What Is Concrete Curing?
What Is
Concrete Curing?
Concrete
curing is the process of maintaining adequate moisture in concrete within a
proper temperature range in order to aid cement hydration at early ages.
Hydration is the chemical reaction between cement and water that results in the
formation of various chemicals contributing to setting and hardening. The
hydration process is affected by the initial concrete temperature, the ambient
air temperature, the dimensions of the concrete, and mix design. Therefore, for
this process to progress well, in-situ concrete must have sufficient moisture
and a temperature that favours this chemical reaction at a rapid and continuous
rate.
The
American Concrete Institute (ACI) recommends a minimum curing period
corresponding to attaining 70% of the compressive strength of concrete. It is
often specified that this can be achieved after seven days of curing. However,
70% strength can be reached quicker when concrete cures at higher temperatures,
or when certain admixtures are used in the concrete mix. Similarly, more time
may be needed for curing when concrete or ambient temperatures are lower.
Typically, 20 degrees Celsius (68°F) is considered an ideal curing temperature.
Why Concrete
Curing Is Important
Careful
control of moisture and temperature of your in-situ concrete during curing is
an essential part of quality control and quality assurance of your concrete
structure. Proper curing techniques will prevent in-situ concrete from drying,
shrinking, and/or cracking, and ultimately affecting the performance of your
structure, particularly at the cover zone. Curing of concrete should occur as
soon as it has been placed. It is also essential that continuous monitoring of
concrete curing conditions be carried out for seven days. If water evaporates from
the concrete before it has attained its maximum strength, there will not be
enough water remaining in the concrete to fully hydrate the cement and achieve
maximum compressive strength. This is especially true during extreme weather
conditions, when your concrete slab is subjected to various environmental
elements and strength development of your concrete can be compromised.
Techniques for
Curing
Many
factors affect the rate at which water evaporates from freshly placed concrete.
This includes air temperature, humidity, concrete temperature, and wind speed.
As a result, many techniques have been developed to help concrete retain
moisture at the early ages. These methods are used to:
1.
Maintain the presence of
water in the concrete during the early hardening period,
2.
Reduce the loss of water
from the surface of the concrete, and
3.
Accelerate concrete
strength gain by supplying heat and additional moisture.
The
method (or combination of methods) chosen depends on factors such as
availability of curing materials, size, shape, and age of concrete, production
facilities (in place or in a plant), aesthetic appearance, and economics. As a
result, curing often involves a series of procedures used at a particular time
as the concrete ages. The timing of each procedure depends on the degree of
hardening of the concrete needed to prevent the procedure from damaging the
concrete surface.
1) Maintaining
Moisture
Ponding and Immersion
Ponding is typically used to cure flat surfaces, such as pavements and floors,
as earth and sand around the perimeter of the concrete surface can retain a
pond of water. Ponding is an ideal method for preventing loss of moisture from
the concrete; it is also effective for maintaining a uniform temperature
throughout the concrete. Immersion of a finished concrete element is commonly
used for curing test specimen.
Spraying and fogging
Spraying and fogging are used when the ambient temperatures are well above
freezing, and the humidity is low. Fog mist is applied through nozzles or
sprayers to raise the relative humidity of the air over flatwork, thus slowing
evaporation from the surface. Fogging is used to minimize plastic shrinkage
cracking. If sprinkling is done at intervals, the concrete must be prevented
from drying between applications of water by using burlap or similar materials;
otherwise, alternate cycles of wetting and drying can cause surface cracking.
Saturated wet coverings
Wet coverings saturated with water, such as burlap, cotton mats, rugs, or other
moisture-retaining fabrics, are commonly used for curing. The materials should
be placed as soon as the concrete has hardened enough to prevent surface
damage. They should be kept constantly wet so that a film of water remains on
the concrete surface throughout the curing period.
Left pin-place forms
Left in place forms usually provide satisfactory protection against moisture
loss for formed concrete surfaces. The forms are usually left in place as long
as the construction schedule allows. If the forms are made of wood, they should
be kept moist, especially during hot, dry weather.
2) Reducing
Loss of Water
Covering concrete with impervious paper or plastic sheets
impervious paper and plastic sheets can be applied on thoroughly wet concrete,
such as polyethylene film. This material is a lightweight, effective moisture
retarder that is easy to apply. During placement, the concrete surface should
be hard enough to prevent surface damage.
Applying membrane-forming curing compounds
Membrane-forming curing compounds are used to retard or reduce evaporation of
moisture from concrete. They can be clear or translucent and white pigmented.
White-pigmented compounds are recommended for hot and sunny weather conditions
to reflect solar radiation. Curing compounds should be applied immediately after
final finishing and must comply with ASTM C3094 or ASTM C13155.
3) Accelerating
Concrete Strength Gain
Live steam
Live steam and high-pressure steam are the two methods of steam curing. The
temperature for live steam should be kept at about 140 degrees Fahrenheit or
less until the desired concrete strength is achieved.
Heating coils
Heating coils are usually used as embedded elements near the surface of
concrete elements. Their purpose is to protect concrete from freezing during
cold-weather concreting.
Electrical heated forms or pads
Electrical heated forms or pads are primarily used by precast concrete
producers.
Concrete blankets
Concrete insulation blankets are used to cover and insulate concrete surfaces
subjected to freezing temperatures during the curing period. When using
concrete blankets, make sure that the concrete is hard enough to prevent
surface damage.
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