Microstructural Study of Different Types of Very High Strength Concretes
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MICROSTRUCTURAL STUDY OF DIFFERENT TYPES OF VERY HIGH STRENGTH CONCRETES
PIERRE-CLAUDE AITCIN, SHONDEEP L. SARKAR, AND YAYA DIATTA Faculty of Applied Sciences, Universit6 de Sherbrooke, Sherbrooke, Quebec, CANADA
ABSTRACT Very high strength concretes with water-cement ratios ranging from 0.21 to 0.27, having compressive strengths varying between 73 and 118 MPa, were prepared. One series was made with only high early strength cement (Type III), and the other series contained 6% to 11% silica fume. In general, the microstructure of very high strength concrete is very dense and is composed mainly of C-S-H in the gel and crystalline phases. Mg, Al, S, Cl, K and Fe were detected in a number of C-S-H locales. The Ca/Si ratio was variable. In concretes without silica fume, the CH content is much lower than in normal concrete, and in the silica fume concretes it is still lower and not well crystallized. A few large, partly reacted and unreacted silica fume particles with surface cracks were present. Strong cement-aggregate bonding is seen in concretes with silica fume containing limestone aggregates, whereas the gravel concretes show microcracks and a weaker bonding.
INTRODUCTION Using superplasticizer, it is currently possible to prepare and deliver ready-mix concrete having very low water-cement ratios (in the range of 0.20 to 0.30). These concretes are mixed using less water than is strictly necessary to fully hydrate the cement particles. The composition of these concretes, however, can be adjusted such that they still flow I hour after mixing [1,21. Substitution by a supplementary cementitious materials, such as silica fume [1,2], fly ash [3], or slag [4], is advantageous because it serves to reduce the amount of cement required and produces a very dense microstructure. The optimum composition of very high strength concrete is still mostly adjusted by trial and error. A series of trial batches is made to find the most suitable local cement, superplasticizer, and aggregates to reach the highest compressive strength possible. It should be mentioned that 100 MPa compressive strength is not the ultimate strength that can be achieved with portland cement. By using a much lower water-cement ratio (in the range of 0.15) with a very high dose of silica fume and superplasticizer and special techniques, Bache [5] was able to produce a 280 MPa concrete in the laboratory. However, very few scientific studies [6] have been undertaken in order to define more precisely the principal characteristics of cement and aggregates necessary to produce very high strength concrete.
EXPERIMENTAL In order either to improve or to introduce changes in the composition of very high strength concrete, it is imperative to understand the microstructure of this product and relate it to the engineering parameters. Therefore, several laboratory-scale experiments were conducted to define more clearly the strength development of very high strength concrete from its microstructure. Concretes were prepared with and without silica fume, and with water-cem
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