scope:

Introduction

Concrete structures are full of cracks. Failure of concrete structures typically involves stable growth of large cracking zones and the formation of large fractures before the maximum load is reached. Yet design is not based on fracture mechanics, even though the basic fracture mechanics theory has been available since the middle of this century. So why has not fracture mechanics been introduced into concrete design? Have concrete engineers been guilty of ignorance? Not at all. The forms of fracture mechanics which were available until recently were applicable only to homogeneous brittle materials such as glass, or to homogeneous brittle-ductile metals. The question of applicability of these classical theories to concrete was explored long ago - the idea of using the stress intensity factor appeared already in the early 1950's (e.g., Bresler and Wollack, 1952) and serious investigations started in the 1960's (e.g., Kaplan, 1961, and others). But the answer was, at that time, negative (e.g., Kesler, Naus and Lott, 1971). As is now understood, the reason was that in concrete structures one must take into account strain-softening due to distributed cracking, localization of cracking into larger fractures prior to failure, and bridging stresses at the fracture front. A form of fracture mechanics that can be applied to such structures has been developed only during the last decade.

Concrete design has already seen two revolutions. The first, which made the technology of concrete structures possible, was the development of the elastic no-tension analysis during 1900-1930. The second revolution, based on a theory conceived chiefly during the 1930's, was the introduction of plastic limit analysis, which occurred during 1940-1970. There are good reasons to believe that the introduction of fracture mechanics into the design of concrete structures, both reinforced and unreinforced, might be the third major revolution. The theory, formulated mostly during the last dozen years, finally appears to be ripe.

Fracture researchers have at the present no doubt that the introduction of fracture mechanics into the design criteria for all brittle failures of reinforced concrete structures (such as diagonal shear, punching shear, torsion or pull out, or for concrete dams), can bring about significant benefits. It will make it possible to achieve more uniform safety margins, especially for structures of different sizes. This, in turn, will improve economy as well as structural reliability. It will make it possible to introduce new designs and utilize new concrete materials. Fracture mechanics will be particularly important for high strength concrete structures, fiber-reinforced concrete structures, concrete structures of unusually large sizes, and for prestressed structures. The application of fracture mechanics is most urgent for structures such as concrete dams and nuclear reactor vessels or containments, for which the safety concerns are particularly high and the consequences of a potential disaster enormous.

Surveys of concrete fracture mechanics have recently been prepared by various committees (Wittmann, 1983, and Elfgren, 1989). However, due to the rapidly advancing research, the contents of the present state-of-the-art report are quite different. A unified, systematic presentation, rather than a compilation of all the contributions by various authors, is attempted in the present state-of-art report. The report is aimed primarily a t researchers, not necessarily specialists in fracture mechanics. However, it should also be of interest to design engineers because it describes a theory that is likely to profoundly influence the design practice in the near future. Subsequent reports dealing with applications in design, finite element analysis of fracture, and dynamic fracture analysis, are in preparation by ACI Committee 446.