PSI - Issue 84

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

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This study presents an analytical investigation into the mechanisms by which fibre-reinforced concrete (FRC) mitigates such risks. While the synergistic benefits of combining steel (S) and polypropylene (PP) fibres are acknowledged in the existing literature, this paper provides a critical review of the state-of-the-art, specifically evaluating how these constituent materials modify the thermo-mechanical response of concrete at elevated temperatures. 2. Thermal stress and mitigation strategies in tunnel fires To comprehend the role of advanced materials in enhancing tunnel safety, it is essential first to analyse the underlying failure mechanisms of concrete at high temperatures. This section provides a detailed examination of the physical processes responsible for fire-induced spalling. It begins by deconstructing the dual phenomena of thermal stress and vapour pressure build-up, before proceeding to an evaluation of fibre reinforcement as the primary mitigation strategy. 2.1. Mechanisms of fire-induced explosive spalling in concrete Although concrete is traditionally recognised for its inherent fire resistance, it remains particularly susceptible to fire-induced spalling – a complex multi-physical phenomenon that continues to be a significant challenge in fire engineering by Qiao et al. (2022). The literature identifies two primary, often interdependent, mechanisms that explain this failure mode. • Thermal stress mechanism , arising from the low thermal diffusivity of concrete. Rapid heating creates steep thermal gradients across the material section. This leads to differential thermal expansion, which generates high compressive stresses near the exposed surface and tensile stresses in the cooler core, thereby compromising the material's mechanical stability by Qiao et al. (2022) and Lo Monte & Felicetti (2019). • Vapour pressure mechanism , as internal moisture vaporises, pore pressure increases significantly. A pivotal factor in this process is the migration of water vapour towards the cooler interior, where it recondenses to form a ‘quasi-saturated layer’ by Qiao et al. (2022). This layer impedes further vapour migration, trapping gas and accelerating the build-up of internal pressure. • The synergy between these mechanisms precipitates explosive failure. Internal pore pressure accelerates crack propagation, while the thermally induced stresses provide the required energy for the violent detachment of concrete layers by Lo Monte & Felicetti (2019). 2.2. The Role of Fibers To counteract the mentioned failure mechanisms, modern mitigation strategies centre on the use of hybrid fibre reinforced concrete (HFRC). While steel fibres (SFRC) provide essential post-cracking ductility and are effective at bridging thermal cracks, they cannot prevent spalling alone and may even exacerbate energy accumulation within the matrix Yao et al., 2003). In contrast, polypropylene (PP) fibres serve as a sacrificial system. Upon reaching their melting point (approximately 170°C), they are absorbed into the cementitious matrix, thereby creating an interconnected network of micro-channels by Hager and Mróz (2019). For this decompression mechanism to be effective, the fibre dosage must exceed the percolation threshold, which ensures a continuous network for vapour evacuation by Kalifa et al. (2001). This synergy between steel and polypropylene fibres optimises the material’s thermo-mechanical response: the latter alleviate internal pressures to prevent explosive failure, while steel fibres maintain the structural load-bearing capacity under extreme thermal stress by Yao et al. (2003) and Kalifa et al. (2001). However, the efficacy of this approach is contingent upon several variables, including fibre dosage and geometry, heating rates, and applied mechanical loads. Accordingly, this review critically characterises FRC behaviour under fire conditions,