Investigation on the free expansive deformation of concrete during the heating process

•The free expansion of concrete during heating process was studied.•The thermal-induced axial strain of concrete increased with a rise in temperature.•The thermal expansion coefficient of concrete increased with a rise in temperature.•The thermal-induced axial strain increased with temperature holding time.•The thermal expansion coefficient increased with temperature holding time. Concrete experiences expansive deformation at high temperatures. The thermal stress resulted in the development of cracks inside the concrete members. To investigate the free expansive deformation of concrete during the heating process, the concrete specimens were heated from room temperature to different target temperatures with an electric furnace. The temperature in the furnace was maintained at the target temperature to provide a thermostatic condition until the center of the specimen also reached the target temperature. The deformation values of the concrete specimens in both of the temperature rising stage and temperature holding stage were measured and analyzed. The experimental results show that the concrete began to deform at 40–60 °C. The thermal-induced axial strain and thermal expansion coefficient of concrete gradually increased with a rise in temperature. After being heated to 800 °C, thermal-induced axial strain and thermal expansion coefficient of concrete M35 reached 1.018% and 13.135 × 10−6 °C, respectively. The thermal-induced axial strain of concrete continued to increase during the temperature holding stage and gradually stabilized after 60–65 min. After the temperature in the center of the specimen reached 800 °C, the thermal-induced axial strain and thermal expansion coefficient of concrete M35 reached 1.252% and 16.155 × 10−6 °C, respectively. For concrete M45 with higher compressive strength than M35, the thermal-induced axial strain and thermal expansion coefficient of concrete M45 show a variation law similar to concrete M35.