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The early-age mechanical properties of ordinary concrete after a thermal cycle are investigated in this paper, to provide information on the residual safety of concrete structures after a fire occurred during the construction phase. The specimens are heated up to 150, 350 and 550 °C, from 3 to 28 days after casting, and tested in compression and in splitting tension after a rest period. The effects of curing period before heating, exposure temperature, cooling methods (in sprayed water or in air) and of the rest period prior to testing are investigated. The rest period after a thermal cycle performed at an early age brings in a marked recovery in terms of final residual compressive and tensile strengths. More than 50–60% of the loss may be recovered which is much lower than that documented in recent studies on fly-ash concrete. The lowest recovery is exhibited by the concretes exposed to high temperature at 7th day. Extending the rest period increases the strength recovery. For the concrete heated at 3rd, 7th and 14th day, cooling the specimens in sprayed water lead to larger strength recoveries than in air, both the strengths immediately after cooling and that after rest periods. [ABSTRACT FROM AUTHOR]

This paper presents the results of an extensive experimental investigation of the thermal and mechanical properties of Q345 steel pipe at elevated temperatures using both the steady-state and transient-state test methods. Under these two test conditions, the thermal expansion coefficient, yield strength and elastic modulus of the specimens at different temperatures were measured. The tested results indicate that both the yield strength and elastic modulus decrease gradually with increasing temperature. However, at the same temperature, both the yield strength and elastic modulus tested using the steady-state test are higher than those tested using the transient-state test. Hence, it is less safe to use the material properties tested using the steady-state test for fire resistance design of the steel structure. Based on the transient-state test results, the models of the mechanical properties of Q345 steel pipe at elevated temperatures were proposed. These models can be used for the design and analysis of Q345 steel pipe structures under fire conditions. [ABSTRACT FROM AUTHOR]

• The free expansion of concrete during cooling process was studied. • The linear expansion rate of concrete decreased with a decrease in temperature. • The coefficient of thermal expansion decreased with a decrease in temperature. • Residual deformation existed in concrete cooled from temperature above 400 °C. High temperatures can cause free expansion deformation of concrete. The deformation remains even when the concrete was cooled down to room temperature. In this paper, to investigate the variation of concrete free expansion during the cooling process, firstly the specimens were heated to the target temperatures of 100, 200, 300, 400, 500, 600, 700 and 800 °C, respectively in a furnace. After the target temperature was attained, the temperature of the furnace was maintained constant for 3 h. Then the specimens were allowed to cool to room temperature. The deformation of concrete in the cooling process were also measured. The results showed that the linear expansion rate (LER) of concrete at 100 and 200 °C was constant in the initial stage of the cooling process. The coefficients of thermal expansion of concrete (CTE) were −9.288 × 10−6/°C after being cooled from 800 °C down to room temperature. The LER of concrete at 300–800 °C increased slightly, and then the LER decreased gradually with the decrease of temperature. The expansion deformation of concrete at the target temperatures of 100–300 °C can be completely recovered during the cooling process. The residual deformation of concrete which was heated at temperatures above 400 °C was obvious. The residual deformation of concrete increased with the heating temperature. The residual LER of 0.036% was generated after being cooled from 400 °C down to room temperature. The residual LER of concrete after being cooled from 800 °C down to room temperature can reach 0.509%. [ABSTRACT FROM AUTHOR]