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.

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.

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.

With the number of 3D printed concrete structures rapidly increasing, the demand for concepts that allow for robust and ductile printed objects becomes increasingly pressing. An obvious solution strategy is the inclusion of fibers in the printed material. In this study, the effect of adding short straight steel fibers on the failure behaviour of Weber 3D 115-1 print mortar has been studied through several CMOD tests on cast and printed concrete, on different scales. The experiments have also been simulated numerically. The research has shown that the fibers cause an important increase in flexural strength, and eliminate the strength difference between cast and printed concrete that exists without fibers. The post-peak behaviour, nevertheless, has to be characterised as strongly strain-softening. In the printed specimens, a strong fiber orientation in the direction of the filament occurs. However, this has no notable effect on the performance in the tested direction: cast and printed concrete with fibers behave similarly in the CMOD test. For the key parameters, no scale effect was found for the specimens with fibers, contrary to the ones without. Numerical modelling of the test by using the Concrete Damage Plasticity material model of Abaqus, with a Thorenfeldt-based constitutive law in compression and a customised constitutive law in tension, results in a reasonable fit with the experimental results.