Effect of micro‐structural changes on concrete properties at elevated temperature: Current knowledge and outlook

Abstract

AbstractThe influence of microstructural changes on concrete properties at elevated temperatures is presented and discussed in this paper, as the exposure to elevated temperatures brings in marked changes in concrete thermal, mechanical and kinematic properties owing to the heat‐induced evolution of material's microstructure. The major microstructural changes consist in increasing pore size, occurrence of microcracking and activation of chemo‐physical processes in the hardened cement‐paste. These processes (like portlandite dissociation and crystalline change in quartzitic aggregate) occur above 400°C and are more pronounced in high‐strength than in normal‐strength concrete. Increasing pore size and microcracking, however, yield the largest contribution to concrete strength‐loss, that occurs mainly between 400 and 800°C, depending on aggregate type and initial moisture content in the mix. On the whole, calcareous (or carbonate) aggregate behaves better than siliceous aggregate face to increasing temperatures. Initial moisture content favors concrete thermal spalling, mostly between 200 and 400°C, but such a complex phenomenon is still open to investigation, as demonstrated by the various theories found in the literature, each with its chemo‐physical mechanisms and limitations. Three theories are discussed in this paper, and these theories together with other critical issues allow to focus on future work.