A numerical framework for fibre orientation prediction in fibre-reinforced concrete tunnel linings

The use of Fibre-Reinforced Self-Compacting Concrete (FR-SCC) has increased significantly in recent decades, due to its superior flowability, workability, and enhanced mechanical performances. These characteristics have made it particularly suitable for both cast-in-place and precast applications, ranging from slabs to tunnel segments. One of the main advantages of FR-SCC is the possibility to control fibre dispersion, which directly influences the material’s structural performance. However, evaluating fibre dispersion within the targeted elements remains a significant challenge, as it depends on both the rheological properties of the mix and the casting process. Structural codes require to account for fibre orientation in the design phase through the definition of an orientation factor. For complex applications, such as the casting of the FR-SCC tunnel lining considered in this study, determining this factor in advance is particularly challenging. To address this issue, a simple yet effective framework for predicting fibre orientation in arbitrary casting processes is proposed. A fluid FEM-based numerical model is employed to accurately compute flow lines during casting. A time-dependent Bingham law is used to reproduce the FR-SCC rheological behaviour, while calibrating the parameters, to account for the effect of fibres, by means of ICAR rheometer tests. Based on the numerically computed velocity field during casting, a novel definition of the orientation factor is introduced. The approach is validated through comparison with experimental results obtained from a non-destructive magnetic survey of the fibre dispersion, performed on full-scale mock-ups of FR-SCC tunnel linings.