing for Structural Applications’, 3 concepts have been explored to achieve the required structural performance: applying steel fiber reinforcement to an existing printable concrete mortar, developing a strain-hardening cementitious composite based on PVA fibers, and embedding high strength steel cable as reinforcement in the concrete filament. Whereas the former produced only an increase in flexural tensile strength, but limited post-peak resistance, the latter two provided promising strain hardening behavior, thus opening the road to a wide range of structural applications of 3D printed concrete.

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Significant attention has been given to the development of new materials and techniques to be employed in the construction market. One of the techniques which has drawn noticeable attention is the additive manufacturing process (a.k.a. 3-dimensional printing (3D printing)). One of the approaches of this construction technique is the extrusion of cementitious composites to form contour of a desired geometry. To achieve high viscosity in cementitious materials, usually viscosity modifying admixtures (VMA) are employed. However, the consequences of using these admixtures at high dosages is still not fully understood. This study characterized the influence of different VMA dosages on Portland cement paste, through a microstructure analysis. Hydration development was assessed, and effect of the admixture was quantified at different curing ages. Techniques such as thermogravimetric analysis, optical and electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, micro computed tomography scan and nanoindentation were employed. Important negative side effects were found such as: VMA increasing the cement setting time, anomalous dispersion of hydration products in the bulk and increasing the void content. On the other hand, positive effects were also found such as: evidence of internal curing, higher degree of hydration and lack of undesired hydration products.@en

The potential of using phase change materials (PCM) in cementitious materials to mitigate damage due to thermal loadings has been recently focus of intensive research. In the case of PCM with transition temperatures near to the freezing point of water, their potential to delay frost in a cementitious matrix has been largely investigated through the monitoring of internal temperature changes when exposed to repeated cycles of subzero and ambient temperature. Yet, the effect of these admixtures to prevent damage in cement-based materials has not been directly studied. In this paper,mortars cylinders of two different sizes and containing 0, 10 and 30%of PCM replacement by volume of aggregates were subjected to frost salt scaling during freeze and thaw cycles. Prior to the start of the weathering and after cycles 1, 3, 7 and 15 the cylindrical specimens were subjected to X-ray microtomography to monitor morphological changes due to frost action, such as chipping and cracks. Compressive and flexural strength, coefficient of thermal expansion and apparent porosity of the undamaged composites were also investigated. Results suggest that the improvement of frost scaling resistance of the mortars with incorporated PCM is a trade-off between resulting mechanical proper-ties, thermal volume stability and porosity of the composite, as evinced from the better performance of mortars with 10%of PCM replacement.@en

Soft inclusions, such as capsules and other particulate admixtures are increasingly being used in cementitious materials for functional purposes (i.e. self-healing and self-sensing of concrete). Yet, their influence on the fracture behaviour of the material is sometimes overlooked and requires in-depth study for the optimization of mechanical and/or smart properties. An experimental investigation is presented herein on the role of bacteria-based lactate-derived particles on the fracture behaviour of cement paste in tensile configuration. These admixtures are currently used for the purpose of self-healing. Digital Image Correlation was used to obtain strain contours on the surface of the samples during the test. The influence of soft particles addition and age of the samples on the fracture mechanics of the composite were investigated.@en

To investigate the effects of viscosity-modifying admixture (VMA) on the extrudability of limestone and calcined clay-based cementitious materials, three mix designs with different dosages of VMA were proposed in this study. The ram extrusion was utilized as an extrusion model for exploring the fresh properties of printable materials. Two methods were used, based on the ram extruder setup—(a) extruding materials with the same extrusion speed at different rest times to determine how the pressure changes with time; (b) extruding materials with different extrusion speeds at the same rest time to investigate the material flow parameters using the Basterfield et al. model. The main findings of this study could be summarized as—(1) the extrusion pressure of all mix designs exhibited an increasing trend with time. At the same tested age, the extrusion pressure under 0.25 mm/s of piston speed was increased and the shape retention of the extruded filaments was enhanced by increasing the dosage of VMA; (2) the correlation between the experimental results and the Basterfield et al. model was excellent (R-squared value: 0.99). The mixture with a higher content of VMA showed an increased elongational yield stress, flow consistency, and shear yield stress.@en

Recently the concept of crack self-sealing has been investigated as a method to prevent degradation and/or loss of functionality of cracked concrete elements. To obtain self-sealing effect in the crack, water swelling admixtures such as superabsorbent polymers (SAP) are added into the cementitious mix. In order to design such self-sealing systems in an efficient way, a three-dimensional mesoscale numerical model is proposed to simulate capillary absorption of water in sound and cracked cement-based materials containing SAP. The numerical results yield the moisture content distribution in cracked and sound domain, as well as the absorption and swelling of SAP embedded in the matrix and in the crack. The performance of the model was validated by using experimental data from the literature, as well as experimentally-informed input parameters. The validated model was then used to investigate the role of SAP properties and dosage in cementitious mixtures, on the water penetration into the material from cracks. Furthermore different crack widths were considered in the simulations. The model shows good agreement with experimental results. From the numerical investigation guidelines are suggested for the design of the studied composites.@en

A method is presented to model deformation and fracture behavior of 3D printed disordered lattice materials under uniaxial tensile load. A lattice model was used to predict crack pattern and load-displacement response of the printed lattice materials. To include the influence of typical layered structures of 3D printed materials in the simulation, two types of printed elements were considered: horizontally and vertically printed elements. Strengths of these elements were measured: 3 mm cubic units consist of lattice elements with two printing directions were printed and their strengths were tested in uniaxial tension. Afterwards, the measured element strengths and bulk material strength, respectively, were used as model input. Uniaxial tensile tests were also performed on the printed lattice materials to obtain their crack pattern and load-displacement curves. Simulations and experimental results were comparatively analyzed. For both levels of disorder considered, only when measured strengths were assigned to the elements with identical printing direction, are the predicted crack patterns and load-displacement curves in agreement with experimental results. The results emphasize the importance of considering printing direction when simulating mechanical performance of 3D printed structures. The influence of disorder on lattice material mechanical properties was discussed based on the experiments and simulations.

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In this work, a numerical model is presented to predict the self-sealing effect provided by superabsorbent polymers (SAP) admixtures in mortar. Firstly, the use of a law of absorption kinetics for SAP embedded in a cementitious matrix was validated with experimental results available in literature. Secondly, two extreme strategies are considered for the swelling of SAP in the crack regarding the variation in its deformation capacity under constraint. The results show the appropriateness of the SAP absorption law and explain the mechanisms of water absorption of mortar with such admixtures. Furthermore, the influence of the deformation capacity of SAP on the water penetration in cracks is studied parametrically. @en

Fibre reinforced cementitious composites (FRCC) may be characterized by their improved performance namely in terms of tensile ductility, accompanied by multiple cracking, and potentially lower permeability to liquid and gas in cracked state. Cracking, which is nearly inevitable, can occur due to applied structural loading, shrinkage, chemical attack, thermal deformations and restrained condition. Even though might not be a structural problem, cracking could be a durability issue, since it considerably modifies the transport properties of the cementitious composite and, as consequence, accelerates the deterioration process, which can significantly impair the long term service life of a structure or element. Literature indicates that, particularly for chloride penetration, the presence of cracks and/or load condition, causes a more deleterious attack compared to standard durability test on sound specimens composites. Contributing to that concern a methodology for accessing chloride attack in loaded and/or cracked FRCC is proposed. Cracking procedure and specimen geometry were selected, considering that cracks produced in laboratory should resemble those in structural elements (beams). Thus, FRCC specimens were firstly pre-loaded under four point bending up to a pre-defined crack width. The crack width was kept using special stainless steel frame. In addition, cracked but not loaded specimens were considered and sound specimens were used as reference. Then, specimens were exposed to wet-dry cycles in a chloride solution. It is argued that the chloride penetration is definitively influenced by the load and cracking conditions, which promoted a higher penetration depth leading to a severe fibres corrosion, which also compromised the mechanical performance of FRCC. @en

The use of fibres to enhance the ductility of cementitious composites has been extensively studied for the past few years. The addition of polymeric or metalic fibres with random orientation to the composite or even natural long and aligned fibres demonstrated a very successful reinforcement capable to reach a high mechanical performance. Other property that has been studied is the use of those composites to work as strain sensors. To develop piezoresistive properties on cementitious composite, the addition of conductive materials is necessary. This research evaluated the incorporation of different volumes of multi-wall carbon nanotubes on the piezoresistive properties of strain hardening cementitious composites (SHCC) reinforced by PVA fibres. Through impedance measurements the opening of the cracks under tensile loading was studied. The characterization of this material can help on the understanding of self-sensing properties, adding value to the SHCC used by the repair industry and will contribute to the continuous infrastructure monitoring.@en

The ability of Superabsorbent Polymers (SAP) to block water flow along cracks in cement-based materials has become an attractive feature of these admixtures. The diminution of fl w rates in such composites are attributed to the capacity of the SAPs to absorb water and swell in the crack, but no evidence exists in literature that indicates one or the other cause. On the other hand, the SAPs present in the bulk matrix might act as distributed sinks through which water is absorbed (water that otherwise would have continued its path into the matrix). In this paper a preliminary effort is made to numerically model the effect of SAPs on the water absorption by mortar. A lattice-type model is proposed to predict both the bulk water absorption and the resulting penetration depth of water into the cementitious matrix. The results of the simulations point out the mechanisms of water absorption in mortar containing SAPs.@en

In the past two decades, much research has been devoted to overcoming the inherent brittleness of cementitious materials. To that end, several solutions have been proposed, mainly utilizing fibres. One of the most promising classes of materials is strain hardening cementitious composite (SHCC). It utilizes PVA fibres, and it relatively costly compared to regular concrete, so it is commonly used only in surface layers. This paper presents a multi-functional ductile cementitious composite based on SHCC. It uses microencapsulated phase change materials (PCMs), capable of reducing temperature fluctuations in the material due to their high heat of fusion. It is shown that, although addition of microencapsulated PCMs are detrimental to compressive strength, they have very little effect on the flexural strength and deflection capacity. This material has potential to reduce temperature effects on concrete surfaces, while at the same time being extremely ductile.@en

This paper presents a validation procedure of multi-scale modelling scheme by making, testing and modelling deformation and fracture of cement paste beam at sub-meso scale (between micro and meso scale). Miniaturized three-point bending testing concepts were adopted. Cement paste beams with a cross-section of 500. μm. ×. 500. μm were fabricated using a micro dicing saw and subjected to bending using nanoindenter. Simultaneously, fracture behaviour of the same size specimens was predicted by a microstructure informed multi-scale lattice fracture model in which input microstructures are obtained by X-ray computed tomography at two scales (micro and sub-meso). Uncoupled upscaling approach was used to relate the material's mechanical behaviour at sub-meso scale with the microstructures at lower scale. A good agreement between experimental and numerical results was observed which shows that starting from micro scale and with relatively simple mechanical considerations it is possible to correctly reproduce behaviour at upper scale.

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Abstract In the past two decades, much research has been devoted to overcoming the inherent brittleness of cementitious materials. To that end, several solutions have been proposed, mainly utilizing fibres. One of the most promising classes of materials is strain hardening cementitious composite (SHCC). It utilizes PVA fibres, and it is relatively costly compared to regular concrete, so it is commonly used only in surface layers. In this paper, a multi-functional ductile cementitious composite based on SHCC has been developed. It uses microencapsulated phase change materials (PCMs), capable of reducing temperature fluctuations in the material due to their high heat of fusion. It is shown that, although addition of microencapsulated PCMs are detrimental to compressive strength, they have very little effect on the flexural strength and deflection capacity. In the future work, mixtures with higher PCM contents will be developed in order to exploit their heat storage capability better. This material has potential to reduce temperature effects on concrete surfaces, while at the same time being extremely ductile. @en

Cracking in concrete structures compromises the durability and functionality of the structures themselves. Different kinds of self-healing concretes, less or more sophisticated, have been developed in the past ten years to overcome early cracks in structures. An experimental study of a novel self-healing concrete is presented. Bitumen, used as the healing agent, is introduced in fresh fibre-reinforced concrete as the coating of steel fibres. The mechanism exploits induction energy to heat up the steel fibres inside the cracked concrete matrix; the bitumen then melts and finally flows into the cracks, sealing them. The aim of the research is to set up the main parameter affecting the performance of the healing mechanism as well as its efficiency. In order to achieve this goal, the microstructure of healed specimens has been studied through Light Microscope. Mechanical behaviour and permeability of the samples, before and after healing, were also checked. Fiber content is studied in the paper amongst the many parameters affecting the mechanism. Results point out the potential of the proposed self-healing mechanism to contrast early cracking (i.e. due to shrinkage). Presence of a certain amount of fibres bridging the crack highly influenced the healing efficiency, and so a uniform distribution inside the concrete matrix, which was directly related to fibre amount and its optimum concrete matrix. @en

This work aims to provide a method for numerically and experimentally investigating the fracture mechanism of cement paste at the microscale. For this purpose, a new procedure was proposed to prepare micro cement paste cubes (100 × 100 × 100 µm3) and beams with a square cross section of 400 × 400 µm2. By loading the cubes to failure with a Berkovich indenter, the global mechanical properties of cement paste were obtained with the aid of a nano-indenter. Simultaneously the 3D images of cement paste with a resolution of 2 µm3/voxel were generated by applying X-ray microcomputed tomography to a micro beam. After image segmentation, a cubic volume with the same size as the experimental tested specimen was extracted from the segmented images and used as input in the lattice model to simulate the fracture process of this heterogeneous microstructure under indenter loading. The input parameters for lattice elements are local mechanical properties of different phases. These properties were calibrated from experimental measured load displacement diagrams and failure modes in which the same boundary condition as in simulation were applied. Finally, the modified lattice model was applied to predict the global performance of this microcube under uniaxial tension. The simulated Young’s modulus agrees well with the experimental data. With the method presented in this paper the framework for fitting and validation of the modelling at microscale was created, which forms a basis for multi-scale analysis of concrete.@en