International Concrete Abstracts Portal

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.

A new method to predict the debonding behavior of fiber-matrix interface has been proposed by applying the principles of the micromechanics of inclusion and fracture mechanics. The validity of the mathematical model is further verified by uniaxial tension tests carried out on steel fiber reinforced cementitious composite specimens by employing a digitally controlled closed-loop MTS testing machine. It was demonstrated that the debonding occurs before the bend over point; the debonded lengths are largely influenced by the sequence of the occurrence of transverse matrix cracks and the loading stage. A stable growth of debonding was observed in the investigation. The measured debonded lengths were compared with the theoretical prediction of the proposed model. A reasonable agreement was observed.

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.

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.