PSI - Issue 84

Nunzia Gargiulo et al. / Procedia Structural Integrity 84 (2026) 1334–1338

1338

failure mode from an unpredictable, explosive event to a more controlled process of thermal degradation. This transition towards deterministic behaviour is fundamental for reliable structural fire engineering design. • Influence of dosage and confinement: The efficacy of polypropylene fibres is dosage-dependent, with experimental data indicating a saturation effect beyond which spalling is virtually eliminated (typically at dosages >2.0 kg/m 3 ). However, it is crucial to note that the operational fibre demand is increased by mechanical confinement and biaxial stress states, which are typical in tunnel linings. Mix design must therefore be calibrated not only to the anticipated thermal exposure, but also to the specific structural boundary conditions. • Implications for infrastructure resilience: The implementation of hybrid fibre-reinforced concrete (HFRC) substantially reduces the spalled cross-sectional area (often to <0.5%), ensuring that the residual lining thickness remains within safe operational limits. From a broader perspective, this enhancement in material performance is fundamental to improving infrastructure resilience. By minimising repair times and preventing prolonged service disruptions, HFRC directly mitigates the significant socio-economic consequences of tunnel fires. Building upon these findings, the authors' future work will focus on the development and validation of analytical and numerical models for spalling prediction. A dedicated experimental campaign is being designed for the calibration of a multi-physics numerical model capable of accurately capturing the thermo-mechanical behaviour of concrete reinforced with various types of fibres, including both steel and polymer. Chen, B., Liu, J., 2004. Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures. Cement and Concrete Research 34, 1065-1069. Felicetti, R., Cardu, U., Martellozzo, F., Valiante, N., 2025. Confined slab spalling test (CSST): a screening tool to assist concrete mix design in tunnel projects. SiF 2024 -The 13th International Conference on Structures in Fire. Hager, I., Mróz, K., 2019. Role of Polypropylene Fibres in Concrete Spalling Risk Mitigation in Fire and Test Methods of Fibres Effectiveness Evaluation. Materials 12, 4333. Kalifa, P., Chéné, G., Gallé, C., 2001. High-temperature behaviour of hpc with polypropylene fibres from spalling to microstructure. Cement and Concrete Research 31, 1487-1499. Kim, S., Shim, J., Rhee, J., Jung, D., Park, C., 2019. Temperature Distribution Characteristics of Concrete during Fire Occurrence in a Tunnel. Applied Sciences 9, 4740. Lo Monte, F., Felicetti, R., 2019. Assessment of concrete sensitivity to fire spalling: A multi-scale experimental approach. Construction and Building Materials 204, 584-598. Maluk, C., Tignard, J., Ridout, A., Clarke, T., Winterberg, R., 2021. Experimental study on the fire behaviour of fibre reinforced concrete used in tunnel applications. Fire Safety Journal 120, 103031. McNamee, R., Sjostrom, J., Boström, L., 2021. Reduction of fire spalling of concrete with small doses of polypropylene fibres. Fire and Materials 45, 2. Qiao, R., Guo, Y., Zhou, H., Xi, H., 2022. Explosive Spalling Mechanism and Modeling of Concrete Lining Exposed to Fire. Materials 15, 3131. Wang, F., Xiong, T., Liu, J., Zeng, J., Luo, C., 2025. Experimental study on temperature characteristics and multi‐field anal ysis for concrete spalling of tunnel linings exposed to high temperatures. Structural Concrete 26, 6860-6878. Yan, Z., Shen, Y., Zhu, H., Li, X., Lua, Y., 2015. Experimental investigation of reinforced concrete and hybrid fibre reinforced concrete shield tunnel segments subjected to elevated temperature. Fire Safety Journal 71, 86–99. Yao, W., Li, J., Wu, K., 2003. Mechanical properties of hybrid fiber-reinforced concrete at low fiber volume fraction. Cement and Concrete Research 33, 27-30. Yoshitake, I., Baba, K., Ito, T., Nakagawa, K., 2005. Behavior of fiber reinforced concrete under fire temperature. International Workshop on High Performance Fiber Reinforced Cementitious Composites in Structural Applications, Honolulu, HW. References