Materials and Structures/Mat~riaux et Constructions, Vol. 32, April 1999, pp 203-209 The role of silica fume in the direct tensile strength of cement-based materials H. A. Toutanji1, L. Liu I and T. El-KorchV (i) University ofAlabama in Huntsville, Huntsville, AL 35899 (USA) (2) Worcester PolytechnicInstitute, Worcester,MA 01609 (USA) Paper received: November 11, i997; Paperaccepted: April 7, 1998 ABSTRACT The direct tensile strength of silica fume cement paste and mortar were evaluated at various water-cementitious content ratios. Four different water-cementitious content ratios of 0.22, 0.25, 0.28, and 0.31 were used, and three contents of silica fume, 8%, 16%, and 25% by mass of cement. Superplasticizer content was adjusted for each mix to ensure that no segregation would occur. Results show that partial replacement of cement by 8% of silica fume resulted in an increase in the tensile strength of mortar, but showed no effect on the tensile strength of cement paste. The replacement of cement by a higher dosage of silica fume (16 and 25%) resulted in a decrease in the tensile strength of both cement paste and mortar. However, this reduction was higher in cement paste than in mortar. It was also demonstrated that superplasticizer in combination with silica fume plays a more effective role in mortar than in paste mixes. This can be attributed to a more efficient utilization of superplasticizer in the mortar mixes due to better dispersion of the silica fume particles. The direct tensile strength was evaluated using a new hydraulic tensile testing technique. The technique mea- sures the intrinsic tensile strength of the cement-based composites by producing a uniform tensile stress along the length of the specimen, and thus minimizes misalignment and stress concentration at gripping. A brief description of this technique is presented in this paper. RESUME La r&istancea la traction directe de la pdte de ciment-fume'e de silice et de mortier a ~t~ &alu& pour diff&ents rapports eau/ciment. Quatre rapports eau/ciment ont {t~ utilise's, 0,22, 0,25, 0,28, 0,31, avec un dosage de fume'e de silice en pourcentage de ciment de 8%, 16%, et 25%. Pour chaque formulation, le dosage du super- plastfant a ~t8 ajustOafin d'&iter la s@Ogation. Les r&ultats ont montr~ qu'un remplacement partiel de ciment de 8% de fum& de silice engendre une augmentation de la re'sistance a la traction du mortier, mais aucun effet sur la r&is- tance a la traction de la p&e de ciment. Le remplacement de ciment par un important dosage de fum& de silice (16% et 25%) r&ulte une diminution de la r&istance h traction du mortier ainsi que la pdte du ciment. Cette diminution a {tO plus importante dans le cas de ciment que dam celui du mortier. I1 est aussi d{montr~ que l' utilisation du superplastifiant avec fum& de silice joue un role plus efficace dam le mortier que dam la p8te de ciment. Cela peut &re attribu~ h une meilIeure dispersion des particules defum& de silice dans le mortier. La re'sistancea la traction directe a ~t~ d&rmin& en utilisant un nouveau systkme hydraulique , hydraulique tensile testion technique ~. Cette technique permet de mesurer la contrainte intrinskque de traction du composite en exerqant une contrainte de traction uniforme le long de l'@ouvette, qui permet de minimiser la concentration des contraintes h I'ancrage de l'@ouvette. Cette nouvelle technique est d&rite dam cet article. ~i{ii{i{iiiii 1. INTRODUCTION Silica fume (SF) is accepted as an important con- stituent in many concrete mixtures. It is a relatively new member in the family of pozzolans that are currently used in concrete. Silica fume was first used in 1969 in Norway, but only began to be systematically employed in North America and Europe in the early 1980s. Since then, the use of silica fume has been increasing rapidly, either as a partial replacement for cement or as an addi- tive when special properties are desired. Silica fume is known to improve both the mechanical characteristics and durability of concrete [1-5]. One of the benefits of using silica fume in Portland cement-based composites is its performance as a filler in capillary pores and in the cement paste-aggregate inter- face. Some maintain that the influence of silica fume on concrete properties is not primarily pozzolanic [6], but rather that it fundamentally alters the nature of the tran- sition zone between paste and aggregate [7]. The influ- ence of silica fume on interracial bond and fracture prop- erties has been shown to be important. It strengthens 1359-5997/99 9 RILEM 203