In the field of Digital Fabrication with Concrete (DFC), to date in most publications and pilot projects the greatest attention has been focused on concrete shaping processes, especially on Additive Manufacturing (AM) approaches, also called 3D Concrete Printing (3DCP), while the solutions for incorporating reinforcement are still rudimentary in many instances. This article offers a comprehensive analysis of the existing solutions for integrating reinforcement in digital concrete technologies into DFC with particular emphasis on 3DCP. It is based on the resent publication by a group of international experts in this field . The major focus is on the technological aspects. A generic classification and process description outline was developed for reinforcement integration, which is regarded as an extension of the RILEM process classification framework for Digital Fabrication with Concrete .
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Viktor Mechtcherine is Director of the Institute for Construction Mate- rials and Professor at the TU Dresden, Germany. He is Coordinator of the German Research Foundation (DFG) Priority Program SPP 2005 „Opus Fluidum Futurum – Rheology of reactive, multiscale, multi-phase construction materials“ and Speaker of the DFG Research Training Group GRK 2250 „Mineral-bonded composites for enhanced structural impact safety“. Prof. Mechtcherine is Editor of the Journals “Cement and Concrete Composites” and “Materials and Structures”, Member of the Science Academy of Saxony and the Russian Engineering Academy, Chair of RILEM TC ADC “Assessment of Digitally Fabricated Concrete Materials and Structures” and Chair of the WG “Digital concrete construction by additive manufacturing” and Member of the Research Advisory Board of the German Committee for Reinforced Concrete (DAfStb). He is RILEM Fellow and awardee of Wason Medal for Materials Research by American Concrete Institute (ACI) and Innovation Award of the bauma 2016 Munich.
Role of reinforcement
For most applications of concrete parts, elements, and structures reinforcement is indispensable in attaining the required mechanical performance. While key features of concrete with respect to its use in construction are compressive strength and durability, the main functions of reinforcing material are to carry tensile forces and to impart structural ductility. The various reinforcements differ in terms of their capacity to carry tensile forces and their direction of action; see Figure 1. From the technological perspective in the overwhelming majority of existing approaches and applications, concrete processes provide for the shape of a manufactured element independently if a formative shaping process such as casting or additive shaping process like, e.g., material extrusion is used. Only in rare cases does the arrangement of reinforcement create scaffolding for concrete deposition and “dictate” the shape of the element. Thus, the key question with respect to manufacturing with reinforced concrete is how to integrate reinforcement into concrete with minimum interference in the concrete shaping process.
Not every type of existing reinforcement integration in 3DCP can be used for static-constructive purposes. In this respect material characteristics, geometric dimensions, installation position, the bond with concrete, durability, etc. are decisive. When using steel, it is possible to fall back on well-established material parameters. The material characteristics of yarns and textiles made of carbon, glass or basalt fibres are subject to greater scattering as these are composites whose behaviour depends on the materials used for impregnation. Furthermore, the bond between the reinforcement and the concrete matrix is crucial to the effectiveness of the reinforcement. The bond is significantly influenced by the form fit using the surface condition of the reinforcement and its complete embedding into the matrix.
By using different types of reinforcement, the failure form of the components can be influenced. However, while the failure behaviour of reinforcement ranges from brittle, abrupt failure as, for example, in the case of carbon reinforcements, to good-natured, slow-onset failure as with steel reinforcements. Failure on the composite and component levels is affected by a number of additional parameters such as the degree of reinforcement and the geometry of the component. The ductility of components can be increased by the additional use of short fibre reinforcement.
Process-specific characteristics of reinforcement integration
While the structural functionalities of reinforcement are critical to structural design, they are indirectly relevant also in manufacturing processes. The choice of material and the position of reinforcement in an element to be printed certainly affects possible integration scenarios for reinforcement and associated process characteristics. Establishing the links between structural design and technological implementation is certainly essential from a general perspective. However, detailed deliberations are beyond the scope of this paper. Thus, the process characteristics are presented predominantly from the technological perspective.
Continuity of reinforcement
The continuity of the reinforcing entities is of elementary importance to the global load-bearing capacity of a structural element or of an entire structure. In particular, achieving continuity of reinforcement either orthogonal or inclined to the deposited concrete layers represents a major challenge. Reinforcement strategies offering such reinforcement arrangements can – in addition to providing the necessary vertical reinforcement – also improve cross-layer force transfer and so make for less anisotropic mechanical behaviour.
Starting from the automation of the concrete shaping process, the automation of reinforcement integration is essential to the enabling of seamless digital fabrication in the future. To arrange the respective reinforcement elements in an automated process, the automation capability assesses the process engineering effort required. The automation of approaches in a single process step together with the shaping of the concrete seems particularly demanding.
The geometric freedom that additive concrete application allows can be limited by the choice of reinforcement technique. Thus, the technology of reinforcement integration exhibits some restrictions with respect to both structural and architectural design.
An important criterion both for the technical applicability and economic viability of any reinforcement technique is the achievable process speed. This is particularly relevant for single step processes in which reinforcement is introduced simultaneously or congruously to the shaping of the concrete. In such cases an insufficient process speed or insufficient ease of the process – both together defining the overall process speed – can delay the concrete printing process, possibly leading to long time intervals between subsequent layers, which may result in insufficient interlayer bonds, generally leading to an undesired slowdown of the entire AM process. However, also in two-step processes, i.e., where the reinforcing process is decoupled from the concrete shaping, the production efficiency is of high relevance.
Robustness of the process
Various approaches to reinforcement implementation impose various levels of technical sophistication when being implemented. Generally, the robustness of a process tends to decrease with an increasing level of complexity, e.g., the number of necessary process sub-steps or high requirement on precision in timing or positioning. Some types of reinforcement require additional installation aids during their integration. These temporarily used devices increase the complexity of automation and in turn have an influence on the robustness of the entire process, increasing the infrastructural requirements. Additionally, the robustness of the reinforcement material as such plays a role with respect to range of its handling scenarios.
Technological maturity level
The technological maturity level is not really a process-specific characteristic, but rather the indicator of the current state of development for the given reinforcement type. This indicator is supposed to express the effort required to implement a new technology successfully, here a reinforcement approach. The assessment is based on the so-called Technology Readiness Level (TRL), which is defined in nine levels and ranges from “the observation and description of the functional principle" to "a qualified system with proof of successful use"; see, for example, .
Indeed, the quantification or at least comprehensive qualitative description of the process specific characteristics listed above are crucial for a comparative assessment of various approaches to incorporating reinforcement into digital fabrication with concrete. In the following review, however, the authors will not be particular in respect of these characteristics due to the very limited amount of available qualitative and quantitative information as yet.
The classification framework and process description
The classification framework suggested in  focuses on the processes for integration of reinforcement into DFC, and it is designed as an extension of the RILEM process classification framework for DFC technologies . The RILEM framework is an over-arching scheme that helps to define, describe, and classify DFC processes using well-defined intersection terms in interdisciplinary field where construction meets manufacturing and automation; see Figure 2.
It is important to note that the RILEM framework in many instances provides description of a single process step. While DFC uses many different manufacturing operations methods and approaches to shape the material into the form it maintains in its hardened state, often more than one process step is required to manufacture an end-use product such as a structural element. It holds especially true with respect to the integration of reinforcement. Identifying these steps helps to define boundaries and so helps in clearly defining the principal operations involved in a process. Thus, the classification considers the sequence of distinct processes according to the manufacturing timeline of product as a starting point; see Figure 3. While building upon the RILEM framework, the classification also provides a link to structural design issues by naming corresponding options for the choice of reinforcement according to the following primary categories: cage, mesh/textile, bar, cable/yarn, and short fibre. Indeed, the proposed classification begins right there where the RILEM framework ends, i.e., at the level of the DFC process subclass for shaping concrete either additively, i.e., particle bed binding, material extrusion, material jetting, or formatively, i.e., solidification, deformation; cf. Figure 2.
For Additive Manufacturing methods with concrete, reinforcement can be integrated within a single process step as a sub-process occurring during concrete shaping. This is not feasible for formative processes. However, the integration of reinforcement prior to or after concrete shaping, i.e., in a separate step, can be performed both with additive and formative concrete processes in a similar manner. These options are indicated in the classification as two-step processes. Additionally, concrete mixing is defined as a pre-process preceding any concrete shaping process. During mixing short fibre may be added as dispersed reinforcement to produce either ready mix or dry mix for further use in both single-step and two-step DFC processes. A prominent example for AM processes is material extrusion with Strain-Hardening Cement-based Composites (SHCC) .
There are four categories for integration of reinforcement during the concrete shaping process; see Figure 3. The first is entrainment into concrete bulk before material deposition. For extrusion-based processes entrainment of cables or yarns can be realised as a part of printhead process; see Figure 4. Short fibre and textile/fine mesh can be entrained as well. Note that the dispersion of short fibre requires energy for intermixing with the concrete matrix, which can be done both in the material extrusion process, mixing of fibre as a part of the printhead process, and in the material jetting process, in- or outside the nozzle.
The second category is the placement of reinforcement between layers of concrete. In contrast to the entrainment where the deposition of concrete and the entrained reinforcement occur simultaneously, the process is contiguous in this case. In the examples given in Figure 5, all reinforcement types are positioned horizontally in the longitudinal direction of concrete filaments arranged vertically one over another. However, other arrangements are technically possible as well, e.g., deposition of a yarn or stripe of textile on the vertical face of a concrete filament and depositing the next filament laterally onto the yarn or textile and previously deposited concrete filament. This applies also for short fibre, which can be sprinkled on both horizontal and vertical concrete surfaces. The deposition of reinforcement between horizontal layers can be used in all three subclasses of AM incl. particle-bed binding.
Cross-layer encasement is also a contiguous process. Vertical or inclined fragments of reinforcement as well as the attendant horizontal components are placed before the next concrete layer is deposited. The concrete layer encases the fragments but still does not cover their tops, since further reinforcement fragments will be attached there and / or in order to establish cross-layer reinforcement. Vertical or inclined stripes of mesh/textile can be used in this category as well as vertical or inclined bars or little cage fragments locally assembled as in Figure 6a or additively produced. The fourth category also addresses cross-layer arrangement of reinforcement; however, the key feature here is that the reinforcement is induced by penetration while the concrete is still in the fresh/plastic state. Typically, straight, one-dimensional pieces of reinforcement are used for the purpose, either pins or screws; see Figure 6b. They can be placed perpendicular to the layers’ plane or inclined to it.
The two-step processes are subdivided into two categories according to the time of the reinforcement integration, i.e., prior to or after concrete shaping. The key feature of reinforcement placed prior to concrete shaping is its support for the concrete or the absence of such support. If reinforcement provides support to concrete, the concrete shaping process is a formative one since the shape of the element is defined by the supporting reinforcement, which acts as a mould or sheathing; cf. Figure 7. In the no-support-case, both formative and additive concrete shaping approaches are applicable.
Finally, in the category ‘after concrete shaping’ we distinguish between 1) placement of reinforcement in or on hardened concrete as a process step to complete a structural or non-structural element; see e.g., Figure 8a. Assembling elements/parts to a structure are illustrated in Figure 8b. In such cases, also post-tensioned cables can be used efficiently.
Note that the suggested classification for the integration of reinforcement does not cover the step of assembling or shaping of reinforcement either in a conventional or digital manner. Indeed, it would be a separate process step to be considered and described. However, this can be done by using RILEM Classification Framework for DFC as basis. For example, the manufacturing process of Mesh Mould reinforcement  can be described by operation assembly, with joining as the principal process step and welding as the primary process class.
Finally, two examples of applying the new classification to describe digital fabrication processes with reinforced concrete are provided here in form of simple process flow charts; see Figure 9. The first example shows the multi-step fabrication process of the bicycle bridge in Gemert, The Netherlands. The first process step is a 3DCP process in which reinforcing cable is entrained into the concrete filament and thus deposited simultaneously with concrete shaped by extrusion. This purely digital process continues until the printed part/element is finished, while the sub-process of the cable entrainment can be interrupted on demand by cutting the cable and stopping its feed; and the entrainment can be eventually resumed by restarting the feed. Eventually, further parts / segments are produced in the same way. The second process is assembling of the parts / segments. The final, third process is placing of prestressing strands and post-tensioning them. Note that the second and third processes a) were performed in a conventional manner in the given example but can in principle be digitised and automated, and b) do not depend on the process of segment production, i.e., if it is additive or formative. The final product of the process chain is the bridge itself.
The second example presents a relatively rare case, where reinforcement is produced first in a distinct process step by assembling steel mats and bars. This step is followed by progressing encasement of reinforcement mats or cage with concrete in an additive extrusion-based shaping process. Since the printhead dimensions limit the height of the reinforcing elements, which can be encased in the approach under consideration, several repetitions of this sequence, i.e., assembling reinforcement/concrete printing, are required before the product, here an in-situ printed wall is finished. In this way Huashang Tengda fabricated a two-story villa in Beijing. Note that in the given example the assembling of reinforcement was performed in conventional manner, but this process step can be potentially automated and digitised as well.
The ongoing development of digital fabrication with concrete has led to an increasing number of projects appearing in practice over the course of the past few years. With the aim of realising structural applications, the need for reinforcement integration in DFC is obvious and thus, is increasingly being addressed by industry and academia across the globe. Although the functional requirements for reinforcement in DFC are similar to those in conventional concrete construction, the particular process characteristics of DFC render traditional reinforcement solutions unsuitable. As such, a new range of reinforcement solutions is being developed, targeting specifically the integration in DFC, presented in various stages of development.
To facilitate comparison between solutions and indicate the performance and suitability of each reinforcement method, a common language is desirable. To this end a classification framework for reinforcement in digital fabrication with concrete was suggested  having focus on additive digital concrete technologies. As such, it connects with a previous publication  in which DFC technologies themselves have been classified. Firstly, a distinction is made between process type, i.e., a pre-process, a single step process or a two-step process.
· For pre-process applications, reinforcement is typically integrated during mixing. This concerns mainly short fibres, added into the ready mix or during dry mixing of the printable composition.
· In single-step processes, reinforcement is integrated during concrete shaping. Here, reinforcement is either entrained simultaneously with concrete, or placed between or across layers. For single-step processes, a wide variety of reinforcement types is available, spanning from bars and meshes to cables and yarns.
· Finally, for two-step processes a distinction can be made between reinforcement placed prior to concrete shaping and after concrete shaping. In the first case, the reinforcement solution can provide a support to the fresh concrete. In the latter, the reinforcement is placed in or on the hardened concrete member or used to assemble multiple parts into a reinforced structure.
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