Admixtures
Admixtures
Admixtures
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Admixtures for Concrete
Admixtures are those ingredients in concrete other than Portland cement, water, and
aggregates that are added to the mixture immediately before or during mixing.
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. Colouring admixtures
11. Miscellaneous admixtures such as workability, bonding, damp-proofing,
permeability reducing, grouting, gas-forming, anti-washout, foaming, and pumping
admixtures
Concrete should be workable, finish able, 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
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Air-Entraining Admixtures
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. Rapid slump loss
results in reduced workability and less time to place concrete.
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more significant factors that cause shrinkage cracks in concrete. Using a water reducer
to reduce the cement and water content of a concrete mixture—while maintaining a
constant water-cement ratio—can result in equal or reduced compressive strength, and
can increase slump loss by a factor of two or more(Whiting and Dziedzic 1992).
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70 MPa (10,000 psi), (2) increased early strength gain, (3) reduced chloride-ion
penetration, and (4) other beneficial properties associated with low water-cement ratio
concrete.
High-range water reducers are generally more effective than regular water
reducing admixtures in producing workable concrete. A significant reduction of
bleeding can result with large reductions of water content; this can result in finishing
difficulties on flat surfaces when rapid drying conditions are present. Some of these
admixtures can cause significant slump loss. Significant retardation is also possible, but
can aggravate plastic shrinkage cracking without proper protection and curing. Drying
shrinkage, chloride permeability, air retention, and strength development of concretes
with high-range water reducers are comparable to concretes without them when
compared at constant water-cement ratios (reduced cement and water contents).
Concretes with high-range water reducers can have larger entrained air voids and
higher void-spacing factors than normal air-entrained concrete. This would generally
indicate a reduced resistance to freezing and thawing; however, laboratory tests have
shown that concretes with a moderate slump using high-range water reducers have good
freeze-thaw durability, even with slightly higher void-spacing factors. This may be the
result of lower water cement ratios often associated with these concretes. When the
same chemicals used for high-range water reducers are used to make flowing concrete,
they are often called plasticizers or super-plasticizers
The addition of a plasticizer to a 75-mm (3-in.) slump concrete can easily produce a
concrete with a 230-mm (9-in.) slump. Flowing concrete is defined by ASTM C 1017 as
a concrete having a slump greater than 190 mm (71⁄2 in.), yet maintaining cohesive
properties. ASTM C 1017 has provisions for two types of admixtures: Type 1—
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plasticizing, and Type 2—plasticizing and retarding. Plasticizers are generally more
effective than regular or mid-range water-reducing admixtures in producing flowing
concrete. The effect of certain plasticizers in increasing workability or making flowing
concrete is short-lived, 30 to 60 minutes; this period is followed by a rapid loss in
workability or slump loss.
RETARDING ADMIXTURES
Retarding admixtures are used to delay the rate of setting of concrete. High
temperatures of fresh concrete (30°C [86°F]) are often the cause of an increased rate of
hardening that makes placing and finishing difficult. One of the most practical methods
of counteracting this effect is to reduce the temperature of the concrete by cooling the
mixing water and/or the aggregates. Retarders do not decrease the initial temperature of
concrete. The bleeding rate and bleeding capacity of concrete is increased with
retarders.
Retarding admixtures are useful in extending the setting time of concrete, but
they are often also used in attempts to decrease slump loss and extend workability,
especially prior to placement at elevated temperatures. Retarders are sometimes used to:
(1) offset the accelerating effect of hot weather on the setting of concrete; (2) delay the
initial set of concrete or grout when difficult or unusual conditions of placement occur,
such as placing concrete in large piers and foundations, cementing oil wells, or pumping
grout or concrete over considerable distances; or (3) delay the set for special finishing
techniques, such as an exposed aggregate surface.
ACCELERATING ADMIXTURES
An accelerating admixture is used to accelerate the rate of hydration (setting) and
strength development of concrete at an early age. The strength development of concrete
can also be accelerated by other methods: (1) using Type III or Type HE high-early-
strength cement, (2) lowering the water-cement ratio by adding 60 to 120 kg/m3 (100 to
200
lb/yd3) of additional cement to the concrete, (3) using a water reducer, or (4) curing at
higher temperatures. Accelerators are designated as Type C admixtures under ASTM C
494 (AASHTO M 194).
Calcium chloride (CaCl2) is the chemical most commonly used in accelerating
admixtures, especially for non-reinforced concrete. It should conform to the
requirements of ASTM D 98 (AASHTO M 144) and should be sampled and tested in
accordance with ASTM D 345.
The widespread use of calcium chloride as an accelerating admixtures has
provided much data and experience on the effect of this chemical on the properties of
concrete. Besides accelerating strength gain, calcium chloride causes an increase in
drying shrinkage, potential reinforcement corrosion, discoloration (a darkening of
concrete), and an increase in the potential for scaling.
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Calcium chloride is not an antifreeze agent. When used in allowable amounts, it
will not reduce the freezing point of concrete by more than a few degrees. Attempts to
protect concrete from freezing by this method are foolhardy. Instead, proven reliable
precautions should be taken during cold weather.
When used, calcium chloride should be added to the concrete mixture in solution
form as part of the mixing water. If added to the concrete in dry flake form, all of the
dry particles may not be completely dissolved during mixing. Undissolved lumps in the
mix can cause popouts or dark spots in hardened concrete.
The amount of calcium chloride added to concrete should be no more than is
necessary to produce the desired results and in no case exceed 2% by mass of cementing
material. When calculating the chloride content of commercially available calcium
chloride, it can be assumed that:
1. Regular flake contains a minimum of 77% CaCl2
2. Concentrated flake, pellet, or granular forms contain a minimum of 94% CaCl2
CORROSION INHIBITORS
Corrosion inhibitors are used in concrete for parking structures, marine structures,
and bridges where chloride salts are present. The chlorides can cause corrosion of steel
reinforcement in concrete. Ferrous oxide and ferric oxide form on the surface of
reinforcing steel in concrete. Ferrous oxide, though stable in concrete’s alkaline
environment, reacts with chlorides to form complexes that move away from the steel to
form rust. The chloride ions continue to attack the steel until the passivating oxide layer
is destroyed. Corrosion-inhibiting admixtures chemically arrest the corrosion reaction.
SHRINKAGE-REDUCING ADMIXTURES
Shrinkage-reducing admixtures, introduced in the 1980s, have potential uses in
bridge decks, critical floor slabs, and buildings where cracks and curling must be
minimized for durability or aesthetic reasons. Propylene glycol and poly-oxyalkylene
alkyl ether have been used as shrinkage reducers. Drying shrinkage reductions of
between 25% and 50% have been demonstrated in laboratory tests. These admixtures
have negligible effects on slump and air loss, but can delay setting. They are generally
compatible with other admixtures (Nmai, Tomita, Hondo and Buffenbarger 1998 and
Shah, Weiss and Yang 1998).
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COLORING ADMIXTURES (PIGMENTS)
Natural and synthetic materials are used to colour concrete for aesthetic and
safety reasons. Red concrete is used around buried electrical or gas lines as a warning
to anyone near these facilities. Yellow concrete safety curbs are used in paving
applications. Generally, the amount of pigments used in concrete should not exceed
10% by weight of the cement. Pigments used in amounts less than 6% generally do not
affect concrete properties.
Unmodified carbon black substantially reduces air content. Most carbon black for
colouring concrete contains an admixture to offset this effect on air. Before a colouring
admixture is used on a project, it should be tested for colour fastness in sunlight and
autoclaving, chemical stability in cement, and effects on concrete properties. Calcium
chloride should not be used with pigments to avoid colour distortions. Pigments should
conform to ASTM C 979.
DAMPPROOFING ADMIXTURES
The passage of water through concrete can usually be traced to the existence of
cracks or areas of incomplete consolidation. Sound, dense concrete made with a water-
cement ratio of less than 0.50 by mass will be watertight if it is properly placed and
cured.
Admixtures known as damp-proofing agents include certain soaps, stearates, and
petroleum products. They may, but generally do not, reduce the permeability of
concretes that have low cement contents, high water-cement ratios, or a deficiency of
fines in the aggregate. Their use in well-proportioned mixes may increase the mixing
water required and actually result in increased rather than reduced permeability.
Damp-proofing admixtures are sometimes used to reduce the transmission of
moisture through concrete that is in contact with water or damp earth. Many so-called
damp-proofers are not effective, especially when used in concretes that are in contact
with water under pressure.