The 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.

中文翻译：

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高温下微观结构变化对混凝土性能的影响：当前知识与展望

本文介绍并讨论了高温下微观结构变化对混凝土性能的影响，因为由于材料微观结构的热诱导演化，暴露在高温下会导致混凝土热、力学和运动学性能发生显着变化。主要的微观结构变化包括孔径增加、微裂纹的出现和硬化水泥浆中化学物理过程的激活。这些过程（如石英质骨料中的硅酸盐解离和结晶变化）发生在 400°C 以上，并且在高强度混凝土中比在正常强度混凝土中更为明显。然而，增加孔径和微裂纹对混凝土强度损失的贡献最大，主要发生在 400 到 800°C 之间，取决于骨料类型和混合物中的初始水分含量。总体而言，钙质（或碳酸盐）骨料在温度升高时表现优于硅质骨料。初始含水量有利于混凝土热剥落，主要是在 200 到 400°C 之间，但这种复杂的现象仍有待研究，正如文献中发现的各种理论所证明的那样，每种理论都有其化学物理机制和局限性。本文讨论了三种理论，这些理论与其他关键问题一起可以专注于未来的工作。但如此复杂的现象仍有待研究，正如文献中发现的各种理论所证明的那样，每种理论都有其化学物理机制和局限性。本文讨论了三种理论，这些理论与其他关键问题一起可以专注于未来的工作。但如此复杂的现象仍有待研究，正如文献中发现的各种理论所证明的那样，每种理论都有其化学物理机制和局限性。本文讨论了三种理论，这些理论与其他关键问题一起可以专注于未来的工作。