Civil Engineering Reference
In-Depth Information
CHAPTER 6
Admixtures for Concrete
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
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.
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
