Silica Fume Effects on the Pull-Out Behavior of Randomly Oriented Steel Fibers from Concrete
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SILICA FUME EFFECTS ON THE PULL-OUT BEHAVIOR OF RANDOMLY ORIENTED STEEL FIBERS FROM CONCRETE PARVIZ SOROUSHIAN* AND ZIAD BAYASI** *Department of Civil Engineering, Michigan State University, MI 48824 **Department of Civil Engineering, Michigan State University, MI 48824
East Lansing, East Lansing,
ABSTRACT Silica fume is known to improve the pore system of cementitious pastes and their adhesion capacity to fibers and other mix inclusions. This study has been mainly concerned with the effect of silica fume on the pull-out strength of randomly oriented steel fibers from concrete. Silica fume effects on the fresh mix workability and the overall flexural and compressive behavior of steel fiber reinforced and plain concretes were also assessed. The only variable in different fibrous and plain mixes was the fraction of portland cement substituted with silica fume. This fraction ranged from 0% to 20%. The workability of plain and fibrous mixes were obtained by slump and inverted slump cone tests, respectively. In both the flexure and compression tests on fiber concrete, the complete load-deformation relationship was obtained. The pull-out strength of randomly oriented fibers was assessed through analysis of the flexural test data. All aspects of the fresh and hardened steel fiber reinforced concrete performance considered in this study, especially the pull-out strength of randomly oriented fibers from concrete, were observed to improve with the increase in silica fume-cementitious ratio up to a certain limit, after which the trends in silica fume effects were reversed. INTRODUCTION Silica fume, because of its extreme fineness and high pozzolanic activity, is effective in reacting with the Calcium Hydroxide crystals concentrated around the mix inclusions and dispersed throughout the bulk of the hydrated cement paste [1-3]. This action of silica fume results in a denser structure with an improved pore system, especially in the transition zone where the concentration of Calcium Hydroxide crystals is relatively high. The reaction of silica fume with the relatively large Calcium Hydroxide crystals results in the formation of Calcium Silicate Hydrate which is a stable and fine cementitious compound with a desirable adhesion capacity [4,5]. The above action of silica fume in portland cement concrete results in a denser microstructure in the cement paste and especially in the transition zone around the mix inclusions, with enhanced matrix-inclusion bond characteristics. The paper presented herein intends to experimentally confirm and quantify these effects of silica fume on randomly oriented steel fibers, the pull-out action of which involves a complex interaction between the fiber-matrix interfacial bond and the bulk of cement paste. EXPERIMENTAL PROGRAM In order to assess the effects of partial substitution of portland cement with silica fume on the pull-out performance of randomly oriented steel fibers, and also on the fresh mix and hardened material properties of the composite material, tests were performed on concretes wit
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