Flash vaporization next to an opening crack: a possible explanation of the explosive nature of concrete spalling

In this paper, the very quick vaporization of water in the pores of hot concrete next to an opening crack is regarded as a way to convert the considerable amount of thermal energy stored in the material into kinetic energy of the fractured splinters. To this purpose, a special setup has been developed in order to suddenly release the vapor pressure accumulated at a sealed edge of a heated sample. The quick local cooling observed confirmed that a sizeable loss of thermal energy occurs in a rather thin layer of concrete facing the depressurized surface. Fast sequences of Neutron Radiographic images allowed to recognize that the influenced zone doesn't exceed 1mm thickness. This has a direct impact on the time scale of thermal and hygral transients and corroborates the idea that a significant contribution to the explosive nature of spalling may be provided by water vaporization.