Study on Freezing-Thawing damage mechanism and evolution model of concrete

Freezing-thawing (F-T) cycle is one of the main reason of concrete damage in cold regions. F-T cycle mainly lead to degradation of macroscopic mechanical properties of concrete by changing the number of micropores, mesopores and macropores, and causing severely damage to its internal texture. The variations of the physical and mechanical properties of concrete sample with different inclination angle of internal flaw exposed to different FT cycles (0, 10, 30 and 50 cycles) under unconfined compression condition was deeply investigated based on the acoustic emission (AE) and nuclear magnetic resonance (NMR) technology. The evolution of the internal cracks are tracked in real-time by AE monitoring systems during the entire test process. Meanwhile, the micro-structure of the concrete sample after F-T cycle treatment is microscopically observed with T-2 distribution by low-field nuclear magnetic resonance. The results show that the peak strength of concrete decreases with the increasing F-T cycles and increases as the increasing inclination angle of prefabricated flaw. The elastic modulus decreased gradually with the increase of F-T cycles. The elastic modulus of the sample without F-T cycles treatment increases with the increase of inclination angle of prefabricated flaw. As the flaw inclination angle increases from 0 degrees to 90 degrees, the F-T cycles increases from 0 to 50, the peak strain shows an obvious discreteness without monotone change. When the number of F-T cycle reaches 50, concrete blocks fall off from the sample surface. A continuum damage model considering F-T damage from different pore size is finally introduced to describe the damage evolution of concrete during compression test. The research results can be used to quantitatively evaluate the strength of concrete samples treated by F-T cycle without mechanical tests.