| The structures of the actual engineering often exist defects because of the disadvantages of the concrete itself. These defects can be extremely easy to result the concrete cracking. With cracks in the structure will not lose the ability to work. Crack propagation process of concrete materials can be divided into three distinct phases:crack initiation, stable propagation and unstable failure. Double-K fracture models think that the increasing fracture toughness when the cracks expand stably will increase the cohesion which have the role of crack closure. Therefore, it is very important to improve the fracture toughness of concrete for engineering structures durability and practicability.In the hydraulic concrete structures, not just by virtue of the method to improve the strength of concrete structures to ensure that cracks resistance ability. The most effective way is to research the materials from the concrete aspects of the design and construction technology and to find the degree of the best mix which can improve the fracture toughness and to ensure good performance of concrete under the premise of the cost saving,From the concrete three aspects of mixing speed, fly ash and the maximum size of coarse aggregate to study the crack resistance of concrete with cracks, analyze the influences of various factors on the fracture behavior and the reasons for changing initiation toughness values and provide effective and feasible schemes for the study of hydraulic concrete anti-cracking. Wedge splitting specimens are applied to determined the double-K fracture parameters. According to the information provided by the ascending branch of the load-crack tip opening displacement (P-CTOD) curve recorded in the test, one can evaluate the initial cracking load PQ. The initiation toughness KICQ which is obtained by inserting PQ and the preformed notch length ao. KICQ is called unstable fracture toughness and its value is determined by inserting the maximum load Pmax, and the critical effective crack length aC measured from the load-crack mouth opening displacement (P-CMOD) curve. By comparing compressive strength values in the 28 days and the test days, elastic modulus of calculated values and empirical formula values, the average values of the initiation load and the unstable load, the average values of fracture toughness and unstable fracture toughness in each group, concluded: 1. it is a feasible way to get PQ by the displacement sudden-increase line of the ascending branch of P-CTOD curve.2. Fly ash can improve effectively the fracture properties of hydraulic concrete dams. When the dosage of fly ash is by 30%, the cube compressive strength, initiation toughness and unstable toughness are the highest values.3. When the speed is at 1.4 m/s, concrete cube compressive strength is the highest value. When the speed is small, concrete crack is not easy to initiate. When the speed is large, unstable fracture is not easy to turn out. Mixing speed in concrete has no effects on the critical effective crack length. The elastic modulus values which calculated by empirical formula and the experimental values have very good agreement degrees.4. With the maximum size of coarse aggregate increasing, the strength of concrete was a generally increasing. The elastic modulus values which calculated by empirical formula and the experimental values have little difference. The maximum size of coarse aggregate Dmax has no effects on the critical effective crack length. When the Dmax≤40 mm, KICQ,KICS with Dmax size increases; When Dmax>40 mm, KICQ,KICS with Dmax decreases.Keywords:concrete; mixing speed; fly ash; the maximum size of coarse aggregate; fracture toughness... |
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