Historical Perspective

Concrete structures are, of course, 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. So why has not the design of concrete structures been based on fracture mechanics, a theory whose principles have been available since the 1950s? Have concrete engineers been guilty of ignorance?

Not really. The forms of fracture mechanics that were available until recently were applicable only to homogeneous brittle materials, such as glass or to homogeneous typical structural metals. The question of applicability of these classical theories to concrete was explored long ago, beginning with Kaplan (1961) and others, but the answer was negative (e. g., Kesler, Naus and Lott 1972). Now, we understand that the reason for the negative answer was that the physical processes occurring in concrete fracture are very different from those taking place in the fracture of the aforementioned materials and, especially, that the material internal length scale for these fracture processes is much larger for concrete than for most materials. A form of fracture mechanics that can be applied to this kind of fracture has appeared only during the late 1970s and the 1980s.

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 1930s, was the introduction of plastic limit analysis during 1940-1970. There are now good reasons to believe that introduction of fracture mechanics into the design of concrete structures might be the third revolution. The theory, formulated mostly after 1980, finally appears to be ripe.