This study investigates the stress-strain relationship and the microstructure, in addition to visual observations of color change, cracking, and spalling, of normal-strength concrete (NSC) and high-strength concrete (HSC) after exposure to various temperatures. An electric furnace was used to heat cylindrical specimens with dimensions of 100×200 mm to the temperatures of 400°C, 600°C, and 800°C to measure the mechanical and microstructural properties of concrete at those extremes. Both NSC and HSC maintain their structural integrity with slight surface damage at 400°C. By 600°C, thermal damage is clearer, with C-35 developing cracks and rough surfaces and C-55 showing signs of internal microcracking. NSC spalls and suffers significant surface cracking at 800°C, while HSC experiences internal damage because of its dense structure. Both compressive strength and axial strain results shown a significant degradation with increasing the temperature. The compressive strength decreased by 22.4%, 61.5%, 79.3%, at 400°C, 600°C, and 800°C; respectively. Regarding Stress-strain curves demonstrate reduced stiffness and strength as temperature increases. At 800°C, the strength and ductility of both types of concrete decreased significantly; HSC showed a greater reduction. These findings highlight the need for temperature effects to be accounted for in the design of structures made of concrete and subjected to high temperatures. This investigation can help with the design of structures that are resistant to fire and the evaluation of the structural integrity after exposure to fire.
