Deterioration Of Mechanical Property And Identification Of Instability Precursor Of Concrete Under Freeze-thaw Cycles

The freeze-thaw performance of concrete is one of the important factors affecting the durability of building structures in cold areas.It is of great significance to reveal the deterioration of mechanical properties and the precursor characteristics of instability after freeze-thaw cycle of concrete.In this thesis,the bearing capacity,fracture characteristics and acoustic emission activity of concrete deteriorated are systematically studied by means of theoretical research and laboratory test.Based on the critical moderation theory,the critical moderation characteristics of acoustic emission information of freeze-thaw deteriorated concrete damage were analyzed.The precursory information of deteriorated concrete damage is obtained.The research results are as follows:(1)The programming constant temperature testing machine was used to freezethaw the concrete specimens.The physical change characteristics of the freeze-thaw deterioration process of concrete specimens are obtained.It is found that the average mass loss rate of specimens gradually increases as the number of freeze-thaw cycles increases.The surface of the specimen appears pockmarked,desizing and even block falling.The macroscopic cracks gradually increase,the ultrasonic wave velocity decreases,and the overall damage deepens.(2)Based on uniaxial and triaxial compression tests,the mechanical property changes and fracture characteristics of freeze-thaw deteriorated concrete in the whole process were studied.It is found that in the uniaxial compression test,with the increase of the number of freeze-thaw cycles,the bearing capacity and elastic modulus of concrete decrease,and the Poisson’s ratio increases.In the triaxial compression test,the deformation ability of concrete is enhanced with the increase of the number of freezethaw cycles.And under high confinement pressure,the specimen presents a "drum" shaped failure mode.(3)Acoustic emission information of the whole process of the damage of freezethaw deteriorated concrete was collected.The damage evolution process of concrete was characterized based on the characteristics of concrete acoustic emission parameters.The results show that the acoustic emission signal can better characterize the damage state of the specimen.There is a certain commonality between the amplitude,peak frequency,and impact of acoustic emission.The change in the b value corresponds to the damage process.The positioning point is consistent with the fracture form of the specimen.(4)Based on the critical slow down theory,the acoustic emission signal of the loading process of the compression test of the deteriorated concrete is analyzed.The precursory characteristics of the curves of acoustic emission duration,count and rise time are studied.The results show that the accuracy of the precursor points obtained by this method can reach more than 80% of the peak stress.The variance may be the main judgment basis,and the autocorrelation coefficient may be the secondary judgment basis.It is of great significance to identify the instability precursor information of freeze-thaw deteriorated concrete.