Introduction

I.1. Context of the book

The ability to generate a three-dimensional (3D) object from a single image would seem like an idea pulled from a work of fantasy or science fiction. Nevertheless, starting from the mid-1980s, with the first patents on additive manufacturing and 3D printing, this future possibility has become a reality. Initially limited to polymers, additive manufacturing has now expanded to an ever-increasing number of materials [HUL 86, AND 84]. In the 2000s, the development of fused deposition modeling, or the rapid prototyping through the deposition of polymer strands, led to a rapid democratization of the process and gave the general public a glimpse of the ample possibilities offered by 3D printing, in terms of economic and industrial development. In addition, this technology is perfectly suited to the societal environmental issues currently faced: in that it first enables us to save materials for the manufacturing of parts with complex geometry, and second, to consider the "on demand" manufacturing of spare parts.

Naturally, the possibility of transferring these technologies into the field of construction, including concrete, was initially studied by Pegna in 1997 [PEG 97] and then by Professor B. Khoshevnis of the University of Southern California in the first half of the 2000s [KHO 04]. At the same time, the computer-aided design of structures has undergone significant advances with the introduction of the first digital models (building information modeling - BIM) [HEG 01].

As a result, traditional building methods may now find themselves overthrown by the Third Industrial Revolution and by the introduction of computerization and digital technologies. In this context, the design and monitoring of projects have already been influenced by the use of BIM: the creation of complete digital models of buildings compelled further action to be taken in the period leading up to the execution of a construction project, allowing for further steps towards both an optimization of the execution methods and optimal construction quality (referred to as lean building).

Nevertheless, the use of digital technologies in production methods is still in its infancy (prototypes, feasibility and reliability in the laboratory). However, the opportunity to take advantage of the complete digitization of construction projects, starting from the time they are designed, enables us to envision the automation of construction methods and to make greater progress towards the objectives of lean building.

Thus, the application of additive manufacturing methods, originally developed for plastics, to concrete is now the subject of numerous academic studies and private initiatives around the world. As a result, the number of initiatives and projects related to the 3D printing of concrete has grown exponentially since 2015. For example, Figure I.1 shows the growth of the number of publications on the topic of concrete additive manufacturing in the 10 most influential scientific journals in the field of civil engineering (source: Google Scholar, date: July 1, 2018). The late 2000s and early 2010s saw the publication of pioneering works by Professor Khoshnevis of the University of Southern California and the team of Professor Buswell of Loughborough University in England [KHO 04, KHO 06, BUS 07, LE 12, LEA 12]. Since 2016, there has been an explosion in the number of publications that show the current nature of this research area and the need for knowledge related to the area of concrete 3D printing in construction works. While there were four such publications in 2016, 16 were found in 2017 and 33 in the first six months of 2018.

The motivation for these studies can be found in:

• - the economic advantage offered by 3D printing, which could offer the potential of avoiding the use of concrete forms, representing up to 50% of the cost of cast concrete;
• - unprecedented freedom for architects in the shapes they can create;
• - a reduction in environmental impacts - the ability to place the material exclusively where it is needed (known as the concept of topological optimization);
• - improvements in working conditions - elimination of heavy handling tasks and the vibration of concrete.

These initial works have made it possible to validate the technical feasibility of the process of 3D printing with concrete, and small-scale demonstrators have been carried out around the world (individual homes, walkways).

Figure I.1. Number of publications in the top 10 most influential science journals in civil engineering (source: Google Scholar, as of July 1, 2018) over the past 15 years

The market for printed concrete is now worth nearly ?30 million, and is now growing at a rate of 15% per year. The exponential growth in the number of projects has made it possible to imagine an extremely rapid increase in the revenue of this market.

As a result, the application of printed concrete in structural material no longer looks like a utopian vision, and it is now important to lay the foundations for these new manufacturing techniques by carrying out a comprehensive compendium of the knowledge and technologies developed in the field.

I.2. Current research topics and scientific challenges

Currently, the research topics related to 3D printing are:

• - Shift towards a 100% digital construction industry

The design of a construction project involves the production of a digital mock-up, which leads to both the anticipation of construction problems and the optimization of the interactions between the different professional occupations and stages of construction. This anticipation tool enables us to improve the quality of the constructions and to optimize the methods of execution.

In addition, these digital models make up a raw material that can be implemented using robots and automation schemes, allowing construction projects to be carried out faster, more accurately and more reliably. The construction of concrete structures using 3D printing fits perfectly within this framework. An efficient transfer interface between the digital model and the trajectory of the robot placing the concrete is expected to be achieved while taking into account the configuration of the construction site and its constraints.

• - Processes: optimization and mastery of the rheology of the concrete for the purpose of using it for printing

To be able to be printed, the concrete must not only be fluid enough to be transferred (pumpable concrete) but also rigid enough to hold up under its own weight once extruded, without deforming. Similarly, it must "quickly" bear the weight of the layers placed on top of it. The competition between the rate of mechanical structural build-up and that of the elevation of the printed structure is therefore a critical parameter to be controlled in order to ensure that the process is carried out smoothly. This involves controlling not only the behavior of concrete in the state of being freshly placed but also the changes it undergoes over time. It is also important to gain control over the additives used, which allows the material to be put in place "on demand" (many processes involve the addition of an accelerator in the nozzle of the printer). Works on the mechanical behavior of concrete at a very early age will be necessary to describe the behavior of the material up to the end of its placement, and thus to allow its transition with the initial rheological behavior. It is important not only to work on experimental methods for describing the evolution of the rheological behavior (shear yield stress) of concrete over time in a simple and reliable way, but also to be able to control and follow the process inline.

It should also be noted that other innovative processes, such as deposition on a support, injection into aggregate beds or through meshes or porous structures and "intelligent" sliding forms, are conceptually similar technologies that will need to be studied.

• - Structural design of printed structures

- Characterizing and reinforcing an anisotropic material

Printed concrete set in layers may exhibit anisotropic behavior, induced by its layered structure, which should be qualified. The interface between layers, depending on the process, remains a sensitive area, which may represent the mechanically weak points of the structure. It will therefore be necessary to establish a study methodology to characterize the complex and anisotropic behavior of this type of material.

Furthermore, printed concrete, similar to poured concrete, has a tensile weakness that will have to be compensated for by the addition of steel reinforcements, following the same principles of the reinforcement traditionally used for poured concrete.

Several approaches are currently being tested: the addition of fibers within the layers, the casting of steel bars in dedicated spaces and the addition of a metal wire to the concrete. The effectiveness of these reinforcement methods remains to be evaluated, and strategies for their scaling are to be developed accordingly.

- Topological optimization

Additive manufacturing technologies enable us to envision a new level of freedom in structural design inspired by nature (biomimicry) that optimizes the management of resources by using materials only where they are mechanically necessary. This leads to the possibility of lean manufacturing and mechanically optimized...