Study On The Determination Of Mode Ⅰ And Mode Ⅱ Fracture Parameters For Concrete

Due to inherent defects of concrete materials and various influences from construction and operation periods of structures, concrete is inevitable to crack, especially for mass concrete structures. Thus the appropriate appraisal of crack stability, safety and consequent maintenance and reinforcement of concrete structures, and the improvement of concrete materials and mass concrete structure design have always been the issues of concern in the engineering and academic fields. For crack-resistance design during concrete mixing stages and structure pattern choices, together with the evaluation of cracks occurring in existing concrete buildings, the first thing is to determine the concrete fracture toughness. Considering these facts, the author carried out the researches on determination of modeâ… and modeâ…¡fracture parameters for concrete shown as follows. And this research is part of the professional standard of water conservancy and hydro power profession: Norm for Fracture Test of Hydraulic Concrete (DL/T5332-2005) and National Nature Science Foundation project "The modeâ…¡and mixed mode double-K fracture criterion and the determination of the corresponding double-K fracture parameters of concrete based on the cohesive force (50178015)" .1. The determination of crack initiation load, the size effect of fracture toughness as well as geometry influence are key issues in the norm. To provide sufficient test data for the norm, a total of 140 specimens were tested on three-point bending beams and wedge-splitting specimen fabricated with various grading aggregates and wet-screened components of dam concrete. The maximum size for three-point bending beams isS×D×B = 2200×550×240mm³ , and wedge-splitting specimens with 2H×D×S = 1200×1200×250mm³. The results show that its is feasible to use electric resistance strain gauges to determine the crack initiation load; no apparent size effect is observed in the double-K fracture toughness from three-point bending beams, while the double-K fracture toughness from wedge-splitting testing according to the norm shows a certain size effect; and the double-K fracture toughness from these geometries are almost of the same magnitude for the same aggregate grading and same specimen height.2. The size effect in the fracture toughness from wedge-splitting tests could be attributed to the influence of vertical component of load P_v and additional moment M_v on the stress field around the crack tip. Accordingly, a novel wedge-splitting test on compact tension specimen for true fracture parameters is proposed where the load is imposed at quarter-point of the specimen width to counteract the above-mentioned influences.3. In three-parameter law of fracture toughness for quasi-brittle materials, the cohesive force-induced fracture toughness K_Ic^c is difficult to be obtained whether by using integration or simplified method. For practical applications of the norm, an alternative determining method for K_Ic^c is put forward using a bivariate Lagrange interpolation function. By comparison withthe simplified method, the interpolation method is shown to have a higher accuracy. And still this much more simple calculating procedure can be achieved on a pocket calculator and it is very convenient for practical use.4. Failure mechanism for concrete is explored based on experiments on double-edge notched specimens. The measured initiation fracture angle is approximately O^-|o and the crack propagates along the ligament line. The presence of concrete fragments typical of shear fracture at the crack tip shows that the specimen is experiencing modeâ…¡fracture. Feature points on the experimental curve could be used to determine the critical load, and in turn the fracture toughness K_IIc is calculated by using the corresponding stress intensity factorformula for the given specimen. The proposed method is shown to be robust and applicable to predict modeâ…¡fracture.5. Based on the author's recent research, a symmetrically non-notched specimen is recommended to conduct modeâ…¡fracture tests of concrete and its stress intensity factor analytical formula is presented. A mirco-shear zone is developed in the front of the nominal ligament as a result of high stress concentration. As the modeâ…¡stress intensity factor in the tip of mirco-shear zone reaches the modeâ…¡fracture toughness, the specimen will fail in modeâ…¡fracture. Consequently, failure mechanism for this specimen is essentially the same with double-edge notched specimens6. Modeâ…¡fracture process for concrete could be described by three observable stages: crack initiation, stable propagation and unstable failure. The initiation fracture toughness K_IIcⁱⁿⁱ and unstable fracture toughness K_IIc^un are introduced here to differentiate these three stages. Thus a modeâ…¡fracture double-K fracture criterion for concrete is established, i.e., when K_II = K_IIcⁱⁿⁱ the preformed crack begins to crack initially; when K_IIcⁱⁿⁱ< K_II < K_IIc^un thepropagating crack develops steadily; when K_II≥K_IIc^un the crack propagates unsteadily.