Evaluation of Concrete-Steel Friction for Automated Tunnel Segment Extrusion

Italy is a country, that due to its tomography, has one of the highest number of tunnels in its roadway network. Due to the limited technology available at the time of construction and the years elapsed since then, maintenance interventions are needed in order to avoid their collapse. An academia-industry collaboration between Politecnico di Milano and the start-up company Hinfra is active in this field, validating a new technology for the refurbishment of existing tunnels through extrusion of a Steel Fibre Reinforced Concrete (SFRC) lining against the existing old one. This new technology is asking for tailored material and process performance requirements, in order to boost the productivity through the use of a dedicated mechanical extrusion system. In designing the latter, it is fundamental to correctly assess the extrusion resistance due to the friction of concrete against the surface of the moving steel mould. This is of fundamental importance as the mechanical engine has to be designed in order to be capable to slip the extruded SFRC layers during the slip-forming process. The goal of this paper is to present an experimental campaign aimed at evaluating the aforesaid property for an extrudable concrete mix at early ages and speeds both calibrated on the basis of the full-scale extrusion equipment and target productivity rate (between 1 and 3 m per hour). The influence of mix design variables including water binder ratio and retarder admixture dosage have been investigated through a factorial design methodology