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This paper provides a comprehensive review of the material behaviour for extrusion-based concrete 3D printing spanning from the early age to long-term performance. We begin with a discussion on the recent progress on the understanding of early-age behaviour. Following this, the mechanical response in the hardened state, and the different strategies to introduce reinforcement are reviewed. Finally, we present insights and perspectives on the transport mechanisms in printed concrete to assess the long-term performance, and also discuss the overall impact of construction by concrete 3D printing on sustainability. [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]

3d printing of cementitious material is a relatively new additive manufacturing process whose growing interest and fast development is mainly due to the digitalised manufacturing, allowing the disposition of material where it pleases. Yet, due to the properties of the fresh material and the difficulty to generate paths for the robots, the printed geometries have remained simple. In this regard, this papers longs to broaden the range of printable shapes by proposing a process-aware exploration of the 3d printing design space. This is done by looking at historic strategies that have been developed to build cantilevers, vaults and domes in masonry - a more ancient additive manufacturing process. Similarities and main differences between the two processes are pointed out, at the scale of the component, the layer and the global structure. From that a classification of masonry strategies to build cantilevers is proposed, facilitating the identification of parameters for 3d printing that will allow to reproduce such structures. Later, some guidelines for the design of printable geometries and the generation of robotic toolpaths are given, in the light of previous findings. • Stability during 3d printing must be verified at the scale of the lace section, the layer and the global structure • Cantilevers in additive manufacturing can be done by creating local corbels or by inclining the layers • For a similar global cantilever, inclining layers will create less stress in the material than a local corbel • A structure printability depends on the material used but also on the printing set-up - pump capacity and the maximum speed of the robot. [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]

Developments in the automation of construction processes, observable in recent years, is focused on speeding up the construction of buildings and structures. Additive manufacturing using concrete mixes are among the most promising technologies in this respect. 3D concrete printing allows the building up of structure by extruding a mix layer by layer. However, the mix initially has low capacity to transfer loads, which can be particularly troublesome in cases of external components that need to be placed on top such as precast lintels or floor beams. This article describes the application of additive manufacturing technology in the fabrication of a building wall model, in which the door opening was finished with automatic lintel installation. The research adjusts the wall design and printing process, accounting for the rheological and mechanical properties of the fresh concrete, as well as design requirements of Eurocode. The article demonstrates that the process can be planned precisely and how the growth of stress in fresh concrete can be simulated, against the strength level developed. The conclusions drawn from this research will be of use in designing larger civil structures. Furthermore, the adverse effects of concrete shrinkage on structures is also presented, together with appropriate methods of control. [ABSTRACT FROM AUTHOR]

Today, the majority of research in 3D concrete printing focuses on one of the three methods: firstly, material extrusion; secondly, particle-bed binding; and thirdly, material jetting. Common to all these technologies is that the material is applied in horizontal layers. In this paper, a novel 3D concrete printing technology is presented which challenges this principle: the so-called Injection 3D Concrete Printing (I3DCP) technology is based on the concept that a fluid material (M1) is robotically injected into a material (M2) with specific rheological properties, causing material M1 to maintain a stable position within material M2. Different to the layered deposition of horizontal strands, intricate concrete structures can be created through printing spatially free trajectories, that are unconstrained by gravitational forces during printing. In this paper, three versions of this method were investigated, described, and evaluated for their potential in construction: A) injecting a fine grain concrete into a non-hardening suspension; B) injecting a non-hardening suspension into a fine grain concrete; and C) injecting a fine grain concrete with specific properties into a fine grain concrete with different properties. In an interdisciplinary research approach, various material combinations were developed and validated through physical experiments. For each of the three versions, first architectural applications were developed and functional prototypes were fabricated. These initial results confirmed both the technological and economic feasibility of the I3DCP process, and demonstrate the potential to further expand the scope of this novel technology. [ABSTRACT FROM AUTHOR]

At a time when construction projects are increasing in complexity and costs are constantly rising, 3D concrete printing have emerged as a revolutionary technology in construction. However, reinforcing 3D printed concrete elements has posed technical issues in research and practice. This paper introduces a methodology for 3D printing nonstandard steel rebar shapes together with 3D concrete printing. The new process can revolutionize the construction industry's approach to reinforced concrete design, as it provides a high degree of flexibility in both concrete and steel free-form creation, which is a key difference from traditional methods that only utilize standard steel reinforcement shapes. In addition, the proposed method will provide an even higher degree of automation in the concrete 3D printing construction process. An agent based model using Any Logic© was developed to simulate and optimize the printing of retaining and shear walls for a floor in a reinforced concrete building. Results show the optimization ratios of steel printing heads to concrete printing heads using the current technology and promise significant reductions in time and cost while providing a cleaner, safer, more automated, and an unbounded construction process. Findings from this research call for an in-depth investigation of the capabilities of steel 3D printing and its utilization in construction. It also highlights the importance of considering the application of new construction tools that would cope with the rapid growth of computational capabilities, and their adoption in design practices. • A new automated process for reinforcing 3D printed concrete elements is presented. • Agent based modeling is used to optimize the process. • A clear understanding of limitations and areas that can be enhanced is introduced. • Results show that steel 3D printers are the bottleneck of the process. • Compared to the traditional workflow, the proposed method can save time and money. [ABSTRACT FROM AUTHOR]