Fire-induced mechanical transformations in Hawkesbury sandstone: Experimental observations and analysis

Abstract

A series of laboratory experiments were conducted on Hawkesbury sandstone specimens that were exposed to a simulated fire following the modified hydrocarbon fire curve (MHC). For comparison, a similar test program was performed on specimens heated at four rates (2 °C/min, 5 °C/min, 10 °C/min, and 20 °C/min) up to 1100 °C and untreated specimens. Both uniaxial compressive strength (UCS) and elastic modulus showed a significant decrease due to heating, with a more striking loss of integrity for those heated following the MHC curve; the average decreases of UCS and elastic modulus across all four linear heating rates were 31.5 % and 57.1 %, whereas those for MHC were 79.2 % and 90.0 %, respectively. A similar observation was made for the tensile strength, where the linear heating rates showed an average decrease of 58.1 %, while that for MHC was 84.7 %. Among the linear heating rates, 20 °C/min displayed the most significant weakening due to heating. The microstructural observations exhibited pervasive heat-induced inter- and intra-granular cracking within heated specimens, which were more pronounced for 20 °C/min and MHC cases due to the thermal shock, leading to increased porosity and greater deformation when loaded as observed through digital image correlation analyses. The different heating conditions proved to produce different mineralogical changes within the specimens. The quartz content increased for all heated specimens, with the greatest increase for MHC, and the increasing heating rate caused a consistent increase in mullite content. Overall, the results of this study suggest that the rocks exposed to realistic fire scenarios can undergo significant degradation of mechanical properties, which cannot be reliably replicated by testing specimens that were heated at slower, linear heating rates, and stress the importance of assessing rock performances under real fire conditions to inform the designs of fire-resilient rock structures.