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The stilling basin of Kinzua Dam on the Allegheny River in western Pennsylvania has experienced severe abrasion-erosion damage since the structure was put into operation in 1967. The basin was repaired in 1973-74 using a steel fiber-reinforced concrete overlay. Deterioration continued to the extent that repairs were again necessary in 1983. A laboratory program was undertaken to evaluate the abrasion-erosion resistance of several concrete mixtures proposed for the 1983 repairs. This program showed that high-strength concrete made with silica fume and limestone aggregates available near the project site would provide suitable abrasion-erosion resistance at a reasonable price. The Corps of Engineers, owner of the structure, required potential suppliers of silica fume to conduct full-sized placements to demonstrate that this concrete could be made and placed outside the laboratory. Based upon these demonstrations and the laboratory program, the repair concrete was specified with a compressive strength of 86 MPa (12,500 psi) at 28 days as a means of obtaining the required abrasion-erosion resistance. Approximately 1500 m* (2000 yd3) of 250-mm (9 3/4-in.) slump concrete were placed using silica fume delivered as a slurry that included water-reducing admixtures. The average 28-day compressive strength was over 90 MPa (13,000 psi). Diver inspection of the concrete after one year in service, including a period with a very large volume of debris in the stilling basin, has indicated that the silica-fume concrete is performing as intended.

This paper reports results from an on-going research program on the use of condensed silica fume and superplasticizers in concrete. The program is being performed at different laboratories andvariations caused by the local conditions are reported. The major objective of the research program has been to establish influence charts for variable dosage rates of mineral and chemical admixtures under conditions which are as close to the field conditions as possible. The relative increases in compressive and bending strengths of concrete for various dosage rates of condensed silica fume and superplasticizer are presented. It is concluded that important admixtures such as silica fume and superplasticizers do not influence strength at a constant or linear ratio; the influence is more complex and is governed by the variables involved.

The effect of condensed silica fume on the strength deve-lopment in concrete was studied by replacement of 0, 5,10, and 20% cement by silica fume. No admixtures was used. For all the mixes,about the same strength was obtained up to 7 days. From then on,the mixes with silica fume produced an increased strength development compared to that of pure cement. At a partial replacement of cement by 20%, the compressive strength at 28 and 90 days was increased by 43 and 55%, respectively, with respect to the strength at 7 days.

Drying shrinkage and creep of concrete with condensed silica fume are experimentally analyzed and compared with those of concrete without condensed silica fume. Specimens for drying shrinkage tests and creep tests are manufactured by two kind of curing methods, namely, standard and autoclave curing, and are measured in air at 20°C and 50% R.H.. Some specimens are measured in water at 20°C. Measurements for volume change and weight change are continued for 800 days. Within the scope of this research the following conclusions may be drawn: (1) Drying shrinkage of concrete with condensed silica fume is lower than that of concrete without condensed silica fume at the same water-cement ratio. But in case of standard curing, the values of drying shrinkage per unit cement paste volume are roughly the same for both concretes with and without condensed silica fume at the same compressive strength; while in case of autoclave curing, the values are higher for concrete with condensed silica fume at the same compressive strength. (2) Creep in air is high for concrete with condensed silica fume in both cases of standard curing and autoclave curing. Creep in water is high for concrete with condensed silica fume in case of standard curing, but creep in water is low for concrete with condensed silica fume in case of autoclave curing.

In order to reduce energy, save raw materials and improve mechanical properties, different pozzolans are now commonly used in cement and concrete production. A comprehensive research program was undertaken where cement and concrete properties, influenced by fly ash and condensed silica fume in different combinations, were investigated. This paper presents their influence on strength and heat development. The program included an ordinary portland cement and two blended cements with 10 % and 25 % fly ash respectively. The three cements were combined with 0 %, 5 % and 10 % condensed silica fume. Curing temperatures used were 5°C, 20°C and 35°C. Condensed silica fume is very finely graded and the content of amorphous Si02 is very high. The pozzolana reaction starts therefore early, at 20°C from around 7 days, at 35°C from around 2 days. At 5°C, no pozzolana reaction was observed for the first 28 days. The pozzolana reaction from fly ash was found to be slower than the reaction from condensed silica fume, probably due to the coarser grinding and the lower Si02-content. The compressive strength results indicated that the pozzo-lana reaction was more sensitive to the temperature than the reaction involving cement hydration alone. The slow strength development of concrete when using fly ash in blended cements can be avoided by grinding the cements to a higher fineness. The effect on strength development when using condensed silica fume was approximately the same in all three types of cement investigated. The heat development was higher in pure portland cement than in blended cements. However, when adding condensed silica fume, the heat development increased. Maturity functions were found to be valid up to maturities corresponding to curing in 20°C for approximately 2 days.