Stochastic Characterization of Postfire Concrete: Quantifying Residual Mechanical Variability via Extensive Uniaxial Compression Testing

This study presents a large-sample experimental investigation on the residual mechanical properties and statistical variability of normal- and high-strength concrete after exposure to elevated temperatures. A total of 240 prism specimens was tested under uniaxial compression following consistent procedures for mix design, curing, heating, and natural cooling. This consistent protocol enabled reliable evaluation of temperature-induced degradation and variability in compressive strength, elastic modulus, and deformation behavior. In addition to extracting key mechanical parameters, the study systematically captured full stress–strain and stress–volumetric-strain responses from ambient to 800°C. Results reveal temperature-dependent reductions in stiffness and strength, coupled with increasing peak strain and post-peak ductility. Volumetric strain curves showed distinct inflection points, indicating a clear transition from compaction to dilation, which becomes more pronounced with temperature. Compared to normal-strength concrete, high-strength concrete exhibited more significant stiffness loss but relatively milder strength degradation. In addition to revealing average trends, the study quantified the statistical variability of key mechanical parameters, with coefficients of variation for compressive strength reaching 15.5% and 17.1% at 600°C and 800°C, respectively, values representing lower-bound variability under controlled laboratory conditions. Empirical trend models were then developed to characterize how this variability evolves with temperature. By providing a large-sample and internally consistent dataset, this study fills a significant gap in the literature. The results lay a solid foundation for developing stochastic constitutive models, calibrating uncertainty parameters, and informing future fire-resistance design guidelines for concrete structures exposed to high temperatures.