Admixtures are those ingredients in concrete other than
portland cement, water, and aggregates 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 workability, bonding, damp proofing, permeability reducing,
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
effectively than by other means
3. To maintain the
quality of concrete during the stages of mixing, transporting, placing, and
curing in adverse weather conditions
4. To overcome certain emergencies during concreting
operations.
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 admixture 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. 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, waterinsoluble esters of
carbonic and boric acid, silicones Air-entraining admixtures Improve durability
in freeze-thaw, Salts of wood resins (Vinsol resin), some synthetic (ASTM C 260
and deicer, sulfate, and alkali- detergents, salts of sulfonated lignin, salts
of petroleum AASHTO M 154) reactive environments acids, salts of proteinaceous
material, fatty and resinous Improve workability acids and their salts,
alkylbenzene sulfonates, salts of sulfonated hydrocarbons Alkali-aggregate
reactivity Reduce alkali-aggregate reactivity Barium salts, lithium nitrate,
inhibitors expansion lithium carbonate, lithium hydroxide Antiwashout
admixtures Cohesive concrete for underwater Cellulose, acrylic polymer
placements Bonding admixtures Increase bond strength Polyvinyl chloride,
polyvinyl acetate, acrylics, butadiene-styrene copolymers Coloring admixtures
Colored concrete Modified carbon black, iron oxide, phthalocyanine, umber,
(ASTM C 979) chromium oxide, titanium oxide, cobalt blue Corrosion inhibitors
Reduce steel corrosion activity in a Calcium nitrite, sodium nitrite, sodium
benzoate, certain chloride-laden environment phosphates or fluosilicates,
fluoaluminates, ester amines Dampproofing admixtures Retard moisture
penetration into dry Soaps of calcium or ammonium stearate or oleate concrete Butyl
stearate Petroleum products Foaming agents Produce lightweight, foamed Cationic
and anionic surfactants concrete with low density Hydrolized protein
Fungicides, germicides, Inhibit or control bacterial and fungal Polyhalogenated
phenols and insecticides growth Dieldrin emulsions Copper compounds Gas formers
Cause expansion before setting Aluminum powder Grouting admixtures Adjust grout
properties for specific See Air-entraining admixtures, Accelerators, Retarders,
applications and Water reducers Hydration control Suspend and reactivate cement
Carboxylic acids admixtures hydration with stabilizer and activator
Phosphorus-containing organic acid salts Permeability reducers Decrease
permeability Latex Calcium stearate Pumping aids Improve pumpability Organic
and synthetic polymers Organic flocculents Organic emulsions of paraffin, coal
tar, asphalt, acrylics Bentonite and pyrogenic silicas Hydrated lime (ASTM C
141) Retarders (ASTM C 494 Retard setting time Lignin and AASHTO M 194, Borax
Type B) Sugars Tartaric acid and salts Shrinkage reducers Reduce drying
shrinkage Polyoxyalkylene alkyl ether Propylene glycol Superplasticizers*
Increase flowability of concrete Sulfonated melamine formaldehyde condensates
(ASTM C 1017, Type 1) Reduce water-cement ratio Sulfonated naphthalene
formaldehyde condensates Lignosulfonates Polycarboxylates AIR-ENTRAINING
ADMIXTURES Air-entraining admixtures are used to purposely introduce and
stabilize microscopic air bubbles in concrete. Airentrainment will dramatically
improve the durability of concrete exposed to cycles of freezing and thawing
(Fig. 6-2). Entrained air greatly improves concrete’s resistance to surface
scaling caused by chemical deicers (Fig. 6-3). Furthermore, the workability of
fresh concrete is improved significantly, and segregation and bleeding are
reduced or eliminated. Air-entrained concrete contains minute air bubbles that
are distributed uniformly throughout the cement paste. Entrained air can be
produced in concrete by use of an air-entraining cement, by introduction of an
airentraining admixture, or by a combination of both methods. An air-entraining
cement is a portland cement with an air-entraining addition interground with
the clinker during manufacture. An air-entraining admixture, on the other hand,
is added directly to the concrete materials either before or during mixing. The
primary ingredients used in air-entraining admixtures are listed in Table 6-1.
Specifications and methods of testing air-entraining admixtures are given in
ASTM C 260 and C 233 (AASHTO M 154 and T 157). Airentraining additions for use
in the manufacture of airentraining cements must meet requirements of ASTM C
226. Applicable requirements for air-entraining cements are given in ASTM C 150
and AASHTO M 85. See Chapter 8, Air-Entrained Concrete, Klieger (1966), and
Whiting and Nagi (1998) for more information. WATER-REDUCING ADMIXTURES
Water-reducing admixtures are used to reduce the quantity of mixing water
required to produce concrete of a certain slump, reduce water-cement ratio,
reduce cement content, or increase slump. Typical water reducers reduce the
water content by approximately 5% to 10%. Adding a water-reducing admixture to
concrete without reducing the water content can produce a mixture with a higher
slump. The rate of slump loss, however, is not reduced and in most cases is
increased (Fig. 6-4). Rapid slump loss results in reduced workability and less
time to place concrete. An increase in strength is generally obtained with
water-reducing admixtures as the water-cement ratio is 107 Chapter 6 ◆
Admixtures for Concrete Table 6-1. Concrete Admixtures by Classification
(Continued) Type of admixture Desired effect Material Superplasticizer* and
Increase flowability with retarded set See superplasticizers and also water
reducers retarder (ASTM C 1017, Reduce water–cement ratio Type 2) Water reducer
Reduce water content at least 5% Lignosulfonates (ASTM C 494 and Hydroxylated
carboxylic acids AASHTO M 194, Type A) Carbohydrates (Also tend to retard set
so accelerator is often added) Water reducer and Reduce water content (minimum
5%) See water reducer, Type A (accelerator is added) accelerator (ASTM C 494
and accelerate set and AASHTO M 194, Type E) Water reducer and Reduce water
content (minimum 5%) See water reducer, Type A (retarder is added) retarder
(ASTM C 494 and and retard set AASHTO M 194, Type D) Water reducer—high Reduce
water content (minimum See superplasticizers range (ASTM C 494 and 12%) AASHTO
M 194, Type F) Water reducer—high Reduce water content (minimum See superplasticizers
and also water reducers range—and retarder 12%) and retard set (ASTM C 494 and
AASHTO M 194, Type G) Water reducer—mid Reduce water content (between
Lignosulfonates range 6 and 12%) without retarding Polycarboxylates *
Superplasticizers are also referred to as high-range water reducers or
plasticizers. These admixtures often meet both ASTM C 494 (AASHTO
AIR-ENTRAINING ADMIXTURES
Air-entraining admixtures are used to purposely introduce
and stabilize microscopic air bubbles in concrete. Airentrainment will
dramatically improve the durability of concrete exposed to cycles of freezing
and thawing (Fig. 6-2). Entrained air greatly improves concrete’s resistance to
surface scaling caused by chemical deicers (Fig. 6-3). Furthermore, the workability
of fresh concrete is improved significantly, and segregation and bleeding are
reduced or eliminated. Air-entrained concrete contains minute air bubbles that
are distributed uniformly throughout the cement paste. Entrained air can be
produced in concrete by use of an air-entraining cement, by introduction of an
airentraining admixture, or by a combination of both methods. An air-entraining
cement is a portland cement with an air-entraining addition interground with
the clinker during manufacture. An air-entraining admixture, on the other hand,
is added directly to the concrete materials either before or during mixing. The
primary ingredients used in air-entraining admixtures are listed in Table 6-1.
Specifications and methods of testing air-entraining admixtures are given in ASTM
C 260 and C 233 (AASHTO M 154 and T 157). Airentraining additions for use in
the manufacture of airentraining cements must meet requirements of ASTM C 226.
Applicable requirements for air-entraining cements are given in ASTM C 150 and
AASHTO M 85. See Chapter 8, Air-Entrained Concrete,
WATER-REDUCING ADMIXTURES
Water-reducing admixtures are used to reduce the quantity of
mixing water required to produce concrete of a certain slump, reduce
water-cement ratio, reduce cement content, or increase slump. Typical water
reducers reduce the water content by approximately 5% to 10%. Adding a
water-reducing admixture to concrete without reducing the water content can
produce a mixture with a higher slump. The rate of slump loss, however, is not
reduced and in most cases is increased (Fig. 6-4). Rapid slump loss results in
reduced workability and less time to place concrete. An increase in strength is
generally obtained with water-reducing admixtures as the water-cement ratio is
Nitrate
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