Study On Fracture Mechanics And Acoustic Emission Characteristics Of RCC Interlayer Interface

Roller compacted concrete dam combines the structural advantages of normal concrete dams and the construction advantages of roller compacted earth rockfill dams,and is recognized as one of the most promising representative dam types in the construction of high dams and large reservoirs.However,because the RCC dam is rolled in layers and has a large number of paving layers,if the layer is not treated properly,it will become the weak link of the structure.The interface between rolling layers is easy to produce initial microcracks due to improper construction,external environmental impact and interface stress concentration,and then develop into macro cracks under external load.The interface cracks may expand along the interface or deflect to penetrate the dam body.This will not only affect the overall strength and anti-seepage effect of the dam,but also affect the construction progress,and even endanger the dam safety.Therefore,the problem of interface between roller compacted concrete(RCC)has always been a subject of great concern to the academic and engineering circles of water conservancy,and the research on the interface fracture of RCC has attracted the attention of many scholars.In order to explore the fracture mechanics behavior and fracture damage mechanism of RCC interlayer interface,this paper uses hydraulic servo testing machine and advanced non-destructive acoustic emission monitoring means to carry out three-point bending and four point shear fracture tests on RCC composite specimens,and compares the I-mode fracture performance of RCC under different time intervals,layer treatment methods and maintenance conditions,The failure mode and crack propagation behavior of I-II composite mode fracture of roller compacted concrete(RCC)under complex stress state are studied,and various acoustic emission characteristics in the process of RCC fracture are analyzed.The conclusions are as follows:1.The results of three-point bending fracture test show that the time interval is the main factor affecting the fracture performance of RCC interlayer interface.The fracture initiation toughness and fracture energy of hot joint formed by three-hour interval have reduced by 20.8% and 31.7% compared with that of concrete body.The treatment mode of layer surface is an important factor affecting the fracture performance of roller compacted layer The results show that the initial fracture toughness and fracture energy of the specimens can be increased by 18.4% and 26.7% respectively by means of concave convex treatment,which is a very effective method for strengthening and toughening the rolled layer.The non-standard curing with high humidity and air can reduce the fracture toughness and fracture energy by 13.1% and 14.8% respectively.2.The results of four point shear fracture test show that the failure mode and crack propagation behavior of RCC I-II composite fracture are mainly affected by the toughness ratio of interface and concrete body and the mode ratio of crack tip.The higher the ratio of interface toughness and mode ratio is,the higher the possibility of deflection and propagation of interface crack is,otherwise,the failure along the interface is easy to occur.In this test,due to the short interlayer interval time and relatively high interface toughness ratio,the specimens are generally subject to deflection and expansion failure.3.The analysis of acoustic emission characteristic parameters and tensile shear fracture mechanism shows that the whole fracture process of roller compacted concrete can be divided into three stages: crack initiation,crack stable propagation and instability failure.The critical state of crack initiation and instability can be effectively judged by the mutation of AE impact number,AE energy and AE absolute energy accumulation,and based on AF and RA The change of the tensile shear failure mode of microcracks under different stress states is obtained by the ratio,thus revealing the fracture damage evolution law of crack development from discrete to concentrated,from micro to macro,from stable growth to unstable growth.It can be seen that AE technology can be used as an effective means to analyze and predict the interface fracture of engineering structures.4.Based on the spectrum analysis of acoustic emission signals,it is found that many high amplitude signals begin to emerge at the time of failure.After entering the instability stage,AE activity intensifies,and the dominant frequency changes from discontinuous strip distribution to continuous sheet distribution.During the fracture process,the dominant frequency band will transition from low frequency band to high frequency band.In the post peak stage,the dominant frequency band with frequency range of 39.1k Hz ～ 78.1k Hz will reach the peak.In the state of I-II composite stress field,there is not a single dominant frequency band as the dominant frequency band,which indicates that the failure modes inside the specimen are more diverse.These failure precursory information has a certain reference value for the analysis and prediction of RCC interface fracture.