PSI - Issue 84

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

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systematically evaluating these influencing factors to establish a comprehensive understanding of the material's structural response. 3. Experimental investigations on FRC fire performance The analytical characterisation of FRC under thermal actions is founded upon experimental campaigns designed to simulate the extreme conditions of tunnel fires. The literature demonstrates that spalling is not a standalone material property, but a multi-physical phenomenon governed by the interplay between the cementitious matrix, fibre properties, and external boundary conditions. This review provides a comparative analysis of existing research, examining various experimental scales to identify the key parameters that dictate fibre effectiveness in mitigating explosive spalling. 3.1. Small- to medium-scale investigations: influence of fibre type Small- and intermediate-scale tests are typically focused on validating mix designs by monitoring thermal gradients and pore pressure evolution. At the material level, small-scale specimens (e.g., prisms and small blocks) are primarily used to evaluate the intrinsic effectiveness of different fibre combinations. Yoshitake et al. (2005) demonstrated that under severe RWS heating conditions, both plain concrete and steel fibre-reinforced concrete (SFRC) exhibited frequent and explosive spalling events. In contrast, synthetic fibres (polypropylene- and vinyl-based) provided superior mitigation by melting and creating a sacrificial network for vapour transport. This material-scale efficacy was further examined by McNamee et al. (2019), whose results indicated a saturation effect in fibre dosage: while a minimal content (0.2 kg/m 3 ) reduced spalling depth by 50%, the effect plateaued beyond a dosage of 0.6 kg/m 3 . However, medium-scale tests highlight that material properties are not the only governing parameters of spalling behaviour. Felicetti et al. (2024), using the Confined Slab Spalling Test (CSST), demonstrated that biaxial mechanical confinement (e.g., 5 MPa) significantly increases spalling severity by closing micro-cracks that would otherwise facilitate vapour migration. In such conditions, conventional polypropylene fibres may prove insufficient, necessitating the use of high-performance ones with high melt-flow rates to ensure complete protection. Thus, medium-scale tests reveal that the demand for fibre-induced permeability increases in proportion to the applied mechanical stress. 3.2. Advanced tunnel simulation: large-scale and structural testing. Large-scale tests shift the focus towards the structural performance of elements such as tunnel linings. Maluk et al. (2021) observed that in large, unconstrained slabs, spalling tended to be progressive rather than purely explosive, with material detaching progressively during the first 18 minutes of fire exposure. This structural complexity was fully captured in the study by Yan et al. (2015) on 1:3 scale tunnel segments. This research highlighted the critical role of hybrid fibre-reinforced concrete (HFRC). By combining steel fibres (for structural ductility and bending resistance) with polypropylene fibres (for decompression), the spalled area was limited to less than 0.5%, in contrast to 8.9% observed in conventionally reinforced concrete. Interestingly, large-scale curved elements may exhibit slightly more favourable behaviour than flat slabs, owing to a specific distribution of radial stresses which can impart a stabilising effect on the heated surface layers. 4. Trends in experimental evidence The synthesis of the experimental data collated in this review establishes a coherent framework for understanding the efficacy of various reinforcement systems in mitigating fire-induced spalling. As illustrated in Fig. 1, a distinct transition in material behaviour emerges when moving from plain concrete to fibre-reinforced composites. Plain concrete (PC) and steel fibre-reinforced concrete (SFRC) exhibit the most severe damage, with mean spalling depths typically exceeding 80-100 mm. These findings confirm that metallic reinforcement alone, while beneficial for post-cracking ductility, is insufficient to counteract the build-up of interstitial pore pressure during thermal exposure. In contrast, the introduction of polypropylene (PP) fibres leads to a substantial and systematic reduction in spalling