Describes a small scale flexure test for the determination of cement- aggregate bond strength. Cylindrical test specimens were prepared by drilling cores in a perpendicular direction through slabs of rock against which mortar had been cast. A special casting procedure eliminated many sources of experimental variation and allowed the bond strengths of different mortars and rock types to be compared directly. Long term tests were conducted by coring the mortar/aggregate slabs at a number of curing times and coefficients of variation of 5 to 10 percent for bond and mortar strengths were obtained. The effect on cement-aggregate bond strength of partial cement replacement by silica fume was evaluated for a number of aggregate types. For siliceous aggregates (glass, obsidian, and quartzite), bond strength was increased significantly by the addition of silica fume; failure tended to occur away from the interface particularly in long term tests. For carbonate rocks (limestone and dolostone), similar bond strengths were obtained at seven days with and without the addition of silica fume. At later ages, silica fume interfered with strengthening of the cement-carbonate rock interface and lower bond strengths were obtained. For specimens not containing silica fume, bond strength increased more rapidly to glass and obsidian than to quartzite, which showed essentially "inert" behavior. This was tentatively attributed to strengthening of the transition zone by a pozzolanic mechanism involving reactive silica from the aggregate. A marked reduction in bond strength occurred with glass specimens containing boosted alkali content. This was attributed to alkali- silica reaction at the interface and was suppressed by the addition of 15 percent silica fume.

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

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.

In many practical engineering applications of fiber reinforced concretes (FRC), fibers are subjected to significant lateral loading. The lateral stress may have a significant effect on fiber debonding and pullout, thus affecting the performance of FRC. In this investigation, a novel experimental set-up was developed to carry out fiber pullout tests under various levels of lateral compression. Interfacial properties for steel fiber in mortar were derived from the measured fiber load versus displacement curves, based on a unified fiber debonding theory. As expected, the interfacial friction at the onset of sliding increases with applied compressive stress. Surprisingly, the pre-sliding interfacial resistance (which can be either the interfacial strength or the interfacial fracture energy, depending on whether interfacial debonding is strength or fracture governed) is also found to increase with lateral compressive stress. Also, with higher compression, there is a more rapid decay of interfacial friction when the fiber is sliding out of its groove. As a result, while lateral compression can significantly increase the peak pullout load, the increase in total energy absorbed during the pullout process is much less drastic.

The global behavior of concrete is influenced by various scenarios of crack initiation and crack propagation. Recently, the study of the interface fracture and cracking in interfacial regions has emerged as an important research field, especially in the context of the development of high performance concrete composites. For a rigorous study, the use and further development of fracture mechanics based concepts are needed. The crack path criterion for elastically homogeneous materials is not valid when the crack advances at an interface because, in this case, the consideration of the relative magnitudes of the fracture toughnesses between the constituent materials and the interface is involved. In this paper, criteria based on energy release rate concepts are considered for the prediction of crack growth at the interfaces and an experimental/numerical study is presented on two-phase composite models of concrete to investigate the cracking scenarios in interfacial regions. From the testing and numerical analysis on physical models, the interface fracture and the crack propagation in concrete composites are studied, and the role of interface fracture toughness is discussed.