Using Central-notched Splitting-tension Cube Specimens To Study Double-K Fracture Toughness Parameters Of Concrete And Their Rate Dependence

Concrete is a heterogeneous, multiphase materials, which has the characteristics of high compressive strength, rich in raw materials, simple production process. So far, concrete had been widely used in various types of structures. But, due to the heterogeneous and multiphase features of concrete, there inevitably exits micro-cracks and gaps between cement matrix and aggregates. Therefore, for concrete structures, the whole failure process when subjected to external load is intimately associated with appearance and propagation of crack.Fracture test is an effective method to study concrete fracture behavior. The traditional methods for fracture test are three-point bend beam, compact tension specimen and wedge splitting test and so on. Recently, some researchers proposed to apply the Brazilian split-tension test into concrete fracture test and the central-notched cube or cylinder geometry of specimen had been adopted. Compared with traditional fracture tests, the Brazilian split-tension fracture test has several advantages, such as compactness, lightness, the influence of self-weight can be ignored, specimens can be drilled from existing structures and so on.Double-K fracture theory have described the whole process of concrete nonlinear fracture behavior, there are three stages for the whole process of crack development:crack initial, crack stable development and crack unstable development. The studies on double-K fracture parameters almost all based on static loading conditions in the past. However, in the range of dynamic loading rate, the study on double-K fracture parameters’rate dependence has not been carried out. In addition, there are some researchers already have studied the dynamic fracture properties of concrete, but a unified conclusion has not been obtained.Based on the above two problems, details of the present study are introduced as follow:(1) Central-notched split-tension cubes have been used to carry out fracture tests under static loading. The tested parameters are the relative widths of compression load-distribution (0.05,0.10,0.15,0.20) and initial crack/depth ratio (0.1,0.3,0.5). Also, the double-K fracture toughness, initial fracture toughness and unstable fracture toughness are calculated. Results show that when the relative widths of compression increases, the double-K fracture toughness have a trend of increasing, and the increasing in unstable fracture toughness is larger than initial fracture toughness. In addition, the results also indicate that when the initial crack/depth ratio (0.1～0.5) increases, the measured unstable fracture toughness increases, while the initial fracture toughness increases first and then decreases.(2) Based on Cohesive Crack Model and weight function method, the value of cohesive fracture toughness, which is caused by cohesive stress distributed along the fracture process zone, has been calculated analytical. Also, the unstable fracture toughness has been calculated analytical through three-parameter law which is proposed in double-K fracture model. Results show that the analytical calculation results of unstable fracture toughness agree well with the experimental results, which have about5%difference in the average relative error and the maximum difference is less than11%. The above results confirm that the double-K fracture theory could be used to analyze the fracture behavior of central-notched split-tension cubes.(3) Four different strain rates (10-5/s、10-4/s.10-3/s and10-2/s) have been adopted in the present central-notched split-tension cubes test and the unstable fracture toughness has been calculated experimental and analytical. Results show that a good agreement between the analytically calculated initiation fracture toughness and the experimental ones was demonstrated, confirming that double-K fracture model could be further extended to characterize fracture failure of concrete structures subjected to load with high loading rate such as earthquake excitation.(4) The rate dependence of concrete double-K fracture parameters have been studied preliminary. Results show that both initiation fracture toughness and unstable fracture toughness increase when the strain rates increase from10-5/s to10-2/s. Subsequently, the rate dependence of measured unstable fracture toughness could be described using both Baznat model and the revised Shah model. Finally, based on the measured results, the present study tries to establish a relationship between double-K fracture toughness (unstable fracture toughness and initial fracture toughness) and the velocity of crack length before crack unstable development.