Acoustic emission characteristics of full-graded concrete fracture process after cyclic damage
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Abstract:
As the main infrastructure of the water conservancy project, the concrete dam undertakes the functions of power generation, flood control and shipping. However, the fatigue damage inside the concrete, caused from repeated loads such as water, temperature and earthquake, accumulates continuously. The durability of the dam is seriously reduced and the stiffness is rapidly degraded, which endangers the normal operation of the dam. However, previous studies mainly focused on single static and dynamic load conditions. In addition, scholars often used wet screening concrete specimens, which were made of coarse aggregates with a particle size greater than 40 mm slowly after screening, to indirectly obtain the basic properties of dam concrete. Due to the influence of aggregate chain effect, it was difficult to accurately reflect the mechanical properties of dam concrete. Therefore, it is helpful to the construction of dam engineering and improves the safety of concrete dams, considering the influence of initial cyclic damage on the fracture characteristics of full-graded dams.In order to study the effects of increased cyclic amplitude (2.5 kN, 5.0 kN, 7.5 kN) on the fracture characteristics of full-graded concrete, three-point bending fracture test was carried out on fully-graded concrete specimens subjected to initial cyclic damage. The fracture damage process was monitored by acoustic emission technology. The results showed that the greater the increased amplitude of the initial cyclic load, the greater the cumulative damage of concrete beams in the initial cyclic damage stage. Moreover, the peak load after damage decreases correspondingly, and the brittleness characteristic is more obvious. Through the analysis of AE b-value, it is found that after the peak load, b-value had a significant decline stage. At this stage, the microcracks in the beam rapidly converged and expanded into macroscopic cracks. Late in the loading phase, the b-value fluctuated slowly around 1.0. It indicated that new micro-cracks were continuously generated inside the specimen and expanded into macroscopic cracks. The formation of macroscopic cracks in the loading process can be well characterized by the change of b-value. By comparing the AE damage index of energy, ringing count and impact number, it could be found that the damage index curves of the three groups of specimens appeared multiple points of intersection with the advance of the loading process. The damage index of impact number was more sensitive to the overall damage, while the damage index of the energy was more sensitive to the local damage. Through the analysis of RA-AF values, it could be seen that the proportion of shear cracks increases with the loading time, which is mainly due to the presence of large aggregate and boundary effect. But the proportion of tensile cracks always dominates. By comparing the shear crack proportion curves of the three groups of specimens, it could be found that the larger the amplitude with the initial cycle, the larger the proportion of shear cracks during failure.It can be concluded that the dynamic damage process of full-graded concrete specimens could be well characterized by analyzing acoustic emission parameters such as ringing number, energy and amplitude. It is showed that the greater the increase of initial cyclic load, the more serious the stiffness attenuation of dam concrete and the more obvious brittleness characteristics. Besides, in the middle and late loading period, the fracture characteristics of the sample were mainly affected by the distribution of large aggregates, but not by the initial cyclic damage. In addition, the effect of different initial cyclic damage on the fracture behavior of full-graded concrete was obtained, which could provide practical value for predicting the fracture failure of concrete dam.
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