Fracture Mechanics of Concrete: Concepts, Models and Determination of Material Properties

Description

Fracture Mechanics of Concrete: Concepts, Models and Determination of Material Properties. 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 recentlywere applicable only to homogeneous brittle materials such as glass, or to homogeneousbrittle-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 theearly 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 developrnent of the elastic no-tension analysis during1900-1930. The second revolution, based on a theory conceived chiefly during the 1930’s, wasthe 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 concretestructures, both reinforced and unreinforced, might be the third major revolution. Thetheory, formulated mostly during the last dozen years, finally appears to be ripe.