Curing Concrete in Construction
By Jerzy Z. Zemajtis, Ph.D., PE (WA)*
plays an important role on strength development and durability of
concrete. Curing takes place immediately after concrete placing
and finishing, and involves maintenance of desired moisture and
temperature conditions, both at depth and near the surface, for
extended periods of time. Properly cured concrete has an adequate
amount of moisture for continued hydration and development of strength,
volume stability, resistance to freezing and thawing, and abrasion
and scaling resistance.
The length of adequate curing time is dependent on the following
- Type of cementitious materials used
- Mixture proportions
- Specified strength
- Size and shape of concrete member
- Ambient weather conditions
- Future exposure conditions
Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways,
floors, canal linings) and structural concrete (e.g. bridge decks,
piers, columns, beams, slabs, small footings, cast-in-place walls,
retaining walls) require a minimum curing period of seven days for
ambient temperatures above 5°C (40°F)1.
American Concrete Institute (ACI) Committee 301 recommends a minimum
curing period corresponding to concrete attaining 70% of the specified
compressive strength2. The often specified 7-day curing commonly
corresponds to approximately 70% of the specified compressive strengths.
The 70% strength level can be reached sooner when concrete cures
at higher temperatures or when certain cement/admixture combinations
are used. Similarly, longer time may be needed for different material
combinations and/or lower curing temperatures. For this reason,
ACI Committee 308 recommends the following minimum curing periods3:
- ASTM C 150 Type I cement 7 days
- ASTM C 150 Type II cement 10 days
- ASTM C 150 Type III cement 3 days
- ASTM C 150 Type IV or V cement 14 days
- ASTM C 595, C 845, C 1157 cements variable
Effect of curing duration on compressive strength development is
presented in Figure 11.
Figure 1. Moist Curing Time and Compressive Strength Gain
Higher curing temperatures promote an early strength gain in concrete
but may decrease its 28-day strength. Effect of curing temperature
on compressive strength development is presented in Figure 21.
Figure 2. Effect of Curing Temperature on
There are three main functions of curing:
1) Maintaining mixing water in concrete during the early hardening
a. Ponding and immersion
Ponding is typically used to cure flat surfaces on smaller jobs.
Care should be taken to maintain curing water temperature at not
more than 11°C (20°F) cooler than the concrete to prevent
cracking due to thermal stresses.
Immersion is mainly used in the laboratory for curing concrete
b. Spraying and fogging
Spraying and fogging are used when the ambient temperatures are
well above freezing and the humidity is low. Fogging can minimize
plastic shrinkage cracking until the concrete attains final set.
c. Saturated wet coverings
Wet coverings saturated with water should be used after concrete
has hardened enough to prevent surface damage. They should be
kept constantly wet.
d. Left in 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 the loss of mixing water from the surface of the concrete
a. Covering concrete with impervious paper or plastic sheets
Impervious paper and plastic sheets can be applied on thoroughly
wetted concrete. The concrete surface should be hard enough to
prevent surface damage from placement activities.
b. 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.
Curing compound shall comply with ASTM C3094 or ASTM C13155.
3) Accelerating strength gain using heat and additional moisture
a. Live steam
Live steam at atmospheric pressure and high-pressure steam in
autoclaves are the two methods of steam curing. Steam temperature
for live steam at atmospheric pressure should be kept at about
60°C (140°F) or less until the desired concrete strength
b. 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.
c. Electrical heated forms or pads
Electrical heated forms or pads are primarily used by precast
d. Concrete blankets
Concrete insulation blankets are used to cover and insulate concrete
surfaces subjected to freezing temperatures during the curing
period. The concrete should be hard enough to prevent surface
damage when covering with concrete blankets.
Other forms of curing include internal moist curing with lightweight
aggregates or absorbent polymer particles. For mass concrete elements
(usually thicker than 3 ft.), a thermal control plan is usually
developed to help control thermal stresses. Additional information
can be found in ACI Committee 308 report Guide to Curing Concrete3.
For specialty concretes, it is recommended to refer to other ACI
reports as follows:
- Refractory concrete ACI 547.1R
- Insulating concrete ACI 523.1R
- Expansive cement concrete ACI 223
- Roller-compacted concrete ACI 207.5R
- Architectural concrete ACI 303R
- Shotcrete ACI 506.2
- Fiber-reinforced concrete ACI 544.3R
- Vertical slipform construction ACI 313
Curing in either cold or hot weather requires additional attention.
In cold weather, some of the procedures include heated enclosures,
evaporation reducers, curing compounds, and insulating blankets.
The temperature of fresh concrete shall be above 10°C (50°F).
The curing period for cold weather concrete is longer than the standard
period due to reduced rate of strength gain. Compressive strength
of concrete cured and maintained at 10°C (50°F) is expected
to gain strength half as quickly as concrete cured at 23°C (73°F).
In hot weather, curing and protection are critical due to rapid
moisture loss from fresh concrete. The curing actually starts before
concrete is placed by wetting substrate surfaces with water. Sunscreens,
windscreens, fogging, and evaporation retardants can be used for
hot weather concrete placements. Since concrete strength gain in
hot weather is faster, curing period may be reduced. Additional
information can be found in ACI 306.1, Standard Specification
for Cold Weather Concreting, ACI 306R, Cold Weather Concreting, ACI
305.1, Specification for Hot Weather Concreting, and ACI
305R, Hot Weather Concreting.
Curing Concrete Test Specimens
Curing of concrete test specimens is usually different from concrete
placed during construction. American Society for Testing and Materials
(ASTM) has developed two standards for making and curing concrete
specimens. ASTM C1926 is intended for laboratory samples while ASTM
C317 is intended for field samples. Both documents provide standardized
requirements for making, curing, protecting, and transporting concrete
test specimens under field or laboratory conditions, respectively.
ASTM C192 provides procedures for evaluation of different mixtures
in laboratory conditions. It is usually used in the initial stage
of the project, or for research purposes.
ASTM C31 is used for acceptance testing and can also be used as
a decision tool for form or shoring removal. Depending on its intended
purpose, the standard defines two curing regimes: standard curing
for acceptance testing and field curing for form/shoring removal.
Variation in standard curing of test specimens can dramatically
affect measured concrete properties. According to the National Ready
Mix Concrete Association8 (NRMCA), strength for concrete air cured
for one day followed by 27 days moist cured will be approximately
8% lower than for concrete moist cured for the entire period. The
strength reduction is 11% and 18% for concrete specimens initially
cured in air for 3 days and 7 days, respectively. For the same air/moist
curing combinations, but 38°C (100°F) air curing temperature,
the 28-day strength will be approximately 11%, 22%, and 26% lower,
* Jerzy Z. Zemajtis, Ph.D., PE (WA)
Senior Engineer, CTLGroup, Skokie, IL
(847) 832-0260, email@example.com
1S. Kosmatka et al, Design
and Control of Concrete Mixtures, 14th Edition, PCA Engineering
Bulletin EB 001, Portland Cement Association , Skokie, IL 2002
2 Specifications for Structural Concrete, ACI
3 Guide to Curing Concrete, ACI 308R-01 (www.concrete.org)
4 ASTM C309, Standard Specification for Liquid Membrane-Forming
Compounds for Curing Concrete (www.astm.org)
5 ASTM C1315, Standard Specification for Liquid Membrane-Forming
Compounds Having Special Properties for Curing and Sealing Concrete
6ASTM C192 / C192M, Standard Practice for Making
and Curing Concrete Test Specimens in the Laboratory (www.astm.org)
7ASTM C31 / C31M, Standard Practice for Making and
Curing Concrete Test Specimens in the Field (www.astm.org)
8 David N. Richardson, Review of Variables that Influence
Measured Concrete Compressive Strength, NRMCA Publication 179,
NRMCA, Silver Spring, MD, 1991.
The Link Between Concrete Sustainability and
Sustainability, according to the Bruntland Report and adopted by
many experts, is development that meets the needs of the present
without compromising the ability of future generations to meet their
own needs. This can be accomplished in one of two ways: either by
using recyclable, reusable, or so little resources that future generations
have the same access to them; or by producing development that meets
our needs as well as the needs of future generations. We can use
proper curing of concrete to advance towards the reduction of resource
A concrete element is expected to last a certain number of years.
In order to meet this expected service life, it must be able to
withstand structural loading, fatigue, weathering, abrasion, and
chemical attack. The duration and type of curing plays a big role
in determining the required materials necessary to achieve the high
level of quality.
is the process in which the concrete is protected from loss of moisture
and kept within a reasonable temperature range. The result of this
process is increased strength and decreased permeability. Curing
is also a key player in mitigating cracks in the concrete, which
severely impacts durability. Cracks allow open access for harmful
materials to bypass the low permeability concrete near the surface.
Good curing can help mitigate the appearance of unplanned cracking.
When smart, suitable, and practical curing is used, the amount of
cement required to achieve a given strength and durability can be
reduced by either omission or replacement with supplementary cementitious
materials. Since the cement is the most expensive and energy intensive
portion of a concrete mixture, this leads to a reduction in the
cost as well as the absolute carbon footprint of the concrete mixture.
Additionally, being practical with curing methods can enhance sustainability
by reducing the need for resource intensive conditioning treatments,
should the curing method be incompatible with the intended service
Curing Pavements and
While curing of concrete is an important issue with all concrete
applications concrete pavements and bridge decks require careful
consideration and have significantly different needs with regard
to curing of the concrete of these structures. Both categories have
basic requirements for the durability of the structures including
strength, abrasion resistance, freezing and thawing and deicer resistance,
and, in the case of bridges, low permeability for corrosion protection
of the reinforcement of the structure.
Typical recommendations for curing of pavements allow the use of
sheet curing, moist curing, or application of a film forming curing
compound. Due to the large surface areas typical of concrete paving
the application of curing compound to all exposed surfaces is the
most common curing method. Moist curing and sheet curing of large
surface areas may become cost prohibitive due to the large quantity
of materials required to cover the full surface of concrete placed
in any single day. In addition moist curing and sheet curing require
maintenance to assure the curing method is properly completed for
the full time duration chosen for paving (typically 7 days). Moist
coverings require rewetting and sheet goods are prone to being disturbed
by wind, either of which would reduce the effectiveness of the curing
Curing compounds should be applied to pavements as soon as possible
after bleed water has left the surface of the concrete at a rate
of 5 m2/L (200 ft2/gal) for standard mixtures
and application, 3.75 m2/L (150 ft2/gal) for
fast track paving, and 2 1/2 m2/L (100 ft2/gal)
for slabs thinner than 125 mm (5.0 in.)
In contrast concrete bridges require a higher standard of curing
to achieve the low permeability required for protection of steel
reinforcement. Standard recommendations for curing bridge decks
is moist curing for a minimum of 7 days for concrete mixtures containing
only portland cement and as long as 14 days when supplementary cementing
materials are included in the concrete mixture. Some states also
require the application of curing compound upon removal of the moist
Typical moist curing for bridge decks requires the application of
adequate quality water saturated burlap or other approved absorptive
material covered with minimum 6 mil plastic covering. The temperature
of the saturated materials should be within 11°C (20°F)
of the temperature of the in-place concrete. In most cases plastic
will be specified to be white in color to reflect solar radiation,
reducing the temperature rise beneath the plastic, while cold temperatures
(less than 10°C (50°F)) may allow the use of black plastic
to add heat to the system. Proper moist curing will also require
uncovering and rewetting the absorptive material to assure that
there is a constant supply of water available to satisfy the evaporation
rate at the project site.
Design and Control
of Concrete Mixtures, 14th Edition, EB001
Guidelines for Concrete Streets and Local Roads, IS119
Issue No. 45, Fall 2006
Read an FAQ on "What
should be considered when choosing a curing method for slabs that
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