AbstractIt is well known that fire-induced spalling usually occurs to high-strength and ultra-high performance concretes. However, experiments have shown that thermal spalling can also occur to normal strength concrete (NSC) and the high moisture content is conducive to the occurrence of spalling of specimens and structures of NSC. In view of the fact that the spalling analysis of NSC is still lacking and the spalling mechanism of NSC is still not well understood, in this paper, the thermo-chemo-hydro-mechanical behavior of a NSC cube specimen with 90% moisture content exposed to fire is numerically studied at a mesoscale based on the experiment reported in the literature. By analyzing the nonlinear mechanical effects of the vapor pressure and the thermal stress, the spalling mechanism is investigated. It is concluded that, instead of explosive spalling, surface spalling induced by the vapor pressure occurs to the specimen. This conclusion is also confirmed by a strain energy analysis. Furthermore, with the energy analysis, the feature of non-explosive spalling of NSC under fire heating is also explained.