Trends in Fracture of Quasibrittle Materials

The research activity in fracture mechanics of quasibrittle materials —concrete, rocks, ceramics, com­posites, ice, and some polymers— experienced a burst of activity during the 1980s, Much research effort was —and still is— devoted to refine the foregoing models, to improve the analytical and numerical tools required to handle the models, to develop experimental methods to measure the parameters entering the various theories, and to relate the macroscopic fracture behavior to the microstructural features of the materials. In this respect, idealized models reflecting the heterogeneous nature of concrete have been developed to help understanding of the macroscopic behavior (see Chapter 14 for details). Extensive bibliographies and historical reviews of concrete fracture mechanics have recently appeared in the reports of various committees (Wittmann 1983; Elfgren 1989; ACI Committee 446 1992).

Recently, it is being recognized that fractures of concrete and of modern toughened ceramics exhibit strong similarities. Their exploitation should benefit both disciplines. In fact, the way to toughen ceramics is to make them behave more like concrete, especially reinforced concrete.

At present, we are entering a period in which introduction of fracture mechanics into concrete design is becoming possible (see Chapter 10). This will help achieve more uniform safety margins, especially for structures of different sizes. This, in turn, will improve economy as well as structure reliability. It will make it possible to introduce new designs and utilize new concrete materials. Fracture mechanics will no doubt be especially important for high-strength concrete structures, fiber-reinforced concrete structures, concrete structures of unusually large sizes, and other novel structures. Applications of fracture mechanics are 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.

One of the simplest ways to incorporate fracture mechanics into design practice is through the size effect, or modification of structural strength with the size of the structure. The analysis of size effect starts later in this chapter and permeates most of the book.