Anisotropic Concrete (2024-11)¶

As they are abundant, inexpensive and relatively strong, cementitious materials are massively used to build and renovate, from housing to infrastructure. However, the large environmental impact of cement requires us to reinvent the way we use it, to develop more material-efficient techniques than the traditional, massive concrete casting construction. In this regard, the optimization possibilities offered by additive manufacturing are now well understood: the toolpaths traced by the printing machines open a new, lighter formal vocabulary of surfaces and grids, the potential of which is clear to see in comparison to the massiveness of most contemporary cast concrete structures. But it is also a new requirement for the material, which still needs to be reinforced, and to which the steel rebar-reinforced concrete technique cannot respond in the best way, because of the great thicknesses of concrete that must encase each rebar, and its complexity (the steels must be cut, shaped and installed in parallel with the printing). Long-fiber composite materials, which have already taken on a major role in automotive, aviation and the naval industries, are characterized by very fine-scale reinforcement, which is much more suited to printing, but also by their anisotropy, an additional parameter that gives designers the opportunity to place not only the right material in the right place (which is common to all additive manufacturing processes) but also the right reinforcement in the right direction. “Thinking composite” in structural design, by optimizing both the layout of continuous reinforcement orientations and material distributions, has already led to breakthroughs across the aforementioned industries riding the “composite wave” : construction of the fastest sailboats, reduction of aircraft structure weight to 20% compared to aluminum designs according to Boeing [1], manufacturing more than 40-m long wind turbine blades. While these milestones certainly cannot be directly translated into the field of construction, being driven by very different constraints, it is however clear that the time is right to explore how composite engineering can bridge into the emerging field of concrete 3D printing construction. While it has been already shown that short-fiber addition is possible for extrusion-based 3D printing, the reinforcement rate remains relatively low, because of the pumpability requirements of such materials, which undoubtedly limits the attainable performances. On the other hand, several research teams, including ours, worked on the development of long-fiber composite inspired technologies. In this essay, we introduce the patented Flow-based Pultrusion process, developed at the Navier Laboratory. It applies to extrusion-based 3D printing, and enables to print ductile structural concrete components, thanks to the unidirectional reinforcement of laces in their printing direction with long fibers. In the first Section, the process and apparatus are outlined. In a second Section, the obtained “anisotropic concrete” is presented, along with its potential and limitations compared to traditional steelreinforced concrete. The third Section presents the first large-scale prototypes and provides some ideas on future anisotropic concrete applications.