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

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.

Fracture of the interfacial zone between a fiber and the cementitious matrix plays a significant role in the mechanical behavior of fiber reinforced cementitious composites. For better understanding of debonding characteristics of a fiber in the composites, the behavior of the extension of cracks along the interface was examined under the fluorescence microscope. The correspondence between the features of fracture zones obtained by the microscopic study and the fracture toughness for the interfacial zone is discussed in this paper. Examinations under the microscope revealed that the debonding and the extension of interfacial cracks were not caused by a simple shear failure at the actual interface, but were accompanied by local failures over relatively wide regions surrounding the steel fibers. The incorporation of silica fume resulted in the reduction in areas containing local failures along the interface. Fewer local failures in the interfacial zone in the steel fiber-silica fume-bearing cement composite were reflected by the decrease in the fracture toughness for the interfacial zone.

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.

The bond between reinforcing bars and concrete under impact loading was studied both experimentally and by the finite element method. The experiments consisted of pullout tests and push-in tests, under three different types of loading: static, medium rate, and impact. Different concrete strengths (normal and high), types of fibers (polypropylene and steel), and fiber contents were considered. The study focused on the bond-slip relationships and the fracture energy in bond failure. The experimental results were compared with those obtained by the finite element method, in which a special "bond-link element" that was able to transmit both shear and normal forces was adopted to model the connection between the reinforcing bar and the concrete. It was found that higher loading rates, higher concrete compressive strengths, and the addition of steel fibers had significant effects on the bond resistance, the fracture energy, and the bond stress-slip relationship, especially for the push-in case. Reasonably good correspondence in the results between the two methods was also found, and a bond-stress-slip relationship under high rate loading could be established analytically.