Exploring Architectural Units Through Robotic 3D Concrete Printing of Space-Filling Geometries

Abstract

The integration of 3D concrete printing (3DCP) into architectural design and production offers a solution to challenges in the construction industry. This technology presents benefits such as mass customization, waste reduction, and support for complex designs. However, its adoption in construction faces various limitations, including technical, logistical, and legal barriers. This study provides insights relevant to architecture, engineering, and construction practices, guiding future developments in the field. The methodology involves fabricating closed architectural units using 3DCP, emphasizing space-filling geometries and ensuring structural strength. Across three production trials, iterative improvements were made, revealing challenges and insights into design optimization and fabrication techniques. Prioritizing controlled filling of the unit’s internal volume ensures portability and ease of assembly. Leveraging 3D robotic concrete printing technology enables precise fabrication of closed units with controlled voids, enhancing speed and accuracy in production. Experimentation with varying unit sizes and internal support mechanisms, such as sand infill and central supports, enhances performance and viability, addressing geometric capabilities and fabrication efficiency. Among these strategies, sand filling has emerged as an effective solution for internal support as it reduces unit weight, simplifies fabrication, and maintains structural integrity. This approach highlights the potential of lightweight and adaptable modular constructions in the use of 3DCP technologies for architectural applications.

1. Introduction

Architectural design and production are constantly evolving with technological advancements and the needs of society. In today’s era characterized by mass production and robotics, architecture has to adapt to new design and fabrication methods [1]. This adaptation is possible through ongoing research efforts and the ability of the industry to adapt to these technological advancements [2]. Among these, 3D concrete printing (3DCP) stands out as a promising fabrication method, offering potential with innovative design solutions, high-precision manufacturing, and the capability to enable freeform production [3]. With additive manufacturing and cement-based materials, 3DCP has opened new perspectives in building production, enabling the construction industry to discover possibilities that are generally limited by traditional methods [4]. While 3DCP has been developing for over twenty years, it is now more accessible than ever for both industrial and research fields [5,6,7]. One of its main advantages is the ability to produce complex structures without molds, making it particularly interesting for customized construction [3,8,9]. The potential of 3DCP in the construction industry is becoming increasingly evident as research in these topics expands. The industry is growing in the adoption of 3DCP technology with its expanding potential in production on multiple scales [9]. 3DCP offers many benefits, including mass customization, waste reduction, and the facilitation of complex designs tailored to specific needs. This flexibility increases both efficiency and creative freedom in construction projects [10].

Examples of live projects demonstrate the transformative potential of 3DCP in addressing real-world challenges [11]. Many companies, such as CyBe [12], SQ4D [13], PERI [14], COBOD [15], and Iston [16], have established themselves in the market by leveraging 3DCP for large-scale building production. While university labs continue to drive foundational research, creating a knowledge base that supports further innovations. As a pioneer in this field, ICON has completed several single-story structures using 3DCP [17]. Projects like Tecla by Wasp emphasize on-site production with local materials and demonstrate the adaptability of robotic fabrication to tailor structures to unique demands and locations. This project introduces a new form of housing produced entirely from reusable and recyclable materials derived from local soil, making it carbon-neutral and adaptable to any climate or setting [18]. These applications underline the versatility of 3DCP in diverse contexts, from large-scale housing developments to ecological construction solutions. Additionally, some studies have used 3DCP technology to produce prefabricated elements such as walls [19,20] and columns [10,21]. ETH Zurich’s Concrete Choreography project demonstrates the fabrication of complex building elements that deviate from traditional post-production building practices [3]. Despite these advances, the inherent material properties of concrete present challenges in constructing certain architectural forms, especially curved surfaces and completely closed geometries. Researchers are investigating solutions for holistic structures and the production of small building elements and units. Serendix, for example, completed the production of a shelter smaller than 10 m² in less than 24 h [22]. However, the production of smaller units offers more efficient logistics. As another example, the DE: Stress Pavilion project exemplifies the application of parametric design and robotic fabrication to create a pavilion with tailoring structural units with different dimensions and geometric forms, all produced using 3DCP [23]. Attempts to generate small-scale enclosed structural units are still being investigated as a separate area of research [24]. However, despite these advantages, the full integration of 3DCP into the construction industry still faces many challenges. These challenges span the technical, logistical, and legal domains and present limitations for the active use of new construction methodologies. Researchers are actively seeking to address these issues, focusing on seamlessly integrating 3DCP into construction processes. Within this research, four different thematic areas emerge as focal points of research: material detail, mechanical properties, printer properties, and construction planning properties [25].

Material detail research in 3DCP involves examining critical aspects that support the successful additive manufacturing of concrete structures [26,27,28]. This research domain covers a wide range of topics, including the exploration of sustainable alternatives [29,30,31,32,33] and integrating reinforcements to improve mechanical performance [34,35,36]. Further studies investigate various materials and mixtures suitable to optimize 3D printing applications [37,38,39]. The mechanical properties of 3D concrete print are another vital area, focusing on properties such as compressive strength [40,41,42], flexural strength [43,44,45], tensile strength [46], and fracture toughness [47,48], which are critical for the material’s structural integrity and performance. Researchers are exploring the effects of additives and reinforcement strategies to enhance these properties. The studies on printer properties primarily examine the technological aspects underlying the layered manufacturing process, including nozzles [49,50], extrusion speed [51], and overall machine efficiency. These studies aim to improve printing speed, precision, and efficiency, pushing the boundaries of 3DCP capabilities and expanding the applicability of the method to a broader range of architectural projects. Lastly, research on the design and planning aspects of additive manufacturing is centered around the construction plan. This includes examining the robot’s production path [52,53,54], production area [55], and support structure [56] to optimize material flow, enabling the precise execution of digital designs. These planning strategies are critical for enabling controlled and efficient fabrication workflows, especially when creating complex forms.

While approaches in 3DCP often focus on large-scale structures, this paper introduces a new approach by using a space-filling polyhedral, the Bisymmetric Hendecahedron, to explore its potential in modular construction. With the ability to seamlessly tessellate and fill spaces, the Bisymmetric Hendecahedron offers unique opportunities to create stackable units, prioritizing material reduction and spatial efficiency. The presented research aims to fill a gap in the 3DCP literature by shifting the focus from large-scale fabrication to the design and production of modular units optimized for material efficiency and geometric adaptability. By exploring the versatility of space-filling polyhedra, this research proposes a modular construction framework that meets the demands of contemporary architecture. The use of these architectural units enables a scalable and resource-efficient approach that balances structural performance and logistical flexibility, making it suitable for applications for adaptive architectural systems like temporary housing and emergency shelters. Furthermore, the shift from large-scale architectural elements to modular units highlights the importance of portability and rapid assembly in construction, addressing the challenges related to on-site customization and efficient resource utilization.

2. Methodology and Production Setup

The global construction industry is undergoing increasing complexity due to environmental factors and rising demands, resulting in a need for innovation and technological integration [57]. While existing literature often focuses on large-scale building units [10,21], this study aims to explore 3DCP technology for small-scale enclosed units, fostering sustainable construction solutions. The selection of unit geometry prioritizes space-filling 3D geometric forms and enhances units’ geometric scalability and flexibility. Concrete is chosen for its structural resilience and strength, which is crucial for closed units. To fabricate closed units with controlled inner voids, 3DCP was selected as the production technique to optimize construction processes through speed and precision. Designing lightweight units with efficient inner structural frameworks is essential for mobility and assembly simplicity. Following the geometry research, the internal structures of the building units are tailored for 3DCP. In the final phase, three proposals are produced using a 6-axis robotic arm and 3D printing to analyze geometric capabilities, feasibility, and production efficiency, addressing weight and scale concerns. The inclusion of this information improves understanding of the research methodology and progress.

The methodology designed for the research aims to investigate, through experimentation and analysis, the precise and efficient production of enclosed architectural units, exploring innovative and sustainable solutions in the building industry, with a particular focus on the potential of 3DCP technology. For this purpose, the project examines many aspects of the design and construction process of close, small-scale architectural elements. The process’s fundamental component is the choice of space-filling 3D geometric shapes, which promote the envisioned structural systems expansion in all directions and improve geometric flexibility and scalability. A polyhedron, Bisymmetric Hendecahedron, is selected as the fabrication geometry (Figure 1). As for the material, concrete is chosen for its structural strength and resistance, providing sound and thermal insulation with controlled internal voids. Within the framework of the research, using 3D robotic concrete printing for precise and rapid construction process design, attention was directed towards achieving an optimal balance between the design of the internal structures, material utilization, fabrication path length, and structural integrity, as it is important to fill the unit volumes in a controlled manner for portability and ease of assembly. The three prototypes of the unit were fabricated using a 6-axis robotic arm and 3DCP, iterating to improve efficiency. Throughout the production trials, iterative improvements were made to address challenges and increase efficiency. Experiments with varying sizes and internal support mechanisms, such as sand infill and central supports, were conducted to enhance the unit’s performance and viability, evaluate geometric capabilities, fabrication efficiency, and feasibility, and address scale and weight issues.

2.1. Choice of Material and Tool

To facilitate closed geometry production through 3DCP, a close collaboration with Iston was made to provide focus on material and robotic production support. The robot used to manufacture the units is the 6-axis Kuka KR 210-L150 robotic arm (Figure 2 and Figure 3), offering extended motion range and multi-axis traveling capability for freeform geometries. This versatility grants designers significant freedom. Additionally, 3D printable concrete properties, crucial for successful printing, include ease of mixing, pouring, and spreading, maintaining workability during printing, and post-printing resistance to external loads and environmental conditions without dimensional changes or cracking [27].

In this research, the articulated robotic arm was operated in a controlled environment to ensure precision in the experimental phase. However, site-specific applications will require adaptations for the transportation and installation of such robotic systems. Mobile platforms or crane-assisted installations can efficiently transport the robotic arm to construction sites. For large-scale field operations, gantry systems offer greater mobility and adaptability, supporting larger productions and ensuring seamless workflows [58]. Furthermore, the independence of the units produced by 3DCP as part of this research from each other and the production site allows for off-site production and the subsequent transportation of modules to the construction site, eliminating the need for robotic installations on-site. This flexibility, resulting from the practical scalability of the technology, is supported by modular production in the construction process.

Material plays an essential role in achieving successful results with the use of the 3D concrete printing technique. Evaluating the optimum mixture for 3D printing requires multiple tests [59]. Within the scope of this research, the material research was not conducted; it was provided by Iston as part of a collaborative agreement for research purposes. Therefore, the mixture properties, including the inclusion of polypropylene fibers and admixtures, were predetermined and were not subject to experimental optimization or variation within the scope of this study. The provided material is presented for the project based on its compatibility with the 3DCP technique and its ability to meet the overall performance criteria for structural stability and printability. Although the focus of the study did not include material formulation experiments, the fixed composition ensured consistency across all trials, allowing geometric and fabrication variables to be examined in a controlled approach.

Typically, concrete mixtures consist of binders, admixtures, fibers, and particles like aggregates like crushed stone, sand, or gravel, adjustable for specific product requirements [38]. Additives such as fly ash, slag, and micro silica enhance structural properties. The concrete mixture used in this research contains crushed sand with a maximum grain size of 0.3 mm, Ordinary Portland Cement (CEM|42.5 N) at a ratio of water to cement of 0.56, water-reducing admixture, and setting duration modifier admixture. The choice of sand with a maximum gain size of 0.3 mm has been made deliberately to optimize the 3D printing process. The fine sand provides smooth material flow through the nozzle, reducing blockages that can disrupt print continuity. Moreover, smaller grain sizes contribute to a more homogeneous mixture and improve the surface quality and precision of the printed layers. This is especially significant for achieving the required precision in the production of closed geometries with complex internal support structures. Additionally, polypropylene fibers are added to control and minimize the formation of cracks. Polypropylene fibers bridge microcracks during curing, increasing tensile strength, reducing shrinkage, and improving long-term durability. Material delivery to the robotic arm’s nozzle end effector is facilitated by a mortar pump, ensuring uninterrupted vertical structure and building with a printing resolution of 15 mm layer height and 30 mm/s printing speed. The printing speed of 30 mm/s was chosen to achieve a balance between layer adhesion and structural accuracy. The production environment and material recipe used in the research process, as well as slower production speeds, allow sufficient time for the material to adhere properly, minimizing the risk of delamination between layers. Furthermore, this speed ensures precise deposition, which is important for the successful production of complex geometries with internal voids. While faster speeds can increase productivity, they often require compromising the quality and stability of the printed structure. As for the pump, it operates independently from the robotic arm, necessitating design adjustments for continuous robot movement coordination [60].

2.2. Design of Infill Structures

In concrete 3D printing, material flow management differs significantly from plastic-based printing. Continuous flow necessitates seamless transitions between layers to avoid weak points, with challenges arising from the time-dependent curing of cement-based materials. Furthermore, due to the curing duration and fluidity of the material, internal support strategies are crucial in 3DCP productions. Despite complexities, ongoing research seeks to optimize infill structures and refine techniques for large-scale projects. Several of these studies utilize sand as an internal support strategy in fabrications [55,61,62]. As sand provides a larger support area than concrete internal support structures, it is an efficient alternative.

Within this research scope, various infill structure designs were explored for both whole- and half-units (Figure 4), considering factors like production path length, material utilization, production time, and support efficiency within the constraints of a 30 mm nozzle diameter and 15 mm layer thickness. After analysis and discussions, a centralized structure design was deemed optimal for both. Due to the 15 mm thick nozzle, trajectory calculations were necessary to prevent material accumulation in the designed infill structures (Figure 5).

Infill structures contribute to the stability of 3D-printed objects by providing internal support. The variety of infill patterns available in 3D printing gives designers and fabricators a wide range of options, allowing them to choose the most suitable infill structure for each project, such as grid, triangle, honeycomb, linear, centric, and gyroid are patterns frequently preferred by the 3D printing community [63,64,65]. The research aims to identify the most efficient infill pattern, with studies suggesting honeycomb for high tensile strength [66], while others propose linear patterns [67,68]. Moreover, the Archimedean chord [65] or centric patterns are noted for their mechanical properties [69]. There is a need for continuous print path-imposed constraints on infill structure design, with reference patterns like linear, honeycomb, grid, triangular grid, and central grid informing 3DCP. These patterns, chosen for their effectiveness in plastic-based printing, were pragmatically selected to leverage existing knowledge and insights. The linear grid pattern, incorporating parallel lines, facilitates a continuous flow in the printing process but increases production path length and unit weight while lacking sufficient structural support. The curvilinear pattern, departing from straight lines, offers an aesthetic alternative but risks material dragging and lacks support for upper surfaces and was thus excluded from this research. The square grid pattern, with intersecting lines, was considered for its simplicity and adaptability, yet its weight increase on convex surfaces deemed it impractical. The honeycomb pattern, known for structural efficiency, was adapted to the continuous print path requirement but shares limitations with the grid design. Similarly, the triangular grid pattern, balancing integrity and efficiency, was not used due to its contribution to weight and production path increase. Finally, central infill patterns radiating from a center point were found optimal, providing both structural integrity and aesthetic appeal for large-scale additive manufacturing (Figure 6).

3. Results

Throughout the fabrication, various adjustments in unit size, geometry, and internal support techniques were implemented to optimize production processes and address novel challenges. The overriding aims of geometric clarity, mobility, and structural integrity impacted decisions about unit size, support structures, and production characteristics. Despite obstacles, each fabrication experiment provided valuable insights into the approach’s limitations and strengths, influencing subsequent iterations and contributing to a broader understanding of 3DCP for architectural applications. This systematic approach to problem-solving and innovation exemplifies the experimental process undertaken in this research, seeking to pave the way for more efficient and sustainable construction practices in the future through novel design solutions and exploration of 3DCP’s potential. All production trials were conducted at the Iston fabrication workshop, utilizing its facilities and expertise. The first trial was executed on 21 June 2023, while the second and third trials were conducted on 8 January 2024.

3.1. First Trial: Exploring Geometry and Ensuring Structural Integrity

Prototype models for fabrication trials adhere to 3D printing principles, layering forms incrementally. Depending on geometric capability, forms are either directly placed on a suitable surface or supported by ground structures. In this research on fabricating the Bisymmetric Hendecahedron via 3DCP, surface selection prioritizes production efficiency. To avoid the need for external supports, a lightweight, easy-to-assemble half-unit is produced, utilizing a production surface alternative (Figure 7), dividing the unit into two symmetrical pieces, and using the cross-sectional surface as the production surface simplifies fabrication. However, joining the two half-units poses challenges, risking structural weakening. To address this, the cross-sectional surface remains unfilled, aiding unit connection and reinforcing structural performance. Using a 30 mm nozzle diameter and 15 mm layer height ensures manageable unit weight and geometric clarity. The digital model is converted to additive manufacturing code using SprutCAM.

The unit was successfully fabricated without deformation (Figure 8 and Figure 9), but its weight posed challenges for single-person assembly due to the lengthy 21 m production path, exceeding expectations at 20 kg. Additionally, the resolution was insufficient, with a 30 mm nozzle radius and 15 mm layer height, resulting in a lack of clarity. The infill structure occupied most of the inner volume, hindering the desired solidity (Figure 10). Proposed solutions included reducing nozzle radius or infill path length or scaling up, but limitations on nozzle size constrained adjustments. The increasing scale was deemed optimal to achieve a clearer geometry and a more balanced void-to-solid ratio within the inner volume. However, scaling up would reduce portability and assembly efficiency, requiring a redesign for the second trial to address these challenges. Based on the post-trial analysis, the unit was redesigned for the second production trial to optimize performance.

3.2. Second Trial: Scaling up and Addressing Mobility Challenges

Based on observations from the initial trial, a new prototype model was designed to overcome manufacturing limitations, utilizing the same 30 mm radius nozzle and 15 mm layer height. Unlike the first prototype, the geometry was extensively re-evaluated, and a complete unit was fabricated. This trial involved positioning the model in an 80 × 80 × 60 cm bounding box, with one of the largest surfaces placed on the ground, necessitating an assembly logistics plan. Regarding the need for external support for the two surfaces of the unit, two wooden mold structures were crafted to match the surface angles of the geometry precisely (Figure 11). While these support structures were intended to prevent the prototype from collapsing during fabrication, concerns were raised about their positioning and potential obstructions to the fabrication process at the intersection with the nozzle. To enhance insulation, the unit’s meeting surface with the fabrication area was left open except for infill structures, facilitating accessibility to the inner volume. This open surface also allowed for the installation of structural reinforcements, augmenting load-carrying capacity and connections between units. Various infill designs were explored, with a central infill design ultimately chosen for its optimal balance of structural support, production path length, and weight. This design provided internal support to each unit surface while creating space to reinforce the top surfaces through its central branches, shortening the manufacturing route length. The form was then converted to production code using SprutCAM.

The larger-scale production trial marked a notable improvement in geometric clarity, as the increased layers contributed to a more discernible form. Despite this advancement, the upsizing of the unit resulted in longer production times and paths, leading to a substantial increase in the production path length, measuring 197 m, and a corresponding weight escalation to 186 kg. While the external support structures effectively prevented material flow disruptions during fabrication, there were instances of distortion along inner surfaces due to insufficient support. This highlighted the need for more comprehensive support mechanisms, particularly in areas prone to deformation (Figure 12).

Moreover, the resolution of the form significantly improved at the new scale, maintaining success in fabricating all edges and surfaces, except the closed top surfaces and their meeting edges (Figure 13). This enhancement in geometric quality, although at the expense of unit efficiency in terms of weight, underscored the importance of balancing production considerations with geometric precision. Looking ahead to future trials, it becomes imperative to devise a fabrication strategy that ensures adequate support for every surface of the unit while maintaining the current scale comprehensively.

Several potential solutions were explored to address the challenge of preventing deformation on the top surfaces of the unit. These included augmenting the branching of the infill structure, enhancing adhesive properties, and accelerating curing speed through material recipe modifications. However, altering the unit’s orientation, fabricating it in two parts, or implementing a holistic alternative for the inner structural system was also considered. Each option presented its own set of advantages and limitations, with careful consideration required to strike the optimal balance between geometric precision and production efficiency.

Dividing the unit into two parts and supporting it with sand filling in the inner volume emerged as a promising solution. This approach not only addressed weight concerns but also facilitated more efficient fabrication. However, the convexity of all unit surfaces during fabrication posed a challenge in terms of blocking the fabrication path with the support mold. To mitigate this issue, replacing the additional mold with sand fill in the inner volume was proposed, albeit with the caveat of potential load increase on convex surfaces. Nonetheless, it was deemed the most viable solution for realizing an efficient prototype fabrication, resulting in lighter and more manageable units compared to previous trials.

3.3. Third Trial: Sand Filling for Lightweight Units

In the third prototype trial, production was designed as a half-unit with a 40 × 80 × 60 cm bounding box filled with sand as an infill structure. Half-unit construction eliminates the need for external support while using sand for inner support, which prevents the need for any infill structure (Figure 14). This approach allows the production of a lighter unit with less fabrication path length. The particle structure of the sand infill is crucial due to the cohesive nature of the material, so fine-grained sand was preferred. To increase the unit’s resistance against horizontal and vertical pressures, the unit’s perimeter will be fabricated as two rows. This method is expected to make the unit more robust against external forces.

By the end of the production process, the sand-filled prototype geometry was successfully fabricated without collapse, achieving high-quality surface geometry. Its weight, in a 40 × 80 × 60 cm bounding box, reduced by 59.1% to 76 kg compared to the previous trial, with a shorter production path of 79 m. The sand infill was gradually added as the internal support, ensuring each layer was adequately filled. However, uneven distribution and sand accumulation occurred due to filling the sand manually (Figure 15). While this did not impair material adhesion or hinder production, it affected some surfaces. The inability to add infill in the last four layers due to the reduced production path caused uneven material accumulation and hindered top-edge production.

Ultimately, the third trial confirmed sand filling as the most effective method for inner support despite uneven distribution deforming the unit’s form (Figure 16). Solutions include acquiring a continuous sand addition system for proportional support or reinforcing sand filling to support the top edge. The former requires specialized tooling, while the latter may increase unit weight. Re-evaluating unit scale or exploring changes in nozzle diameter or material composition could address weight concerns.

3.4. Reflection and Comparison of the Three Trials

The fabrication experiments demonstrated the intricate relationship between geometric design and manufacturing technique in producing the intended structural and aesthetic results. The iterative nature of the fabrication process has underlined the significance of continuous optimization and refinement to confront emerging obstacles and capitalize on possibilities for advancement. The geometric complexity presented significant challenges during fabrication, particularly in ensuring surface alignment and sufficient support, as demonstrated by the trials. The accuracy of 3D-printed elements in this study was evaluated through geometric resolution and structural alignment in three production trials. Geometric precision was optimized for layer height and nozzle dimensions to balance clarity and structural integrity. Alignment was aimed to be maintained during manufacturing by designing support structures and minimizing surface deformations. Although the prototypes did not achieve perfect accuracy, the iterative process demonstrated that complex geometries, such as the Bisymmetric Hendecahedron, can be produced using 3DCP. These experiments present an important step towards refining the technique and achieving higher levels of accuracy. Throughout the prototype production process, a variety of challenges arise, ranging from geometric resolution issues to structural integrity concerns and mobility. However, these challenges present opportunities for innovative thinking and creative problem-solving. The investigation of alternate forms of support, such as sand filling, demonstrates a motivation experiment with novel approaches, emphasizing the dynamic aspect of the research process.

To ensure feasible production across all three trials, factors such as scale, geometry, and infill support were reassessed based on insights gained from the preceding processes (Figure 17). The first trial revealed challenges in fabricating closed-shaped architectural units using 3DCP, particularly in ensuring geometric clarity and structural integrity. The production path length for the first trial was 21 m, resulting in a 20 kg unit with poor geometric resolution due to printing parameter constraints and design complexities. These limitations underscored the need for resolution and weight optimization improvements, including exploring alternative support structures and refining printing parameters. In the second trial, designed with the knowledge gained from these findings, progress was made in geometric legibility. However, this improvement led to an increase in the weight of the unit to 186 kg and the length of the production path to 197 m. Furthermore, challenges remain in material flow management and support structure optimization. Concerns about weight and assembly efficiency highlighted the need for further optimization. In the third trial, the sand infill was introduced as a support strategy, reducing the unit’s weight to 76 kg and decreasing the production path length to 79 m. Despite the satisfactory achievement of the surface geometry, difficulties arose with the support distribution and filling homogeneity. The sand-filling technique showed promise for future iterations, emphasizing the importance of ongoing improvement and optimization. The insights gained from these trials have significant implications for future research in 3DCP for architectural applications, highlighting the need for continued exploration and refinement of fabrication techniques, support structures, and material formulations to unlock the technology’s full potential. Interdisciplinary collaboration will be crucial in driving innovation and advancing the field of 3DCPin architecture.

The iterative prototype productions conducted in this research revealed various key lessons about the challenges and opportunities associated with the fabrication of enclosed architectural units using 3DCP. Each trial builds on the findings of the previous one, demonstrating the interaction between geometric design and fabrication techniques. The first trial served as an initial exploration into the feasibility of fabricating the Bisymmetric Hendecahedron geometry. While the unit was successfully fabricated without failure, problems with geometric resolution emerged. The chosen infill structure occupied most of the internal volume and reduced material efficiency. Additionally, insufficient resolution reduced the clarity of the geometry. These problems increased from the unbalanced nozzle size and geometric scale interaction. Based on these findings, the second trial’s design and fabrication process were improved to address the weaknesses identified in the first trial. The scale of the unit increased to improve geometric resolution, resulting in clearer edges and surfaces. Also, unlike the first trial, the complete geometry was produced as the unit fabricated with external support structures. However, the large size of the unit reduced its portability and caused difficulties in the assembly strategy. Moreover, deformation in areas where internal support was insufficient required more stable and evenly distributed support strategies. In the third trial, the key lessons from the previous trials were used to develop a lighter and more efficient manufacturing strategy. The sand infill eliminated the need for infill structures and external supports. However, the uneven spread of sand created new challenges in achieving uniform support, mainly for the last several layers. Despite these limitations, the trial demonstrated the potential of sand filling to optimize both geometric precision and structural performance in 3DCP units.

4. Discussion

This presented research seeks to critically evaluate how 3D concrete printing (3DCP) technologies adapt to the traditional construction sector while exploring alternative methods for addressing and questioning this adaptation. One key aspect of this evaluation is understanding how new technologies like 3DCP find their place within traditional building practices, requiring ongoing refinement of printing techniques, material compositions, and equipment capabilities. Moreover, this research explores how space-filling geometries, particularly the Bisymmetric Hendecahedron, enable modular units optimized for structural efficiency, scalability, and sustainability.

3DCP has been referred to as an innovative technology in construction, enabling the production of complex geometries that were previously unattainable or too expensive using traditional methods. Compared to conventional construction, 3DCP offers flexibility in design modification and integration with other construction systems by eliminating the need for formwork or additional reinforcement during production. Traditional casting methods often prefer simple designs due to cost and material limitations, allowing limited flexibility in architectural forms and minimizing financial and logistical needs. However, projects without cost constraints require the use of unique molds to produce complex forms, leading to material waste and construction delays. In contrast, 3DCP enables the production of complex forms at a lower cost by significantly reducing waste and accelerating production [38,70]. Another advantage of 3DCP is its geometric versatility, which improves the performance of structural elements. Optimized geometries can improve energy efficiency by increasing thermal resistance by 30–40% compared to conventional methods [70,71]. In addition, 3DCP can reduce water consumption during construction, potentially saving up to 20% compared to conventional methods [72]. These benefits are supported by the fact that 3DCP has the potential to reduce construction durations, making it possible to construct small-scale productions in a single day compared to five to ten days using manual methods [38]. Compared to manual methods, 3DCP offers significant advantages. The precision and ability to produce geometrically complex structures without the need for molds ensure long-term efficiency [73]. While the setup and calibration needed for robotic arms and material transport systems may initially cause delays, the ability to automate repetitive tasks and quickly integrate design changes makes 3DCP faster for scenarios requiring customization or scalability. 3DCP eliminates the long drying time for large material batches required in conventional concrete molding processes, as drying continues throughout the continuous layering process. Moreover, the absence of the need for molds reduces the costs associated with material supply and storage, especially in large-scale production [11]. By eliminating conventional molds and associated labor-demanding processes, 3DCP is seen as a time-efficient production strategy [27]. In this context, a comparison of 3DCP and manual casting methods in terms of their efficiency, suitability, and potential for integration into contemporary construction workflows suggests that the 3DCP method is more efficient (

Table 1. Comparison of two production methods.

Production Method	Setup Time	Production Time	Drying Time	Material Efficiency	Cost	Complex Geometry Predictability
3D Concrete Printing	4	5	5	5	3	5
Manual Casting	2	2	3	3	4	2

1 = poor; 2 = below average; 3 = Average; 4 = above average; 5 = excellent.

).

Understanding the needs of the building sector is paramount in driving the adoption of 3DCP technologies [74]. These needs include increased efficiency, reduced labor requirements, enhanced design flexibility, and improved sustainability. By addressing these requirements through innovative designs and material optimization, this research demonstrates the feasibility of integrating modular 3DCP units into mainstream construction.

The results of this study also reveal the trade-off between increased unit size for higher structural clarity and resolution and the associated challenges of reduced portability and assembly efficiency. Larger units demonstrate better geometric accuracy and stability, but their increased weight and size create more complex transportation and in situ handling needs. As a response to these challenges, the outcome of this research provides a foundation for the availability of modular assembly techniques, where smaller and lightweight units are produced and assembled in situ to achieve the desired architectural scale. The modular construction approach not only increases portability but also offers flexibility in assembly configurations and functional requirements. The tessellation of space-filling geometries provides seamless stacking and interlocking, minimizing the need for additional fasteners or supports.

The comparison of central infill pattern and sand-infill techniques reveals distinct advantages and limitations, highlighting the importance of selecting the appropriate support strategy based on specific project requirements. The investigation of two specific internal support strategies, central infill patterns, and sand-fill techniques, offers a valuable contribution to balance structural stability and material efficiency by eliminating the need for additional reinforcing elements while maintaining structural integrity. Central infill patterns would provide load-bearing capacity while minimizing material usage, whereas sand-filling techniques stabilize lightweight units. This approach enables the production of lightweight yet flexible units while reducing overall material consumption and aligning with sustainable building practices. The interpretative comparison of these strategies reveals that central infills are particularly effective for permanent modular structures, while sand-filling techniques are better suited for temporary installations that require rapid deployment (

Table 2. Comparison of two support strategies.

Support Strategy	Advantages	Disadvantages	Application Scenarios
Central Infill	High stability, optimized material use	High cost, longer production path	Permanent structures with high load-bearing capacity
Sand Infill	Cost-effective, lightweight, easy to deploy	Not suitable for long-term use, low stability	Temporal housing, emergency shelters

).

Moreover, 3DCP’s ability to customize architectural elements and integrate functional features offers the potential to transform the construction sector. The adaptation process itself is multifaceted, requiring not only technological advancements but also a shift in industry mindset and skill development. Strategies such as pilot projects and educational initiatives can facilitate this process, fostering collaboration among stakeholders and promoting a culture of innovation. As an example, prototype trials conducted in this research emphasized iterative improvements, refining the design of modular units to increase portability, assembly simplicity, and structural feasibility.

Assessing the cost-effectiveness of implementing 3DCP technology in architecture involves weighing initial investment costs against long-term benefits such as reduced labor, material efficiency, and faster construction timelines. However, challenges like scalability, maintenance, material costs, and regulatory compliance remain critical. Future advancements in technology, materials, and process optimization are essential for enhancing cost-effectiveness and ensuring widespread adoption in the construction industry. Articulated robotic arms demonstrate high precision and flexibility in experimental setups. These robotic arms are useful for performing complex movements under controlled conditions and maintaining geometric accuracy during production. However, their efficiency is limited by workspace size and installation requirements. For larger scale or on-site applications, alternative devices such as gantry 3D printers offer better efficiency. Gantry systems accommodate larger production areas and reduce the need for repositioning during production, making them well-suited for large projects [58].

Ultimately, this research showcases how integrating advanced geometric design and additive manufacturing techniques can streamline construction workflows, reduce material waste, and accelerate project timelines. The ability to produce complex geometries without molds enhances sustainability while improving architectural challenges like adaptability and rapid assembly. These insights contribute significantly to advancing 3DCP technologies as a transformative instrument in architecture and construction. As future research progresses, the use of lightweight concrete and optimized infill structures may enable unit weight and size reduction without compromising structural integrity. The use of filling materials for inner support, like sand, would allow the units to minimize material use while maintaining their load-bearing capacity. This is particularly advantageous in applications where rapid production and assembly are critical, such as disaster aid or any other temporary housing [75].

5. Conclusions

Recent advances in 3D concrete printing research are increasingly focused on modular units and innovative geometries to enhance architectural adaptability, material efficiency, and structural robustness. This research explores the usability and fabrication of modular, enclosed architectural units based on a Bisymmetric Hendecahedron, with space-filling properties that enable seamless tessellation, optimizing spatial efficiency and minimizing material use, aligning with applications requiring portability, scalability, and rapid assembly. These features are advantageous in scenarios such as temporary housing, adaptable partition systems, and emergency shelter construction.

The trials presented in this research aim to encourage 3DCP as an efficient alternative to conventional construction techniques. While conventional methods are cost-effective for basic structural requirements, they often lack the flexibility required for customized designs. In contrast, the versatility of 3DCP allows for the production of customized architectural elements while minimizing labor use and material waste [76]. Furthermore, traditional methods often rely on a significant amount of labor and prefabrication logistics that 3DCP does not require due to its on-site manufacturing capabilities. While capital costs for 3DCP equipment are higher, long-term benefits such as shortened construction times and improved adaptability make these methods viable for a variety of architectural designs [11].

This research investigates two specific internal support strategies, sand-infill and central infill pattern, to increase load-bearing capacity without conventional reinforcements. These techniques aim to balance structural stability and material efficiency, enabling lightweight but rigid units while reducing overall material consumption. The three iterative production trials were conducted to refine the design and evaluate the units’ performance, scalability, and material efficiency, providing valuable insights into the application of 3DCP for modular architectural systems.

Through the optimization of the design and manufacturing process, this study demonstrates the potential of 3DCP technology in producing efficient, precise, and scalable modular units. The selection of concrete for its structural strength and resistance to external forces, combined with innovative geometric designs, facilitates high-precision construction processes. Furthermore, the robotic 3DCP production was strategically used to realize the infill system to be added to support the structural strength of the inner volumes of the envisioned closed units. Unit geometry selection is crucial in enhancing flexibility and scalability, allowing units to grow in multiple directions. During the design phase, unit scale and weight considerations were emphasized to increase portability and assembly simplicity. Infill structure optimization further ensures the optimum balance between material utilization and unit weight and ensures both structural stability and mobility.

Three prototype production trials were conducted with iterative improvements throughout the research process to address the challenges and increase efficiency. Experiments with different unit sizes and internal support systems, including central supports and sand fill, aimed to enhance the feasibility and performance of the units. In light of the research objectives, the comparison of trials reveals a progressive evolution in design and technique and in advancing the state-of-the-art 3DCP technology. The first trial serves as the foundational exploration, establishing fundamental principles and focusing on basic geometric configurations and material compositions. Building upon the insights gained from the first trial, the second trial introduced refinements in structural supports and material flow, addressing identified challenges. The third trial finalized the integration of advanced infill support by using sand. Ultimately, progressive iterations in design and technique contribute to the state-of-the-art advancements in 3DCP.

The findings of this study demonstrate the potential of 3DCP to address architectural and construction challenges through geometric and support strategies. Central infill patterns would be used for permanent modular housing with their structural stability and scalable architectural units in urban developments. On the other hand, the sand-fill technique could offer a practical solution for temporary or emergency scenarios, allowing for rapid deployment. These approaches highlight 3DCP’s versatility in creating resource-efficient and adaptable solutions, paving the way for future applications like prefabricated components and on-site eco-housing. By leveraging insights gained from this research, practitioners can refine design parameters, select appropriate materials, and implement efficient fabrication techniques to improve the quality and performance of printed structures. The results of this research extend beyond academia to practical applications in architecture, engineering, and construction, laying the groundwork for future advances in sustainable modular systems.