Fourth RILEM International Conference on Concrete and Digital Fabrication

To address the environmental challenges associated with Construction and Demolition Wastes (CDW) disposal and the depletion of natural sand resources by the construction industry, this paper investigates the potential use of fine aggregates from CDW as a complete replacement for natural sand in concrete formulations tailored for 3D printing applications. The study begins by physically characterizing fine aggregates produced by crushing and sieving CDW from concrete and fired clay brick residues. This stage includes water content and water absorption capacity tests, specific gravity tests and unit weight tests, and particle size analysis. Then, a 3D printable concrete mix formulated entirely with CDW fine aggregates, replacing 100% of natural sand, is developed using mortar flow and rotational rheology tests. This formulation is validated by printing a medium-sized wall using a 3D printing system developed in-house. Finally, compression tests are performed on printed filaments to examine mechanical properties such as compressive strength and modulus of elasticity. Fresh-state and hardened-state properties are compared with control concrete samples made with natural sand (0% of CDW fine aggregates). The study demonstrates the feasibility of formulating printable concretes with a total replacement of sand by CDW for real-size applications. However, special attention must be given in large-scale projects to the rate of workability loss caused by the high water absorption capacity of CDW fine aggregates. The research findings offer valuable insights into the potential and performance of CDW aggregates in 3D-printed concrete applications within the context of a circular economy.

The Brundtland Report defined sustainable development as a development that meets the needs of the present without compromising the ability of future generations to meet their own needs. If this definition is applied to the construction sector, a new, circular approach to cementitious materials is required. Additive manufacturing (AM) enables to design material-efficient and monolithic structures that meet physical and chemical requirements of building standards while allowing reuse or high-quality recycling of concrete.Here, the environmental impact as well as the circularity potential of an additively manufactured exterior wall, produced by 3D concrete extrusion, are determined. To classify the results, the additively manufactured exterior wall is compared with a conventional concrete construction. Finally, various options for optimizing the printed wall are considered and evaluated. Additive manufacturing enabled significant material savings compared to conventional construction. The environmental impacts strongly depend on the used concrete composition. For example, greenhouse gas emissions could be reduced by 20% when changing the type of lightweight aggregate. The additively manufactured exterior wall exhibited advantages in terms of circularity. Through further development and research, additive manufacturing of concrete can promote a paradigm shift towards sustainable practices.

In recent years, digital construction methods, such as 3D-printing with concrete, have seen a surge in interest. They address many challenges currently encountered in the construction industry. However, for the digital construction to be successful, both in terms of sustainability and cost-effectiveness, the 3D printable materials must fulfil high requirements, particularly regarding their rheological and mechanical properties. The use of environmentally friendly materials with low clinker content, such as Limestone-Calcined-Clay-Cement (LC3), could further reduce the carbon footprint of these processes. Additionally, integrating recycled aggregate into the 3D printable mix (Printable Recycled Aggregate Concrete: PRAC) could promote resource conservation, environmental protection, and energy efficiency. This study investigates the time-dependent development of the static yield stress and the structural build-up by means of a rapid penetration test and a newly proposed modified cone geometry. These tests enable to realistically describe the material behaviour of new, environmentally friendly 3D printable mixtures with coarse aggregates. The results attained provide a foundation for future efforts to use demolition materials more efficiently in 3D concrete printing.

The construction industry faces numerous challenges, including low productivity, a skilled labor shortage, safety concerns, and excessive waste generation. Various construction technologies have emerged to address these issues, with 3D printing and prefinished volumetric construction (PPVC) proving promising solutions, as demonstrated in the first 3D-printed PPVC project built in 2022, “CPAC-PP3DVC building”.In 2023, the second 3D Printed-Prefabricated Prefinished Volumetric Construction building for commercial mixed-use was proposed with a usage area of 58 square meters and six modules. In this project, to further reduce the embodied carbon of the building, we utilized lightweight carbon 3D-printed mortar, which has 51% lower carbon emission than normal-weight 3D-printed mortar but has comparable strength. Another significant project challenge was the module's weight, less than 10 tons, for easy transportation and assembly at a public location. Therefore, a lightweight low-carbon 3D-printed mortar, a lightweight floor system, and optimized structural strengthening of the 3D-printed walls approaches were combined to reduce the module’s weight. The maximum module weight of this project was only 6.6 tons, which facilitated the ease of assembly at the site. The assembly time of the building took only 5:30 h, and the total project's construction time was approximately 41 days, which is 54 percent faster than traditional brick-and-mortar construction. In addition, the amount of waste generated from the building's structure using the innovative 3D-printed PPVC approach was up to 97% less than that generated using brick-and-mortar construction.This project shows that 3D-printed PPVC is a promising construction technology with high potential. This technology can address the industry's challenges and provide a more sustainable and efficient way to build.

Digital 3D printing has become a viable technique in construction that can increase productivity while reducing costs. Substitution of conventional cementitious materials by innovative cement matrices, containing natural raw materials and industrial wastes, might help to reduce environmental issues associated to carbon emissions. Magnesium phosphate cements are rapid setting binders with low water content and capacity to include large amounts of industrial by-products in their formulation. Rheological properties, setting time and mechanical strength displayed by magnesium phosphate cement formulation are reported in this study with the objective to limit the conditions related to pumping, extrudability and deposition, which are key functional parameters for 3D printing by material extrusion technology. An optimized magnesium potassium phosphate cement formulation was proposed, prior to a 3D printing test, containing a blend of high- and low-grade MgO, a magnesium to phosphate molar ratio equal to 4, the minimum stoichiometric water content to produce K-struvite, a content of retarder equal to 3wt.% and a filler to binder ratio equal to 0.5.

A critical aspect of sustainable manufacture is the minimisation of waste, and this is especially important due to the relatively high embodied carbon content of the materials used in 3DCP, however, waste production is largely unreported. This article addresses this gap by presenting a case study based on the manufacture of three similar 250–300 kg panels, where material mass was tracked through all stages of production. A new methodology for systematic waste monitoring is presented and for the case studied, the total waste material was found to be 11–14%, and observed that problems during manufacture, can also lead to excessive machine run time and associated energy consumption. The work suggests that waste could be reduced to 7% to 10% in a ‘best-case’ production setting. The work also highlights the sensitivity to human error/judgement in increasing waste, as well as utilising recycling of printed material during printing to reduce waste, which is straight forward in 1K systems, but suggests that 2K approaches with rapid setting could be much more sensitive to poor operation.

Digitalisation and automation approaches are increasingly relevant for the construction sector. Especially additive manufacturing processes based on extrusion and material jetting are expected to reduce manual labour and production costs. Additionally, with the inherent design freedom construction elements with complex free-form shapes can be created. However, high material application rates lead to coarse surfaces with the stacking of layers resulting in a staircase effect, especially for overhangs and cavities. In order to increase the overall surface quality of freely designed construction elements, the development of an automated process for post-processing surfaces is necessary. Therefore, this article proposes a novel approach for the automated finishing of free-form surfaces using robotic end effectors. By dividing a component surface into smaller sections consisting of various spine functions, a form-flexible trowel can be used for the finishing of freshly-printed components. By continuously deforming the tool edge with two linear axes, freely adjustable spline curves are achievable, which enable a processing of free-form surface. Besides the development and an initial concept for the control of the end effector, a path planning concept for a synchronous movement with a robot is presented. First results on the deformability are analysed and discussed. By minimizing lateral forces due to an elongation of the tool edge, an improvement in the adaptability to cubic splines was achieved, even for higher elongations. This enhancement increases the usablility of the developed end effector by reducing the processable radii of free-form surfaces. The article concludes by discussing application scenarios of the end effector in the construction industry as well as its limitations. By implementing a surface finishing process using the presented tool concept of a flexible trowel to reduce the staircase effect, additive manufacturing in construction can become a reality in the near future, even for components with high surface requirements.

Over the last decades, lightweight foamed concrete has gained recognition and widespread adoption in the construction industry, owing to its intrinsic multifunctionality and versatility. Notably, the ability to achieve a broad range of densities through mix design adjustments makes this material appealing for fulfilling different essential functions, including mechanical strength and thermal insulation. Moreover, recent studies exploring the application of foamed concrete in Additive Manufacturing processes underline the considerable advantage of combining the peculiar properties of foamed concrete with the benefits associated with automated procedures.In the present study the application of multi-density foamed concretes in the fabrication of multifunctional engineered building components through 3D Concrete Printing (3DCP) processes is investigated. The possibility of employing medium-density foamed concrete for 3D printing topologically optimized structural sections and ultra-lightweight foamed concrete for filling these sections with thermal insulation purpose is proposed. This innovative solution allows for the fulfillment of multiple performance requirements - high mechanical performance and excellent thermal insulation - within a single cohesive cementitious element, thus eliminating the need to assemble numerous monofunctional layers of different materials.The primary properties of the two proposed foamed concrete mixes were investigated. Compressive strengths of 7.04 MPa and 5.40 MPa were achieved for cast and 3D-printed medium-density foamed concrete, respectively. Thermal conductivities of 0.205 W/mK and 0.072 W/mK were obtained for medium-density and ultralight-density foamed concrete, respectively. A successful 3D printing application with medium-density foamed concrete was executed using a collaborative robotic arm, and the possible pouring of ultralight-density foamed concrete to produce multi-density building components was assessed.

Internal cavities or channels have the potential to enhance a component’s insulation capacity, allow for fluid transport and heat exchange, and/or reduce material use without structural disadvantages. However, extrusion-based 3D-printing of built-in cavities or channels with cementitious materials has remained challenging due to several limitations, including the lack of support materials, early-age deformations, and slicing algorithms that exclusively generate layered toolpaths parallel to build plate. The objectives of this research were to identify opportunities to 3D-print mm-scale internal channels with cement paste and develop an accessible, high-throughput workflow for designing and printing components with channel networks.The research examined channel stability by considering channel design variables, including channel diameter and channel inclination, and proposes the use of angled planar toolpaths to increase the stability of the 3D-printed channels. Our new parametric modeling script (Grasshopper, Rhinoceros) rapidly models the filament geometry, identifies its unsupported sections, and, from those, indicates the degree of channel stability. The samples were 3D-printed and evaluated on channel stability using fluid flow analyses and micro-CT imaging.The developed Grasshopper-based design workflow [1] offers new opportunities for extrusion-based 3D-printing of cementitious components with internal channels. It allows for rapid 3D-modeling of complex internal channel networks (of various types, and with the option to apply Murray’s law), toolpath angle optimization for channel stability, and automated g-code generation. The workflow capabilities were demonstrated by 3D-printing several designs with complex 2D channel networks inspired by biological vascular systems.

The application of digital manufacturing technologies in the construction industry has shown rapid development in recent years. Digital concrete fabrication enables efficient design options with the increased shape of freedom by removing traditional formwork. Also, unlike the traditional concrete process, which is labor-intensive, digital fabrication technology primarily aims to enhance the productivity rate through digitalization. Most digital fabrication processes require pumping, placement, and then the subsequent buildup of strength after placement. This final requirement, often called buildability, often relies on controlling hydration to increase building rates through the addition of an activator or set accelerator. Adding an activator into ‘an inline mixing system’ introduces the additional requirement of controlling the mixing inside the inline mixer. In inline mixing, the distribution of the accelerator through mixing gains importance in monitoring building rates and selecting proper parameters such as accelerator dosage, flow rate of concrete, and inline mixing speed. To comprehend the mixing patterns within the inline mixer, it is essential to assess the residence time distributions of the accelerator. This research introduces a novel approach employing cost-effective and user-friendly image analysis. In contrast to techniques using μXRF and SEM-EDX, which require a higher degree of sample preparation and enable investigation of only small surface areas, the image analysis proposed allows for the examination of the entire sample area, giving a faster and more complete picture of mixing behavior.

3D Concrete Printing (3DCP) in many off-site manufacturing environments use serial manipulator robots as positioning devices due to their low relative cost, high working volume to footprint ratio, and ease of use. These devices are responsible for replicating the tool paths generated that translate a digital representation of a solid object into the precise location of deposited material in the physical world by material extrusion. Defects such as interfacial voids, which affect the mechanical performance of material and poor compliance of the external geometry are both well recognised quality issues that are a function of both the rheology of the material and the path of the extrusion nozzle. While there has been significant effort in understanding the rheology of the material, there has been far less, if any, published work exploring the effect of precision of the robotic positioning systems, which are known to exhibit positional errors statically and variable dynamic behaviour, on printed parts. This article addresses this gap by presenting methods and a case study assessment of a robotic system used for full-scale 3DCP. From a dataset of 1575 paths taken for a sample artefact, subtle changes in the control strategy of the robot are identified that cause considerable errors in the path completion time, the dwell time at corners, and the amount of positional overshoot. Finally, the relationship between these parameters and the geometry of printed parts is shown empirically on printed samples based on the RILEM TC 304-ADC test artefact.

The production of freeform building panels in concrete is often limited by the fabrication capacity of custom formworks. Creating such formworks is time and labor-intensive as well as wasteful as the formworks are commonly discarded after use. This research develops a novel approach for the production of freeform panels through coupling robotic 3D Concrete Printing (3DCP) and a CNC Dynamic Bed Device (DBD). This enables the production of waste-free, freeform, and highly detailed concrete panels without requiring individualized formwork. The DBD features a composite printing surface made of silicone and a TPU grid, achieving two paradoxical aspects: maximum geometric flexibility while maintaining structural integrity and preventing buckling under the material’s weight in the span between actuating pins. The geometric fidelity of the DBD is verified through a series of laser scans of the bed in convex, concave, complex saddle, and single-curved geometries. The production of complex freeform panels with textures through 3DCP and DBD involved nonplanar print path strategies. This is key in achieving consistent filament deposition and avoiding under/over-extrusion. The results included 5 unique 3DCP freeform panels. Each panel measured 50×50 cm.

Digital fabrication with concrete requires set-on-demand and self-support of fresh material without formwork. To this end, the twin-pipe pumping strategy has been proposed, employing a motionless static mixer to introduce a chemical trigger into fresh concrete. In this short paper, we present the concept and recent progress in twin-pipe pumping. We start with the characterization and optimization of the static mixing process in both numerical and experimental approaches. This is followed by an overview of various twin-mixture designs based on different hydration reaction mechanisms. Finally, the mechanical behavior of twin-pipe concrete is discussed.

Employing force-flow-oriented designs in structural components holds a significant potential to achieve material savings. This potential is of particular interest to the construction industry due to the substantial component size and the high energy demands in the production of raw materials. However, manufacturing the intricate free-form shapes generated by topology optimizations using conventional construction techniques is costly. Consequently, concrete-based additive manufacturing (AM) processes are researched for construction applications. Utilizing concrete as a 3D printing material faces several challenges such as its susceptibility to environmental influences, including humidity, temperature, and sunlight. While the influence on the individual layer is neglectable, the deviations add up due to the layer-by-layer production and can lead to component collapse. Previous research indicates that large-scale AM’s reproducibility and stability improve using inline process control. This publication introduces a spatial multi-layer approach wherein the measured inline data is stored within the machines’ coordinate system. This approach enables designing a process control algorithm based on current measurements as well as incorporating underlying deviations. This allows the implementation of integral components into the control algorithm to enhance controller performance and stabilize printing processes. The present experiments prove stabilization of the layer width and the spray distance, even in the attendance of multi-layer defects.

Additive manufacturing of concrete structures is a novel and emerging technology. Free contouring in civil engineering, which allows for entirely new designs, is a significant advantage. Although several techniques and approaches demonstrate these advantages, quality control during printing is highly challenging and rarely applied. Due to the continuous mixing process used in 3D concrete printing, it is impossible to exclude variations in the dry mixture or water content, and a single test sample is insufficient to represent the whole structure. A defect in one layer during printing can affect the integrity of the entire structure. Therefore, continuous and real-time process monitoring is required to record and document the printing process.At the Bundesanstalt für Materialforschung und -prüfung (BAM), a test rig for 3D concrete printing was developed to monitor the properties during the printing process. This study investigates the relationship between geometric accuracy and process parameters (pressure, pump torque, print speed, layer height, water content) in extrusion-based 3D concrete printing. Using a high-precision laser scanner, the geometric measurements of printed concrete elements are evaluated in real-time. The concrete elements are printed under controlled conditions with varied process parameters. Preliminary findings indicate a significant correlation between process variables and geometric accuracy.

Interlocking mechanisms enable modular designs of structures and offer additional energy absorption capacities. These types of designs with often complex toolpaths can benefit from the utilization of digital fabrication techniques. Two-component (2-K) concrete robotic additive manufacturing (AM) provides an enhanced ability to achieve geometrically complex designs. However, tailored designs enabled by robotic AM techniques require a customized toolpath, involving both rectilinear and curvilinear trajectories. Thus, development of assessment methods and reliable benchmarks is crucial. Here, two interlocking mechanisms, namely suture and dovetail have been introduced to assess the geometric accuracy (error) of individual and pairwise interlocking bodies. The impacts of non-linearity in toolpath trajectory (i.e., rectilinear vs. curvilinear) on geometric accuracies were examined for a range of geometric features (wide/narrow neck, length, and width), toolpath design (spiral, and zigzag) and printing speeds. A Grasshopper algorithm was developed to generate the toolpath trajectories for the ABB industrial robot (in RAPID commands). A 2-K robotic AM process with accelerated hydration was used to fabricate the proposed interlocking benchmarks. Image analysis results show that toolpath trajectory remarkably alters the geometric accuracy given the higher error observed in the curvilinear cases compared to the rectilinear counterparts in both individual and pairwise bodies. Additionally, decreasing printing speed leads to exacerbating effect on error of the individual and pairwise benchmarks. The findings provide initial insight that a calibration of extrusion flow rate is necessary in curvilinear segments of the toolpath in design of complex components such as modular elements. This approach can be used to improve the efficiency of large-scale additive manufacturing in an accuracy-critical context such as modular 3D-printed structures that otherwise may need downstream subtractive manufacturing process.

3D concrete printing technology is gaining attention in smart construction due to its automation and digitization advantages. However, issues in the material supply chain during the extrusion 3D printing process can cause defects or halt printing. To address this, recording and analyzing the stresses exerted by the material on the printhead is proposed to monitor and correct its behavior in the fresh state. Implementing a system to measure the force on the die during material extrusion offers precise quantification of pressure variations from process and materials parameters. This device, comprising a die mounted freely on an extruder and connected by three load cells, can serve as a rheological and/or quality control tool. Two parametric studies are conducted. The first evaluates pressure variations induced by viscosity at the extruder die via viscosity-modifying admixture dosage variations. The second focuses on printing process parameters to characterize time-dependent material properties like cement hydration or thixotropic properties. This instrumentation allows precise assessment of the stress on concrete by the printhead during extrusion. The study's results will provide valuable insights for optimizing printing parameters, ensuring the quality of 3D printed concrete structures and their durability.

Irregularities and defects that often occur during the concrete 3D printing process decrease the reliability and limit wider-scale adoption for this emerging digital fabrication technique. In this project, an augmented reality visualization and machine learning driven approach is developed for inspection of concrete 3D printed structures. It aims to provide an inspector with easy access to data and analysis gathered during the manufacturing process and combined into a digital twin. Using this tool, an inspector has additional means to assess defects that may compromise structural stability and durability. Therefore, he/she can ensure the safety and longevity of a 3D printed structure, which would make the technology more reliable and increase trust.To obtain manufacturing-related data for the inspection we suggest a three-step procedure consisting of monitoring, processing and synthesizing data. The print procedure is first monitored using a camera mounted on the printing nozzle. The data is then processed using convolutional neural networks and other image analysis methods. This provides an understanding of the 3D printing process including whether a manufacturing defect has occurred and how significant it may be. Finally, the data is synthesized in a digital twin of the concrete structure which combines defect-specific data as well as other relevant sensory data. The introduced three-step procedure is demonstrated with filament tearing as the defect of concern.This paper presents initial experiments to support visual perception of an inspection procedure using Microsoft HoloLens 2 were conducted. A software application was developed to superimpose a digital twin upon a structural element. This digital twin highlights defects onto the physical structure and provides an overview of what an inspection procedure could look like.In summary, this paper introduces an AR application granting an inspector access to data and analysis collected during the manufacturing process of a 3D printed structure therefore gaining insights into previously hidden details of its past.

In the digital age, 3D as-built monitoring of structures and buildings is becoming increasingly important. It provides the basis for automated quality control and progress monitoring through as-built and as-planned model-comparisons, as well as 3D modelling for automated repair, refurbishment and reuse in existing construction. Especially for 3D model-based manufacturing processes such as additive manufacturing, efficient as-built monitoring is essential to ensure the quality and accuracy of buildings and products.Therefore, three different 3D scanning devices have been evaluated for 3D as-built monitoring of concrete structures with different characteristics: An industrial handheld laser scanner commonly used in automotive and mechanical engineering, a mobile device with lidar for commercial public use, and a terrestrial laser scanner commonly used for surveying. The devices were tested on 3D printed, cast, and assembled structures of different sizes and geometries.The scan results are compared to the as-planned models based on their dimensional distances using Cloud-to-Cloud Comparison (C2C). The results show that the applicability is highly dependent on the size, geometric complexity and surface texture of the concrete structures. While small and flat structures can be captured completely and with micrometer accuracy using the handheld industrial laser scanner, terrestrial laser scanners are suitable for capturing large structures and environments. The handheld, photogrammetry-based mobile device has demonstrated the broadest applicability for concrete structures. The accuracy and coverage of the point cloud results were respectively high. Therefore, photogrammetry-based scanning with advanced mobile devices is a cost-effective, fast and portable alternative for 3D as-built monitoring and data acquisition of a wide range of concrete structures.

A novel experimental method is developed that allows for on-line high-frequency measurement of the material’s yield stress and stiffness under compressive loading. This loading condition corresponds to the dominating condition directly after extrusion, and the obtained properties are thus essential to controlling the extruded material’s shape and stability. Since these measurements can be conducted at high frequency, they could also be used for quality control. A theory for the design of the experimental setup is developed and applied in a case study. In this case study, the material’s stiffness remains relatively stable after four seconds after the last moment of applied shear. Furthermore, its dependency on nozzle width correlates to the model estimation, which validates the measurement principle. It is concluded that the inverse slugs test is less suitable for measuring process variations since it is relatively labor-intensive. It can, however, be used for an in-depth analysis to quantify the effect of the nozzle design.

In the evolving landscape of construction technology, accurately determining the rheological parameters for fresh concrete has become increasingly important, driven by technical advancements and economic considerations. The traditional correlation of standard test methods, such as the slump test and flow table tests, with rheological parameters is a subject of ongoing interest. Established estimations based on slump values and spread diameters are now recognized as having limitations in precision. This has led to a growing interest in non-invasive optical measurement methods, which, along with the widespread availability of standard equipment, could potentially replace costly and complex laboratory tools like rotational rheometers, often absent at construction sites.This paper presents a novel semi-automated approach for efficient collecting and preparing a dataset for machine learning (ML), specifically artificial neural network (ANN) training. This methodology aims to predict key rheological parameters of fresh concrete from its overall 3D slump geometry. The method involves conducting standard slump tests and measuring rheological parameters using a rotational rheometer across a comprehensive set of mixtures. These tests are paired with capturing the 3D geometry of the slumps. Each slump is also subjected to dynamic action on a standard flow table, followed by 3D scanning. Automation is achieved through 3D photogrammetric reconstruction using a gantry printer-mounted camera, which continuously captures images around the slump. A scripted process then transforms coordinates and slices 3D models to derive an array of height and diameter pairs, representing the slump's geometry before and after dynamic action. Automating geometric data collection and processing boosts robustness and overcomes manual acquisition challenges, enhancing scalability and reproducibility of the method. The slump geometries are prepared to be further used as training data, with yield stress and viscosity serving as the training targets.

3D concrete printing (3DCP) remains vulnerable to ambient conditions, variations in material properties, and unforeseen interruptions. This vulnerability is largely due to poor robot control and rudimentary decision-making processes, which substantially constrain the implementation of adaptive printing algorithms. 3D Vision, a subfield of computer vision, enables machines to perceive depth and three-dimensional structures from visual data. It has been widely applied in numerous robotics applications and constitutes an effective, reliable, and often even the sole method for sensing the environment. The realization of 3D perception differs greatly across the diverse landscape of modern robotic systems. This also applies to 3D printing, although 3D Vision is so far grossly underrepresented in 3DCP.This article sheds light on the integration of 3D Vision into 3DCP and elaborates on particular technical issues and challenges. In particular, a range of individual aspects is discussed, including exposure and spatial resolution, signal transfer, and corresponding latencies. It concerns printing conditions and their influence on depth image quality. The paper also addresses the ways to estimate and handle signal latencies. Special attention is paid to the application specificities related to concrete printing.

This study introduces a new and simple test method to measure the structural build-up of printing concrete. The proposed test draws inspiration from the food industry and uses a knife to cut the concrete and measures its resistance to the cutting as an indicator of its strength. This method evaluates the structural build-up of printing concrete by assessing the resistance to cutting with the increasing age of the concrete. For the proof of concept, a device was customized to conduct the knife cut test upon printing concrete. An accelerated printable concrete mixture was tested with the developed device. For the measurement of the structural build-up, prismatic specimens of printing concrete were cut in slices at the speed of 2.5 mm/sec with the increasing age of concrete. The cutting force versus cut depth profiles were used for measuring the peak cut strength and firmness of concrete over time. Rapid penetration tests were also conducted alongside. Analysis of the results showed the high capacity of the developed tool to measure the structural build-up of printing concrete. This simple test can be conducted by attaching a cutting blade with the compression testing machines at a concrete or soil mechanics laboratory and utilizes minimum concrete material. The developed testing system can be applied to the multiple aspects of 3DCP such as material design, printing process design and also as a daily quality control test method.

Extrusion-based 3D concrete printing has become increasingly widespread in recent years. While using this technology, it is important to monitor the fresh properties of concrete in its fresh state, as the extruded walls must retain their shape without formwork support. Unless automated testing methods for fresh concrete are utilized, it is impossible to achieve full automation of 3D concrete printing. The penetration test (PT) is one of the most suitable methods for this purpose: It does not require manual sample preparation and can be performed directly on a printed structure without destroying it.This article proposes a method of automating PT by mounting a penetration tip on a robotic arm equipped with a load cell, allowing automatic movement from one test location to another. Cleaning the cone between tests is also done automatically by wiping the penetrator against a sponge soaked in water or oil. In this study, experiments were conducted to investigate the effect of coating the cone with these fluids, which showed that in both cases it led to a slight decrease in the results.The use of a robotic arm allows PT to be performed not only vertically, but also from any other orientation, enabling testing not only of the uppermost but also of the layers beneath it. The test results presented in this research showed that the values obtained with PT in the horizontal direction were higher than the values obtained with PT in the vertical direction. Furthermore, the results of PT in the horizontal direction increased proportionally to the height of the wall above the penetration point.

Digital fabrication of concrete has increasingly gained attention from scholars and industrial investors during the past several years. However, the proper concrete mix for such applications needs to meet stricter criteria compared to conventional concrete casting, due to the higher process demands. This leads to a shortcoming in this technology, which is that digital concrete mixes are more sensitive to minor variations in mix proportioning, properties of the incoming materials, and environmental conditions such as temperature. 3D printing of concrete, which is one of the main subdivisions of digital concrete, can be further categorized into 1k and 2k systems. This work focuses on 2k systems, which includes a secondary mixing step for the addition of an accelerator, omitted in 1k systems. In this study, the robustness of different accelerating systems was investigated and compared. Two accelerating systems were studied respectively based on Calcium Aluminate Cement pastes (CAC) and Aluminum Sulfate solutions (A$). The accelerated mixes are subjected to dosage as well as temperature variations. The obtained results are discussed in view of enhancing the robustness of digital concrete.

Due to the absence of formwork, additive manufacturing with cement-based materials requires a sufficient structural build-up of the materials used to allow vertical construction progress without failure of the printed structure. Especially in applications with rapid layer-by-layer build rates, such as Shotcrete 3D Printing, material systems with controllable rheological properties offer significant advantages. Active and precise rheology control enhances efficiency and flexibility in the manufacturing process and minimises the susceptibility of the system to failure.This paper focuses on the design and investigation of a controllable two-component fine-grained concrete system. The key aspect of the system is the blending of two retarded mortars, inducing an accelerated synergistic reaction between them just before leaving the nozzle. This enables precise process control and targeted regulation of the material properties immediately before and during application. To reduce the carbon footprint compared to established 3D mix designs, which typically contain high levels of ordinary Portland cement, we use CO2-reduced binders (limestone calcined clay cement, calcium sulfoaluminate cement) in our approach.The developed two-component material system is investigated in terms of the hydration properties as well as the structural build-up, comparing it with an established Shotcrete 3D Printing system. The systems are analysed using isothermal calorimetry and penetrometer tests. Additionally, the influence of different mixing ratios (25:75, 50:50, 75:25) of the individual mortar systems on the performance of the combined system is investigated. The studies show that the two individual systems are characterised in particular by long workability for several hours and delayed hydration reaction due to the retarders. In contrast, the combined two-component system shows an accelerated reaction and a fast increase in green and early strength. The two-component system analysed was finally tested in an application using a lab-scale SC3DP unit.

Efforts toward decarbonizing construction materials and industrial processes related to cement and concrete can be aided via multifaceted approaches that target alternative admixtures as well as precision control of fabrication. Chemical admixtures for water reduction have played a crucial role in the development of advanced concrete mixtures. Newer biomass processing techniques developed for aviation fuel production from corn stover biomass produce a more reactive lignin byproduct that is suitable for chemical modifications to mimic the properties of polycarboxylate ether admixtures with a smaller carbon footprint. The present study examines the use of lignin-based water-reducing admixture in cement pastes and mortar mixtures for 3D printing. The experimental program explores the use of different dosages of lignin-based admixture to produce 3D-printed samples with appropriate extrudability and buildability. The rheological characterization was performed to determine the flow curve of various mixtures. Finally, the heat of hydration of cement pastes was monitored via isothermal calorimetry to assess the impact of lignin-based admixtures on the hydration process of cement. The results of this study indicate that the use of biomass by-products, such as lignin-based admixtures have great potential to effectively control the fresh-state properties of cement-based materials.

This study investigates the potential of using steel slag blended grouts for 3D concrete printing applications. Steel slag, a by-product of the steel-making industry, is considered chemically less reactive compared to other supplementary cementitious materials. However, finely ground steel slag can influence the fresh-state properties of Portland cement mortars used for 3D printing. The effect of steel slag addition on the flow and static yield strength of mortars made with different Portland cement to steel slag ratios was evaluated. Effectiveness of commercial chemical admixtures enhancing the extrusion properties of Portland cement–steel slag blended mortar at the mixing stage, and further for the on-set of mortar after extrusion from the printing nozzle was determined for one selected mix design. Results indicate that adding steel slag reduces the static yield strength of the mix design required for structure build-up. Nevertheless, the accelerating admixture at the printing nozzle of the 3D printer (bi-component, 2K) helps gain the static yield strength required for buildability. Findings reveal that accelerator admixture has no impact on the mechanical performance of mortar containing 50 wt.% steel slag of the solids. However, superplasticizers can influence the flow retention property like the open time of the mortar. On the other hand, a higher proportion of steel slag has proven beneficial in inducing better extrusion properties. Printing of laboratory-scale hollow cylindrical geometry with same selected mix design has shown better buildability without any yielding of printed layer or buckling. In general, this study gives a perspective on using Portland cement–steel slag blended mortar for 3D printing applications.

This paper presents a new technology of on-demand delivery of set accelerators in printable concrete to meet the conflicting rheological requirements in 3D concrete printing (3DCP) . The on-demand delivery is achieved by encapsulating the set accelerators with thermally responsive polymers and mixing the encapsulated accelerators in the initial mixing followed by print head activation using heating to release the accelerators from the capsules. Various encapsulation methods including the gelatine capsules and phase change material (PCM) based capsule and accelerators including shotcrete accelerators, NaOH and Na2SiO3 were investigated. It was found that all encapsulation methods showed excellent pumpability properties before print head activation. On the other hand, the static yield stress (SYS) of printable mixes were significantly enhanced after the print head activation with different set-accelerators resulting in different degree of enhancement. For instance, the SYS at 25 min was increased by 4 times, 4 times and 9 times for the shotcrete accelerator, NaOH and Na2SiO3 respectively. On the contrary, the mechanical properties of the printed concrete were affected differently by the proposed encapsulated accelerator technologies. The corresponding 7-day compressive strength was reduced by 31% and 2% in perpendicular direction for NaOH accelerator and Na2SiO3 accelerator respectively. In contrast, the 7-day strength of the mixture with liquid set accelerator were increased by 26% in perpendicular direction. Finally, a brief discussion on the development of a prototype print head for the proposed method and large-scale print validation was presented.

This research investigates the early-age development of Young’s modulus in 3D printable mortar mixes, emphasizing the critical role of material properties in the success of 3D concrete printing technologies. Through a synthesis of findings, the study utilizes ultrasonic pulse velocity (UPV), compression, and microindentation tests to examine how water-to-cement ratio, aggregate size, paste volume, and the use of accelerators influence stiffness development within the first 24 h after mixing. The analysis highlights significant difference in measured Young’s moduli across the three testing techniques. Particularly at early stages, ultrasonic test shows considerably higher values compared to compression test. These differences underscore the need for careful selection of testing methods and cautious interpretation of data when assessing the mechanical properties of 3D printed concrete. The research also reveals that lower water-to-cement ratios and the addition of accelerators lead to quicker development of yield strength, compressive strength, and Young’s modulus, delivering valuable insights into optimizing 3D printing parameters for enhanced construction processes. This study advocates for further investigation into the observed discrepancies between measurement techniques at early age, aiming to refine our understanding of material behavior and improve the application of digital fabrication in construction.

Rewetting 3D-printed elements is an active mitigation approach aimed at preventing rapid increases in capillary pressure. The method lowers the capillary pressure in freshly placed concrete layers, reducing internal stresses and minimizing the risk of cracking. In the present study, the concrete surface was rewetted at a constant application rate at different times following specimen production. The results demonstrate that rewetting can significantly reduce shrinkage and the tendency of printed elements to crack. However, inline spreading of water during extrusion, such as attaching water jets directly to the printing nozzle, proves ineffective. The study results confirm that rewetting the concrete surface efficiently prevents shrinkage when performed during the phase of decreasing evaporation in the concrete element, i.e., shortly after extrusion.

Application of 3D concrete printing (3DCP) in tunnel invert construction presents a solution to the challenges inherent in conventional methods. This study proposes using 3DCP for tunnel invert construction to improve productivity. This method can reduce the number of concrete workers from six to two, a 67% reduction. This paper reports on two experiments testing the proposed method. The first experiment tested the shape accuracy of a mock-up arch of the tunnel invert. The mock-up member was constructed by printing a grid-shaped permanent formwork (3DCP-formwork) and pouring concrete into it. The shape of the mock-up member was measured using 3D scanning, and the shape accuracy of the lateral surface of the 3DCP-formwork met the specification requirements. The 3DCP-formwork also aided in the uniform finishing of the top curved surface. The second experiment examined the mechanical integrity of hybrid sections consisting of a 3D-printed component and internal concrete. Hybrid specimens with different printing patterns (straight and staggered) and plain concrete specimens were prepared for flexural and compressive strength tests. In the flexural strength tests, the staggered printing pattern improved mechanical integrity and exhibited greater flexural strength than plain concrete with a vertical joint, a known weakness in conventional construction. In the compressive strength tests, the compressive strengths of the straight and staggered printing patterns were 36% and 44% higher than that of plain concrete, respectively, indicating good mechanical integrity.

With the increasing technological development of 3D concrete printing processes, there has been an increased focus on researching the durability of 3D printed concrete and how the processes compare to conventionally cast concrete. Conventionally produced concrete is considered to be isotropic in terms of its hardened concrete properties (e.g. density, porosity and durability). In 3D concrete printing, the layer-by-layer nature of the process may cause an inhomogeneous distribution of concrete properties.This study compares the durability characterized by the penetration behaviour of pollutants (gaseous CO2 and liquid NaCl solution) for three manufacturing techniques: (1) Extrusion 3D Printing, (2) Shotcrete 3D printing, and (3) conventional concrete casting. For this purpose, the specimens are produced under the same conditions (material composition, storage). The specimens are tested for carbonation (accelerated carbonation method) and chloride migration resistance (rapid chloride migration test). The resulting chloride and carbonation depths are analysed using a Matlab tool. The results show a higher chloride migration resistance of the SC3DP samples compared to the extruded and conventionally cast samples. In contrast, the extruded and conventionally cast samples show an increased resistance to carbonation.In general, it is found that the 3D printed samples exhibit a distinct inhomogeneous penetration behaviour of the pollutants; however, the location of the greatest penetration depths varies depending on the 3D printing processes. The highest penetration depths of extruded samples were achieved in the bulk, whereas it occurred in the interface region for SC3DP samples. The results indicate that the durability resistance and the penetration behaviour of harmful substances in 3D printed concrete are not only determined by the layer-by-layer application itself, but also by process-specific influences from the 3D printing process, such as pumping or spraying.

Extrusion-based 3D-printing techniques with concrete have been studied for two decades and gained a lot of interest from both academia and industry. To produce mixtures capable of promoting fast hardening after extrusion, new approaches such as the use of accelerated blends have been recently developed. Two-component systems (2K), designed by partially replacing ordinary Portland cement with a fast-setting constituent – such as the combination of calcium-aluminate cement with anhydrite (CAC + CS) – were proven efficient for printing processes. However, there is a lack of literature regarding the durability of such mixtures, despite the increased interest in assembling structures with that process. This paper reports on the performance of 3D concrete printing, produced with a two-component system accelerated with CAC and CS, when submitted to water absorption, carbonation, and chloride ingress tests. The performance of printed and cast samples is compared, and the results are discussed. The findings of this study underline the main challenges to improve the performance of 3D printed concrete produced with two-component systems and fill an important gap of knowledge in the current literature.

As the printing speed, the material properties, and the printing quality of additively manufactured concrete, also called 3D printed concrete (3DPC), are vastly improving, the technology is increasingly finding its way towards real-life applications such as dwellings and bridges. However, to reach the technology readiness level this technique is aiming for, two large challenges remain, namely the introduction of reinforcement into 3DPC, and the reduction of volume used to achieve sustainable and visually pleasing structures. The former concerns the application of 3DPC as structural elements that have a certain degree of ductility, and the latter concerns the sustainability of 3DPC, which can only outperform cast concrete in the case of geometrically complex structures where volume is reduced. The current contribution investigates a solution to the reinforcement problem by introducing a hybrid beam system, in which an unreinforced additively manufactured concrete component is combined with a traditionally reinforced cast concrete component to form a functional hybrid 3DPC-cast beam system. A common volume reduction algorithm is applied to the 3DPC component of the hybrid system. A maximum stiffness objective is combined with a volume constraint, after which the structural capacity is determined considering the interface stresses observed for each of the volume-reduced geometries. Herewith, a complex geometry is obtained to be manufactured with 3D concrete printing. A numerical analysis follows in which challenges for structural application, such as loss of bearing capacity and loss of interface bonding are identified and further elaborated.

The field of 3D concrete printing and the integration of load-bearing reinforcement remains a significant challenge. This paper introduces three innovative methods for constructing monolithic, steel-reinforced concrete walls using the CONPrint3D technology, which was previously developed for large-scale, monolithic, on-site 3D concrete printing. These methods include: 1) the additive application of reinforcement meshes hung in front of a printed core wall, 2) the lateral additive filling within a reinforcement cage, and 3) the fully automated construction of a welded reinforcement structure followed immediately by layer-by-layer encasement with extruded concrete. In the paper, the material challenges regarding concrete and the mechanical engineering development approaches are discussed. A comparative evaluation of these proposed and in lab scale tested new processes against established construction methods is included.

In Digital Fabrication with Concrete (DFC), reinforcement integration is one of the key challenges to enable large-scale application, and further adoption of DFC by industry. This paper presents an investigation on green state milling as a novel approach to reinforce 3D printed concrete elements, in particular for reinforcement integration during fabrication utilising Shotcrete 3D Printing (SC3DP). To enable force-flow oriented freeform reinforcement installation, grooves providing space for reinforcement are robotically milled in green state. Subsequently, standard steel rebars or other reinforcing materials are installed, and a cover layer is sprayed. Two key parameters influencing the bond quality and integrity of reinforced elements are explored here: time of milling after SC3DP, and groove geometry. The results are qualitatively assessed in terms of the presence of voids and imperfections, in particular around rebars.The identified optimal set of parameters is then used for the fabrication of a second set of specimens: 1) elements for rebar pull-out testing and 2) full-scale beams for structural testing and quantitative analysis.

Additive manufacturing is gaining popularity in the construction industry due to its advantages in producing complex concrete components. However, current techniques often focus on unreinforced concrete, which poses a challenge for structural elements, that usually require reinforcement. A promising solution is the integration of short rebars, where concrete layers are first printed and then reinforced with inserted rebars. Thus, the bond quality of the inserted rebar is highly dependent on the rheological properties of the printed concrete. This study investigates if vibrating the short rebar during insertion can improve the bond zone by locally fluidising the surrounding concrete. Rebars are inserted into fine-grained 3D printing concrete both with and without vibration. To study the effect of the concrete structural build-up, i.e. rheology, rebars are integrated at three different time steps (10, 30 and 60 min). The yield stress at each integration time is measured using penetrometer tests. The bond strength is evaluated by pull-out tests according to RILEM RC 6. For reference, conventional specimens are produced in moulds. The results show a decrease in bond strength with increasing integration time for all specimens. A negative correlation between the concrete yield stress and the resulting bond strength is observed. However, the vibration of the rebar significantly improves the bond strength, increasing it at early integration times to the bond quality of the cast reference specimen. Even at an integration time of 60 min, vibration increases bond strength by 166% compared to direct insertion without any vibration.

In recent years, 3D concrete printing (3DCP) systems have attracted great attention in the construction field and is in fierce development competition. 3DCP systems cause discontinuities between layers and results in mechanical performance degradation of the laminated structures. Therefore, numerous reinforcement methods have been proposed, however, many of proposals include problems such as limited effective reinforcement directions and the necessity of combining conventional labor-intensive construction site work. In this study, we proposed a novel reinforcement mechanism which the combines fiber-reinforced cementitious composites (FRCC), for reinforcing the horizontal direction, and an automated insertion mechanism of reinforcement steel pins, penetrating between layers for the vertical direction. FRCC layers with reinforcing pins inserted by the automated insertion system were prepared to realize this proposal. A bending test was conducted on the obtained specimens by cutting from the laminates. The results showed the direction-independent reinforcing effect of both parallel and vertical to the printing direction. In the horizontal direction, strength and toughness were increased due to the FRCC characteristics, and in the strength in vertical direction, in which the FRCC effect was limited according to previous studies, was increased up to approximately 50% by the inserted reinforcing pins between layers.

3D printing in construction allows for diverse, lightweight flexural members with enhanced performance. This study investigated the effects of materials (two cementitious mixtures), reinforcement (barbed wire vs. none), and cross-sections (including Full section, T section, U section, and Hollow section) on 3D printed concrete beam strength. While the mixture with higher compressive strength offered greater moment capacity, the increase in moment capacity was lower than that in compressive strength, suggesting other factors like printing quality and bond strength can influence flexural performance. Barbed wire reinforcement was explored as a viable method for reinforcing concrete in 3D printing and it substantially improved moment capacity for both materials compared to Plain sections, even exceeding ACI standard expectations for cast beams. Utilizing T, U, and H cross-sections achieved material reduction (up to 43%) compared to full sections. The T section exhibited the greatest deflection and ductile failure with the highest material reduction. Full sections offered the highest moment capacity but with high material consumption and a brittle failure mode. H sections offered a balance between moment capacity, material efficiency, and ductile failure, making them suitable for specific applications. However, T and U sections showed reduced moment capacity, likely due to delamination between layers. This research emphasizes the importance of material optimization, strategic reinforcement, and tailored cross-sectional design to achieve superior flexural performance and efficient material usage in 3D printed concrete structures.

This paper introduces a novel methodology for the design and Additive Manufacturing (AM) of functionally hybridised, lightweight concrete building components. Grounded in simplified, monolithic, and single-origin construction principles, the proposed approach leverages geometric freedom and the use of lightweight aggregates provided by the AM technology Selective Paste Intrusion (SPI). This method enables the seamless integration of insulating and load-bearing functions within unreinforced, prefabricated concrete elements. The concept envisions an interlocking three-dimensional puzzle, wherein manufactured components are transported to the site, assembled, and connected through custom interlocking joints, with the option of incorporating post-tensioning cables when structurally required. The SPI method involves a repeated dual-step process, including depositing a loose layer of a lightweight aggregate of expanded clay (LECA) in a particle bed, followed by selective binding with penetrating cement paste. This research explores the potential of trapping loose aggregate within the cavities of the bound structure, which exhibits lower thermal conductivity compared to the fully bound structure. Consequently, areas containing trapped lightweight aggregates enhance thermal insulation, while regions with bound aggregates provide structural support. Compared to cast lightweight concrete with uniform material properties, lightweight SPI allows for creating an internal closed-cell composition that can be functionally graded to meet individualised structural and thermal requirements within a single freeform component. To experimentally validate this concept, a section of an outer wall on a 1:1 scale was manufactured using an SPI printer. With this demonstrator, we highlight the key conceptual features of the design method, including the manufacturing strategy of large-scale AM components with interlocking joints and a functionally graded internal structure.

This paper presents the experimental results of a 1:1 scale prototype of a segmented pedestrian bridge produced by the Additive Manufacturing (AM) technology Selective Paste Intrusion (SPI). SPI employs a technique where aggregates are selectively bound layer-by-layer in a particle bed with cement paste, enabling the 3D printing of large-scale, free-formed geometries with high shape accuracy and resolution while maintaining material properties comparable to conventionally cast unreinforced concrete. Traditional reinforced concrete structures, like bridges, often face challenges during their end-of-life phase, including limited potential for recoverability, reconstruction difficulties, or, if recycled, time-consuming material separation processes. This paper presents a novel methodology for the fabrication-aware design and additive manufacturing of a bridge structure using unreinforced concrete elements within a post-tensioned system to address these challenges. Leveraging SPI’s geometric versatility, mechanical properties, and material separability, a system of prefabricated, dry-assembled, and post-tensioned concrete elements is proposed, resulting in a bridge structure that is fully demountable, reusable, or recyclable at the end of its life cycle. While briefly outlining the fabrication-aware digital design workflow, this paper focuses on the material testing, the manufacturing process, and the experimental results of the assembly and disassembly workflow of the bridge structure. By presenting the potentials, challenges, and limitations of the proposed methods, this research aims to contribute to the understanding of SPI's applicability for automated building construction, offering insights into potential refinements and directions for future exploration.

Selective Cement Activation (SCA) is a powder bed 3D printing technique that offers the possibility to precisely manufacture free-form building elements with a high degree of complexity and surface quality. Expanding upon initial findings, we aim to further improve the dimensional accuracy. This fundamental research will explore the effect of vertically adjusting the water content over height to account for the often observed uneven dimensional deviations across the height of a component printed with SCA. Therefore, we investigated three types of w/c-ratio gradation: (a) core-shell gradation, (b) core-shell gradation with shell thickness adaption and (c) core-shell gradation with core water content adaption. The samples produced in this way are compared with samples printed with a uniform w/c-ratio in terms of dimensional accuracy, compressive strength and homogeneity. The results show that grading the w/c-ratio, in general, can increase the dimensional accuracy compared to specimens printed with a high uniform w/c-ratio while simultaneously not significantly decreasing the compressive strength. The adapted gradation concept with core water content adaptation further increased the dimensional accuracy, resulting in just slightly less accuracy than the reference specimens printed with a low uniform w/c-ratio of 0.3. However, these specimens achieved 51% higher compressive strength compared with the specimens printed with a constant w/c-ratio of 0.3 while simultaneously having the highest homogeneity in density over their cross-section.

In recent years, there has been notable progress in implementing digital manufacturing technologies in the construction industry. Digital concrete fabrication, which eliminates the requirement for traditional formwork, offers increased design flexibility as one of its key benefits. The innovative concept of Digital Casting Systems (DCS) introduces a different approach to digital concrete, allowing the use of traditional and thin formworks alongside precise reinforcement placement. The key to DCS lies in utilizing self-compacting concrete (SCC). The rheology of the SCC is controlled in real time during production by adding admixtures into an inline mixing system during the buildability phase. This effectively reduces formwork pressure and substantially accelerates building rates. However, the inclusion of inline mixing imposes challenges in the scale-up process. These include finding viable methods to increase aggregate content and use coarse aggregates, achieving fast construction rates, and extending processing time. This study seeks to transition DCS from the laboratory to the industry scale by overcoming the challenges addressed by inline mixing.The scaling-up strategy in DCS involves identifying optimal values for comprehensive system parameters, including processing and material characteristics, while carefully considering specific system requirements. This research underlines the advantages of DCS in industrial applications while delving into the optimal parameters for upscaling such automated digital casting systems.

The Nubian Slab is a real-world 3D-printed structural concrete element for a residential building in Zürich. The vaulted slab proposes an innovative material-efficient construction method based on digitally fabricated thin shell stay-in-place concrete formworks. The method targets structural slabs, which contribute up to 60% of concrete consumption in architectural applications. The fabrication process is based on the ancient Nubian vaults. These roof structures feature self-supporting inclined masonry courses that can be built without temporary support. This traditional building technique inspired the proposed layered concrete extrusion process, with the 3D-printed concrete layers being analogous to Nubian brick courses. The key difference to conventional 3D concrete printing is the inclination of the extrusion layers, allowing shallow vaults to be produced suspended in thin air without additional supports. The paper presents the robotic 3D-printing setup with a custom nozzle, the fabrication-informed design considerations, and the current limitations of the process, focusing on the case study of a 16 m2 Nubian slab with an irregular perimeter installed in a residential building. Based on this case study, the paper outlines a comprehensive construction sequence for Nubian slabs, considering discrete prefabricated 3D-printed Nubian formworks, assembly details, reinforcement strategies, functional integration, and in-situ monolithic casting. The proposed 3D-printed Nubian slab system enables innovative material-efficient architectural design solutions that may accelerate construction times on site, facilitate mass customisation, automation and integration, and enhance structural performance while remaining compatible with traditional building practices.

As Additive Manufacturing in Construction (AMC) continues to gain attention across the building industry, challenges regarding its application and quality assessment are increasingly relevant. The surface quality of printed elements is particularly important, as it determines the aesthetics and is among others, a crucial aspect when compared to traditional cast concrete components. However, these aesthetics vary for individual AMC processes. For deposition processes, the surface is characterised by a clearly visible layer structure This layer structure is additionally characterised by an overlaid rough structure for material jetting processes such as Shotcrete 3D Printing (SC3DP). This surface can be finished through post-processing to achieve a smooth surface or a defined texture for functional integration. For this purpose the present research investigates an innovative surface post-processing technique based on a rotating trowel disc. Experiments are performed on a wall segment produced with SC3DP, focussing on evaluating the influence of three different waiting times after printing before starting surface post-processing (10, 45 and 90 min). Penetration tests are used to determine the yield stress of the printed material for each post-processing time. The resulting surface roughness is analysed by determining the mean profile depth and the estimated texture depth. Results indicate an overall feasibility of the investigated post-processing technique. Especially the reduced structural build-up at the edges compared to the centre of the shotcrete 3D-printed strands allows for a large process window for surface finishing. In this context, delayed processing of the surface up to 90 min after material application results in a smoother surface than early processing.

Additive manufacturing (AM) for construction is rapidly developing, with significant effort on the control and measurement of the quality materials, and less directed towards enhancing the surface quality of printed parts. The presence of the staircase effect, a well-known feature of 3DCP, imposes limitations on achieving desired aesthetics and manufacturing tolerances. Secondary process like floating has been demonstrated to show benefit for sprayed concrete surfaces, however, studies of the parameters for application using mix designs for extrusion have not been reported. This paper attempts to address this gap by presenting an investigation of the influence of process on surface quality. Key parameters of power float process in 3DCP are investigated, including pressure, rotational speed, material state, and dwell time and the resulting surfaces are quantifiably evaluated using high-resolution optical roughness measurement. These values were compared to standard surface finishing metrics, and it was found that an applied tool force in the range of 20–40 N and rotational speed in the range of 150–300 Rotational Speed (RPM) yielded the best results.

The rapid development of climate change and the resulting EU climate targets require a significant reduction in CO2 emissions . The usage of fully recyclable, locally sourced low-carbon materials, such as earthen materials, can play a key role in reducing CO2 emissions. Currently, research in the field of additive manufacturing with earthen materials focuses on extrusion-based processes. However, with these new techniques, materials must be precisely tailored to the manufacturing process's requirements to achieve a functional material-process interaction. This results in the requirement for highly specialized materials, hindering the use of local earth resources with inherent varying compositions.As a traditional construction technique for earthen materials, rammed earth construction has been in use for thousands of years. Without adaptations to the ancient process to the current state of the art in industrialized countries, its use results in high construction costs. Therefore, research in advanced processing methods, including digital fabrication and automation approaches, could fully unlock the potential of local earthen construction and bridge the gap between traditional processes and the future demands of construction practices.The present research examines the challenges of sourcing and using local earth materials in a robust, automated, rammed earth manufacturing process. Therefore, investigations on the influence of process parameters such as layer height and impact energy on the material properties such as dry density and compressive strength using a range of raw materials are presented. The aim of this investigation is the identification of robustness criteria regarding the digitally controlled processing of local materials in the robotic rammed earth process on process and material level.

Earthen materials used for centuries are capable of providing effective thermal resistance and good heat absorption in buildings. In addition to materials, passive design strategies that provide natural ventilation have been traditionally employed to achieve thermal comfort levels inside buildings. Several passive cooling and heating systems were used in old buildings such as windcatchers, solar chimneys, and Trombe walls. However, nowadays, these systems and materials are replaced by non-environmentally friendly materials and HVAC systems that consume high energy and emit more carbon emissions, consequently contributing to climate change and the rise of temperature.This paper aims to assess the impact of a designed opening in a large-scale 3D-printed earthen living structure. The opening is operable and has a dual function of natural cooling and heating. The methodology followed utilizes Computational Fluid Dynamic (CFD) analysis simulation to measure the air-flow pattern and movement through the 3D-printed openings in the space considering various parameters while designing such as opening size, shape, and location. A small-scale prototype of the opening mechanism is 3D printed before the large-scale structure. Crane WASP technology is used to 3D print on-site with locally sourced earthen materials excavated from the site. The results record a different performance of airflow patterns and speed in relation to the opening size, shape, and location of both air inlet and outlet. Temperature and pressure are the main factors that allow control of natural movement in space. Further research is to monitor the openings on-site through loggers and sensors at different times. Implementing earthen materials and passive design strategies would enhance the airflow performance and reduce the demand for artificial cooling systems which would enhance the built environment.

This paper presents a novel production concept for casting of material efficient concrete slabs using circular 3D printed earth formworks. While optimized concrete structures exist, their complexity often leads to high waste generation due to single-use forms. Other less known, localized techniques such as clay-filler slabs, popularized in India, successfully reduce concrete consumption by 50% by employing locally crafted pottery as a stay-in-place formwork. Although effective, these analogue methods remain constrained in terms of design flexibility and circularity. This study consequently leverages the sustainability potentials of the `clay-filler slab`— It transforms the technique into a new method for making 3D printed formworks from local earth materials, creating efficient and customizable concrete slabs. This process reduces emissions, improves material efficiency, and expands design options through digital fabrication, promoting circularity. The study investigates material and processing methods for earthen formworks using a slab design initially made in polymer filament. The design is subsequently scaled up into three demonstrators using a customized earth mix, addressing challenges in scalability for real-world construction. Covering the entire process from design to fabrication, it emphasizes digital workflows and tools, showcasing advancements in printing and casting mixtures. Lastly, it discusses the potential for material savings and circularity through digital processes, addresses current limitations, and outlines future development steps.

Arc direct energy deposition (DED-Arc) enables the production of large, optimized components like nodal connectors for the architectural engineering and construction (AEC) sector. High-strength low-alloyed (HSLA) steel wire is particularly suitable for thin-walled and shape-optimized components due to its joining possibilities and strength. As large components for the AEC sector are unique and not meant for serial production, the structural design, prediction of component performance and quality management must be reconsidered. Instead of testing random samples, the process can be monitored and conclusions to the resulting parts can be drawn. With respect to digital fabrication in construction a concept of a digital twin for DED-Arc steel components will be presented. In detail, the manufacturing process has an influence on the design, manufacturing strategy and on the component performance. Therefore, localized information is needed to understand the correlation of as-designed geometry, print strategy, weld data and as-build geometry. This fundamental understanding of design, process and performance enables first time right approaches for sustainable use of DED-Arc technology.To determine the influence of these parameters, extensive studies on thin-walled components and nodal connectors made from HSLA steel wire with DED-Arc were conducted. A workflow will be presented to link point, path and surface wise data in a digital twin structure. In detail: A structured light high-resolution scan of the “as-built” components allowed the 3d-analysis of surface topology and correlated them to the manufacturing process data and print features locally. Understanding these complex interactions in digital twin (DT) data enables adapting the component’s design or manufacturing process to predict the component’s mechanical performance requirements.

In face of the challenge of reducing the carbon footprint of the construction industry, 3D-printing technologies with cement-based materials have gained significant popularity in the past decade. However, ensuring a standard of quality and safety for printed objects and structures is a challenge yet to be overcome. Printability, durability and quality issues often stem from interactions between several physicochemical processes, in the material it-self and its relationship to the surrounding environment. Understanding couplings between mechanical, hydraulic, thermal and chemical processes is thus paramount to the development of predictive simulation tools, which in turn could allow for better process control and understanding of durability properties.Herein, we propose a multi-physics simulation framework, with a fully coupled material constitutive model at its core, thermodynamically derived as an extension of classical unsaturated poromechanics to chemically solidifying media. Experimental procedures from soil mechanics, such as the use of porosimetry measurements, are adapted to fresh cement-based materials and used to determine necessary model properties and their evolutions with hydration. In conjunction with this model, we introduce a finite element (FE) based framework and modelling strategy aimed at recreating the extrusion-based printing process through sequential addition of material. A systematic meshing strategy from toolpath data of the printer is also showcased. Implemented using open-source components, this framework is used to perform 2D and 3D simulations of mortar 3D-printing.This framework allows to investigate various common issues with 3D-printing of cement-based materials along with their coupled origins, ranging from printing collapse to service durability. In particular, the influence of external parameters such as layer-pressing and environmental conditions on buildability and geometric accuracy of prints are shown along with indications to mitigate them. Known durability problems induced by drying processes, such as degraded material properties at interfaces, are also presented in light of a detailed unsaturated behaviour from the constitutive model.

Automating the construction of concrete structures by using 3D printing technology is a novel approach that is rapidly increasing around the world. Compared to conventional construction, 3D printed concrete (3DPC) provides several unique advantages including saving time and money, eliminating the need for formwork, and increasing workers’ safety. Although considerable research has been carried out on the mechanical properties of 3DPC, the number of studies on the structural behavior of 3DPC buildings is limited. In particular, there is no information on the load-carrying capacity of 3DPC buildings when subjected to lateral loads such as wind and earthquakes. A key factor in determining the lateral resistance of 3DPC buildings is the behavior of the interface between the concrete filaments which can be affected by a wide range of parameters and requires careful consideration.This paper presents a three-dimensional finite element modeling approach developed using the LS-DYNA software for performance assessment of 3DPC buildings under lateral loads. The proposed modeling approach is verified against results of an axially loaded wall test reported in the literature. The analytical and experimental results are compared in terms of the load-deflection response and failure mode. The verified model is then employed to assess the seismic performance of a 3DPC building located in Canada that includes walls with different aspect ratios. The ability of walls to resist earthquake loading is investigated by comparing the pushover responses in terms of the lateral load resistance, ductility, and failure mode. The lateral load-carrying capacity of the building is also compared against the seismic design force calculated according to the Canadian Building Code for two cities in Eastern and Western Canada. The findings of the study will help engineers to gain insight into the structural performance of 3DPC buildings under lateral loads.

Recent developments in 3D printing in the construction sector have highlighted the importance of reducing the impact of the materials used. One of the main levers is to reduce the amount of cement by maximizing the aggregates content. However, increasing the amount of aggregates could lead to problems during the pumping or transport step to the printhead, which is limited by the power of the pumping equipment and the viscosity of the cement-based materials, which has a strong effect on the pumping pressure. As the addition of aggregates generally increases the viscosity of the mixture, the influence of the addition of these large aggregates on the properties of high-viscosity printable cement-based materials during extrusion needs to be analysed.The aim of this study is to investigate the influence of the pumping process on the self-heating of extruded materials, from paste to mortar and concrete, for both cementitious and clayey matrices. The viscosity of the material is assessed macroscopically by its dynamic viscosity and depends on the amount of aggregates in the mix. Pumping of the material through a 10 m hose and assessment of the associated viscosity will feed a numerical simulation model based on heat transfer for non-isothermal flow in a pipe of non-Newtonian material. Thanks to this dual approach, a comparative study between temperature rise measurements and process simulation will be able to quantify the influence of changing scale and compound design parameters on the extrusion behaviour of printable materials.This study aims to pave the way for new challenges related to the entire supply chain of the 3D printing process for construction materials, taking into account multiphysical contributions such as friction, evaporation, temperature-dependent chemical reactions, etc. and related measurements that can be performed, including the possibility of adding a heat source to control the setting or drive a phase change of a heat sensitive material.

This contribution deals with particle-bed 3D printing and presents a numerical approach to predict and optimize the printing process. The process studied here, Selective Paste Intrusion (SPI), has been successfully used to print small and medium-sized objects, but it has not been widely implemented. Before widespread adoption in the construction industry, fundamental questions regarding process optimization and paste/aggregate properties must be addressed. To optimize the process, the SPI process has been studied numerically and a computational model was developed to predict the propagation of the fluid through the particle bed. The model describes the cement paste as a Bingham fluid and the particle bed as a porous medium. In parallel, the rheology of the paste and the properties of the porous medium were investigated experimentally. The developed numerical tool was validated through printing experiments. It was shown that the tool was able to predict the final penetration depth, which is crucial for the quality of the printed component.

3D Concrete Printing (3DCP) is emerging among additive manufacturing technologies for the construction industry. With 3DCP complex structural components can be built without formwork in a layer-wise fashion, enhancing accuracy, optimizing material use and reducing construction times and waste. However, to fully exploit and control 3DCP it is necessary to develop adequate numerical predictive tools: solid FEM models have been used to predict buildability, while fluid or particle methods are preferred to assess pumpability and extrudability. Currently, a unified numerical framework to simulate the overall 3DCP process is missing. This work intends to take a first step in that direction. A single-phase fluid model of 3DCP based on the Particle Finite Element Method (PFEM) is illustrated. Fresh concrete is modelled with the Bingham law and the static yield stress is increased in time in the layers to reproduce material structuration. In the PFEM framework, a wide range of different phenomena typical of 3DCP can be simulated. In the specific, two 3D printing applications are shown: the virtual printing of a cylindrical object and the prediction of structural failure due to elastic buckling in a rectilinear wall.

Digital fabrication with concrete promises a more sustainable and efficient construction industry. However, its implementation is progressing slowly, with hardly any real-scale, load-bearing applications to date. Large-scale structural projects are thus essential to assess the validity of the promise of digital fabrication with concrete. Tor Alva, a 30 m tall tower in the Swiss Alps, is such a project currently being developed to be built in 2024 using digital fabrication with concrete. The tower is a modular system featuring load-bearing 3D Printed Concrete (3DPC) V-columns connecting conventional prefabricated reinforced concrete slabs serving as base and capital. Due to the innovative nature of the columns, structural testing was required to validate their structural integrity. The experimental campaign consisted of a series of structural tests: (i) standard mortar and concrete tests, (ii) compressive loading of a downscaled version of the structural core, (iii) full-scale load tests on a two-metre-tall V-shaped column subjected to vertical and horizontal loading and (iv) direct tensile tests on reinforced 3DPC chords (currently in progress). The standard mortar and concrete tests were needed for 3DPC material characterisation. The compression tests assessed the behaviour of the cross-section under governing compressive load. The overall integrity of the fabrication concept and the connections between the elements were tested with the full-scale load tests. The tensile tests will be carried out to evaluate the performance of reinforced 3DPC chords, which is relevant for columns subjected to bending or tensile forces in case of lateral loading. This paper presents the details of the first three test setups and highlights the main findings of the 3DPC material characterisation and the compression tests.

Nowadays, 3D concrete printing (3DCP) can build on a broad base of developments concerning all areas along the process. Thanks to increased precision, material performance and reinforcement methods, highly sophisticated concrete components can now be produced using 3DCP.The research presented in this paper integrates various process-related possibilities to design a structural application – a filigree curtain wall façade panel with the dimensions of 135x350 cm. The panel features hollow ribs on the backside, where the two support anchors are situated, and follows the internal force flow by using variable-width printing. To meet the load-bearing requirements, a dual approach is adopted. Areas exposed to tensile stress are strengthened by increasing their thickness and/or printing with embedded wire rope reinforcement. The bending load-bearing capacity is achieved by longitudinally pre-stressing the panel. The panel can be manufactured in one piece or can be assembled from several segments using match-casting method.The paper at hand presents the over-all application concept and shows the results of manufacturing tests, mechanical tests, and numerical investigations. The findings obtained demonstrate the potential of filigree curtain wall façade panels made by 3DCP.

Concrete 3D printing is currently gaining popularity as a scalable technology for construction of large-scale structures for public use. In a similar endeavor, Tvasta constructed a boundary wall of 400 m in length and 3 m in height for an international cricket stadium in Chennai, India. The wall was facing towards one of the major arterial roads of Chennai, within 500 m from the sea beach. The wall was distributed into panels and printed at a factory located about 40 km from the site. A total of 400 panels were printed, with each panel having a length of 2 m, width of 0.3 to 0.35 m, and height of 1.5 m. The criticality of the structure was governed by three factors – a. structural integrity, b. aesthetics, and c. assembly of 3D printed panels. The walls were expected to carry a higher lateral pressure due to cyclones and crowd pressure during cases of riots and vandalism. The mechanical properties were examined at elements (or mortar scale) and components. The mix’s compressive, flexural, and split tensile strength was primarily evaluated in the element scale along with the bond strength between the printed layers. In the component scale, the 2 m length and 1.5 m height panels were tested for axial compression and bending. The achieved ultimate failure axial load was more than 300 tons at an age of 28 days and 550 tons at an age of 180 days. The ultimate transverse load of more than 3 tons was achieved at the age of 28 days. In conclusion, the structure was observed to perform better than the client's stated requirement.

This paper explores how a fast-emerging fabrication process, 3D Concrete Printing (3DCP), challenges conventional workflows for architectural design, and suggests how those workflows could evolve to better incorporate critical parameters related to this fabrication technology. While 3DCP is rapidly being adopted to construct architectural projects, key aspects of design consideration remain unintegrated in the early design stage. In particular, material, printability, fabrication constraints and the assessment of sustainability metrics are considered only at the later stages of design development and fabrication. Consequently, these parameters are not leveraged to their full advantage, missing opportunities for design and material optimization. Further, without being able to evaluate 3DCP using sustainability metrics central to early design decision making, 3DCP faces a fundamental barrier to large scale adoption across the construction industry.This paper introduces a new 3DCP specific workflow that addresses these barriers. This prototypical workflow is composed of new and existing tools and allows for a more holistic design approach. It connects existing optimization, analysis and fabrication tools typically used late in the architectural design process to 3DCP-specific considerations and brings their use forward. It further integrates novel research-based 3DCP specific tools, addressing material recipes and in-process printing behavior. It bridges gaps where future research and development is required.To exemplify the workflow and demonstrate its practical application, it is applied to a case study project ‘The hybrid slab’. The hybrid slab is a series of 3DCP vaulted ceiling arches that utilize material, geometric, and assembly strategies to investigate how 3DCP can be used strategically within a hybrid construction. This application identifies a need for further development of tools that fill ‘missing gaps’ in the workflow and finds that the early inclusion of precise material information is a critical factor across geometry generation, optimization, and evaluation.

The advancing technologies of 3D concrete printing (3DCP) and off-site construction (OSC) signify substantial technological progress in construction, albeit each at distinct stages of evolution. Despite offering enhanced sustainability, reduced labour intensity, and potential quality control improvements, their current utilization remains below their potential, partly due to existing barriers. 3DCP encounters logistical and practical challenges due to its nascent stage, while OSC faces limitations in flexibility, cost implications for late changes, and perceived coordination challenges. This paper explores integrating these technologies within a comprehensive building information modelling (BIM) framework to foster cooperative collaboration and mitigate barriers. A comprehensive survey of contemporary BIM practices among global structural designers, architects, and BIM professionals was conducted and supplemented by a systematic literature review. This provided insights into prevailing BIM management practices in the architectural, engineering, and construction (AEC) industry, as well as the existing project processes in OSC and perceptions of the 3DCP domain. Interviews with industry experts from both OSC and 3DCP further corroborated these findings, unveiling ample opportunities for integrating 3DCP into OSC workflows. Notably, the adaptability of 3DCP for late-stage modifications and its capacity for creating freeform structures could counterbalance the perceived limitations of OSC regarding unique shapes and early design freezes. Moreover, leveraging existing OSC logistics and systems presents a seamless integration pathway for 3DCP into industry. Whether viewed in conjunction or independently, it is evident that both systems necessitate an integrated BIM strategy, preferably implemented during the project's conceptual design phase.In conclusion, the convergence of 3DCP and OSC represents a promising frontier in construction technology. By leveraging the strengths of 3DCP's adaptability and OSC's established logistical frameworks, this integration not only addresses current limitations but also paves the way for a more sustainable, efficient, and architecturally flexible future in construction practices.

Research in digital fabrication, specifically in 3D concrete printing (3DCP), has seen a substantial increase in publication output in the past five years, making it hard to keep up with the latest developments. The 3DCP.fyi database aims to provide the research community with a comprehensive, up-to-date, and manually curated literature data set documenting the development of the field from its early beginnings in the late 1990s to its resurgence in the 2010s until today. The data set is compiled using a systematic approach. A thorough literature search was conducted in scientific databases, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) scheme. This was then enhanced iteratively with non-indexed literature through a snowball citation search. The authors of the articles were assigned unique and persistent identifiers (ORCID® IDs) through a systematic process that combined querying APIs systematically and manually curating data. The works in the data set also include references to other works, as long as those referenced works are also included within the same data set. A citation network graph is created where scientific articles are represented as vertices, and their citations to other scientific articles are the edges. The constructed network graph is subjected to detailed analysis using specific graph-theoretic algorithms, like PageRank. These algorithms evaluate the structure and connections within the graph, yielding quantitative metrics. Currently, the high-quality data set contains more than 2600 manually curated scientific works, including journal articles, conference articles, books, and theses, with more than 40000 cross-references and 2000 authors, opening up the possibility for more detailed analysis. The data is published on https://3dcp.fyi , ready for import into several reference managers, and is continuously updated. We encourage researchers to enrich the database by submitting their publications, adding missing works, or suggesting new features.