Part 7 -
Admixtures for Concrete
An admixture is "a material other than water, aggregate and hydraulic cement that
is used as an ingredient of concrete or mortar and is added to the batch immediately
before or during mixing." A functional addition is an admixture added by the cement
manufacturer at the 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 requirements.
- Mineral
admixtures -- fine solids used to improve workability, durability, and provide
additional cementing properties.
- 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 practices.
When using admixtures, the following precautions should be considered: 1) admixture
should conform to relevant ASTM specifications, 2) follow manufacturers instructions
regarding dosage, 3) ensure that reliable procedures are established for batching of
admixture, 4) consider 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 it is in a saturated or near-saturated state. Without
some admixture, concrete could not be used in pavements, dams, foundations, or other major
applications. However, concrete can be made frost-resistant by using air-entraining
admixtures. Concrete is routinely air-entrained in the Northern U.S. and Canada.
- Entrained
Air-Void System -- The volume of air for optimum frost protection is about 9% by
volume of the mortar. The easiest quantity to measure in the field is the amount of air as
a percentage of concrete volume. The air content should be in the 4 to 8% range for good
frost protection. Some air is naturally entrained in the cement paste, therefore
air-entraining admixtures increase the air voids 3 to 4% by volume of concrete.
Air-entraining admixtures cause the mixing water to foam resulting in literally millions
of tiny air bubbles to be uniformly spaced throughout the paste. These tiny voids are not
visible with the naked eye, but are observable with a microscope. The spacing of the voids
is a critical measure of the effectiveness of the admixture. Also, small voids do not
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 bubbles during agitation. Surface-active agents concentrate at the water-air
interface, lowering the surface tension of the water allowing bubble formation. A typical
dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement - often
requiring predilution before batching.
- Testing of
Air-Entrained Agents -- One way to test the effectiveness of air entrainment is
by counting bubbles in a polished section of concrete under a microscope and calculating
the spacing factor. A more routine test is a performance specification in which an
admixture concrete is tested in rapid freeze-thaw cycles.
- Factors
Affecting Air Entrainment -- Increasing admixture dosage will increase air
content and decrease spacing factors. Finely ground cements entrain less air than do
coarsely around ones. Therefore, the addition of fines into the mix will reduce the air
content. The use of other admixtures can affect the air-entraining potential of
surface-active agents. Low cement content concretes entrain more air than do rich
mixtures. In addition, low w/c ratio mixes entrain less air than do concretes with high
w/c ratios.
- Mixing and
Consolidation -- Air entrainment occurs during mixing. There are several factor
which can effect the air content: type of mixer, rate of mixing, amount of concrete being
mixed, time of mixing, consolidation, temperature, and slump.
- Effects on
Concrete Properties -- Air entrainment increases the workability of fresh
concrete. The tiny bubbles in the cement act like fine aggregates and reduce the
interactions between solid aggregates. The improvement in workability leads to use of air
entrainment admixtures even when freeze-thaw is not a problem. In general, air entrainment
will produce a uniform well-compacted concrete. Air entrained concrete is generally 10 to
20% weaker than non-air-entrained mixtures. Excessive air will lower strength and reduce
freeze-thaw resistance.
Chemical Admixtures
This class of admixtures encompasses all soluble chemicals which affect setting times
and reduce water requirements of concrete mixes. They are classified as follows:
- Type A.
Water-Reducing Admixtures
- Lowers the water
required to obtain a given slump. ASTM classifies an admixture as water-reducing if it
reduces water requirements by 5%. Under this specification, many air-entraining admixtures
are also classified as water-reducing. Most water-reducing admixtures reduce water
requirements by 5 to 10%. Newer admixtures called "superplasticizers" achieve
reductions of 15 to 30% in water requirements. Reducing water requirements while
maintaining cement contents effectively lowers the w/c ratio with an accompanying gain in
strength.
- The main reaction of
all water-reducing admixtures is at the solid-water interface. In general, solid particles
carry a residual surface charge, which may be positive or negative. This causes the
particles to collect together trapping and attracting water. Water-reducing admixtures
neutralize the surface charge so that all surfaces carry a uniform charge of like sign.
The particles now repel each other instead of attracting one another. The water is free to
reduce the viscosity of the paste and improve workability. Most conventional
water-reducing admixtures will also act as retarding admixtures.
- By lowering the water
requirements using water-reducing admixtures, an increase in compressive strengths up to
25% greater than those anticipated from an equivalent mix with a decrease in the w/c ratio
is obtained. This can be attributed to more uniformity throughout the cement paste
structure.
- Superplasticizing
Admixtures -- A linear polymer which can reduce water requirements by 15 to 30%. They are
used to produce flowing concretes with very high slumps (7 to 9 in.) and high strength
concretes with w/c ratios in the range of 0.3 to 0.4. ASTM refers to these admixtures as
"high-range water-reducing admixtures." If added dosages equivalent to normal
water-reducing admixtures are used the results are similar (5 to 10%). However, at higher
dosages, the water reduction increases. The effect of this admixture is that undesirable
side-effects like air-entrainment and retardation are much reduced. High strength can be
obtained for concretes with w/c ratio below 0.4 (incomplete hydration). Superplasticizers
can modify Type I cement resulting in strength gain in excess of Type III. The
accompanying lower cement content reduces the rate of heat generation. Also, lower w/c
ratios lead to better durability, lower creep, and dry shrinkage.
- Type B.
Set-Retarding Admixtures
- Admixtures which
prolong the plasticity of concrete. Useful to counter the effects of high temperature,
eliminate cold joints, and reduce cracking associated with form deflections. Basically,
retarding admixtures increase the dormant stage in the C3S hydration process.
However, subsequent hydration in stages 3 and 4 will be more rapid. Too large a dose of
retarding admixture will cause the reaction never to proceed beyond stage 2 resulting in a
cement that will never set. Over-retarding of concrete has helped many a ready-mix truck
driver out of trouble. The addition of sugar or carbonate drinks to concrete that may have
set up in the truck will return it to a useable form.
- The effective of the
retarder depends on the amount of C3A in the concrete. Retarder is removed from
the solution by the C3A reaction, so less is available to retard C3S
hydration. Less retarder is removed if its addition to fresh concrete is delayed. Even
though this admixture extends setting times, a side effect is loss of workability.
Set-retarding admixtures have been reported to increase ultimate compressive strengths.
Dry shrinkage and creep rate are increased, but ultimate values are unaffected.
- Type C.
Set-Accelerating Admixtures
- There are two types
of set-accelerators, those that accelerate normal setting and strength development and
those that provide rapid-setting concretes by means not associated with normal hydration.
There are many application; shotcreting, plugging leaks under pressure, rapid emergency
repair, or when rapid development of rigidity is required.
- Conventional
accelerators increase the rate of hydration of C3S by shortening the dormant
period and increasing the rate of hydration in stages 3 and 4.
- Set-accelerators do
not generally affect air entrainment. However, handling time is reduced and additional
water or a water-reducing admixture may be necessary to control workability. Early
strength gain can be observed however, ultimate strength is reduced. Effects on dry
shrinkage and creep are similar to those resulting from set-retarding admixtures.
- Some set-accelerating
admixtures have a chloride component which will cause corrosion on reinforcing bars. As
discussed earlier in the course, chloride should never be used in any prestress
application. Alternative admixtures require larger dosages and will be more expensive.
Another solution is to use Type III cement and a concrete with a higher cement content 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 accomplished by introducing finely ground minerals, generally
divided into three groups:
- Materials of Low
Reactivity -- Improve the workability of concrete deficient in fines. Generally,
cementitious or pozzolanic 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 and blast-furnace slags. Most common admixture of this category.
- Pozzolanic Materials
-- A material that reacts with calcium hydroxide (CH) to form C-S-H. The reaction improves
workability and lowers heat of hydration while causing a more impermeable cement. This
reaction is comparable to that of C2S hydration. Type I
cement can be turned into Type IV cement with a pozzolan admixture. Therefore, Type IV
cement is rarely manufactured. A low 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 so far.
- 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 characteristic.
- Expansion-Producing
Admixtures -- convert ordinary cement into expansive cement.
- Grouting
Admixtures -- A variety of admixtures for concrete-based grouts which prevent
bleeding and segregation, increase 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.
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