Thermal spalling is a deterioration phenomenon which is of fundamental importance during durability analysis of concrete structures exposed to high temperature, e.g. during a fire. To assess the risk of this damage mechanism for a real concrete structure, numerical simulations are usually applied since experimental tests are very costly. Some aspects related to predicting thermal spalling by means of numerical modelling of chemo-hygro-thermal and damage processes in heated concrete, are presented in this work. First, we propose a spalling index, validate it with some experimental results and show how it can be used in the quantitative assessment of spalling risk. Then, the results of numerical simulations of a slab, made of two types of concrete (NSC and HPC), heated with three different rates, are discussed from the energetic point of view in order to indicate the main physical causes and predict the nature of thermal spalling: slow, rapid or violent. The presented results allow to assess the contribution of energy due to constrained thermal strains and compressed pore gas into the thermal spalling for different types of concrete heated with different rates.