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Silica fume was first used in shotcrete in Norway in the nineteen seventies. In the early nineteen eighties the use of silica fume developed in North America, first in Western Canada and then in the United States. Silica fume has been added to both wet-mix and dry-mix shotcrete in a variety of different forms, including: as-produced, uncompacted silica fume; compacted low density silica fume; compacted high density silica fume; and as a slurry. This paper  examines the influence of addition of the first three of the above forms of silica fume on the properties of plastic and hardened wet-mix and dry-mix shotcrete, compared to the performance of plain control Portland cement shotcretes. Parameters evaluated included the batching, mixing, conveying and shooting characteristics of the shotcretes. Plastic shotcrete properties evaluated included: slump and air content in the as-batched and applied wet-mix shotcrete; thickness to bond break (sloughing) on shotcrete applied to both vertical and overhead surfaces; and rebound on vertical and overhead surfaces. Properties of the hardened shotcrete evaluated included: compressive strength at 1, 7, 28 and 63 days; flexural strength at 7 and 28 days; boiled absorption and volume of permeable voids; drying shrinkage; rapid chloride permeability, and electrical resistivity. It is shown that all three forms of silica fume can be successfully used to substantially improve both the plastic and hardened properties of the shotcretes studied, relative to plain control Portland cement shotcretes. There are  some differences in the performance characteristics of shotcretes made with the different forms of silica fume, particularly with respect to shooting characteristics; these differences are discussed in the paper.
CANMET/ACI International Workshop on the use of Silica in Concrete in Washington, D.C., USA during April of 1991
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