This research article investigates the thermo-hygro coupled mechanical behavior of high-performance concrete (HPC) under severe transient heating conditions, specifically from ambient temperatures rising to 1000°C for a duration of four hours. The experimental study reveals that HPC mix designs containing supplementary cementitious materials (SCMs) are highly susceptible to thermally induced spalling. It was found that the chemically bonded saturated moisture within the highly durable concrete matrix significantly contributes to explosive spalling, more so than free saturated moisture during transient heating conditions. Under unrestrained conditions, HPC blocks that are subjected to severe heating conditions on one side are especially prone to developing pore pressure at a depth of 20 mm from the heating exposure surface. Nominal spalling has been observed in the concrete matrix at temperatures ranging from 126 to 266°C. This phenomenon is identified by the presence of surface cracks throughout the concrete blocks. In contrast, explosive spalling occurs at higher temperatures, specifically ranging from 325 to 449°C, leading to significant damage to the concrete matrix. It is also observed that within the range of 325 to 449°C in the concrete matrix, the phase change of saturated moisture to vapor increases the heat transmission rate in the concrete matrix. It is essential to address these issues in order to improve the behavior of concrete at elevated temperatures. This research outcome may aid in the development of new techniques to mitigate concrete spalling and enhance the safety and integrity of reinforced concrete (RC) structures, especially under severe fire conditions. Ultimately, this can help reduce the risk of failure or collapse and improve the fire resistance rating of RC structures.