Mathematical modelling and experimental assessment of fire-induced shotcrete spalling

To ensure the structural integrity of underground structures during fires, effective fire safety measures must be incorporated into their design. The fire performance of tunnels depends heavily on their support systems, with shotcrete being a cost-efficient and widely adopted tunnel lining solution. Its ability to be sprayed directly onto various surfaces makes it adaptable to diverse tunnel cross-sections and applications. However, during a fire, shotcrete is susceptible to fire-induced spalling, the sudden ejection of surface layers, which can reduce cross-sectional area, lower fire resistance, and cause severe structural damage. Spalling is generally attributed to two primary mechanisms: thermal stresses and vapour pressure build-up. Understanding the interplay between these mechanisms is essential for developing mitigation strategies. This study employed laboratory-scale fire tests to assess the performance of shotcrete under unrestrained and unloaded conditions, considering different sample sizes and moisture contents. A mathematical framework was developed to quantify the contributions of thermal stress and vapour pressure to spalling and to estimate the associated energy release, enabling the calculation of spalled particle velocities. Advanced imaging techniques were used to observe spalling progression and validate the velocity estimates. The results identified vapour pressure build-up as the dominant energy source during spalling events, with thermal stress promoting micro-crack formation that facilitates vapour escape and particle ejection. Heated surface area and moisture content were also found to be significant factors influencing spalling severity. Future research should examine shotcrete behaviour under mechanical load to evaluate how compressive stresses from restrained thermal expansion interact with vapour pressure and thermal stress mechanisms.