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Curing is an important step in concrete construction that affects performance properties
simply by ensuring there is enough water and maintaining sufficient temperature for the
desired chemical reaction to take place during hydration.
[concrete scooping]
Concrete is composed of aggregates that are bound together by a paste made of
cementitious binder and water.
All hydraulic cementitious binders have one thing in common. They gain strength and other
engineering properties through a hydration reaction that occurs when cement and supplementary
cementitious materials chemically react with the mixing water to form hydration products.
These hydration products partially occupy the space previously filled with water,
binding the cement particles and aggregates together in a rigid matrix known as concrete.
As hydration progresses, the concrete strengthens and becomes less permeable.
Concrete curing continues as long as: cement particles are available and remain accessible to water,
water is available to support the hydration reaction,
the temperature remains high enough to support the hydration reaction.
The most common curing method for highway construction is the application of
membrane-forming curing compounds.
Other methods, such as fogging, plastic sheets, wetted materials,
and insulated blankets may be used, depending on the type of project
and the ambient conditions during and after placement.
Curing takes place in two steps: Initial curing during or immediately after
the concrete is placed, and only if it is placed under less-than-favorable conditions.
Conventional curing is performed on all concrete after the concrete has undergone the initial set.
Initial curing controls the early loss of moisture from the fresh concrete surface,
allowing the concrete to set before the development of plastic shrinkage cracking.
Initial curing is particularly applicable during dry, windy, and hot ambient conditions,
which can result in high rates of moisture evaporation from the concrete surface.
Largely due to increased use of supplementary cementitious materials and low-slump concrete
in slipform paving, modern concrete mixtures often have little to no bleeding, making initial
curing all the more relevant.
The two most common methods to achieve initial curing are
through fogging and evaporation reducers.
Concrete pavements are fogged using specialized fog nozzles to create a fog blanket
on the surface of the fresh concrete.
Fogging minimizes evaporation and reduces plastic shrinkage cracking by raising the
relative humidity immediately above the slab surface
without spraying excessive water on the slab surface.
Evaporation reducers are organic chemical solutions that form a film on the surface,
which reduces evaporation.
Evaporation reducers are applied by spraying them onto the fresh concrete surface when
finishing operations are complete and before conventional curing begins.
Conventional curing begins immediately after finishing is complete.
The purpose of conventional curing is to control the evaporation of moisture from the surface
for the duration of the specified curing period.
During the finishing process, downward pressure from the finishing operations
brings water to the surface.
Along with the high surface area created by texturing operations, this results
in increased evaporation.
The rate of moisture evaporation depends on the air temperature, relative humidity,
concrete temperature, and wind velocity.
Wet coverings or water spraying are the most effective ways to conventionally cure concrete.
However, for practical reasons, concrete pavements are nearly always cured through the application
of liquid membrane-forming curing compounds.
These membrane-forming curing compounds are composed of waxes, resins, and other materials,
that act as a moisture barrier slowing the evaporation of moisture
from the pavement surface.
They must be applied to all exposed concrete surfaces to control moisture evaporation.
Membrane-forming curing compounds are specified under AASHTO M 148
in terms of color and composition,
whereas their water-retention capability is assessed through AASHTO T 155.
For maximum effectiveness, membrane-forming curing compounds must be applied uniformly,
in sufficient quantity per the manufacturer's specifications, and at the proper time.
Poor coverage of the curing compound could result in unacceptable moisture loss.
Any delay in the application of the curing compound under adverse conditions could result
in plastic shrinkage cracking.
Deep texturing such as an aggressive broom finish or tining
may require increased application rates.
To ensure adequate coverage and prevent pooling, the curing compound should be applied in two
passes sprayed at right angles to one another.
To achieve uniform application, it is critical that curing compound spray nozzles and hoses
are cleaned and properly maintained.
Under windy conditions, the spray nozzles may need to be shielded using wind guards
to ensure application uniformity.
White-pigmented curing compounds are used on hot, sunny days as they increase surface
reflectivity, which reduces concrete temperature.
Proper curing is an essential step to ensure that concrete reaches its full potential.
Curing should begin when the surface begins to dry and continue until the desired engineering
properties are achieved to produce a strong, durable, long lasting concrete surface.
For more information on concrete curing, visit our website at fhwa.dot.gov/pavement
or follow FHWA on social media.
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