Evolution of pore pressure in self-compacting concrete with natural fibers at high temperatures

Despite the fact that concrete pore pressures play an important role in structural integrity in realistic fire scenarios, there are still few reports on understanding the evolution of pore pressures during all fire phases, especially during constant temperature and natural cooling. The current gap in understanding still exists due to the past focus on the heating phase. This study investigated pore pressure variations in natural fiber-reinforced self-compacting concrete with varying dosages under high-temperature conditions, expanding upon previous research that focused solely on pore pressure measurements during the heating phase. Novel observations were incorporated for both constant-temperature and natural cooling stages, with particular emphasis on the emergence and dissipation mechanisms of pore pressure arising from thermal expansion mismatch between the matrix and vapor. During the heating phase, pore pressure was primarily attributed to the vapor pressure generated by the evaporation and expansion of free water in cementitious materials and decomposition water from hydration products, accompanied by in-depth analysis of vapor source identification and pore formation dynamics. Notably, a phenomenon contradicting conventional expectations emerged during the constant-temperature phase: The persistent thermal expansion discrepancy between matrix pores and vapor resulted in incomplete vapor release, thereby inducing secondary pore pressure development. Meanwhile, the maximum pore pressure decreased from 1.91 MPa to 1 MPa as the fibre volume doping increased from 0 to 0.3 %. To elucidate these mechanisms, systematic validation was conducted through thermal expansion testing, mass variation, FTIR, and water vapor adsorption experiments. During the cooling phase, synchronized temperature reduction of internal moisture decreased pore pressure, while vapor adsorption from external environment by the matrix led to further mass increase.