Concrete exposed to elevated temperatures introduces a risk of spalling, which can cause significant damage to reinforced concrete (RC) structures. The problem of concrete spalling is complex and involves various mechanisms, with thermo-hydro being particularly influential in causing both normal and explosive spalling at high temperatures. To better understand this behavior, a laboratory-scale experimental study was conducted to assess the effects of thermo-hydro mechanisms on concrete spalling. Specially designed instruments were used to measure vapor pressures and heat transfer in high-performance concrete (HPC) blocks exposed to severe transient heating conditions at 1000 °C for up to 4 h. The results indicated that pressure formation in HPC concerning heat transfer was highest at temperatures from 186 to 424 °C, with maximum pressure observed at 210 kPa between depths 10–40 mm from the fire exposure surface. These findings indicate that structural members made of HPC are more prone to temperature-induced spalling due to the thermo-hydro process within the concrete matrix at temperatures from 186 to 424 °C. This susceptibility may compromise their fire resistance rating and pose a risk of RC structural damage. Designers and engineers must consider the risk of spalling when working with HPC structures and take appropriate measures to ensure their fire resistance rating meets national building codes and practices. By addressing this issue, the safety and integrity of RC structures can be better protected against failure or collapse in the event of a severe fire load.