Concrete Spalling Reframed: A New Theory Built on Known Mechanisms (IWCF RILEM 2025)

Concrete spalling under high-temperature exposure remains an incompletely understood phenomenon. Despite extensive experimental research identifying numerous influencing parameters, a unified theory rooted in the underlying physical mechanisms has yet to be established. This paper revisits hydraulic fracture induced by the thermal expansion of pore liquid as the primary mechanism for crack initiation and extends this concept by incorporating flash vaporization to explain the explosive character of spalling. Within this framework, the spalling criterion reduces to the attainment of full saturation at the drying front resulting from thermal expansion of the liquid phase. Based on this criterion, several representative problems with prototype geometries are analyzed using a simplified approach that combines analytical heat-conduction solutions with an ordinary differential equation governing the evolution of the drying-front position. The results demonstrate how temperature history, initial degree of saturation, and permeability jointly control the location of the drying front and, consequently, the temperature and thermal expansion of the pore liquid. Overall, the proposed theory reframes concrete spalling as a predominantly hygro-thermal process rather than one governed solely by mechanical stress, offering new insight into the interdependence of key parameters. This perspective provides guidance for the design of more meaningful experiments and enhances the potential for predicting full-scale structural behavior from small-scale test results.