The chapter presents results of tests on the effect of the addition of siliceous fly ashes (FA) in the amount of 0, 20, and 30% by weight of cement on the fracture processes in structural concretes. In the course of experiments, measurements of compressive strength of concrete were done as well as fracture toughness for I, II, and III model of cracking was evaluated. During the tests, the effect of age of concretes modified with the additive of fly ashes on analyzed parameters was determined. The experiments were carried out after 3, 7, 28, 90, 180, and 365 days of curing. Fracture toughness of concretes was determined based on the critical stress intensity factors, and then a generalized fracture toughness Kc was determined. The properties of composites with the additive of fly ashes depend on the age of the concrete during tests. Twenty percent additive of fly ashes guarantees high fracture toughness in mature concretes, while the additive of fly ashes in the amount of 30% weight of cement has a beneficial effect on the parameters of concrete only after half a year of curing. Both the 20 and 30% additive of fly ashes significantly reduce the fracture toughness at an early age.
Part of the book: Coal Fly Ash Beneficiation