The elevated temperature behaviours of concrete specimens made of three different concrete grades are presented in this paper. Cylindrical specimens of concrete mixes with 17 MPa (designated as M17), 21 MPa (designated as M21) and 27 MPa (designated as M27) target strengths were employed. Different temperature progression in the core of the concrete cylinders was observed for specimens of different concrete grades. Concrete discoloration started at 300°C, while hairline cracks appeared at 400°C on the surface of the cylinders. Pore pressure generated by steam and carbon dioxide during heating of the cylinders caused explosive spalling at a furnace temperature of 400–600°C. Similar loss of compressive strength of concrete was found for all the employed mixes, which reached to nearly 70% at 800°C. The ultimate strain of concrete increased at higher temperatures. The loss of concrete elastic modulus reached 80%, whereas nearly 50% tensile strength was lost at 600°C. A comparison of observed data with the theoretical predictions based on the analytical models suggested by some of the international codes was made and differences between the predicted and observed data were found. Coefficients of strength reduction and corresponding strains were proposed, which correlated well with the observed stress–strain curves.

中文翻译：

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高温下混凝土的残留特性

本文介绍了三种不同等级混凝土制成的混凝土试样的高温行为。使用具有17 MPa（指定为M17），21 MPa（指定为M21）和27 MPa（指定为M27）目标强度的混凝土混合料的圆柱试样。对于不同等级的混凝土，观察到了混凝土圆柱芯中不同的温度变化。混凝土在300°C时开始变色，而圆柱体表面在400°C时出现细线裂纹。加热钢瓶期间由蒸汽和二氧化碳产生的孔隙压力在400-600°C的炉温下引起爆炸性剥落。对于所有使用的混合物，发现混凝土的抗压强度都有类似的损失，在800°C时达到近70％。高温下混凝土的极限应变增加。混凝土的弹性模量损失达到80％，而在600℃下损失了近50％的拉伸强度。根据一些国际法规提出的分析模型，将观测到的数据与理论预测值进行了比较，发现了预测值和观测值之间的差异。提出了强度折减系数和相应的应变，这与观察到的应力-应变曲线很好地相关。