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Recently, global attention has turned to concrete 3D printing for its potential in structure optimization, life-cycle extension, emission reduction, and cost savings. However, uncertainties persist regarding the printability of ultra-high-performance concrete (UHPC). Previous studies often tailor a mix to a specific printing system and evaluate its printability based on measurements of pumpability, extrudability, and buildability, assuming consistent shape stability regardless of the chosen printing system. To further investigate this assumption, an experimental program was conducted using various printing systems on a nano-modified UHPC mix. Nano clay was incorporated into the self-leveling UHPC mix to increase its viscosity and yield stress, thereby enhancing layer stability post-extrusion. The experimental parameters included the type of robotic system, extrusion system, nozzle design, and material pumping method. Two robotic systems were utilized: a medium-scale gantry and a large-scale ABB robotic arm. Two distinct extrusion systems, a piston-type extruder and an auger system, were employed. Various nozzles, including circular and rectangular designs, were tested. Two pumps were used: a cavity pump and a Thom-Katt pump (piston-type pump). The results indicated that the shape stability of the UHPC mix is significantly influenced by the printing system, suggesting that concrete printability is dependent on the printing system rather than being an inherent material property, as suggested by other researchers. Furthermore, the use of a circular nozzle demonstrated different shape stabilities when the extrusion system was changed from a piston-type extruder to an auger system. Shape stability improved with the auger system due to the lower accumulated pressure within the system. Additionally, the method of material pumping to the extrusion system was found to be critical for the shape stability of the printed layers. The mix failed to maintain its shape post-extrusion when the cavity pump was employed, which was attributed to the higher strain rates imposed on the material during the pumping process. In contrast, this issue was not observed when the piston-type pump was used. • Different printing systems were tested using the same nanomodified mix. • Rectangular horizontal nozzle improves UHPC print shape stability. • Piston-type extrusion affects shape stability more than auger system. • The high strain rates of the cavity pump reduce post-extrusion shape stability. • UHPC 3D printing is more influenced by process than material properties. [ABSTRACT FROM AUTHOR]

Hybrid alkali cement (HAC) is regarded as a sustainable alternative to cementitious materials due to the use of more than 70 % of supplementary cementitious materials while the issues associated with geopolymers are avoided to a great extent. This paper proposes a new method of delivering HAC-based mixtures by a two-part printhead mixing process suited for 3D concrete printing (3DCP). The two-part mixing process addresses the conflicting rheological requirements in 3DCP by facilitating the rapid early-age strength development after placement while showing excellent pumpability prior to the extrusion. The proposed approach is based on introducing the Portland cement in a secondary mixing process to partially replace the fly ash or slag in the printable geopolymer mix. A series of experiments assessing the printability of fresh concrete following the two-part mixing process and the properties of hardened concrete were assessed. The results demonstrate that the replacement of FA with Portland cement (HAC-FA) yielded a more pronounced enhancement than slag-replaced mixes (HAC-S) with the increase in static yield stress (SYS) and modulus of elasticity by 17 times and 3.5 times respectively at 30 min, compared to the control mix. The hardened properties of printable mixes, namely compressive strength and interlayer bond strength at 28 days, revealed an enhancement for the HAC-25FA mix by 20 % and 64 %, respectively, again in comparison to the control mix. The apparent volume of permeable voids (AVPV) and microstructural analysis validated the hardened concrete properties, where the HAC-FA mixes showed the densest microstructure with a reduced apparent porosity, compared to other mixes. • HAC is studied as a sustainable alternative to Portland cement in concrete 3D printing. • A two-part print head mixing technology is proposed to meet the conflicting rheological requirements for 3D printing. • The static yield stress and elastic modulus of fresh printed concrete increased by 17 times and 3.5 times respectively. • HAC mixes showed dense and compacted microstructure. [ABSTRACT FROM AUTHOR]

Among the several geometry and process-related characteristics that determine the ultimate product quality of 3D printed concrete, one that has gained considerable importance is the height/thickness of the individual layers. Layer height dictates the number of interlayer interfaces, and influences the properties of the layer and the interfaces (both interlayer and inter-filament), for a given mixture and printer characteristics. This paper examines the influence of layer height on the constitutive response and failure of 3D printed concrete specimens under compression, investigated along the direction of layer build-up (or perpendicular to the layer interfaces), which is the dominant plane of loading in 3D printed walls and columns. Plain and steel fiber-reinforced wall elements with two different layer heights (7.5 mm and 15 mm) are printed, from which cylindrical and prismatic specimens are extracted and tested. The uniaxial compressive strength is seen to decrease with increasing layer height. The constitutive relationships of the specimens with different layer heights, along with volumetric strains, enable the extraction of crack initiation stress and unstable crack propagation stress, which are influenced by layer height and the presence of fiber reinforcement. The unstable crack propagation stress is lower than the peak stress, and it decreases with increasing layer height. Digital image correlation (DIC) is used to demonstrate the dominant effect of inter-filament interfaces on the crack propagation response and resultant failure. • Examination of the influence of layer heights on compressive response. • Dependence of critical stress states on layer heights. • Dominant effect of inter-filament interfaces on crack propagation. • Accounting for layer heights in conjunction with material design for better performance. [ABSTRACT FROM AUTHOR]

The control of mortar rheology is of paramount importance in 3D printing concrete by extrusion. This is particularly sensitive for two-component (2K) processes that use an accelerator to switch the printed mortar very quickly from a liquid behavior to a sufficiently solid behavior to be able to be printed (i.e. structuration). After some main key points about measuring the structuration of printed mortars, we propose an original and simple inline test using a pocket shear vane tester. The test is able to measure over several decades of yield stress range, mobilizes small quantities of mortar and enables in-situ measurements on freshly printed layers. The results highlight the difference of structuration between a benchtop-prepared printable material and a truly printed material, fostering the importance of inline measurements, and demonstrating the quality of the method. This pocket shear vane test appears as good contender for a light and affordable rheometer for 2K printable mortars. • A 3DCP inline pocket vane test that suits fast structuration (100 Pa/s) is proposed. • A linear model fits early structuration of mortars accelerated by aluminum sulfate. • Potential of 2K strategies in adjusting structuration on the fly is demonstrated. • Inline and laboratory measurements differ : Inline testing is a necessity. • A methodology to choose and qualify 3DCP structuration tests is proposed. [ABSTRACT FROM AUTHOR]

• The use of limestone calcined clay binder allows the incorporation of more aggregate in 3D printable mixes. • 3D printable concrete with higher aggregate to binder ratio shows a better structural build up. • Compressive strength of 3D printable concrete is not compromised when aggregate to binder ratio is increased. • The use of higher aggregate to binder ratio makes it possible to produce sustainable 3D printable concrete. The study investigates the effect of increasing aggregate volume on the buildability of printable mixes having ordinary Portland cement and limestone calcined clay, and aggregate-to-binder ratio of 1.5 to 3 (by weight). The experimental program is planned considering the evaluation of extrudability requirements, open time, and buildability requirements (layer compression) while printing. The effect of water-to-binder and aggregate-to-binder ratio on flowability is analysed and the dosage of superplasticizer required to attain 200 mm and 180 mm spread is determined. Further, squeeze flow and step load tests are performed to analyse the compressive resistance of the mix at different time intervals in the fresh state, while compressive strength and drying shrinkage are considered to compare the mixes in the hardened state. The factors of energy consumption and carbon emission are considered to compare the sustainability of the mixes. It is concluded that the binder-aggregate matrix governs the compression of the material in the fresh state, with the paste governing the properties in the first few minutes and then the aggregate skeleton. The compression in the fresh state is observed not to be dependent on the mix's flowability, leading to the possibility of designing a flowable as well as buildable mix with a larger aggregate volume. Further, with an increase in the aggregate ratio from 1.5 to 3, the energy consumption of the mix reduces from 3571 to 2426 MJ and carbon dioxide emission from 462 to 314 kg CO 2 for each m3 of concrete. Hence, it is concluded that a sustainable 3D printable mix with a higher aggregate-to-binder ratio can be designed while satisfying the functional needs of the concrete. [ABSTRACT FROM AUTHOR]

• Artificial reefs built with 3D printing. • Systematisation in the design, material selection and fabrication of artificial reefs. • Sustainable materials in concrete 3D printing. • Sustainable materials for artificial reefs. • LCA and MCDM of concrete for 3D printing. • Numerical simulations of artificial reefs. The recovery of degraded marine coasts and the improvement of natural habitats are current issues of vital importance for the development of life, both marine and terrestrial. In this sense, the immersion of artificial reefs (ARs) in the marine environment is a way to stimulate the recovery of these damaged ecosystems. But it is necessary to have a multidisciplinary approach that analyses the materials, designs and construction process of artificial reefs in order to understand their true impact on the environment. For this reason, this paper presents the manufacture of artificial reefs by 3D printing, proposing designs with a combination of prismatic and random shapes, with different external overhangs as well as inner holes. For the definition of the artificial reef designs, criteria provided by marine biologists and the results obtained from a numerical simulation with ANSYS were taken into account, with which the stability of the artificial reefs on the seabed was analysed. Three dosages of cement mortars and three dosages of geopolymer mortars were studied as impression materials. The studies included determination of the rheological properties of the mortars, to define the printability, determination of the cost of the materials used, and determination of the mechanical strength and biological receptivity in prismatic specimens that were immersed in the sea for 3 months. To evaluate the environmental impact of the materials used in the production of the mortars, a Life Cycle Assessment (LCA) was carried out. In order to choose the mortars that encompassed the best properties studied, Multi-Criteria Decision Making (MCDM) was applied and the two best mortars were used for the manufacture of the artificial reefs. Finally, the advantages and disadvantages of the 3D printing process used were analysed. The results of the studies carried out in this research show that cement mortars have better characteristics for artificial reef applications using 3D printing, and that the technique applied for the manufacture of the artificial reefs allowed the digital models to be faithfully reproduced. [ABSTRACT FROM AUTHOR]

• Recycled crumb rubber is coated with cement paste via a ball milling machine. • Three weight ratios of cement to rubber are designed for different coating quality. • X-ray micro-computed tomography is used to analyse pores and crack propagation. • Degree of increase in compressive strength relies on the coating quality of rubber. • Anisotropic compressive property is primarily related to pore shape and orientation. The modification of a rubber surface, such as cement coating, is an effective method for enhancing the mechanical performance of concrete containing recycled rubber particles. Although this method has been widely investigated for cast concrete, there is limited research on 3D-printed cementitious materials. This study explored the correlation between the compressive strength and microstructural property of 3D-printed rubberised mortar with cement-coated crumb rubber (15 wt% replacement of river sand). Multiple ratios of cement-to-rubber (C/R) were designed to achieve different coating qualities, including C/Rs of 0.25 (CR-0.25), 0.4 (CR-0.4) and 0.55 (CR-0.55). Scanning electron microscopy (SEM) images showed the existence of hardened cementitious shells outside the rubber particles, which effectively improved the interfacial bonding between the rubber surface and the cement matrix. The compressive strengths of the printed specimens did not always improve as the ratio of cement to rubber for coating increased. Moreover, the anisotropy in compressive strength was more obvious in the CR-0.4 and CR-0.55, where the strength in the Y (printing) direction was about 7 % higher than that observed in the Z (layer deposition) direction. X-ray micro-computed tomography (μCT) analysis revealed that the mechanical anisotropy in CR-0.4 and CR-0.55 could be primarily attributed to two factors – pore morphology and pore orientation relative to the external loading direction. For CR-0.25, the rubber-to-matrix interface bonding appeared more critical for the compressive strength. Finally, the printed specimens showed the higher compressive strengths than the cast ones due to the lower fraction of large pores (diameters ≥ 1 mm). [ABSTRACT FROM AUTHOR]

• Highly thixotropic cement-based materials were produced using nanoclay and VMA. • Thixotropic properties of mixtures were quantified by their rheological parameters. • A uniaxial compression test was developed to quantify fresh properties of mortars. • The extrusion-pressure was simulated using the Volume of Fluid (VOF) method. • Higher printing speed and materials's yield stress led to lower width variations. This paper investigates the thixotropic behavior of cement-based materials used in large-scale 3D printing. Five different mortar mixtures containing optimized admixtures, including a polycarboxylate ether-based high-range water reducing agent (PCE-based HRWR), attapulgite nanoclay, a viscosity modifying agent (VMA), fly ash, and silica fume, were subjected to rheological measurements to quantify their thixotropic behavior. The process-induced variations in the static yield stress and the storage modulus of the investigated mixtures were estimated by testing disturbed and undisturbed samples. Uniaxial compression tests were also conducted to simulate the layer-wise pressure increments in extrusion-based 3D printing. Large-scale 3D printing tests were then performed to evaluate and quantify the buildability of the investigated mixtures and validate the scalability of the obtained results during the laboratory tests. The rheological measurements and the large-scale printed elements indicated that incorporating attapulgite nanoclay and a modified starch-based VMA enhanced the thixotropic behavior of the investigated printing mixtures. Moreover, the proposed thixotropy evaluation methods were found to effectively capture the thixotropic evolution of the mixtures and accurately evaluate the impacts of admixtures. Furthermore, the uniaxial compression test method permitted study of the influence of the extrusion pressure (the pressure induced by the 3D-printed layers' deposition) on the buildability of the deposited layers. This was validated using large-scale printing trials. [ABSTRACT FROM AUTHOR]

• 3D printed cob mixture showed higher mechanical strength compared to cast material. • A 3D printed cob component is prone to shrinkage. • 3D printable cob mixtures may experience considerable deformation. • As the percentage of straw increases, the mixture's compressive strength decreases. • 3D printing of cob mixtures shows potential to be used in construction of single story tiny homes. Search for a more environmentally friendly building material has identified earthen construction in recent decades. The current literature focusses mostly on reimplementing historical natural materials into typical construction processes. This study focuses on establishing a 3D printable cob mixture. Unlike adobe that requires molding and the use of bricks in construction, cob does not require any specific form of construction. Its vernacular identity makes it suitable for 3D printing construction. This study focuses on testing and analyzing cob mixtures to establish a 3D printable mixture. The goal of this paper is to provide a new mixture that is printable, buildable, and has the required properties, while utilizing the same materials that were used in traditional cob construction. This paper discusses the process of transitioning a typical cob mixture into a 3D printable version. The mixture's workability and buildability, the effect of different pump systems on the printed structure's quality, and the mixture's fresh and hardened properties are some factors that help direct the process. [ABSTRACT FROM AUTHOR]

• Nanoclay and HRWR were found to be suitable additives to develop 3D printing mixes. • Testing protocols highly affected the structural build-up results of the mixtures. • Morphology of the nanoclay powders affected the thixotropic behaviour of mixtures. • The printability of the mixtures could be predicted using the rheological tests. In this study, the effectiveness of different rheometric methods to evaluate the build-up of the cementitious mortars used for 3D printing was investigated. In the first step, 15 paste mixtures proportioned with various dosages of the nanoclay and the high-range water-reducer were made. The optimized paste mixture was then selected to produce 4 mortar mixtures with different quartz sand contents and gradations. The build-up of the mixtures was evaluated using a non-rheometric method, namely mini-slump cone, as well as the rheometric methods, including the static yield stress, and the storage modulus evolutions. Finally, the printing experiments were carried out using a custom-made 3D printer to evaluate the effectiveness of the investigated methods to assess the build-up of the mortars. The results indicated that different testing methods contributed in understanding different aspects of the build-up such as rigidification at rest or under shear. Each of these build-up aspects provided valuable information on the extrudability and the shape stability of the printed layers. [ABSTRACT FROM AUTHOR]

• Modified Mohr-Coulomb failure criterion was implemented to predict the plastic collapse of 3D printed concrete structures. • Time-dependent material models were experimentally developed for 3D printable concrete. • Developed Mohr-Coulomb based plastic failure criterion is more accurate for 3D printable concrete material. • The friction angle of the concrete mix has a considerable effect on the plastic failure height. 3D concrete printing (3DCP) using the extrusion method is a commonly used additive manufacturing technique to construct the structures layer by layer without using formwork. Therefore, the green (wet or fresh) strength of the printable concrete determines the possible printable height of the structure and the rate of printing to avoid collapse during the printing process. This paper aims to identify the buildability criteria based on the green strength of concrete and the effect of early age material properties on the stability of printed structures. The commonly available strength-based failure models based on the material yield stress and the vertical stresses induced were developed considering higher aspect ratios (height to width ratio). This may be not suitable for lower aspect ratios used in 3DCP layer geometries. Also, the frictional behaviour due to the applied vertical stresses should be considered in the material used for 3DCP applications. In this work, the Mohr-Coulomb based buildability criterion was developed and validated with laboratory experiments and numerical simulations. The laboratory experiments were carried out to establish the time-dependent material and rheological properties of 3D printable concrete mixes. Further, 3D printing of hollow circular sections was carried out to study the failure modes and to obtain the build height at the time of failure. The nonlinear finite difference model of the 3D printed sections was developed with the user-defined time-dependent material models to assess the accuracy of the proposed buildability criterion. Additionally, the proposed buildability criterion was further validated with the 3DCP experimental and numerical data presented in literature and all confirm that the proposed criterion shows higher accuracy over existing methods in assessing the buildability. [ABSTRACT FROM AUTHOR]