On the influence of aggregate alteration on shotcrete fire-induced spalling: A coupled H-TRIS and high-speed photography approach

This study explores the influence of fine aggregate alteration on fire-induced spalling in tunnel lining materials, with a particular focus on the substitution of natural sand with synthetic aggregates. An experimental approach employing the Heat-Transfer Rate Inducing System (H-TRIS) coupled with high-speed imaging, was developed to simulate realistic tunnel fire scenarios, including ISO-standard and hydrocarbon fire curves with varied durations. Using crushed waste glass (CWG) as a representative synthetic fine aggregate, four replacement levels (0%, 10%, 25%, 50%) were tested to evaluate thermal response, spalling severity, and failure mechanisms. The results reveal that aggregate alteration significantly affects spalling behaviour: moderate CWG incorporation (10%–25%) enhanced resistance under prolonged fire exposure due to improved thermal stability and insulation, while short-duration hydrocarbon fires triggered increased spalling susceptibility due to elevated pore pressure from reduced permeability. Post-fire diagnostics, including surface roughness analysis, computed tomography (CT) scanning, and fragment size distribution, confirmed altered cracking and failure modes associated with aggregate type and content. These findings underscore the critical role of fine aggregate selection in fire performance and support the integration of sustainable, recycled materials in performance-based tunnel fire design. • Spalling in tunnel fires was captured using coupled fire testing and imaging. • Fine aggregate changes affect spalling; crushed waste glass improves resistance. • Spalling varies with fire curve and duration due to thermal and pore effects. • CT imaging and fragment analysis assessed mechanisms of spalling failure.