3D-printed fibre-reinforced concrete: a new era for the construction industry?

The building and public works sector is heavily reliant on reinforced con­crete. Reinforced concrete requires formwork, however, and its implementa­tion is long and painstaking. Moreover, its carbon footprint is quite large and repre­sents 2.5% of all CO₂ emissions in France.

And yet an alternative already exists – in the form of fibre-reinforced concrete that uses short metal, glass, plastic or even wood fibres. This alternative offers supe­rior resistance to stress and great tensile strength. But the cost to produce it remains high. In addition, fibre-reinforced concrete as manufactured currently incorporates, at best, three to four percent fibres. This per­centage is insufficient for use in truly struc­tural applications.

Fibre-reinforced concrete ready to hold the flame

That’s where French company XtreeE and its research partners come in. This large-scale 3D printing specialist, together with École des Ponts ParisTech, and the CNRS, have come up with a solution. 3D printing and robots have changed the game by making it possible to pro­duce concrete with a fibre content that is much higher than those currently being produced.bSuch a material can be used to build structures that are more resistant: to stress and bending, to wear and tear, to impact, fire, and abrasion. It also harness­es the best that 3D printing has to offer in terms of design: freedom of form and reduced quantities of materials.

More than just a material, XtreeE’s prod­uct is actually a new process: co-extru­sion of a material with a cement matrix and fibres. This project is perfectly in step with the transformation currently tak­ing place in the construction industry. It aims to speed up the transition from mass concrete structures, which have not been optimised, to ones in thin or hollow-core concrete. XtreeE aspires to provide archi­tects, engineers, and other construction industry stakeholders a brand-new solu­tion that is reliable and sustainable… And at a controlled cost, to boot –to meet the economic and environmental challenges that the industry faces.

Hand-in-hand to innovate and bring transformations

In 2016, a foundation was laid to begin developing a printing process for long fi­bre-based concrete –the mineral version of fibre-reinforced plastics (FRP). Up to that time, none existed. For this purpose, in conjunction with CNRS, XtreeE is co-financing a thesis on the process. An XtreeE employees is working on their PhD on the topic at École des Ponts Paris­Tech in parallel to their job.

As of 2019, an extrusion head was provid­ed to the Navier laboratory through a pro­ject called Build’In, with the goal of testing fibre properties, examining sustainability, and conducting experiments. The project came about following École des Ponts ParisTech’s reflection on the construction sector’s environmental impact.

Since that time, the research has led to development of a plug-in specific to 3D printed fibre-reinforced concrete. The plug-in, when installed to the printer head, enables concrete with a cement ma­trix and fibre reinforcement to be printed by means of a co-extrusion system. The subsequent material contains a percent­age of fibres around three to four times higher than current fibre-based concretes. The material produced by this system has a strength equivalent to that of reinforced concrete. The plug-in has been installed to the 3D print head at the Build’In plat­form and the one at XtreeE’s FabLab in Rungis (Figure 1).

To put it simply, to produce long fibre-re­inforced concrete, a spool unwinds a va­riety of continuous fibres as the machine prints. These fibres are made up of mi­croscopic filaments wound into strands – which in and of itself represents a sig­nificant innovation. Different fibre types have been studied. In particular, carbon, glass, and basalt fibres that meet certain specifications for the desired level of resistance were examined. Nevertheless, the scope of research has been deliber­ately narrowed to focus on carbon fibre, a material for which it is easier to obtain approval and gain acceptance on the con­struction market.

The composite concrete produced with carbon fibre is anisotropic. It possesses intrinsic tensile strength and durability, without necessitating combination with other materials, like steel (Figure 2). The main concern with reinforced concretes is actually that the steel used as reinforce­ment corrodes. Concrete carbonation in­duces corrosion, and affects the durability of such materials. No longer having to use steel rebar allows users to steers clear of any corrosion-related problems.

Ultimately, XtreeE would like to design systems for reinforced printed structures like complex structures, parts for assem­blies, lightweight cladding and coverings, and optimised flooring and crossings. It is also possible to envision replacing mass concrete structures with thin or hollow printed structures. These systems would enhance XtreeE’s catalogue for its ‘print­ing-as-a-service’ platform that gives ac­cess to a set of certified products as well as digital assistance tools to facilitate archi­tectural design and creation of 3D-print­ed parts.

So far, a proof of concept has already been completed, demonstrating tensile strengths along the same lines as for re­inforced concrete. Industrial prototype development is planned for 2024, with commercialisation to take place by the end of the same year.

An alternative with many advantages

The XtreeE solution presents several ad­vantages. The first, and a good one at that, is environmental. When less material is consumed, the carbon footprint is smaller. Manufacturing large quantities of concrete releases quite a lot of CO₂. Continuous long fibre reinforcement enables savings both in terms of the amount of concrete and the clinker used for a component, thereby reducing its footprint. For exam­ple, very hollow-core columns could be fabricated, weighing only one tonne in­stead of four, but with the same strength.

The second advantage is increased sturdi­ness. Long carbon fibre-reinforced con­crete would lead to more design freedom. This would allow structures to be built with shapes that are better-adapted to with­stand natural catastrophes or bad weather. What’s more, freedom of form appeals to architects, who are taking to 3D printing technology more and more. The third and final advantage is financial. Less material is wasted, since the print head was devel­oped to optimise shapes and therefore to use less. This level of precision is the result of a multitude of sensors in the print head. These sensors allow the quantity of cement and water to be controlled, and at the same time, produce very strong concrete. As the long fibre concrete solution requires only a fraction of the amount of material as rein­forced concrete, costs are lower.

High ambitions to match high stakes

In order to achieve the aim of responding to the challenges of the construction in­dustry, ongoing research must lead to op­timised, decarbonised components. Such components are customisable and cost less – or at least the equivalent to those produced conventionally when all direct and indirect costs have been accounted for. But beyond this new process, XtreeE has a broader ambition: industrialising an industry in which productivity has not improved since the 60s, in spite of the fact that the needs and constraints continue to increase. Construction has become more and more complex, particularly because of the new environmental standards. At the same time, the necessary workforce is less available.

And yet, thanks to recent progress in the digital sector (simulation, production, or­ganisation, etc.), it has never been easier to fabricate parts on demand. This in turn should facilitate adoption of 3D printing for construction. To accelerate develop­ment of a 3D printing ecosystem and dig­itisation of building, XtreeE, which has already rolled out 17 units based on its 3D printing technology, intends to create a network of more than 50 units worldwide by 2025. A funding campaign is currently underway to bring this dream to fruition.

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