International Concrete Abstracts Portal

Interfaces, such as mortar-aggregate interfaces and cement matrix-fiber interfaces, affect the mechanical behavior of concrete composites. Significant considerations in understanding the mechanical behavior of concrete are the nature of the deformation and failure of these interfaces and the interaction between the constituent elements of the composite. Development of advanced concrete materials with improved toughness and durability requires a fundamental understanding of the behavior of the interfaces which are intrinsic to the concrete composite. Therefore, there is a need to characterize the interfacial behavior and to study the role of the interfaces on the global material behavior as a basis for the development of high performance cementitious materials. To address this need, ACI International produced the specialized publication based on the technical papers presented during a special session on fracture mechanics. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP156

Reviews the mechanical properties of the cement-aggregate bond, with particular reference to the inherent difficulties in determining these properties. The properties that are determined experimentally appear to be largely artifacts of the specimen preparation and the test procedures. In particular, bleeding effects, the roughness of the rock surface, and the heterogeneity of the interfacial region make it very difficult to compare experimental results among the different investigations that are found in the literature. The authors conclude that useful measurements of the properties of the cement-aggregate interfacial zone still cannot be made. Therefore, the properties of concrete cannot yet be controlled by systematically altering the nature of the interfacial region.

The size effect caused by post-peak softening in the relation of interface shear stress and slip displacement between a fiber or reinforcing bar and the surrounding matrix was analyzed. The problem was simplified as one- dimensional. It was shown that the post-peak softening leads to localization of slip. The larger the bar or fiber size, the stronger the localization. The size effect in geometrically similar pullout tests of different sizes was found to represent a transition from the case of no size effect for small sizes to the case of a size effect of the same type as in linear elastic fracture mechanics, in which the difference of the pullout stress in the fiber and the residual pullout stress corresponding to the residual interface shear stress is proportional to the inverse square root of bar or fiber size. An analytical expression for the transitional size effect was obtained and was found to approximately agree with the generalized form of the size effect law proposed earlier by Bazant. Measurements of the size effect can be used for identifying the interface properties.

The fracture process of high-strength concrete (HSC) submitted to uniaxial compression was analyzed by means of loading tests on cylinders under special closed loop control. Conclusions were drawn from axial and circumferential strain curves, cross sections of specimens impregnated with a fluorescent dye, and visual observations. The evolution of the internal crack extension was revealed and it was shown that under stable progressive fracture, predominantly aggregate bond failure and crack branching occur with the cracks passing around the coarse aggregates. the onset of damage is explained in terms of elementary force transfer concepts. The influence of maximum aggregate size and fiber addition are also discussed in this paper.

This study of the concrete/rock interface addresses primarily the interface of limestone and mortar (since no coarse aggregate was used in the mix) and, to a lesser extent, mortar and rock salt. Uniaxial tensile tests with closed-loop-control were used to determine the stress-crack opening displacement relationship in the softening regime. This relationship is proposed as the constitutive property in an interface cohesive zone model developed for interface fracture. The validity of such a model was investigated through testing and finite element analysis of compact tension specimens. A theoretical investigation of the effect of the complex singularity attributed to an interface crack was performed within the framework of the interface cohesive zone model. Although the theoretical analyses included only a semi-infinite geometry and was, therefore, limited in scope, it was found capable of addressing many of the characteristics of quasi-brittle fracture. Experimental tools used involved a scanning electron microscope to observe microscopic features of the interface that are responsible for strength and toughness. The electronic speckle pattern interferometry technique was used to evaluate pre-peak crack growth. Results indicate that the mechanisms responsible for strength and toughness at the interface are different and that the characteristics of the fracture at the interface is qualitatively similar to that of any other quasi-brittle material.