The Contribution of the Transition Zone to the Strength of High Quality Silica Fume Concretes
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THE CONTRIBUTION OF THE TRANSITION ZONE TO THE STRENGTH OF HIGH QUALITY SILICA FUME CONCRETES A.
BENTUR,*
A. GOLDMAN* AND M.D. COHEN** Research Station, Department of Civil Engineering, Technion, Israel Institute of Technology, Haifa, Israel **School of Civil Engineering, Purdue University, West Lafayette, Indiana, U.S.A. * Building
ABSTRACT The strength of high strength silica fume concretes is usually attributed to the reduction in w/c ratio and the refinement of the pore structure. A study of concretes and pastes, with and without silica fume, suggests that the contribution of the silica fume to strength is also the result of the densification of the transition zone. It is argued here that this influence is as important as the one due to the reduction in w/c ratio. It is suggested that the densification of the transition zone is the result of the effect of the silica fume on the nature of the fresh concrete.
INTRODUCTION The addition of silica fume (SF) in combination with a superplasticizer is an effective means for producing high strength concretes. The combination of the two additives can lead to reduced water demand and an effectively low water to cement ratio (w/c) mix. This influence and the high pozzolanic reactivity of the SF leads to a reduction in total porosity and a finer pore size distribution, and these factors are generally believed to be the main cause for generating high strength. An additional effect of SF is to densify the paste matrix microstructure in the vicinity of the aggregate surface. The transition zone in concrete, which constitutes the -30 Pm wide matrix zone adjacent to the aggregate surface is usually more porous than the bulk paste matrix. The presence of SF has been documented to induce a more compact microstructure in this zone (I1 [2]. Rosenberg and Gaidis [3]
suggested on the basis of theoretical analysis that the densification may play an important role in the strengthening effect of SF. Experimental work also indicated the possible significance of such an effect [4]. The object of the present work was to resolve the role of the interface in high strength concretes prepared with SF. For that purpose the influence of SF was considered in terms of two effects: (a) "Water reducing effect" associated with the reduction in w/c ratio obtained when SF is added in combination with superplasticizer. (b) "Inherent effect" which is associated with the increase in strength in the SF concrete over a similar w/c ratio concrete without SF. The experimental work in this study was intended to resolve the magnitude of these two effects and the possible interpretation of the second effect in terms of densificiation of the transition zone. The results presented in this paper are part of a comprehensive study to evaluate the mechanisms by which SF affects the properties of concrete, considering various engineering characteristics (strength, shrinkage, heat liberation and durability). In this paper, the results obtained with one level of SF content are presented. The influence of the SF conte
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