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N. YAN

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C. CHAN

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C. YAN and A. BENTUR2

'Dept. of Civil Eng., University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 2 National Building Research Institute, Technion, Israel Institute of Technology, Technion City, Haifa, 32000 Israel

ABSTRACT Bond-slip characteristics for steel micro-fibers bonded in cement-based matrices were investigated by conducting single fiber pull-out tests. The influence of the following factors was investigated: fiber inclination, fiber size, fiber embedded length and matrix refinement using silica fume. It was found that the bond-slip characteristics of fibers aligned with respect to the loading direction were necessarily superior than those inclined at an angle. Inclined fibers supported smaller peak pull-out loads and absorbed lesser pull-out energies than the aligned fibers. The use of silica fume in the matrix was found to improve both the average interfacial bond strength and the maximum interfacialbond strength between the fiber and the matrix. Keywords:

fiber reinforced cements, steel micro-fibers, bond-slip, pull-out tests, inclined fibers

INTRODUCTION When a strong and ductile fiber such as steel is dispersed in a brittle cement-based matrix, significant changes in the microfracture processes can be expected. Cracks in the brittle matrix are bridged by the ductile fibers, which, in turn, undergo pull-out processes, and result in macroscopic fracture properties significantly superior to those of the parent matrix. As in any other composite, the influence of fiber-matrix bond-slip behavior on the properties of cement-based composites is fundamental. The effectiveness of a given fiber as a medium of stress transfer is often assessed using a single fiber pull-out test where fiber slip is monitored as a function of the applied fiber load. And while some argue that single fiber pull-out data does not correlate well with the behavior of the real composite (1,2), fiber pull-out tests are routinely conducted as a means of optimizing fiber and matrix characteristics and to understand toughening mechanisms in these composites. Pull-out characteristics of steel fibers embedded in cementitious matrices have been studied as a function of several variables including the rate of load application (3,4), temperature of the environment (5), processing variables and matrix quality (6,7,8) and other test variables such as fiber inclination, etc. (9). In addition, a number of matrix and fiber modifications have been examined as a way of improving the bond-slip characteristics of fibers in a cementitious matrix. These include matrix modifications such as silica fume and polymer additions (10, 11, 12) and fiber surface modifications such as coatings, surface indenting and notching (13, 14, 15). For large fibers of steel (often called macro-fibers), the most effective means of improving the pull-out bond-slip characteristics is "mechanical deforming". Mechanical deformations in the form of a hook, cone or crimp placed at the end or along the fiber length to provide positive endanchor