chapter  15
Fracture of mortar-aggregate interfaces in concrete composites
Pages 10

Abstract Deformation and failure behavior of concrete composites is influenced by the characteristics of mortar-aggregate interfaces. In recent years, there has been a renewed interest in the study of the role of interfaces in composite behavior, particularly, due to the need for the development of high performance cementitious materials. In this paper, an interface fracture mechanics based methodology is described for the characterization of mortar-aggregate interface toughnesses using sandwich specimens. Also, a criterion based on energy release rate considerations is discussed for the study of crack propagation in interface regions. Finally, an application of the method using a two-phase concrete composite beam model is presented. Keywords: Mortar-aggregate Interface, Concrete Composite, Interface Fracture Mechanics, Energy Release Rate, Phase Angle, Cracking

1. I n t r o d u c t i o n

Over the past twenty years, considerable research has been conducted to study the microcrack development, the nonlinear deformation behavior and the failure mechanisms of concrete. It has been generally established that in normal strength concrete the development of bond cracks at the mortaraggregate interfaces plays a significant role in the inelastic deformation behavior and that final failure occurs through the formation of continuous cracks in mortar, bridging the bond cracks (Shah, 1966; Buyukozturk, et al., 1972; Liu, et al., 1972; Struble, et al., 1980). For high strength concrete, however, with respect to the deformation behavior and failure mechanisms, different observations have been made (Gerstle 1979; Carrasquillo, et al., 1981; Zaitsev, 1983; Chen, et al., 1985; Bentur and Mindess, 1986). In some cases cracks through aggregates were observed, indicating a less pronounced effect of crack arrest by the aggregates, while in other cases progression of bond cracks around the aggregates were reported. These different obtained trends are indicative of the importance of relative strength and stiffness properties of mortar, aggregates and the interface in the

characterization of the overall composite behavior. Development of advanced concrete composites with improved

toughness and durability requires a fundamental understanding of the behavior of the interfaces introduced in these materials. Various scenarios of crack initiation and crack propagation in the matrix vs. at the interface, or crack penetration into the aggregate need to be studied in order to engineer the material for an optimum behavior. An essential step, for a systematic study in that respect, is the development of a quantitative methodology for the assessment of the fracture toughness (that is a property) of the interfaces between various mortar/aggregate systems. Furthermore, criteria are needed for the crack, once initiated in one of constituent materials when concrete is modelled as a two-phase composite or in the interfacial region, to propagate along the interface vs. to penetrate into the second material. Solutions for studying such cracking scenarios are rare. Buyukozturk, et al. (1972) and Yamaguchi and Chen (1991) studied the microcrack propagation in two-phase concrete models by the finite element analysis incorporating the smeared cracking model for mortar, and the interface finite elements for bond. However, the results obtained in this way are limited due to the phenomenological nature in which the interface bond property was assumed and the bond cracking criterion was established.