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Chap6 - HOME PAGE CHAPTER 6 Admixtures for Concrete...

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Admixtures are those ingredients in concrete other than portland cement, water, and aggre- gates that are added to the mixture immediately before or during mixing (Fig. 6-1). Admixtures can be classified by function as follows: 1. Air-entraining admixtures 2. Water-reducing admixtures 3. Plasticizers 4. Accelerating admixtures 5. Retarding admixtures 6. Hydration-control admixtures 7. Corrosion inhibitors 8. Shrinkage reducers 9. Alkali-silica reactivity inhibitors 10. Coloring admixtures 11. Miscellaneous admixtures such as workabil- ity, bonding, dampproofing, permeability re- ducing, grouting, gas-forming, antiwashout, foaming, and pumping admixtures Table 6-1 provides a much more extensive classification of admixtures. Concrete should be workable, finishable, strong, durable, watertight, and wear resistant. These qualities can often be obtained easily and economically by the selection of suitable materials rather than by resorting to admixtures (except air-entraining admixtures when needed). The major reasons for using admixtures are: 1. To reduce the cost of concrete construction 2. To achieve certain properties in concrete more effec- tively than by other means 3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in ad- verse weather conditions 4. To overcome certain emergencies during concreting operations CHAPTER 6 Admixtures for Concrete Fig. 6-1. Liquid admixtures, from left to right: antiwashout admixture, shrinkage reducer, water reducer, foaming agent, corrosion inhibitor, and air-entraining admixture. (69795) Despite these considerations, it should be borne in mind that no admixture of any type or amount can be considered a substitute for good concreting practice. The effectiveness of an admixture depends upon factors such as type, brand, and amount of cementing materials; water content; aggregate shape, gradation, and proportions; mixing time; slump; and temperature of the concrete. Admixtures being considered for use in concrete should meet applicable specifications as presented in Table 6-1. Trial mixtures should be made with the admix- ture and the job materials at temperatures and humidities anticipated on the job. In this way the compatibility of the admixture with other admixtures and job materials, as well as the effects of the admixture on the properties of the fresh and hardened concrete, can be observed. The amount of admixture recommended by the manufacturer or the optimum amount determined by laboratory tests should be used. 105 HOME PAGE
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106 Design and Control of Concrete Mixtures EB001 Table 6-1. Concrete Admixtures by Classification Type of admixture Desired effect Material Accelerators Accelerate setting and early-strength Calcium chloride (ASTM D 98 and AASHTO M 144) (ASTM C 494 and development Triethanolamine, sodium thiocyanate, calcium formate, AASHTO M 194, Type C) calcium nitrite, calcium nitrate Air detrainers Decrease air content Tributyl phosphate, dibutyl phthalate, octyl alcohol, water- insoluble esters of carbonic and boric acid, silicones
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