|Violent Spalling is the separation of small or larger pieces of concrete from the cross
section, during which energy is released in the form of popping off of the pieces and
small slices with a certain speed, and also a popping or cracking sound.
This type of spalling is caused by pore pressure and thermal gradients; internal cracking on the meso-level also influences this spalling process.
The surface compression during heating can increase due to lateral restraint, reinforcement, prestressing, large concrete thickness and a high heating rate.
Pore pressures are dependent on heating rate, moisture content, permeability, porosity and the presence of polypropylene fibres (artificial permeability).
Furthermore, an increased ductility of concrete by steel fibres has sometimes been reported to reduce the risk of this type of spalling,
Fellinger and Both (1997).