PSI - Issue 84

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

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depth. For dosages between 0.2 and 0.5 kg/m 3 , the average depth decreases to approximately 60 mm. When the dosage exceeds 2.0 kg/m 3 , the phenomenon is virtually eliminated, with mean values falling below 20 mm. This efficacy is attributable to the thermo-hydro-mechanical mechanisms initiated by the polymer's melting point (approximately 160 170°C): the creation of an interconnected network of micro-channels provides a preferential escape route for water vapour, thereby limiting internal overpressures that would otherwise exceed the tensile strength of the matrix. A particularly pertinent scientific observation, evident from Fig. 1, concerns the statistical dispersion of the experimental data. For PC and SFRC composites, the variability is markedly high, with spalling depths fluctuating between less than 50 mm and over 140 mm. In the absence of a dominant pressure-relief mechanism, the phenomenon appears to be governed by a stochastic interaction of multiple factors, such as local thermal gradients, intrinsic permeability, and residual moisture content. Consequently, minor fluctuations in these parameters can lead to profoundly divergent structural responses. Conversely, the addition of PP fibres serves to regularise the material's behaviour. As the dosage increases, there is not only a reduction in the mean damage depth but also a notable decrease in the dispersion of the results. The formation of this engineered porosity network becomes the prevailing physical mechanism, rendering the concrete’s response less sensitive to environmental variables or compositional uncertainties. In conclusion, the inclusion of polypropylene fibres stabilises the material's performance, transforming an explosive and aleatory phenomenon into a more controlled and predictable process of thermal degradation.

Fig. 1. Spalling depth distribution.

5. Conclusion This systematic review of the thermo-mechanical response of Fibre-Reinforced Concrete (FRC) under conditions representative of tunnel fires has yielded the following key conclusions. • Synergistic mitigation mechanism: The resilience of tunnel linings under fire conditions is critically dependent on a hybrid reinforcement strategy. While steel fibres provide essential post-cracking ductility, thereby preventing brittle collapse, they do not mitigate the build-up of pore pressure. The mitigation of explosive spalling is primarily attributable to the inclusion of polypropylene fibres, which form a sacrificial transport network for vapour evacuation upon reaching their melting point (approximately 170°C). • Transition from stochastic to deterministic behaviour: A principal finding of this review is the regularising effect of polypropylene fibres on the material's fire performance. Whereas spalling in plain and steel-fibre reinforced concrete is a highly aleatory phenomenon, exhibiting significant statistical dispersion, the addition of polypropylene fibres improves the material's response. The formation of an engineered porosity network shifts the