Abstract: In order to study the strength attenuation characteristics and damage effects of microstructure of concrete under high temperatures, the CT scanning, unconfined compression and XRD diffraction tests were carried out. Meanwhile, the mechanism of concrete damage at high temperatures was analyzed. The experimental results show that the unconfined compressive strength fcu and elastic modulus E of concrete gradually decrease with the increase of temperature from 20 ℃ to 1 000 ℃, and the attenuation rate is first slow and then fast. The attenuation amplitude of unconfined compressive strength and elastic modulus increases with the extension of heating time. The damage degree of concrete meso-structure accumulates gradually in the process of fire development. With the increase of fire temperature, the number and size of concrete cracks increase gradually. The porosity of concrete increases exponentially with the increase of temperature, and the unconfined compressive strength has an obvious negative linear correlation with the porosity. The gradual dehydration and decomposition process of gel materials is the essential mechanism of the high temperature damage effects of concrete.

Key words: concrete, high temperature, mesoscopic structure, elasticity modulus