AbstractCracks are one of the factors that can weaken reinforced concrete structures (RCSs), and are associated with a reduction in concrete durability. RCSs could encounter concrete crack issues due to high temperature as a result of fire. Over time, unrepaired concrete cracks can spread on the surface and trigger serious damage to the structure. Therefore, the repair of concrete cracks is essential. In the present laboratory study, cubic concrete specimens with dimensions of 150×150×150 mm were exposed to heat at 600°C and 800°C. Then, the ability of two calcium carbonate-producing bacteria, Sporosarcina pasteurii and Bacillus sphaericus, to repair thermal cracks was evaluated. After aging for 28 days, the specimens were placed in a furnace at 600°C and 800°C for 1 h, with a 5.5°C increase per minute. The efficiency and effectiveness of the two microorganisms on the specimens was then evaluated for compressive strength, water absorption, and water capillary absorption. The tests indicated that the microorganisms had a substantial ability to repair both surface and deep cracks; both types of cracks were completely filled and repaired by the calcium carbonate deposits, and the compressive strength of specimens repaired by the microorganisms increased by 31%–93%. Furthermore, water absorption decreased by about 30%–43%, and capillary water absorption decreased by 15%–31%. Scanning electronic microscope (SEM), X-ray diffraction (XRD), and stereo microscopic images further confirmed the high activity of the microorganisms in the production of calcium carbonate deposits.