From crack initiation to instability at high temperature: a two-stage mechanism for fire-induced spalling in concrete

Concrete's vulnerability to fire-induced spalling poses a critical challenge in structural applications, especially under extreme conditions.This phenomenon is highly material-dependent, with an elevated risk for modern concrete mixes, such as High-Performance Concrete (HPC), due to their dense microstructure.Understanding and mitigating this risk is crucial for enhancing structural safety in fire scenarios.This study investigates the two-stage mechanism underlying spalling: crack initiation and crack instability, both driven by thermal and hygral interactions and strongly influenced by the fracture behaviour of concrete.In the first stage, spalling begins when driving forces-such as pore pressure and thermal stress-surpass the tensile strength of concrete.Biot's coefficient, the shape and dimension of the structure are critical in determining this threshold.The second stage involves rapid crack propagation, driven by thermal energy conversion through vaporisation.This vaporisation occurs adjacent to the cracked region, pressurising the crack and accelerating it for instability.To explore these mechanisms, innovative testing methods were employed, including a frameless direct-tension test setup to evaluate fracture behaviour and a small-scale spalling test to analyse instability thresholds.The results reveal the complex interplay of pore pressure, thermal stresses, and moisture content in spalling dynamics.