The research presented in this paper, motivated by the rapid growth in large-scale 3D concrete printing (3DCP), addresses the current lack of both design tools and integrated design-to-production solutions for this fabrication technology. It is guided by a novel insight regarding the applicability of design and analysis methods developed for unreinforced masonry to large-scale, layered 3D printing with materials favouring compression such as concrete [4, 5, 25]. The paper details a custom toolchain that enabled the integration of shape design, structural engineering, and robotic concrete printing for the design, production and construction of ProjectName - a discrete, dry-assembled, fully unreinforced, bifurcating, arched masonry footbridge composed of 53 3D-concrete-printed blocks and spanning 16 metres.(Figures 1 and 2)
Promise of 3DCP and the masonry design paradigm
The positive aspects of concrete as a construction material include its low cost, ready availability, fire resistance, thermal mass, compressive strength, longevity and low embodied energy [2] and emissions [19] per unit mass. 3D Concrete Printing is generally anticipated to ameliorate the negative aspects of using concrete in construction including labour intensiveness, adverse effects of worker health and safety, excessive wastage due to the casting process, and significant carbon emissions when used in bulk [12, 14, 26]. It is widely recognised that, unlike desktop printing that admits a wide range of arbitrary shapes, only specific types of geometries that adhere to the constraints imposed by large-format 3DCP and incorporate the layer-by-layer deposition of linear filaments in the creation of the shapes, can be printed. Furthermore, the careful design of such shapes is critical to fully deliver the aforementioned benefits of 3DCP [12, 46].
The unreinforced masonry design paradigm and associated design and analysis techniques can specifically meet these requirements and are highly compatible with the compression-dominant, orthotropic material properties of layered 3DCP [5, 25]. Alignment of the printed layers orthogonal to expected compressive force flows engages the compressive strength of 3DCP whilst eliminating the need for tensile reinforcement [5, 7].
Furthermore, the wider benefits of structural geometry and the masonry paradigm to improve recyclability, and the repair and reuse of material and structural components due to dry assembly and clean separation of tensile and compressive materials have also been recently highlighted [10].
In summary, adopting an unreinforced masonry paradigm for the design of 3DCP structures can make it possible to
-
1.
Reduce the amount of concrete used by allowing precise placement of concrete only and precisely where needed along the compressive flow of forces, which additionally reduces the stresses significantly [10].
-
2.
Reduce the amount of steel needed by reducing tensile and flexural strength requirements through a compression-appropriate design of both the global, shape and the block discretization. Interestingly, the rate of carbonation of concrete is inversely proportional to the compressive strength of concrete. Therefore, low-strength concrete, as enabled by the funicular design, is more likely to be fully carbonated and, thus, reabsorb carbondioxide during the lifetime of the structure [34].
-
3.
Repair structures more easily as the separation of concrete and steel allow for straightforward maintenance strategies. A major advantage of the masonry logic is that both structural action (compression versus tension) and the corresponding materials are separated. This offers a maintenance and repair strategy whereby all elements can be easily and separately accessed and inspected. The tension ties can be isolated and directly replaced. Furthermore, the lack of embedded reinforcement in the unreinforced concrete blocks means that corrosion of reinforcement and related long-term deterioration of the structure can be avoided [43, 45]. Importantly, any local damage can be isolated to a specific block, which can be reprinted and replaced. This could be done by propping the structure to relieve the thrust in the arch. Once the new part is placed, the arch can be reactivated. It can be noted that complete material failure and crushing of individual blocks is extremely unlikely and the non-fatal damages to be considered could be local crushing or cracking due to differential settlement of the foundations, or local damage due to impact such as by a vehicle.
-
4.
Reuse components easily, due to the dry-assembled construction, glue-free connections, and thus non-destructive disassembly that masonry structures allow,
-
5.
Recycle material easily and with low energy consumption due to both separation of materials and easy disassembly. Typical recycling of reinforced concrete involves the use of jaw and impact crushers that lead to increased powder by-products, reduced strength and quality of recycled aggregates that then have to be used in down-cycled applications such as road bottoming [20, 42]. Higher quality recycled aggregates and repeatable recycling, are two important parameters in achieving full, closed-loop recycling of concrete, similar to steel and plastics. This requires newer, more refined machines and processes [42]. Separation of materials by design, and thus the lack of embedded steel reinforcement in the concrete blocks is aligned with both these features of closed-loop recycling. On the other hand, dry assembly means discrete blocks can be dismantled with minimal falsework and moved to grinding stations without creating excessive dust. Importantly, dry assembly can lead to an ideal, so-called integrated inverse manufacturing that balances the workload in the construction and disassembly phases [42] (Tomosawa et al, 2005). Integrated inverse manufacturing is considered important to achieve closed-product lifecycles [32]. Lastly, absence of chemical bonding means recycling does not have to contend with material contamination and related complexities [20].
Lack of integrated design-to-production toolkits
To achieve the specific geometries that deliver the benefits of 3DCP, development of computer-aided-design (CAD) tools and design-to-production (DTP) solutions are needed. However, whilst potential features of a CAD pipeline are often discussed, attention is usually only to devoted to material and process aspects of 3DCP. Furthermore, even when shape-design related descriptions are given, they are typically restricted to simple geometries and practical CAD implementation details are absent. This is particularly so for non-parallel, inclined-plane printing [12, 17, 18, 30].
Both early pioneers and recent researchers have emphasised the relevance of the masonry-based design paradigm to address the critical, but often ignored need for a 3DCP-specific, integrated DTP toolkit. In particular, shape-design and analysis methods used for masonry structures along with recent advances in computational masonry and associated geometry processing methods can be combined to create a toolkit.
Furthermore, such a toolkit could provide
-
1.
expressiveness of geometric modelling for designers whilst also being didactic regarding structural and process parameters,
-
2.
possibilities of a rich variety of 3DCP-compatible shapes,
-
3.
constructive guidance about the feasibility constraints imposed by the 3DCP process, and
-
4.
methods to align inclined layers of material filaments orthogonal to compressive forces; [4, 7, 25]
Key contributions
This paper builds on the relevance of the computational masonry paradigm to both delivering the ecological, economical and productivity promises of 3DCP and to the development of a 3DCP-specific, DTP toolkit. Furthermore, the paper focuses attention on the hitherto ignored, but critically necessary aspects of computational design and practical implementation details of a CAD workflow, as described previously.
The main contribution of the paper is the development of a toolchain that enabled the design of an unreinforced, masonry bridge. The project that demonstrated both the concept and the toolchain, was physically realised by the dry assembly of 3DCP blocks that have each of their individual layers of printed concrete aligned orthogonal to the expected dominant compressive force flow. Specifically, the custom toolchain and the constituent, standalone applets enabled
-
the use of the unreinforced masonry paradigm for the computational design of a 3DCP bridge, which is in contrast to both the paradigm and methods of design currently being used in practice and seen in recent examples of 3DCP bridge structures;
-
the integration of user-guided, expressive shape-design, structural engineering, and robotic concrete printing;
-
synthesis of force-aligned, continuously varying, inclined-plane print paths and the generation of robot instructions; and
-
rapid iteration, refinement and collaboration. The novel, complex-geometry, large-scale bridge demonstrator was fully designed, coordinated between remotely located teams and produced on site in less than 6 months.
Limitations of scope
The toolkit is specific to the prefabrication paradigm offered by industrial robotic-arm-based, 6-degree-of-freedom (6-DOF) printing machines. Further, we assume the use of so-called two-component (2K) concrete formulations and printing setups to print along continuously varied inclinations and thicknesses.
We implemented the toolchain using a combination of custom C++ applications and the Python-based open-source framework COMPAS [44].
It can be noted that whilst rigorous computational structural design, verification and approval reports were needed for the realisation of the bridge, the detailed description of those aspects is beyond the focus and scope of this paper. However, a broad overview of the computational design and analysis of unreinforced masonry structures adapted to 3D-concrete-printed masonry bridge design is provided. This includes global form finding and discrete-element modelling for evaluation of structural mechanics and stability. Furthermore, COMPAS already implements all the masonry-related, structural design algorithms as used and outlined in this paper and prior work (Section “Prior work”). Similarly, details regarding material mix and printing processes receive only a cursory description.