Fig. 5 is the ratio of the compressive strength of the test block to the 1H compressive strength of the heating time under the condition of strain rate 10-5/s. From Figure 5, it is known that the strength of the high temperature concrete is 0h to 3h, and its strength decreases linearly with the prolongation of the heating time. This also shows that the decrease of concrete is mainly in the process of temperature rising, and the effect of the heating time is lower than the increase of the heating temperature. At 100 C, the compressive strength of test block decreased with the prolonging of heating time.
At 300 C, with the increase of heating time, the speed of compressive strength of concrete block gradually decreases. This is because the concrete is a hot inert material. Because of the short heating time, the internal temperature of the block is little increased, the temperature stress is produced, the compressive strength of the test block is reduced to a small extent; with the increase of the heating time, the inside of the test block is increased. The enhancement of temperature promotes the unfinished hydration reaction and enhances the strength of the test block. At 500 C, the trend of compression strength decreased at 300 degrees C, and the strength decreased greatly after the heat 2h, and then slowed down. The reason is that the heating temperature is higher, the inner temperature of the block is improved, and the unfinished hydration reaction can be completed, thus the strength of the test block is improved.
In addition, with the increase of the heating time, the moisture in the block is constantly escaping, reducing the hydration reaction and reducing the strength of the test block. Under the combined effect of two factors, the variation of the strength of the block shows a wave . When the heating temperature is 700 C and the heating time is 1H, the temperature gradient in the test block produces a crack, which reduces the strength of the specimen. With the prolongation of the heating time, the internal and external temperature difference decreases gradually and the temperature stress decreases, but the internal temperature of the specimen increases the crystal decomposition and compression of the skeleton in the cement slurry. The intensity dropped sharply. Therefore, when the heating time is 2H ~ 3H at 700 C, the intensity decrease is greater than that of heating time from 1H to 2H.