Freek Bos

Additive manufacturing of concrete in construction: potentials and challenges of 3D concrete printing

ABSTRACT Additive manufacturing is gaining ground in the construction industry. The potential to improve on current construction methods is significant. One of such methods being explored currently, both in academia and in construction practice, is the additive manufacturing of concrete (AMoC). Albeit a steadily growing number of researchers and private enterprises active in this field, AMoC is still in its infancy. Different variants in this family of manufacturing methods are being developed and improved continuously. Fundamental scientific understanding of the relations between design, material, process, and product is being explored. The collective body of work in that area is still very limited. After sketching the potential of AMoC for construction, this paper introduces the variants of AMoC under development around the globe and goes on to describe one of these in detail, the 3D Concrete Printing (3DCP) facility of the Eindhoven University of Technology. It is compared to other AMoC methods as well as to 3D printing in general. Subsequently, the paper will address the characteristics of 3DCP product geometry and structure, and discuss issues on parameter relations and experimental research. Finally, it will present the primary obstacles that stand between the potential of 3DCP and large-scale application in practice, and discuss the expected evolution of AMoC in general.

Experimental Exploration of Metal Cable as Reinforcement in 3D Printed Concrete

The Material Deposition Method (MDM) is enjoying increasing attention as an additive method to create concrete mortar structures characterised by a high degree of form-freedom, a lack of geometrical repetition, and automated construction. Several small-scale structures have been realised around the world, or are under preparation. However, the nature of this construction method is unsuitable for conventional reinforcement methods to achieve ductile failure behaviour. Sometimes, this is solved by combining printing with conventional casting and reinforcing techniques. This study, however, explores an alternative strategy, namely to directly entrain a metal cable in the concrete filament during printing to serve as reinforcement. A device is introduced to apply the reinforcement. Several options for online reinforcement media are compared for printability. Considerations specific to the manufacturing process are discussed. Subsequently, pull-out tests on cast and printed specimens provide an initial characterisation of bond behaviour. Bending tests furthermore show the potential of this reinforcement method. The bond stress of cables in printed concrete was comparable to values reported for smooth rebar but lower than that of the same cables in cast concrete. The scatter in experimental results was high. When sufficient bond length is available, ductile failure behaviour for tension parallel to the filament direction can be achieved, even though cable slip occurs. Further improvements to the process should pave the way to achieve better post-crack resistance, as the concept in itself is feasible.

3D printing concrete with reinforcement

Recent years have seen a rapid growth of additive manufacturing methods for concrete construction. A recurring issue associated with these methods, however, is the lack of ductility in the resulting product. In cases this is solved by combining printing with conventional casting and reinforcing techniques. Alternatively, this paper presents first findings on the development of a system to directly entrain a suitable form of reinforcement during printing. A device is introduced to apply the reinforcement. Several options for online reinforcement medium are compared for printability and structural performance, based printing test runs and 4-point bending tests respectively. It is shown that high-performance steel cables can provide suitable reinforcement characteristics, although improved bond would allow better use of the cable capabilities. Significant post-cracking deformations and post-cracking strength can be achieved. Further research into optimal reinforcement placement and configuration is recommended.

3D concrete printing – a structural engineering perspective

Building codes are a necessity. The first codes were simple, easy to apply for designers and offered residual capacity for a low price. Meanwhile codes are significantly more complicated. This is partly due to the development of more scientific background knowledge and partly to the upcoming significance of regulations in society. National codes disappear and are replaced by continental codes like the Eurocodes, the ACI Code and the Asian Code, with some space for national exceptions, to be negotiated. On the one hand a nostalgic desire develops to go back to more simple and transparent codes, but on the other hand there is a need for more advanced models, in order to deal with the existing infrastructure in an economic way. There is as well a remarkable development of high performance materials, improved calculation and measuring techniques, which ask for modernized codes. Finally new targets are formulated, like life cycle design and sustainable solutions. The development of user-friendly, future-oriented and innovation-stimulating codes is a large challenge.3D Concrete Printing (3DCP) is being developed in an increasing number of places around the globe. The focus is mainly on a trial-and-error based exploration of the possibilities. However, to obtain a viable manufacturing technology and realize the 3DCP potential, a higher level of process control is required. Four levels of control are therefore identified. Research efforts and key results to achieve a higher level of control than the current one are presented. As a final goal, optimization algorithms should be able to define optimum print sessions, based on allowable print strategies and structural analysis models describing both the fresh and hardened concrete state. This will result in new geometries appropriate to this specific manufacturing technique. These geometries, however, can only be applied when structural safety is achieved, either by loading conditions (compression structures), hybrid solutions (combination with conventional reinforced concrete), embedded reinforcement, FRC and prestress. Such solutions are therefore being explored.

A Real-Time Height Measurement and Feedback System for 3D Concrete Printing

Recent years have seen a rapid growth of additive manufacturing methods for concrete construction. Generally, these methods are based on a linear sequence of design → print path definition → actual printer actions in a print environment. However, printing experiments show that a large number of parameters influence the printing process. Not all of these can be predicted accurate on forehand. Therefore, a method is introduced that allows real-time adjustment of the print process. As a proof-of-concept, a measurement system for the nozzle height has been developed and tested. Because this variable relates to machine properties, environmental conditions as well as material behaviour, it is a crucial parameter to control. In two case study prints, the effectiveness of the device was shown. In one study, the printer could follow a range of irregular curves in the print bed, whereas only a simple flat rectangular print path had been programmed. In the other, it was shown the print path could be adjusted to vertical deformation of the previous layers of printed filament in a tubular object of several dozen layers. Thus, premature failure through irregular loading of the object during printing was avoided. Further expansion of the use of real-time measurement devices may be anticipated in the future. Besides more advanced geometrical measuring, chemical and physical conditions such as concrete temperature (both before and after deposition), surface wetness, and environment humidity, can be recorded. Combined with the machine action log, this should result in a detailed set of as-built data of the printed object, allowing e.g. for a geometrical clash control with the design as well as other quality controls.

Design of a 3D printed concrete bridge by testing

ABSTRACT The current state of research and development into the additive manufacturing of concrete is poised to become a disruptive technology in the construction industry. Although many academic and industrial institutions have successfully realised full-scale structures, the limitations in the current codes of practice to evaluate their structural integrity have resulted in most of these structures still not being certified as safe for public utilisation and thus deemed as test prototypes for display purpose only. To realise a 3D concrete printed (3DCP) structure which could be certified safe for public use, a bridge was realised using the print facility of the Eindhoven University of Technology (TU/e) based on the concept of ‘Design by Testing’. This paper holistically discusses the complications encountered while realising a reinforced 3DCP bridge in a public traffic network and decisions taken to find solutions for overcoming them.

Optimising 3D printed concrete structures using topology optimisation

Additive manufacturing and 3D printing are rapidly developing digital fabrication techniques (Lu et al. 2015). After the first steps in small scale printing of metals (Frazier 2014) and plastics (Gibson et al. 2014) have been made, research from various groups around the world is now also focusing on large scale printing in concrete (Lim et al. 2012) and making this technology more suitable for the construction scale. The potential of using this technology is that it will be possible to create complex and/or customised concrete designs with the expectation that the costs will be low and the construction speeds will be high. Additionally, this new technology will provide opportunities to create more efficient structures. Structures can already be optimised in the early stages of the design for weight and structural performance, but the resulting optimised structures are often difficult to manufacture due to the resulting geometry of the design. Additive manufacturing can address this issue without high costs for moulds and labour.

Large Scale Testing of Digitally Fabricated Concrete (DFC) Elements

Case study projects based on Digitally Fabricated Concrete (DFC) are presented in an increasing pace around the globe. Generally, though, it is not reported what structural requirements (if any) these structures meet and how compliance to these requirements was established. Published material research is often not connected to the presented case studies, and even when it is, it is not necessarily obvious their small scale results can be applied to full scale structures as some scale effects should be anticipated. Caution is required as DFC related material tests are still under development and scale effects in DFC have hardly been studied. Therefore, it is recommendable to perform large scale testing, in the range of 1:5 to 1:1, if DFC is applied to actual use structures. This paper presents such testing for two projects, a pavilion in Denmark (not realized) and a bridge in the Netherlands (realized). In both cases, elements printed with the 3D Concrete Printing facility of the Eindhoven University of Technology were intended for actual load bearing performance. The conservative designs past the test requirements, but nevertheless some important findings with regard to element manufacturing and structural behaviour were experienced. It is concluded that large scale testing remains advisable for DFC structures as long as not all relevant aspects of the technology are quantitatively understood, at least when new concepts are being applied.

Cable-stayed façade structure with welded borosilicate glass tubes

Structurally used tubular glass provides stunning possibilities for architectural applications, which have hardly been explored yet. Over the last decade, only a handful of experimental structures have been presented, such as a 3D lattice structure by the University of Stuttgart [1], a tensigrity structure [2] and columns in a pavilion by the TU Delft [3]. Just one project of significant size has been realized in practice: the hanging glass atrium facade of the Tower Place office building in London, UK, is horizontally supported by a field of glass tubes [4]. All these examples make use of geometrically relatively simple shapes, i.e. tubes. However, welding and hot shaping techniques for glass make it possible to produce much more complex shapes. The possibilities this offers on small scale design of mechanical joints have been explored in a case-study design for facade struts by Bos [5]. However, a glance in an average chemistry laboratory will give you a better impression of what could be possible. Little is known about glass welding for the building industry. Especially the bond strength is an important unknown as is the reliability. Other constraints such as which types of glass can be used and what shapes can be forged are also unknown. The existing projects have already shown the use of glass tubes introduces specific engineering problems compared to ordinary flat glass. They are normally only available in limited size, up to 1.5 m. Force transfer into a glass tube is also complicated as the geometry prohibits drilling holes and complicates adhesive joints. The potential of welding for structural applications of glass tubes has been explored by graduate student C. Giezen, by producing a case-study design alternative for the (existing) facade structure of a curtain facade of the Faculty of Architecture building. This paper is largely based on the results of that study. Theory on welding is reviewed and the practical possibilities of shaping by existing commercial firms explored. Experimental research, carried out on different welded specimens to investigate welded joint strength, is presented. Finally, a facade structure design based on 3D cross-shaped welded glass components, is presented. This study shows welding provides ample design possibilities without significantly compromising the component strength.