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OCR for page 21
21 example, Type E water-reducing and -accelerating admix- of particular importance when HRWRs are being considered, tures being particularly problematic, and Type F HRWR may as difficulties have been reported in obtaining satisfactory air- also retard strength development. Improved paste homo- void systems in mixtures containing Type F HRWR. geneity was observed with the use of a Type F HRWR but not with a Type E water-reducing and -accelerating admix- ture. These latter mixtures had the highest level of paste inho- mogeneity, the highest volume of permeable voids, and a high 4.6 COARSE AGGREGATE degree of microcracking. Various water-reducing admixtures are available for use in The type of coarse aggregate used markedly affects the EOT concrete, and it is impossible to categorize their interac- concrete's density and CTE. The coarse aggregate can also tion with other concrete constituents. However, the final selec- impact some strength properties of the mixtures and scaling tion of the water-reducing admixture must be done only after resistance. Thus, care must be exercised in selecting coarse testing of the job mixture, including evaluation of the impact aggregate that will provide both the desired strength and the on both strength and durability characteristics. This testing is durability properties.