FRACTURE TOUGHNESS OF CONCRETE.

Even though concrete is a relatively brittle material, its fracture toughness is currently not used as a design tool. Concrete structures are normally reinforced so that a designer often neglects the tensile strength of plain concrete, and design is usually based on approximate, empirically based formulae. However with an increase in efficiency and accuracy possible with the numerical methods, a need to rationally and accurately trace cracking in concrete has become evident. The effective critical crack length is assumed to be a function of the two parameters, critical crack tip opening displacement and the critical stress intensity factor. This two parameter fracture model developed at Northwestern is described in detail in this paper. It is shown how one can evaluate the two size-independent parameters from a single notched beam test. The results of the two parameters obtained with different types and geometry of specimens are compared. The theoretically predicted fracture process zone is compared with the data obtained using acoustic emission source location techniques as well as from laser holographic interferometry. The usefulness of the two parameter fracture model is demonstrated by examining size effect, high strength concrete, strain rate effect, and steel fiber reinforced concrete.