Extruded Tunnel Lining Regeneration Project (ETLR): experimental campaign on real scale SFRC tunnel mock-ups

Background Automation in the construction industry is gaining increasing relevance, driven by the need to enhance productivity, foster innovation, and meet evolving societal demands. Slip-forming with extrudable steel fibre-reinforced concrete (SFRC) offers an innovative framework for the maintenance and refurbishment of highway and railway tunnels. However, its structural performance requires full-scale validation before adoption in practice. Methods Four full-scale mock-ups replicating a motorway tunnel lining were constructed using slip-forming with extrudable SFRC and tested under load conditions simulating the relevant action combinations for tunnel linings in the intended scenarios. Both symmetric and non-symmetric loading combinations were examined, including a sequential loading case to assess accumulated damage and residual bearing capacity. Digital Image Correlation (DIC) analysis was performed to study cracking in the mock-ups, enabling computation of crack opening, which was related to the load applied by the hydraulic jacks employed in the experiment. Testing was conducted at the ELSA laboratory of the European Commission, Joint Research Centre in Ispra, as part of the OPENLAB ETLR project. Results The relationship between crack opening and applied load enabled characterisation of the post-cracking residual capacity of the SFRC tunnel lining structure under the tested loading combinations, including its behaviour under sequential loading and the associated accumulation of damage. Conclusions The full-scale tests provide a preliminary validation of the adopted design criteria and an assessment of the effectiveness and structural reliability of the proposed slip-forming SFRC technology for tunnel lining refurbishment.