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Concrete used in paving must possess workability to be mixed, transported, placed, consolidated,
and finished to meet project specifications and constraints without compromising any of
the structural or durability characteristics expected of the concrete once it has hardened.
To achieve these properties for fresh and hardened concrete, it is often necessary to
use chemical admixtures to modify the mixture properties.
The most common types of chemical admixtures used in paving concrete include:
air entraining admixtures,
water-reducing admixtures,
and set-modifying admixtures
that either delay or accelerate the rate of hydration.
Regardless of the type of admixture used, the manufacturer's recommendations for usage
and dosage must be followed.
Trial batching using the admixture and other constituent materials under expected construction
conditions is advised, to assess fresh and hardened concrete properties.
As concrete freezes and ice forms in the pores,
the expansion of the ice causes stress within the concrete.
The presence of uniformly dispersed entrained air bubbles
can provide the needed empty space to relieve stress.
The most common air-entraining admixtures follow AASHTO M154 standard,
and are composed of
salts of wood resins,
organic salts of sulfonated hydrocarbons,
fatty and resinous acids and their salts,
salts of proteinaceous acids,
and synthetic detergents.
Air-entraining admixtures work with the air-water interface
to create stable air bubbles in fresh concrete as it is mixed.
The bubbles remain once the concrete is hardened
to help protect the concrete against damage from freezing and thawing.
Many factors impact the formation of a stable well dispersed air system including the stiffness
of the concrete mixture, type and duration of mixing, and temperature.
The amount of entrained air required depends on the freeze-thaw exposure condition and
the paste content in the concrete.
For paving mixtures exposed to freezing and thawing and where deicers are used,
typical air contents range from 5 to 8 percent.
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The Super Air Meter, SAM, provides a measure of the distribution
or spacing of entrained air within a concrete mixture.
A SAM number less than 0.20 measured in accordance with AASHTO TP 118 indicates 90% probability
that the spacing meets American Concrete Institute durability recommendations.
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Water-reducing admixtures reduce the water required to obtain concrete
with desired workability characteristics.
Freeze-thaw durability of concrete can be improved by reducing the maximum
water-to-cementitious materials ratio.
This reduces the porosity and permeability of the hardened concrete paste.
Water-reducing admixtures conform to AASHTO M194
and can be formulated to have normal,
retarding, or accelerating setting characteristics.
They are classified as follows: Type A, water-reducing.
Type D, water-reducing and retarding.
Type E, water-reducing and accelerating.
Type F, water-reducing, high-range.
Type G, water-reducing, high-range and retarding.
Normal or conventional water-reducing admixtures--
typically classified as Type A, D, or E --
can reduce water content by approximately 5 to 10 percent
without exceeding AASHTO M194 time of set limit.
They act by adhering to the cement grains and dispersing them through electrostatic
and steric repulsion.
Normal water-reducing admixtures can affect setting,
with retardation more common as dosage increases.
Normal water-reducing admixtures are also sensitive to temperature, and interactions
with other mixture constituents can sometimes result in flash setting or severe retardation.
Mid-range water-reducing admixtures provide water reduction between 6 and 12 percent
without the retardation associated with high dosages of normal water-reducing admixtures.
They are compliant with AASHTO M194 Type A and often meet Type F requirements.
High-range water-reducing admixtures provide water reduction
between 12 and 40 percent, without retardation.
They are often used to produce high strength concrete with very good workability and extremely
low water-to-cementitious materials ratio.
They often meet the requirements of AASHTO M194 Type F or G, and are not often used in
paving grade concrete.
Polycarboxylates represent the newest water-reducing admixtures technology.
They are more efficient at dispersing the cement grains and have improved long-term
slump stability.
Hot weather, delivery delays, or extended transport time may make it necessary
to slow down or delay the setting rate of concrete.
Set-retarding admixtures can help slow down the setting time.
Set-retarding admixtures or retarders follow AASTHO M194 standard
and are commonly composed of:
Lignosulfonic acids and their salts.
Hydroxylated carboxylic acids and their salts.
Carbohydrate-based compounds such as sugars, sugar acids, and polysaccharides.
Inorganic salts such as borate and phosphates.
Retarders are specified under AASHTO M194 as Type B, set-retarding without affecting
water requirements, Type D, set-retarding and water-reducing, and Type G, set-retarding
and high-range water-reducing.
Retarders tend to directly impact the hydration of the tricalcium aluminate phase,
the calcium silicate phase,
or both phases, in the cement.
They are sensitive to temperature and other mixture constituents
and must be evaluated for interactions.
On the other hand, set-accelerating admixtures are used to accelerate the
rate of hydration to promote early setting and strength gain with low temperature paving
or to allow early opening to traffic.
Accelerators are specified under AASHTO M194 Type C as set-accelerating without affecting
water requirements and Type E as set-retarding and water-reducing.
The most common accelerator for non-reinforced concrete is calcium chloride,
which can cause corrosion in embedded steel.
Nearly all specifications prohibit the use of chloride-based accelerators
if steel is present.
Non-chloride accelerators are used for reinforced concrete.
Inorganic accelerators act by accelerating the hydration of the tricalcium silicate phase,
whereas organic accelerators accelerate the hydration of tricalcium aluminate phase.
There are also accelerators based on aqueous solutions
of nanoparticles.
The nanoparticles act as nucleation sites for the rapid growth of cement hydration products,
particularly calcium silicate hydrate, away from the cement grains themselves,
resulting in dramatically improved early strength gain.
Accelerators can impact the effectiveness of air-entraining admixtures.
Compatibility of admixtures should be evaluated during mixture design and monitored for changes
during construction.
High dosages of accelerators should be avoided to control performance issues.
Some accelerators can even act as a retarder at high dosages.
The use of accelerators in hot weather may result in very rapid setting as well as placement
and finishing problems.
Accelerator dosage may need to be adjusted as temperatures increase.
There are other admixtures including hydration control admixtures that can stop hydration
and another that will restart it for situations that require extended storage of fresh concrete.
Shrinkage-reducing, alkali-silica reaction inhibiting, and coloring admixtures
are also occasionally used in paving concrete.
Admixtures are an important component of concrete mixtures to achieve desired short-term
and long-term properties.
However, admixtures should not be used to correct for other deficiencies such as poor
material quality, mixture design, or placement operations.
For all admixtures, the supplier's guidance must be followed
for proper dosage and recommended guidelines.
For more information about admixtures or other concrete paving applications and processes,
visit our website at fhwa.dot.gov/pavement
or follow FHWA on social media.
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