Abstract: To reveal the deterioration mechanism of concrete under the action of drying-wetting cycles, the drying-wetting cycles tests of different strength grades of concrete was carried out, and the compressive strength, splitting tensile strength, ultrasonic wave velocity, porosity, and other physical quantities of concrete were measured in the test process. The effect of drying-wetting cycles on the physical and mechanical properties of concrete was explored, and the relationship between concrete strength and pore structure was analyzed from the micro-scale. The results show that with the increase of drying-wetting cycles,the strength of concrete increases at the beginning, and then decreases. After 40 drying-wetting cycles, the relative compressive strength of C20, C30, C40 and C50 concrete decreased 13.33%, 12.47%, 8.45% and 6.58% respectively, and the relative splitting tensile strength decreased by 25.93%, 23.06%, 20.59% and 19.03% respectively, the relative splitting tensile strength is more obvious than the compressive strength attenuation. The ultrasonic velocity and ultrasonic amplitude show different variation laws, with the increase of drying-wetting cycles, the ultrasonic velocity increases first and then decreases, while the ultrasonic amplitude increases continuously. In drying-wetting cycle environment, with the increase of drying-wetting cycles, the porosity, total pore volume, average pore diameter, median pore diameter and most probable pore diameter of concrete samples decrease firstly and then increase. Finally, it shows the characteristics of pore coarsening,which is also the internal reason for the later loss of concrete strength.

Key words: concrete, drying-wetting cycle, strength, ultrasonic wave, microstructure