Introduction to a new extrusion-based technology for the regeneration of existing tunnels

Additive Manufacturing (AM) is process in which a three-dimensional component is produced by the consecutive addition of material. This technology, applied on a large-scale to cementitious materials, is known as 3D Concrete Printing (3DCP). Among the new technologies driving the fourth industrial revolution in the construction industry, 3D Concrete Printing (3DCP) is playing a key role. The typical process is made through robotic arms or gantries equipped with nozzles, similarly to contour crafting in other industries, where the printed object is obtained through the multiple deposition of layers. Despite 3DCP is appealing when addressed to specific items, as complex architectural shapes, the structural behavior and geometrical size are limitations difficult to over come. Upscaling the extrusion process to full sections, introducing a new concept of ultrafast and adaptable slipforming, is the access key to different domains of the industry, as infrastructures, where the increase in productivity results in social, economic and environmental benefits, that are not comparable to the niche where 3DCP is confined. As a matter of fact, the maintenance process of existing infrastructures is a very critical topic in most of the industrialized countries, worldwide. It is commonly recognized by the main players operating in the industry (professional engineers, owners, construction companies etc.) that, despite for new constructions the methodologies are quite evolved (i.e. development of the tunnel boring machines), in the maintenance area there is complete lack of technologies, making still impossible to industrialize the operations. This paper will present the Extruded Tunnel Lining Regeneration (ETLR) technology developed by HINFRA with the scope to automatically regenerate the lining of existing damaged tunnels directly at site. The ETRL processing train is a machinery consisting of several modular units, each solving a specific function. The increasing industrialization of set of operations, typically the demolition, the surface preparation, and the new lining phases, combined with the performances of the special concrete, allow to target productivity rates far from the traditional methods in use in the industry. This is made possible by the development of an extrudable eco-friendly Fiber-Reinforced Concrete (FRC) characterized by high early-age compressive strength and fast setting time, that is the other key aspect of the innovative technology implemented by HINFRA. “Tailored” technological issues, including e.g. the experimental determination of the friction between the extrudable mixes and formworks, will be discussed, together with a design validation related to a FRC tunnel lining, whose use could further exploit, through the significant reduction of ordinary reinforcement, the potentials of 3DCP.