International Concrete Abstracts Portal

Presents the results of a joint project between CANMET, Ottawa, Canada, and NORCHEM Concrete Products, Inc., Hauppauge, New York, on the performance of concretes incorporating various forms of silica fume. Eleven different product forms of silica fume were used in four series of concrete with water-cementitious materials ratio of 0.40, 0.35, 0.30, and 0.22. Test specimens from the above concretes were subjected to varying curing conditions and were tested for compressive and flexural strengths, drying shrinkage, and rapid chloride permeability. It was found that, in general, the performance of the silica fume concretes in terms of mechanical properties was comparable, regardless of the silica fume product form used and the SiO 2 content of the fumes. The Rapid Chloride Permeability values, in coulombs at 41 days, a measure of the resistance of concrete to the penetration of chloride ions (AASHTO T277) for all the silica fume concretes tested except those made with silica fume blended cements, ranged from 94 to 346 coulombs, indicating very low permeability of the concretes. The values for concretes made with the blended silica fume cements were slightly higher.

Presents the results of a study to investigate the effectiveness of ground granulated blast furnace slag, fly ash, and silica fume in controlling chloride penetration into concrete of high water-binder (w/b) ratio. To simulate field conditions, the tests were carried out on 1000 x 500 x 150 mm reinforced concrete slabs. to understand the basic roles of mineral admixtures in controlling chloride penetration, the cement was replaced, mass for mass, by 65 percent slag, 30 percent fly ash, and 10 percent silica fume without either modifying the water-binder ratio or using a water-reducer or superplasticizer. A constant and high water-binder ratio was deliberately used for all the concrete mixtures; the results are compared, where appropriate, with mixtures of lower w/b. the effects on workability, compressive strength, and chloride penetration were then evaluated, the latter two properties over a period of 18 months through cyclic exposure to four percent sodium chloride solution. Irrespective of their effects on workability and compressive strength, all the cement replacement materials reduced both the depth of penetration and the chloride concentration at a given depth from the concrete surface. Silica fume was the most effective and fly ash the least. Even at a very high w/b of 0.75, slag concrete showed a consistently lower chloride concentration for all exposure levels up to 50 cycles, and at all depths from the concrete surface compared to that of portland cement concrete. Both the type of supplementary cementing material and the period of exposure influenced chloride penetration, but the water-binder ratio also had a significant effect at all ages.

Thin concrete overlays are widely used for protection against corrosion during the rehabilitation of concrete bridge decks. These overlays protect the reinforcing steel from the ingress of chlorides from deicing salts or the marine environment. The Virginia Department of Transportation (VDOT) has successfully used thin concrete overlays containing silica fume (minimum thickness 1-1/4 in. [32 mm]) to provide a high resistance to the penetration of chlorides into the concrete. Silica fume concrete (SFC) overlays are now an acceptable alternative to other rehabilitation procedures. In the development of SFC overlays, certain problems were encountered at different phases of production and construction, including proportioning, mixing, bonding, consolidation, and curing. These problems and their solutions are discussed in this paper, and recommendations are made for minimizing problems in future installations.

The sensitivity of strength and deformation of high-strength concrete incorporating silica fume to variations in strain rates were studied experimentally and compared with those of normal strength concrete (without silica fume). The observed phenomena in the experiments were qualitatively interpreted according to an assumed mechanism of strain rate sensitivity of concrete. The differences of the material structure between high-strength concrete with silica fume and normal strength concrete without silica fume are discussed in this paper; emphasis is placed on the change of pore structure and moisture content due to the incorporation of silica fume for high-strength concrete and its influence on the rate sensitivities to strength and deformation of concrete. In particular, the Stefan Effect is believed to play a very important role in the case of rate sensitivity. In general, it was found that high-strength silica fume concrete is more sensitive to the variation of strain rate than normal strength concrete as far as strength and deformation in compression are concerned. However, in tension, this rate sensitivity is less pronounced.

To better understand the behavior of silica fumes (SF) in fresh mortar and concrete, four French silica fumes, with different chemical and textural properties, were characterized with respect to their surface properties and their behavior in aqueous dispersion. Zeta potential measurements were performed as a function of pH and ionic strength. Below about pH=4, all materials behave similarly, whereas at higher pH, the electrochemical potential was found to be related to the Ca content. In suspension, the decrease of the average particle size (bimodal distribution) is directly related to the power input (ultrasonic treatment) and to the intragranular compactness, mass-fractal dimension, and densification treatment. With Ca-rich SF, sedimentation volumes and velocities were found to increase as dispersion proceeds, which suggests that the elementary sub-micrometric silica spheres re- agglomerate after the dispersion treatment, probably due to bridging by calcium silicate hydrates.