Part 7 - Admixtures for Concrete

An admixture is "a material other than water, aggregate and hydraulic cement that is used as an ingredient of con
added to the batch immediately before or during mixing." A functional addition is an admixture added by the cem
cement plant and not at the job site. There are four categories of admixtures:

Air-entraining agents -- added to improve frost resistance of concrete.

Chemical admixtures -- used to control setting and hardening of concrete or reduce concrete water requi
Mineral admixtures -- fine solids used to improve workability, durability, and provide additional cemen
Miscellaneous admixtures -- all other admixtures not in the above categories.

** Admixtures should not be used when similar results can be obtained by following good design and concrete p

When using admixtures, the following precautions should be considered: 1) admixture should conform to relevan
follow manufacturers instructions regarding dosage, 3) ensure that reliable procedures are established for batchin
the effects of the admixture on other concrete properties.

Air-Entraining Admixtures
One major disadvantage of concrete is its susceptibility to damage by single or multiple freeze-thaw cycles when
saturated state. Without some admixture, concrete could not be used in pavements, dams, foundations, or other m
However, concrete can be made frost-resistant by using air-entraining admixtures. Concrete is routinely air-entra
and Canada.

Entrained Air-Void System -- The volume of air for optimum frost protection is about 9% by volume of
quantity to measure in the field is the amount of air as a percentage of concrete volume. The air content sh
range for good frost protection. Some air is naturally entrained in the cement paste, therefore air-entrainin
air voids 3 to 4% by volume of concrete. Air-entraining admixtures cause the mixing water to foam result
tiny air bubbles to be uniformly spaced throughout the paste. These tiny voids are not visible with the nak
with a microscope. The spacing of the voids is a critical measure of the effectiveness of the admixture. Al
easily fill with water even when the concrete is saturated.

Air-Entraining Materials -- A good air-entraining compound will promote the formation of small stable
Surface-active agents concentrate at the water-air interface, lowering the surface tension of the water allo
typical dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement - often requiring pr

Testing of Air-Entrained Agents -- One way to test the effectiveness of air entrainment is by counting b
section of concrete under a microscope and calculating the spacing factor. A more routine test is a perform
which an admixture concrete is tested in rapid freeze-thaw cycles.

Factors Affecting Air Entrainment -- Increasing admixture dosage will increase air content and decreas
ground cements entrain less air than do coarsely around ones. Therefore, the addition of fines into the mix
content. The use of other admixtures can affect the air-entraining potential of surface-active agents. Low
 entrain more air than do rich mixtures. In addition, low w/c ratio mixes entrain less air than do concretes

Mixing and Consolidation -- Air entrainment occurs during mixing. There are several factor which can e
of mixer, rate of mixing, amount of concrete being mixed, time of mixing, consolidation, temperature, an

Effects on Concrete Properties -- Air entrainment increases the workability of fresh concrete. The tiny b
like fine aggregates and reduce the interactions between solid aggregates. The improvement in workabilit
entrainment admixtures even when freeze-thaw is not a problem. In general, air entrainment will produce
concrete. Air entrained concrete is generally 10 to 20% weaker than non-air-entrained mixtures. Excessiv
and reduce freeze-thaw resistance.

Chemical Admixtures
This class of admixtures encompasses all soluble chemicals which affect setting times and reduce water requirem
They are classified as follows:

Type A. Water-Reducing Admixtures

o Lowers the water required to obtain a given slump. ASTM classifies an admixture as water-reduci
requirements by 5%. Under this specification, many air-entraining admixtures are also classified a
water-reducing admixtures reduce water requirements by 5 to 10%. Newer admixtures called "sup
reductions of 15 to 30% in water requirements. Reducing water requirements while maintaining c
lowers the w/c ratio with an accompanying gain in strength.

o The main reaction of all water-reducing admixtures is at the solid-water interface. In general, solid
surface charge, which may be positive or negative. This causes the particles to collect together tra
Water-reducing admixtures neutralize the surface charge so that all surfaces carry a uniform charg
now repel each other instead of attracting one another. The water is free to reduce the viscosity of
workability. Most conventional water-reducing admixtures will also act as retarding admixtures.

o By lowering the water requirements using water-reducing admixtures, an increase in compressive

greater than those anticipated from an equivalent mix with a decrease in the w/c ratio is obtained.
more uniformity throughout the cement paste structure.

o Superplasticizing Admixtures -- A linear polymer which can reduce water requirements by 15 to 3

produce flowing concretes with very high slumps (7 to 9 in.) and high strength concretes with w/c
0.4. ASTM refers to these admixtures as "high-range water-reducing admixtures." If added dosage
water-reducing admixtures are used the results are similar (5 to 10%). However, at higher dosages
increases. The effect of this admixture is that undesirable side-effects like air-entrainment and reta
High strength can be obtained for concretes with w/c ratio below 0.4 (incomplete hydration). Supe
Type I cement resulting in strength gain in excess of Type III. The accompanying lower cement co
heat generation. Also, lower w/c ratios lead to better durability, lower creep, and dry shrinkage.

Type B. Set-Retarding Admixtures

o Admixtures which prolong the plasticity of concrete. Useful to counter the effects of high tempera
and reduce cracking associated with form deflections. Basically, retarding admixtures increase the
 hydration process. However, subsequent hydration in stages 3 and 4 will be more rapid. Too large
admixture will cause the reaction never to proceed beyond stage 2 resulting in a cement that will n
concrete has helped many a ready-mix truck driver out of trouble. The addition of sugar or carbon
may have set up in the truck will return it to a useable form.

o The effective of the retarder depends on the amount of C3A in the concrete. Retarder is removed f
C3A reaction, so less is available to retard C3S hydration. Less retarder is removed if its addition t
Even though this admixture extends setting times, a side effect is loss of workability. Set-retarding
reported to increase ultimate compressive strengths. Dry shrinkage and creep rate are increased, b
unaffected.

Type C. Set-Accelerating Admixtures

o There are two types of set-accelerators, those that accelerate normal setting and strength developm
rapid-setting concretes by means not associated with normal hydration. There are many applicatio
leaks under pressure, rapid emergency repair, or when rapid development of rigidity is required.

o Conventional accelerators increase the rate of hydration of C3S by shortening the dormant period
hydration in stages 3 and 4.

o Set-accelerators do not generally affect air entrainment. However, handling time is reduced and ad
reducing admixture may be necessary to control workability. Early strength gain can be observed
is reduced. Effects on dry shrinkage and creep are similar to those resulting from set-retarding adm

o Some set-accelerating admixtures have a chloride component which will cause corrosion on reinfo
earlier in the course, chloride should never be used in any prestress application. Alternative admix
dosages and will be more expensive. Another solution is to use Type III cement and a concrete wi
in order to get early strength.

Type D. Water-Reducing and -Retarding Admixtures

Type E. Water-Reducing and -Accelerating Admixtures

Mineral Admixtures
Mineral admixtures are used to improve workability and durability and to harden concrete. This can be accompli
ground minerals, generally divided into three groups:

Materials of Low Reactivity -- Improve the workability of concrete deficient in fines. Generally, cementit
materials are preferred due to an additional increase in strength and durability.

Cementitious Materials -- Materials that have hydraulic reactions off their own, like hydraulic limestones
Most common admixture of this category.

Pozzolanic Materials -- A material that reacts with calcium hydroxide (CH) to form C-S-H. The reaction
lowers heat of hydration while causing a more impermeable cement. This reaction is comparable to that o
cement can be turned into Type IV cement with a pozzolan admixture. Therefore, Type IV cement is rare
 early strength is obtained similar to type IV cements.

Miscellaneous Admixtures
Consumption of these admixtures added together is less than the amount used in any of the single types discussed

Bonding Admixtures -- Bonding between old and new concrete or concrete and other materials.

Corrosion Inhibitors -- Generally, an accelerating admixture which is not corrosive to reinforcing bars.

Damp-proofing Admixture -- prevent penetration of rain into porous concrete; provide water-repellent c

Expansion-Producing Admixtures -- convert ordinary cement into expansive cement.

Grouting Admixtures -- A variety of admixtures for concrete-based grouts which prevent bleeding and s
cohesion and retention of water during pumping, and increase set times.

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.

Part 7 - Admixtures for Concrete

An admixture is "a material other than water, aggregate and hydraulic cement that is used as an ingredient of con
added to the batch immediately before or during mixing." A functional addition is an admixture added by the cem
cement plant and not at the job site. There are four categories of admixtures:

Air-entraining agents -- added to improve frost resistance of concrete.

Chemical admixtures -- used to control setting and hardening of concrete or reduce concrete water require
Mineral admixtures -- fine solids used to improve workability, durability, and provide additional cementi
Miscellaneous admixtures -- all other admixtures not in the above categories.

** Admixtures should not be used when similar results can be obtained by following good design and concrete p
When using admixtures, the following precautions should be considered: 1) admixture should conform to relevan
follow manufacturers instructions regarding dosage, 3) ensure that reliable procedures are established for batchin
the effects of the admixture on other concrete properties.
Air-Entraining Admixtures
One major disadvantage of concrete is its susceptibility to damage by single or multiple freeze-thaw cycles when
saturated state. Without some admixture, concrete could not be used in pavements, dams, foundations, or other m
However, concrete can be made frost-resistant by using air-entraining admixtures. Concrete is routinely air-entra
and Canada.

Entrained Air-Void System -- The volume of air for optimum frost protection is about 9% by volume of t
quantity to measure in the field is the amount of air as a percentage of concrete volume. The air content sh
range for good frost protection. Some air is naturally entrained in the cement paste, therefore air-entrainin
 air voids 3 to 4% by volume of concrete. Air-entraining admixtures cause the mixing water to foam result
tiny air bubbles to be uniformly spaced throughout the paste. These tiny voids are not visible with the nak
with a microscope. The spacing of the voids is a critical measure of the effectiveness of the admixture. Al
easily fill with water even when the concrete is saturated.

Air-Entraining Materials -- A good air-entraining compound will promote the formation of small stable b
Surface-active agents concentrate at the water-air interface, lowering the surface tension of the water allo
typical dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement - often requiring pr

Testing of Air-Entrained Agents -- One way to test the effectiveness of air entrainment is by counting bub
of concrete under a microscope and calculating the spacing factor. A more routine test is a performance s
admixture concrete is tested in rapid freeze-thaw cycles.

Factors Affecting Air Entrainment -- Increasing admixture dosage will increase air content and decrease s
ground cements entrain less air than do coarsely around ones. Therefore, the addition of fines into the mix
content. The use of other admixtures can affect the air-entraining potential of surface-active agents. Low
entrain more air than do rich mixtures. In addition, low w/c ratio mixes entrain less air than do concretes

Mixing and Consolidation -- Air entrainment occurs during mixing. There are several factor which can ef
mixer, rate of mixing, amount of concrete being mixed, time of mixing, consolidation, temperature, and s

Effects on Concrete Properties -- Air entrainment increases the workability of fresh concrete. The tiny bu
fine aggregates and reduce the interactions between solid aggregates. The improvement in workability lea
admixtures even when freeze-thaw is not a problem. In general, air entrainment will produce a uniform w
Air entrained concrete is generally 10 to 20% weaker than non-air-entrained mixtures. Excessive air will
freeze-thaw resistance.

Chemical Admixtures
This class of admixtures encompasses all soluble chemicals which affect setting times and reduce water requirem
They are classified as follows:

Type A. Water-Reducing Admixtures

o Lowers the water required to obtain a given slump. ASTM classifies an admixture as water-reduci
requirements by 5%. Under this specification, many air-entraining admixtures are also classified a
water-reducing admixtures reduce water requirements by 5 to 10%. Newer admixtures called "sup
reductions of 15 to 30% in water requirements. Reducing water requirements while maintaining c
lowers the w/c ratio with an accompanying gain in strength.

o The main reaction of all water-reducing admixtures is at the solid-water interface. In general, solid
surface charge, which may be positive or negative. This causes the particles to collect together tra
Water-reducing admixtures neutralize the surface charge so that all surfaces carry a uniform charg
now repel each other instead of attracting one another. The water is free to reduce the viscosity of
workability. Most conventional water-reducing admixtures will also act as retarding admixtures.

o By lowering the water requirements using water-reducing admixtures, an increase in compressive

greater than those anticipated from an equivalent mix with a decrease in the w/c ratio is obtained.
 more uniformity throughout the cement paste structure.

o Superplasticizing Admixtures -- A linear polymer which can reduce water requirements by 15 to 3

produce flowing concretes with very high slumps (7 to 9 in.) and high strength concretes with w/c
0.4. ASTM refers to these admixtures as "high-range water-reducing admixtures." If added dosage
water-reducing admixtures are used the results are similar (5 to 10%). However, at higher dosages
increases. The effect of this admixture is that undesirable side-effects like air-entrainment and reta
High strength can be obtained for concretes with w/c ratio below 0.4 (incomplete hydration). Supe
Type I cement resulting in strength gain in excess of Type III. The accompanying lower cement co
heat generation. Also, lower w/c ratios lead to better durability, lower creep, and dry shrinkage.

Type B. Set-Retarding Admixtures

o Admixtures which prolong the plasticity of concrete. Useful to counter the effects of high tempera
and reduce cracking associated with form deflections. Basically, retarding admixtures increase the
hydration process. However, subsequent hydration in stages 3 and 4 will be more rapid. Too large
admixture will cause the reaction never to proceed beyond stage 2 resulting in a cement that will n
concrete has helped many a ready-mix truck driver out of trouble. The addition of sugar or carbon
may have set up in the truck will return it to a useable form.

o The effective of the retarder depends on the amount of C3A in the concrete. Retarder is removed f
C3A reaction, so less is available to retard C3S hydration. Less retarder is removed if its addition
delayed. Even though this admixture extends setting times, a side effect is loss of workability. Set
been reported to increase ultimate compressive strengths. Dry shrinkage and creep rate are increas
unaffected.

Type C. Set-Accelerating Admixtures

o There are two types of set-accelerators, those that accelerate normal setting and strength developm
rapid-setting concretes by means not associated with normal hydration. There are many applicatio
leaks under pressure, rapid emergency repair, or when rapid development of rigidity is required.

o Conventional accelerators increase the rate of hydration of C3S by shortening the dormant period
hydration in stages 3 and 4.

o Set-accelerators do not generally affect air entrainment. However, handling time is reduced and ad
reducing admixture may be necessary to control workability. Early strength gain can be observed
is reduced. Effects on dry shrinkage and creep are similar to those resulting from set-retarding adm

o Some set-accelerating admixtures have a chloride component which will cause corrosion on reinfo
earlier in the course, chloride should never be used in any prestress application. Alternative admix
dosages and will be more expensive. Another solution is to use Type III cement and a concrete wi
in order to get early strength.

Type D. Water-Reducing and -Retarding Admixtures

Type E. Water-Reducing and -Accelerating Admixtures

Mineral Admixtures
Mineral admixtures are used to improve workability and durability and to harden concrete. This can be accompli
ground minerals, generally divided into three groups:

Materials of Low Reactivity -- Improve the workability of concrete deficient in fines. Generally, cementit
materials are preferred due to an additional increase in strength and durability.

Cementitious Materials -- Materials that have hydraulic reactions off their own, like hydraulic limestones
Most common admixture of this category.

Pozzolanic Materials -- A material that reacts with calcium hydroxide (CH) to form C-S-H. The reaction
lowers heat of hydration while causing a more impermeable cement. This reaction is comparable to that o
cement can be turned into Type IV cement with a pozzolan admixture. Therefore, Type IV cement is rare
early strength is obtained similar to type IV cements.

Miscellaneous Admixtures
Consumption of these admixtures added together is less than the amount used in any of the single types discussed

Bonding Admixtures -- Bonding between old and new concrete or concrete and other materials.

Corrosion Inhibitors -- Generally, an accelerating admixture which is not corrosive to reinforcing bars.

Damp-proofing Admixture -- prevent penetration of rain into porous concrete; provide water-repellent ch

Expansion-Producing Admixtures -- convert ordinary cement into expansive cement.

Grouting Admixtures -- A variety of admixtures for concrete-based grouts which prevent bleeding and se
cohesion and retention of water during pumping, and increase set times.

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.

Part 7 - Admixtures for Concrete

An admixture is "a material other than water, aggregate and hydraulic cement that is used as an ingredient of con
added to the batch immediately before or during mixing." A functional addition is an admixture added by the cem
cement plant and not at the job site. There are four categories of admixtures:

Air-entraining agents -- added to improve frost resistance of concrete.

Chemical admixtures -- used to control setting and hardening of concrete or reduce concrete water require
Mineral admixtures -- fine solids used to improve workability, durability, and provide additional cementi
Miscellaneous admixtures -- all other admixtures not in the above categories.

** Admixtures should not be used when similar results can be obtained by following good design and concrete p
When using admixtures, the following precautions should be considered: 1) admixture should conform to relevan
follow manufacturers instructions regarding dosage, 3) ensure that reliable procedures are established for batchin
the effects of the admixture on other concrete properties.
Air-Entraining Admixtures
One major disadvantage of concrete is its susceptibility to damage by single or multiple freeze-thaw cycles when
saturated state. Without some admixture, concrete could not be used in pavements, dams, foundations, or other m
However, concrete can be made frost-resistant by using air-entraining admixtures. Concrete is routinely air-entra
and Canada.

Entrained Air-Void System -- The volume of air for optimum frost protection is about 9% by volume of t
quantity to measure in the field is the amount of air as a percentage of concrete volume. The air content s
range for good frost protection. Some air is naturally entrained in the cement paste, therefore air-entrainin
air voids 3 to 4% by volume of concrete. Air-entraining admixtures cause the mixing water to foam result
tiny air bubbles to be uniformly spaced throughout the paste. These tiny voids are not visible with the nak
with a microscope. The spacing of the voids is a critical measure of the effectiveness of the admixture. Al
easily fill with water even when the concrete is saturated.

Air-Entraining Materials -- A good air-entraining compound will promote the formation of small stable b
Surface-active agents concentrate at the water-air interface, lowering the surface tension of the water allo
typical dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement - often requiring pr

Testing of Air-Entrained Agents -- One way to test the effectiveness of air entrainment is by counting bub
of concrete under a microscope and calculating the spacing factor. A more routine test is a performance s
admixture concrete is tested in rapid freeze-thaw cycles.

Factors Affecting Air Entrainment -- Increasing admixture dosage will increase air content and decrease s
ground cements entrain less air than do coarsely around ones. Therefore, the addition of fines into the mix
content. The use of other admixtures can affect the air-entraining potential of surface-active agents. Low
entrain more air than do rich mixtures. In addition, low w/c ratio mixes entrain less air than do concretes w

Mixing and Consolidation -- Air entrainment occurs during mixing. There are several factor which can ef
mixer, rate of mixing, amount of concrete being mixed, time of mixing, consolidation, temperature, and s

Effects on Concrete Properties -- Air entrainment increases the workability of fresh concrete. The tiny bu
fine aggregates and reduce the interactions between solid aggregates. The improvement in workability lea
admixtures even when freeze-thaw is not a problem. In general, air entrainment will produce a uniform w
Air entrained concrete is generally 10 to 20% weaker than non-air-entrained mixtures. Excessive air will
freeze-thaw resistance.

Chemical Admixtures
This class of admixtures encompasses all soluble chemicals which affect setting times and reduce water requirem
They are classified as follows:

Type A. Water-Reducing Admixtures

o Lowers the water required to obtain a given slump. ASTM classifies an admixture as water-reduci
requirements by 5%. Under this specification, many air-entraining admixtures are also classified a
water-reducing admixtures reduce water requirements by 5 to 10%. Newer admixtures called "sup
 reductions of 15 to 30% in water requirements. Reducing water requirements while maintaining c
lowers the w/c ratio with an accompanying gain in strength.

o The main reaction of all water-reducing admixtures is at the solid-water interface. In general, solid
surface charge, which may be positive or negative. This causes the particles to collect together tra
Water-reducing admixtures neutralize the surface charge so that all surfaces carry a uniform charg
now repel each other instead of attracting one another. The water is free to reduce the viscosity of
workability. Most conventional water-reducing admixtures will also act as retarding admixtures.

o By lowering the water requirements using water-reducing admixtures, an increase in compressive

greater than those anticipated from an equivalent mix with a decrease in the w/c ratio is obtained.
more uniformity throughout the cement paste structure.

o Superplasticizing Admixtures -- A linear polymer which can reduce water requirements by 15 to 3

produce flowing concretes with very high slumps (7 to 9 in.) and high strength concretes with w/c
0.4. ASTM refers to these admixtures as "high-range water-reducing admixtures." If added dosage
water-reducing admixtures are used the results are similar (5 to 10%). However, at higher dosages
increases. The effect of this admixture is that undesirable side-effects like air-entrainment and reta
High strength can be obtained for concretes with w/c ratio below 0.4 (incomplete hydration). Supe
Type I cement resulting in strength gain in excess of Type III. The accompanying lower cement co
heat generation. Also, lower w/c ratios lead to better durability, lower creep, and dry shrinkage.

Type B. Set-Retarding Admixtures

o Admixtures which prolong the plasticity of concrete. Useful to counter the effects of high tempera
and reduce cracking associated with form deflections. Basically, retarding admixtures increase the
hydration process. However, subsequent hydration in stages 3 and 4 will be more rapid. Too large
admixture will cause the reaction never to proceed beyond stage 2 resulting in a cement that will n
concrete has helped many a ready-mix truck driver out of trouble. The addition of sugar or carbon
may have set up in the truck will return it to a useable form.

o The effective of the retarder depends on the amount of C3A in the concrete. Retarder is removed f
C3A reaction, so less is available to retard C3S hydration. Less retarder is removed if its addition
delayed. Even though this admixture extends setting times, a side effect is loss of workability. Set
been reported to increase ultimate compressive strengths. Dry shrinkage and creep rate are increas
unaffected.

Type C. Set-Accelerating Admixtures

o There are two types of set-accelerators, those that accelerate normal setting and strength developm
rapid-setting concretes by means not associated with normal hydration. There are many applicatio
leaks under pressure, rapid emergency repair, or when rapid development of rigidity is required.

o Conventional accelerators increase the rate of hydration of C3S by shortening the dormant period
hydration in stages 3 and 4.

o Set-accelerators do not generally affect air entrainment. However, handling time is reduced and ad
reducing admixture may be necessary to control workability. Early strength gain can be observed
 is reduced. Effects on dry shrinkage and creep are similar to those resulting from set-retarding adm

o Some set-accelerating admixtures have a chloride component which will cause corrosion on reinfo
earlier in the course, chloride should never be used in any prestress application. Alternative admix
dosages and will be more expensive. Another solution is to use Type III cement and a concrete wi
in order to get early strength.

Type D. Water-Reducing and -Retarding Admixtures

Type E. Water-Reducing and -Accelerating Admixtures

Mineral Admixtures
Mineral admixtures are used to improve workability and durability and to harden concrete. This can be accompli
ground minerals, generally divided into three groups:

Materials of Low Reactivity -- Improve the workability of concrete deficient in fines. Generally, cementit
materials are preferred due to an additional increase in strength and durability.

Cementitious Materials -- Materials that have hydraulic reactions off their own, like hydraulic limestones
Most common admixture of this category.

Pozzolanic Materials -- A material that reacts with calcium hydroxide (CH) to form C-S-H. The reaction
lowers heat of hydration while causing a more impermeable cement. This reaction is comparable to that o
cement can be turned into Type IV cement with a pozzolan admixture. Therefore, Type IV cement is rare
early strength is obtained similar to type IV cements.

Miscellaneous Admixtures
Consumption of these admixtures added together is less than the amount used in any of the single types discussed

Bonding Admixtures -- Bonding between old and new concrete or concrete and other materials.

Corrosion Inhibitors -- Generally, an accelerating admixture which is not corrosive to reinforcing bars.

Damp-proofing Admixture -- prevent penetration of rain into porous concrete; provide water-repellent ch

Expansion-Producing Admixtures -- convert ordinary cement into expansive cement.

Grouting Admixtures -- A variety of admixtures for concrete-based grouts which prevent bleeding and se
cohesion and retention of water during pumping, and increase set times.

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.