Fast vapour migration next to a depressurizing interface: A possible driving mechanism of explosive spalling revealed by neutron imaging

When exposed to high temperatures, concrete is prone to explosive spalling, resulting in the projection of concrete flakes and the reduction of the structural element cross-section. Elastic energy alone cannot justify the explosive nature of the process: accumulated thermal energy serves as a supplementary source, together with the water in the pores, which can generate kinetic energy through vaporization. The objective of this paper is to study the fast thermal and hygral transients occurring upon depressurization of an internal interface, emulating a crack developing in hot moist concrete. One possible mechanism that could justify this contribution is the flash vaporization of water. This paper presents a new experimental approach to directly measure flash vaporization through rapid-neutron imaging. Additionally, a thermo-hygral model assesses key parameters influencing fasttransient phenomena. Experimental observations reveal moisture loss in a 1-mm-thick layer, equating to a 90 degrees C temperature drop, justifying a significant amount of released energy.