Fracture Mechanics of High Strength Concrete and Size Effect

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Crack resistance and size effect in high strength concrete were studied from the view point of fracture mechanics of concrete. The following results were obtained.1) Although fracture energy of concrete increases up to about 80MPa of the compressive strength, it turns to decrease beyound 100MPa of the compressive strength when RILEM standard notched beams of 100x100x8400 (mm) were used. On the other hand, wedge splitting compact tension specimens with a six times larger size give a higher values of the fracture energy than that of notched beams even for the very high strength concrete of about 100MPa.To investigate the mechanism to couse the difference, the changing rates of tangential stiffness load-CMOD curves were analyzed. It was shown that the fracture process zone in front of the crack tip was fully developed before the fracture in large specimens but not in smaller ones.2) While high strength concrete is realized by making the microstructure finer, it changes strongly the tension softening properties and especially increases the maximum cohesive stress. It was clarified that the maximum cohesive stress is empirically described as a function of the porosity.3) Brittle index defined with three parameters determined by standard tests, can well charcterize the crack resistance properties of concrete with various strength and sizes of specimens.4) Steel fiber reinforcement provides high strength concrete with higher crack resistance.