aci code

1. ANSI/ACI 308-92(11/24/93)(Reapproved 1997)
Roger E. Carrier
Richard H. Danforth
L Blake Fentress*
Jerome H. Ford
Steven H. Gebler
Gilbert Haddad
Richard E. Hay
Samuel B. Helms
Standard Practice for
Curing Concrete (ACI 308-92)(Reapproved 1997)
Reported by ACI Committee 308
Bryant Mather
Chairman
R.W. Kriner
AT. Livingood
R.H. Mills
Glenn E. Noble
Dixon O’Brien, Jr.
H. Celik Ozyildirim
William S. Phelan
Members of the committee voting on the 1991 revisions:
Richard W. Kriner
Chairman
Ronald L. Dilly Kenneth C. Hover
Jerome H. Ford Leonard M. Johnson
Steven H. Gebler Irvin S. Kaufman, Jr.
Gilbert Haddad Frank A. Kozeliski
Samuel B. Helms James A. Lee
Edward P. Holub Bryant Mather
Curing is the maintaining of a satisfactory moisture content and tem-
perature in concrete during its early stages so that desited properties may
develop.
Basic principles of curing are stated; commonly accepted methods pro-
cedures, and materials are described . Requirements are given for curing
pavements and other slabs on ground; for structures and buildings; and for
mass concrete For each of these categories, methods, materials time, and
temperature of curing are stated Curing reqirementsfor precast produ cts,
shotcrete, preplaced-aggregate concrete, refractory concrete, plaster, and
other applications are given.
Keywords: bridges (structures); builings cement-base paints; cold-weather
construction; concrete construction; concrete pavements; concretes; curing;curing
compounds; curing films and sheets; hot-weather construction; insulating
concrete; insulation; mass concrete; moist curing; plaster; precast concrete;
refractory concretes; reinforced concrete; sealers; shells (stuctural forms);
shotcrete; slab-on-ground construction; slipform construction; standards; steam
curing; stucco.
ACI Committee Reports, Guides, Standard Practices, and
Commentaries are intended for guidance in designing, plan-
ning, executing, or inspecting construction and in preparing
specifications. References to these documents shall not be made
in the Project Documents. If items found in these documents
are desired to be a part of the Project Documents, they should
be phrased in mandatory language and incorporated into the
Project Documents.
Owen Richards
Arthur P. Seyler
Luke M. Snell
William L Trimm
Lewis H. Tuthill
Robert J. Van Epps
Frank T. Wagner
Laverne R. Mertz
H. Celik Ozyildirim
Gary D. Pfuehler
William S. Phelan
Ephraim Senbetta
Luke M. Snell
CONTENTS
Chapter l-Introduction and referenced standards, pg.
308-2
l.l-Scope
1.2-Need for curing
1.2.1-Satisfactory moisture content
1.2.2-Favorable temperature
1.3-Referenced standards
1.3.1-ASTM Standards
1.3.2-ACI Standards and Reports
1.3.3-AASHTO Materials Standards
Chapter 2-Curing methods and materials, pg. 308-4
l Adopted as a standard of the American Concrete Institute August 1981 to
supersede ACI Standard “Recommended Practice for Curing Concrete (ACI
308.71),” in accordance with the Institute’s standardization procedure. Revised by
Expedited Standardization Procedure effective July 1, 1986, and Mar. 1, 1992.
Copyright 1980 American Concrete Institute.
All rights reserved including rights of reproduction and use in any form or by
any means, including the making of copies by any photo process, or by any elec-
tronic or mechanical device, printed or written or oral, or recording for sound or
visual reproduction or for use in any knowledge or retrieval system or device,
unless permission in writing is obtained from the copyright proprietors.

2. 308-2
2.1-Scope
2.2-Water curing
2.2.1-Ponding or immersion
2.2.2-Fog spraying or sprinkling
2.2.3-Burlap, cotton mats, and rugs
2.2.4-Earth curing
2.2.5-Sand and sawdust
2.2.6-Straw or hay
2.2.7-Termination
2.3-Sealing materials
2.3. l-Plastic film
2.3.2-Reinforced paper
2.3.3-Liquid membrane-forming curing
compounds
2.4-Cold weather protection and curing
2.5-Hot-weather curing
2.6-High-pressure steam curing
2.7-Low-pressure (or atmospheric-pressure)
steam curing
2.8--Evaluation of curing procedures
2.9-Criteria for effectiveness of curing
2.9. l-General
2.9.2-Strength basis
2.9.3-Maturity-factor basis
2.10-Minimum curing requirements
Chapter 3-Curing for different constructions,
pg. 308-9
3.1-Pavements and other slabs on the ground
3.1.1-General
3.1.2-Curing procedures
3.1.3-Duration of curing
3.2-Structures and buildings
3.2. l-Scope
3.2.2-Curing procedures
3.2.3-Duration of curing and protection
3.3-Mass concrete
3.3.1-Scope
3.3.2-Temperature control
3.3.3-Methods and duration of curing
3.4-Other construction
3.4.1-Precast units
3.4.2-Vertical slipform construction
3.4.3-Shotcrete
3.4.4-Refractory concrete
3.4.5-Cement paint, stucco, and plaster
3.4.6-Shell structures
3.4.7-Insulating concrete
3.4.8-Concrete with colored or metallic surfaces
CHAPTER l-INTRODUCTION AND
REFERENCED STANDARDS
l.l-Scope
This standard describes practices to be followed in
curing concrete. Since concrete is used for many pur-
poses and under widely varying conditions of service,
requirements are given for curing according to curing
methods and materials, according to method of construc-
tion, and according to the use to be made of the hard-
ened concrete.
1.2-Need for curing
Curing is the maintaining of a satisfactory moisture
content and temperature in concrete during its early
stages so that desired properties may develop. Curing is
essential in the production of concrete that will have the
desired properties. The strength and durability of con-
crete will be fully developed only if it is cured. No action
to this end is required, however, when ambient con-
ditions of moisture, humidity, and temperature are
sufficiently favorable to curing. Otherwise, specified
curing measures shall start as soon as required. Tem-
perature control must be exercised to prevent freezing of
the concrete until it develops a compressive strength of
at least 500 psi (3.4 MPa).* Following this the concrete
must be kept sufficiently warm so as to produce the
required strength at a specified age. Other aspects of
favorable temperature are given in Section 1.2.2.
1.2.1 Satisfactory moisture content-The amount of
mixing water in the concrete at the time of placement is
normally more than must be retained for curing. How-
ever, excessive loss of water by evaporation may reduce
the amount of retained water below that necessary for
development of desired properties. The potentially harm-
ful effects of evaporation shall be prevented either by
applying water or preventing excessive evaporation. Fig.
1 shows the effect of air temperature, concrete tem-
perature, relative humidity, and wind velocity, on surface
evaporation rate. When these factors combine to cause
excessive evaporation of some of the mixing water,
shrinkage cracks may occur in the plastic concrete. Fig.
1 will aid in evaluating these effects and guidance on
evaporation rate levels is given in its caption. Failure to
prevent such excessive evaporation frequently causes
plastic shrinkage cracks and loss of strength in the
material near the surface.
1.2.2 Favorable temperature-The rate of cement
hydration varies with temperature, proceeding slowly at
cooler temperatures down to 14 F (-10 C) and more
rapidly at warmer temperatures up to somewhat below
212 F (100 C). Concrete temperatures below 50 F (10 C)
are unfavorable for the development of early strength.
Below 40 F (5 C) the development of early strength is
greatly retarded and at 32 F (0 C) little strength
develops. There is some evidence that curing for a
shorter time at a higher temperature will not be as
beneficial as longer curing at a lower temperature in
terms of final strength (see Fig. 2). Autoclaving at
temperatures above 325 F (163 C) greatly accelerates
hydration and may produce strengths in a few hours
*See Powers T.C., “Prevention of Frost Damage to Green Concrete,” Reunion
Internationale des Laboratoires d’ Essais et de Recherches sur les Materiaux et
les Constructions RILEM Bulletin 14, 120-124 (March, 1962), (PCA Res. Bull.
No. 145).

3. STANDARD PRACTICE FOR CURING CONCRETE
308-3
deg C
15 25 35
4 0 50 60 70 8 0 90
Air temperature, deg F
To use this chart:
I. Enter with air temp-
0 7
‘*
erature, move up to
G O . 6relative humidity
2. Move right to concrete 0.5
temperature
3. Move down to wind
“ 0 4
velocity
0.3
4. Move left: read approx
rate of evaporation
Fig. l-Effect of concrete and air temperatures, relative humidity, and wind velocity on the rate of evaporation of surface
moisture from concrete. This chart provides a graphic method of estimating the loss of surface moisture for various weather
conditions. To use the chart, follow the four steps outlined above. When the evaporation rate exceeds 0.2 (1.0
measures shall be taken to prevent excessive moisture loss from the surface of unhardened concrete; when the
rate exceeds 0.1 (0.5 such measures may be needed. When excessive moisture loss is not prevented, plastic
shrinkage cracking is likely to occur
equal to those obtained at 28 days of curing at 70 F (21 The temperature of the concrete during its early
C). However, autoclaving is a special case, since at such stages is affected by various factors such as: the ambient
elevated temperatures and pressures there are chemical temperature, the absorption of solar heat, the heat re-
reactions which produce products that are not formed at leased by the hydration of the cement, and the initial
lower temperatures (see 1.3.2.6). Methods of accelerated temperature of the materials. Evaporation of mixing or
curing of concrete compression test specimens are given curing water at the surface of the concrete can produce
in ASTM C 684 (1.3.1.11). a significant cooling effect, which is beneficial as long as

4. 308-4 ACI STANDARD
6000
deg C
40 60 80 100 120
Curing temperature, deg F
34.5
0
27.6
69.
Fig. 2-One-day strength increases with increasing curing
temperature but 28-day strength decreases with increasing
curing temperature. Reference: “Structures and Physical
Properties of Cement Pastes” (Verbeck and Helmuth, Pro-
ceedings, Fifth International Symposium on the Chemistry
of Cement, 1968, V 3, p. 9)
evaporation is not so great as to cause lower ultimate
strength or cracking either due to plastic shrinkage or
excessive surface cooling.
It is preferable to avoid curing temperatures very
much higher than the average temperature of the con-
crete anticipated during its period of service and to
maintain a reasonably uniform temperature throughout
the whole mass of concrete (see 3.3.2).
1.3-Referenced standards
1.3.1ASTM Standards-These are found in the Annual
Book of ASTM Standards
1.3.1.1 C 31-Standard Practice for Making and
Curing Concrete Test Specimens in the Field
1.3.1.2 C 39-Test Method for Compressive
Strength of Cylindrical Concrete Specimens
1.3.1.3 C 42-Methods for Obtaining and Testing
Drilled Cores and Sawed Beams of Concrete
1.3.1.4 C 78-Test Method for Flexural Strength of
Concrete (Using Simple Beam with Third-Point Loading)
1.3.1.5 C 94-Specification for Ready Mixed Con-
crete
1.3.1.6 C 156-Test Method for Water Retention
by Concrete Curing Materials
1.3.1.7 C 171-Specification for Sheet Materials for
Curing Concrete
1.3.1.8 C 192-Standard Practice for Making and
Curing Concrete Test Specimens in the Laboratory
1.3.1.9 C 309-Specification for Liquid Membrane-
Forming Compounds for Curing Concrete
1.3.1.10 C 597-Test Method for Pulse Velocity
Through Concrete
1.3.1.11 C 684-Test Method for Making, Ac-
celerated Curing, and Testing of Concrete Compression
Test Specimens
1.3.1.12 C 803-Test Method for Penetration
Resistance of Hardened Concrete
1.3.1.13 C 805-Test Method for Rebound Number
of Hardened Concrete
1.3.1.14 C 873-Test Method for Compressive
Strength of Concrete Cylinders Cast-in-Place in
Cylindrical Molds
1.3.2 ACI Standards and Reports-These are found in
the ACI Manual of Concrete Practice
1.3.2.1 207.1R-Mass Concrete
1.3.2.2 302.1R-Guide for Concrete Floor and Slab
Construction
1.3.2.3 305R-Hot-Weather Concreting
1.3.2.4 306R-Cold-Weather Concreting
1.3.2.5 506R-Guide to Shotcrete
1.3.2.6 506.2-Specification for Materials, Pro-
portioning, and Application of Shotcrete
1.3.2.7 517.2R-Accelerated Curing of Concrete at
Atmospheric Pressure
1.3.3 AASHTO Materials Standards-These are found
in AASHTO Materials Standards, 13th Edition, 1982,
Parts I and II, respectively
1.3.3.1 M182-Specification for Burlap Cloth Made
From Jute or Kenaf
1.3.3.2 T26-Method of Test for Quality of Water
to be Used in Concrete
CHAPTER 2-CURING METHODS
AND MATERIALS
2.1-Scope
Various materials, methods, and procedures for curing
concrete are available but the principles involved are the
same; to insure the maintenance of a satisfactory mois-
ture content and temperature so that desired properties
may develop.
The two systems of maintaining a satisfactory moisture
content are: 1) the continuous or frequent application of
water through ponding, sprays, steam, or saturated cover
materials such as burlap or cotton mats, rugs, earth, sand,
sawdust, and straw or hay, and 2) the prevention of ex-
cessive loss of water from the concrete by means of
materials such as sheets of reinforced paper or plastic, or
by the application of a membrane-forming curing com-
pound to the freshly placed concrete.

5. STANDARD PRACTICE FOR CURING CONCRETE
2.2-Water curing rain.
If application of water is selected, the economics of Cotton mats and rugs hold water longer than burlap
the particular method should be considered for each job
since the availability of water, labor, curing materials, and
other items will influence the cost. The method selected
must provide a complete and continuous cover of water
that is free of harmful amounts of deleterious materials.
Where appearance is a factor, the water must be free of
harmful amounts of substances that will attack, stain, or
discolor the concrete. * Care needs to be taken to avoid
thermal shock or excessively steep thermal gradients due
to use of cold curing water or high rates of evaporative
cooling. Several methods of water curing are described
below:
2.2.1 Ponding or immersion-Though seldom used, the
most thorough method of water curing consists of total
immersion of the finished concrete unit in water. Ponding
is sometimes used for slabs such as culvert or bridge
floors, pavements, flat roofs, or wherever a pond of water
can be created by a ridge or dike of earth or other
material at the edge of the slab, or where there is a
stream of water as through a culvert. Damage from
premature or sudden release of ponded water should be
avoided. For example, if the ponded water leaks out, the
slab might not get proper curing and the water might
soften the supporting soil, or damage the surroundings.
Curing water should not be more than about 20 F (11 C)
cooler than the concrete, because of surface temperature
stresses which could cause cracking.
2.2.2 Fog spraying or Sprinkling-Fog spraying or
sprinkling with nozzles or sprays provides excellent curing
when the temperature is well above freezing. So long as
the concrete surface is cooler than the atmosphere in the
enclosure, steam at atmospheric pressure will cause a
film of moisture to be present on the surface. Lawn
sprinklers are effective where water runoff is of no con-
cern. A disadvantage of sprinkling is the cost of the water
unless there is an ample supply available for the cost of
pumping. Intermittent sprinkling is not acceptable if
there is drying of the concrete surface. Soaking hoses are
useful, especially on surfaces that are vertical or nearly
so. Care must be taken that erosion of the surface does
not occur.
2.2.3 Burlap, cotton mats, and rugs-Burlap, cotton
mats, rugs, and other coverings of absorbent materials
will hold water on the surface, whether horizontal or
vertical. These materials must be free of injurious
amounts of substances+ such as sugar or fertilizer that do
harm to the concrete or cause discoloration. Burlap
should be thoroughly rinsed in water to remove soluble
substances or to make it more absorbent. Burlap that has
been treated to resist rot and fire should be considered
when it is to be stored between jobs. The heavier the
burlap the more water it will hold and the less frequently
it will need to be wetted. Double thicknesses may be
used advantageously. Lapping the strips by half widths
when placing will give greater moisture retention and aid
in preventing displacement during high wind or heavy
with less risk of drying out. They are handled much the
same as burlap except that due to their greater mass,
application to a
the concrete has
burlap.
freshly finished surface must wait until
hardened to a greater degree than for
2.2.4 Earth curing-Wet earth curing has been used ef-
fectively, especially on comparatively small jobs of slab or
floor work. The earth should be essentially free of par-
ticles larger than 1 in. (25 mm) and should not have
injurious amounts of organic matter or other substances
that will damage the concrete?
2.2.5 Sand and sawdust-Wet clean sand and sawdust
are used in the same manner as earth curing. Sawdust
containing excessive amounts of tannic acid should not be
used. Sand and sawdust are especially useful where car-
penters and form setters must work on the surface since
such coverings
and stains.
help to protect the surface against scars
2.2.6 Straw or hay-Wet straw or hay can be used but
there is the danger that wind may remove it unless it is
held down with screen wire, burlap, or other means.
There is also the danger of fire if the straw or hay is
allowed to become dry. Such materials may cause dis-
coloration of the surface for several months after
removal. If these materials are used the layer should be
at least 6 in. (150 mm) thick.
2.2.7 Termination-Saturated cover materials shall not
be allowed to dry out and absorb water from the con-
crete, but at the end of the required period of wetness
shall be allowed to dry thoroughly before removal so that
the concrete will dry slowly.
2.3-Sealing materials
Sealing materials are sheets or membranes placed on
concrete to reduce the loss of water from the concrete by
evaporation. There are advantages in the use of sealing
materials for curing that make their use preferable under
many conditions. For example, if the moisture is sealed
in, there is less likelihood of harmful drying due to
failure to keep the covering wet. Also sealing materials
are often less costly and are easier to handle and can be
applied earlier, often without any other initial curing. In
arid regions they are particularly useful for curing flat
work on a moist subgrade and for massive structural
concrete. Common sealing materials are described in the
following sections. Forms left in place serve to reduce
loss of moisture from surfaces in contact with the forms.
2.3.1 Plastic film-Plastic film is light in weight and is
available in clear, white, or black sheets. The film should
meet the requirements of ASTM C 171 (1.3.1.7) which
specifies a 0.0040 in. (0.10 mm) thickness. This spe-
l See McCoy, W. J., “Mixing and Curing Water for Concrete.” Chapter 43 of
Significance of Tests and Properties of Concrete and Concrete-Making Materials,
ASTM STP 169B, 1978, pp. 765-773 with 24 references.
See also ACI 201 “Guide to Durable Concrete,” Chapter 2, for more infor-
mation on substances that attack concrete.

6. cification does not mention black sheeting, but black
issatisfactory under some conditions. White is more ex-
pensive but gives considerable reflection of the sun’s rays,
whereas, clear films have little effect on heat absorption.
Black should be avoided during warm weather except for
interiors, but has advantages in cold weather because of
its heat absorption. Care must be taken not to tear or
otherwise interrupt the continuity of the film curing.
Plastic film reinforced with glass or other fibers is more
durable and is less likely to be torn.
Where appearance is of critical importance, concrete
should be cured by other means because the use of
smooth plastic film usually results in a mottled
appearance. This may not be serious in pavements, roof
slabs, and curb and gutter, and may be prevented by
occasional flooding under the film. Combinations of
plastic film bonded to absorbent fabric help to retain and
distribute the moisture released from the concrete and
condensed on the curing cover.
The plastic film should be placed over the wet surface
of the fresh concrete as soon as possible without marring
the surface, and should cover all exposed surfaces of the
concrete. It should be placed and weighted so that it
remains in contact with the concrete during the specified
length of curing. On flat surfaces such as pavements, the
film should extend beyond the edges of the slab at least
twice the thickness of the slab. The film should be placed
flat on the concrete surface, without wrinkles, to min-
imize mottled discoloration. Windrows of sand or earth,
or strips of wood should be placed along all edges and
joints in the film to retain moisture in the concrete and
prevent wind from getting under the film and removing
it. In lieu of this procedure, it is acceptable and generally
more economical to use a narrow strip of plastic film
along the vertical edges, placing it over the sheet on the
horizontal surface and securing all edges with windrows
or strips of wood. When the covering is to be removed,
the strip can be pulled away easily leaving the horizontal
sheet to be rolled up without damage from tears or
creases.
2.3.2 Reinforced paper- Reinforced paper should
comply with ASTM C 171 (1.3.1.7). It is composed of
two sheets of kraft paper cemented together with a
bituminous adhesive and reinforced with fiber. Most
paper sheets for curing have been treated to reduce the
amount of expansion and shrinkage when wetted and
dried. The sheets can be cemented together with bit-
uminous cement as desired to meet width requirements.
Paper sheets with one white surface to give reflectance
and reduce absorption of heat are available. A reflec-
tance requirement is included in ASTM C 171. Rein-
forced paper is applied in the same manner as plastic
film (see Section 2.3.1). It is permissible to reuse
reinforced paper as long as it efficiently retards loss of
moisture. Tears are readily discernible and can be
repaired with a patch of paper cemented with a suitable
glue or bituminous cement. Pin holes resulting from
walking on the paper or from deterioration of the paper
through repeated use, are evident if the paper is held up
to the light. When the condition of the paper is ques-
tionable, it should be used in double thickness.
2.3.3 Liquid membrane-forming curing compounds-
Liquid membrane-forming compounds for curing con-
crete should comply with the requirements of ASTM C
309 (1.3.1.9), when tested at the rate of coverage to be
used on the job. Such compounds consist essentially of
waxes, natural and synthetic resins, and solvents of high
volatility at atmospheric temperatures. Adequate ven-
tilation should be provided and other safety precautions
should be taken. The formulation must be such as to
form a moisture-retentive film shortly after being applied
and must not be injurious to portland-cement paste.
White or gray pigments are often incorporated to provide
heat reflectance, and to make the compound visible on
the structure for inspection purposes. Curing compounds
should not be used on surfaces that are to receive ad-
ditional concrete, paint, or tile that requires a positive
bond, unless it has been demonstrated that the mem-
brane can be satisfactorily removed before the sub-
sequent application is made, or that the membrane can
serve satisfactorily as a base for the later application.
The compound should be applied at a uniform rate.
The usual values for coverage range from 150 to 200 sq
ft per gal. (0.20 to 0.25 Tests to determine
compliance with the requirements of ASTM C 309 are
made at the coverage to be used in the field, or if not
stipulated, at 200 (0.20 When feasible,
two applications at right angles to each other are sug-
gested for complete coverage. On very deeply textured
surfaces, such as used on some pavements to improve
surface friction properties, there may need to be two
separate applications each at 200 (0.20
with the first being allowed to become tacky before the
second is applied. Curing compound can be applied by
hand or power sprayer, usually at about 75 to 100 psi (0.5
to 0.7 MPa) pressure. If the job size warrants, mechanical
application is preferred because of speed and uniformity
of distribution. For very small areas such as repairs, the
compound can be applied with a wide, soft-bristled brush
or paint roller.
For maximum beneficial effect, liquid membrane-
forming compounds must be applied after finishing and
as soon as the free water on the surface has disappeared
and no water sheen is visible, but not so late that the
liquid curing compound will be absorbed into the con-
crete. If the ambient evaporation rate exceeds 0.2
hr (1.0 (See Fig. 1) the concrete may still be
bleeding even though the surface water sheen has dis-
appeared and steps must be taken to avoid excessive eva-
poration. If membrane-forming compound is applied to
a dry-appearing surface, one or the other of two un-
desirable conditions may follow: a) evaporation will be
effectively stopped but bleeding may continue, resulting
in a layer of water forming below the layer of cement
paste to which the membrane is attached; such a con-
dition promotes scaling; b) evaporation will be tem-

7. STANDARD PRACTICE FOR CURING CONCRETE 308-7
porarily stopped but bleeding may continue resulting in
map cracking of the membrane film, requiring reap-
plication of the curing compound. In some highway work,
the applicable specifications may allow water-soluble
linseed-oil base membrane-forming compounds to be ap-
plied before the water sheen has gone. When forms are
removed, the exposed concrete surface should be wet
with water immediately and kept moist until the curing
compound is applied. Just prior to application, the
concrete should be allowed to reach a uniformly damp
appearance with no free water on the surface and then
application of the compound should be begun at once.
Pigmented compounds must be stirred to assure even dis-
tribution of the pigment during application, unless the
formulation contains a thixotropic agent to prevent
settlement.
2.4-Cold-weather protection and curing
In cold weather concrete should be cured and pro-
tected from freezing in accordance with ACI Committee
306 (1.3.2.4). Although concrete exposed to cold weather
is not likely to dry at an undesirable rate, particular
attention should be given to maintaining satisfactory
moisture in concrete that is undergoing the protection
required by ACI 306. Concrete should be protected from
freezing at least until it develops a compressive strength
of 500 psi (3.4 MPa); nonair-entrained concrete should
never be allowed to freeze and thaw in a saturated
condition. Air-entrained concrete should not be allowed
to freeze and thaw in a saturated condition before
developing a compressive strength of 3500 psi (24 MPa).
These factors should be considered especially for
concrete placed late in the fall.
2.5-Hot-weather curing
Concrete should be cured in hot weather in ac-
cordance with the provisions of the report of ACI
Committee 305 (1.3.2.3.). Since hot weather leads to
more rapid drying of concrete, protection and curing are
critical. Water curing, if used, should be continuous to
avoid volume changes due to alternate wetting and
drying. The need for adequate continuous curing is
greatest during the first few days after placement of
concrete in hot weather. During hot weather, provided
favorable moisture conditions are continuously main-
tained, concrete may attain a high degree of maturity in
a very short time.
2.6-High-pressure steam curing
High-pressure steam curing, or autoclaving is covered
in detail in the report prepared by ACI Committee 516
(1.3.2.6). This curing process is used in the production of
some concrete masonry units, asbestos-cement pipe, and
lightweight cellular concrete. Products made with ap-
propriate mixtures and cured by autoclaving are char-
acterized by reduced drying shrinkage and increased
sulfate resistance.
2.7-Low-pressure (or atmospheric-pressure) steam
curing
Low-pressure or atmospheric-pressure steam curing is
covered in detail in ACI Standard 517 (1.3.2.7). Atmo-
spheric-pressure steam curing is commonly used in the
manufacturing of concrete products to accelerate early
strength development.
2.8-Evaluation of curing procedures
ASTM C 156 (1.3.1.6) may be used for comparing the
water-retention effectiveness of concrete curing mater-
ials.* Maintenance of a satisfactory moisture conten t by
direct application of water, either by spraying, ponding,
or wet covers, has often been stated to be the ideal
method. Such methods are satisfactory only so long as
the presence of water is continuous and the concrete
does not dry out to such a degree that the development
of desired properties is prevented. Intermittent wetting,
especially after an initial 2 or 3 days of satisfactory
curing, will allow continued strength gain although not as
rapid as continuous curing. Intermittent curing during
early stages of curing is likely to result in surface cracks
and reduced service durability.
The efficiency of curing with plastic or reinforced
paper sheets depends on the extent to which they seal
water in, or are in contact with the concrete. Any leakage
at the edges or joints between the sheets, or through
tears or pin holes will reduce the efficiency. The same is
true for liquid membrane-forming compounds, if the ap-
plication is not uniform or not at the proper rate; loss of
moisture through thin or uncovered spots reduces the
curing efficiency. Also, if the application is delayed too
long there may be substantial water loss before the sur-
face is sealed.
It is not always possible to determine the degree of
curing efficiency since the atmospheric conditions during
the time of curing play a major role in curing. During
rainy or foggy weather little or no effort is needed to
achieve curing, although protection of the surface against
washout or erosion in heavy rainfall may be needed. For
a very low humidity environment particular care should
be taken to prevent excessive moisture loss from the con-
crete.
2.9-Criteria for effectiveness of curing
2.9.1 General-Curing will be effective, by definition,
if the moisture content and temperature that were main-
tained allowed the desired levels of concrete properties
to develop and prevented the undesirable cracking, dust-
ing, scaling, and crazing that can result from failure to do
so. Such consequences, if the result of improper curing,
usually are caused by failure to maintain a satisfactory
moisture content in the concrete immediately adjacent to
surfaces. Therefore, this Chapter has primarily dealt with
methods and materials for preventing concrete surfaces
l *See also Carrier, R E., Turing Materials,” Chapter 44 of Significance Tests
and Properties of Concrete and Concrete-Making Materials, ASTM STP 169B, 1978,
pp. 774-786 with 9 references.

9. STANDARD PRACTICE FOR CURING CONCRETE 308-9
Studies have shown that the transformation is
reasonably correct when adequate information is avail-
able about the concrete mixture, moisture loss does not
occur from the concrete, air temperatures are not ex-
treme, and the concrete temperature remains relatively
constant. The following information must be available in
order that an estimate of the in-place concrete strength
can be made: a) The strength-time relationship of the
concrete under standard laboratory conditions; b) A
time-temperature record of the in-place concrete. These
may be obtained by use of expendable thermistors or
thermocouples cast at varying depths in the concrete. The
location giving the lowest values should be the source of
the temperatures used in the computations.
Other properties of concrete, such as the degree of
permeability, resistance to abrasion, resistance to freezing
and thawing, and resistance to sulfate attack, are also
improved by curing. Consequently, curing beyond that
needed to develop a certain strength is often desirable.
It should not be surprising that the length of curing
prescribed for different types of concrete varies. In each
instance, the recommended length of curing is based on
what is practical and yet sufficient.
2.10-Minimum curing requirements
Natural curing from rain, mist, high humidity, low
temperature, moist backfill, etc., may be regarded as
sufficient to provide ample curing when its effect is at
least the equivalent of keeping the concrete moist for the
first 14 days if made with Type II cement, 7 days if made
with Type I cement, or 3 days with Type III cement, if
kept above 50 F (10 C), unless otherwise prescribed in
the project specifications.
CHAPTER 3-CURING FOR
DIFFERENT CONSTRUCTIONS
3.1-Pavements and other slabs on the ground
3.1.1 General-Slabs on the ground include highway
and airfield pavements, canal linings, parking-lot slabs,
driveways, sidewalks, and floor slabs on grade in build-
ings. Slabs have a high ratio of exposed surface area to
volume of concrete, and, without measures to prevent it,
the moisture loss due to evaporation from the concrete
can be so large and so rapid as to result in plastic
shrinkage cracking, and have a deleterious effect on
strength, abrasion resistance, and frost resistance. Rapid
loss of moisture from the fresh concrete may also result
from inadequate moistening of some subgrades prior to
placement of slabs. To prevent such loss of moisture
from fresh concrete in slabs, and to provide reserve
moisture for curing, the subgrade should be prewetted
and, after finishing the slab, curing should be begun as
soon as possible.
The high ratio of exposed surface area to volume of
concrete can also result in subjecting inadequately cured
concrete to excessive variations in temperature. If stresses
due to variations in temperature exceed tensile strength,
cracking of the slabs will occur. The selected method of
curing will affect the variation in temperature of the
concrete; therefore curing methods should be selected to
minimize early variations in temperature under condi-
tions normally encountered.
3.1.2 Curing procedures- If needed in order to main-
tain a satisfactory moisture content and temperature, the
entire surface of the newly placed concrete should be
treated in accordance with one of the water curing or
sealing methods described in Chapter 2 or a combination
thereof, beginning after finishing operations have been
completed and as soon as marring of the concrete will
not occur.
Under usual placing conditions either sealing materials
or continuous curing under wet burlap, cotton mats, rugs,
or other similar material may be used.
If plastic shrinkage cracking starts to develop, the
concrete should be initially cured by fog spraying (Sec-
tion 2.2.2), sprayed with an evaporation retarding com-
pound, covered with pre-soaked burlap or cotton mats,
or measures taken to reduce the effective temperature or
wind velocity or both. Exposed surfaces of the slab
should be entirely covered, and kept wet or sealed until
firm enough to permit foot traffic without damage.
Mats used for curing may either be left in place and
kept saturated for completion of the curing, or may be
removed at the end of an initial curing period and the
concrete surface covered with liquid membrane-forming
curing compounds, plastic sheeting, reinforced paper, wet
earth, or straw, or water.
3.1.3 Duration of curing-For daily mean ambient
temperatures above 40 F (5 C) the recommended mini-
mum period of maintenance of moisture and temperature
for all procedures is 7 days or the time necessary to
attain 70 percent of the specified compressive or flexural
strength, whichever period is less. If concrete is placed
with daily mean ambient temperature 40 F (5 C) or
lower, precautions should be taken to prevent damage by
freezing as recommended by ACI 306 (1.3.2.4).
3.2-Structures and buildings
3.2.1 Scope-Concrete in structures and buildings in-
cludes cast-in-place walls, columns, slabs, beams, and all
other portions of buildings except slabs on ground which
are covered in 3.1. It also includes small footings, piers,
retaining walls, bridge decks, railings, wing walls, and
tunnel linings and conduits. Not included are mass con-
crete, precast concrete, and special constructions as
described in Section 3.4.
3.2.2 Curing procedures-Under usual placing condi-
tions, curing should be accomplished by one or a com-
bination of methods from Chapter 2.
When additional curing of underside surfaces is
required after removal of forms, either apply liquid
membrane-forming curing compound promptly or
sprinkle sufficiently to keep continuously moist.

10. 308-10 ACI STANDARD
For vertical and other formed surfaces, after the
concrete has hardened and while the forms are still in
place, form ties may be loosened and water should be
applied to run down on the inside of the form if
necessary to keep the concrete wet. Immediately fol-
lowing form removal, the surfaces should be kept
continuously wet by a water spray or water-saturated
fabric. Liquid membrane-curing compound may be used
if authorized by the specifications for the work or
otherwise properly approved. Such authorization or
approval should not be given when the concrete has a
water-cement ratio of 0.4 or less by weight.*
3.2.3 Duration of curing and protection-When the
daily mean ambient temperature is above 40 F (5 C),
curing should be continuous for a minimum of 7 days or
for the time necessary to attain 70 percent of the
specified compressive or flexural strength, whichever
period is less. If concrete is placed with daily mean
ambient temperature 40 F (5 C) or lower, precautions
should be taken as recommended by ACI 306. For some
structural members, such as columns where high strength
[6000 psi (41 MPa) or greater] is required curing periods
may be increased to 28 days or greater to allow develop-
ment of the required strength of the concrete.
3.3-Mass concrete
3.3.1 Scope-Mass concrete is any volume of cast-in-
place concrete with dimensions large enough to require
that measures be taken to cope with the generation of
heat and attendant volume change to minimize cracking.
Its most frequent occurrence is in piers, abutments, dams,
heavy footings, and similar massive constructions. Usually
the cement content (or total cementitious material con-
tent) will range from about 200 to 500 lb per (about
120 to 300 Mass concrete also includes some
tremie concrete placements and some large girders and
columns where high strength, high cement content, and
moderate sized aggregates are required. Due to the heat
generated in such large masses, temperature control
assumes considerable importance if harmful thermal
stresses are to be prevented. The curing practices
described below should be followed.
3.3.2 Temperature control-For very large unreinforced
structures, such as dams, where the design criteria are
such that it is necessary to establish a reasonably stable
and uniform temperature throughout the mass as soon as
practicable after placement, particularly to avoid
cracking, the internal temperature during hydration
should not rise more than 20 to 25 F (11 to 14 C) above
the mean annual ambient temperature. To achieve this,
various steps may be taken including:
a) Use a low cement content
b) Use a pozzolan or other mineral admixtures
c) Cool the concrete materials
d) Use ice instead of mixing water
e) Use embedded cooling pipe in the concrete
f) Use low-heat cement
Such procedures have been described by ACI
Committee 207 (1.3.2.1).
For heavily reinforced concrete elements such as
blast-off pads, heavy machinery foundations, and load-
transfer girders, it is desirable to avoid high temperature
rise during the first few days, but internal concrete tem-
peratures as high as 130 F (55 C) are frequently found in
such elements. However, due to the large amount of re-
inforcement in such construction, these high temper-
atures may not be harmful.
3.3.3 Methods and duration of curing-Water curing
may be used to keep horizontal or sloping unformed
surfaces of mass concrete continuously wet. Water
spraying, wet sand, or water-saturated fabrics can be
used. The use of a liquid membrane-forming curing
compound may be permitted if the surface is not a con-
struction joint, or provided the membrane is removed by
sandblasting before casting the adjacent concrete. The
appearance of a membrane-coated exposed surface may
also be a factor to consider.
For vertical and other formed surfaces, after the
concrete has hardened and the forms are still in place,
the form ties may be loosened and water should be sup-
plied to run down on the inside of the form as necessary
to keep the concrete wet. Immediately following form
removal, the surfaces should be kept continuously wet by
a water spray or water-saturated fabric.
Curing should start as soon as the concrete has
hardened sufficiently to prevent surface damage. For
unreinforced massive sections not containing pozzolan,
curing should be continued for not less than two weeks.
Where pozzolan is included as one of the cementing
materials, the minimum time for curing should be not
less than three weeks. For construction joints, curing
should be continued until resumption of concrete place-
ment or until the required curing period is completed.
For heavily reinforced massive sections, curing should be
continuous for a minimum of 7 days as described in Sec-
tion 3.2.2.
3.4-Other construction
3.4.1 Precast units-A precast concrete unit is one that
is cast, cured and finished in a place or position other
than that which it occupies in service. Typical precast
concrete units are pipe, block, brick, and structural
members such as channels, single- and double-tees,
columns, and floor and wall panels. These units are
generally given accelerated curing in order to achieve
economical reuse of forms and casting space.
Due to the variety of units and methods of manu-
facture, different curing procedures are used. Concrete
block, brick, precast pipe, and other units are removed
from the forms immediately after casting, allowing most
of the surface of the unit to be exposed to ambient con-
* See Klieger, Paul, “Early High-Strength Concrete for Prestressing,”
Proceedings, World of Conference on Prestressed Concrete, San Francisco, July
1957, A5-1 to AS-14 (PCA Res. Bull. No. 91).

11. STANDARD PRACTICE FOR CURING CONCRETE
ditions. Some precast pipe, and vertically cast panels
remain almost completely enclosed in their forms for 12
to 24 hr before they are stripped. Channels, single- and
double-tees, and horizontally cast panels represent an
intermediate condition of exposure; although the units
remain in the forms, large areas are not covered or
enclosed. Curing of such concrete units with large
surfaces exposed requires considerable care to assure that
an excessive amount of water is not lost from the surface
throughout the curing cycle.
Although they could be cured at normal temperatures,
most precast units are cured at temperatures between
125 and 185 F (52 to 85 C) for periods of 12 to 72 hr.
Autoclaved units are cured at temperatures above 325 F
(160 C) for 5 to 36 hr. Recommendations regarding
curing procedures are discussed by ACI Committees 516
and 517 dealing with high pressure and atmospheric
pressure steam curing, respectively (1.3.2.6 and 1.3.2.7).
3.4.2 Vertical slipform construction-Chimneys , silos,
elevator shafts, and other structures erected using vertical
slipforming methods should be cured in accordance with
the procedures used in curing other vertical surfaces,
recognizing the particular problem of slipform construc-
tion. A wet skirt of suitable length in contact with the
concrete can be carried up by attachment under the fin-
isher’s platform, as could a system of sprays or fog
nozzles. The walls for slipform construction for example,
receive a short initial cure by the form. The use of a
curing compound is often necessary due to the short time
the concrete is protected by the form and wet skirt. How-
ever, the use of curing compound may not be desirable
on the inside of certain silos due to possible fire hazard,
toxicity, or contamination of material to be stored there-
in, and on the outside of the silos because of color vari-
ations that could result from uneven application of curing
compound. The inside of a silo can usually be kept above
40 F (5 C) during cold weather and enclosed to maintain
a high humidity for curing. In some methods of construc-
tion, the inside of a silo may need to be ventilated to
avoid excessive buildup of heat. When this is done, the
vents should be arranged to keep drafts from the walls,
which would tend to dry the inside of walls excessively
unless they are, in some way, water cured.
3.4.3 Shotcrete-Shotcrete is usually placed in thin
layers and has rough surfaces. Shotcrete surfaces should
be kept continuously wet for at least 7 days. Liquid
membrane curing is satisfactory where no additional
shotcrete or paint is to be applied and the appearance is
acceptable. Because of the rough surface, liquid mem-
brane-forming curing compound should be applied at a
higher rate than on ordinary concrete surfaces, usually at
about 100 per gal. (0.40 as recommended by
ACI 506 (1.3.2.5) (see 2.3.3).
3.4.4 Refractory concrete*-Refractory concrete that
uses portland cement as the binder should be cured in
accordance with the procedures described in Chapter 2.
Refractory concrete that employs calcium-aluminate
cement as the binder should be cured in accordance with
the instructions of the manufacturer of the calcium-
aluminate cement used. Normally, for such concrete,
curing wouId be complete in 24 hr after mixing.
3.4.5 Cement paint, stucco, and plaster-The same fog
spray device used for dampening the surfaces to which
these materials are applied may be used to moisten the
applied cement paint, stucco, or plaster after application.
Such water spray should be applied be,tween coats where
more than a single coat is used, and then two or three
times a day for at least 2 days following completion of
the paint, stucco, or plaster application. Required
frequency of moistening depends on weather conditions.
The curing should be started as soon as the applied
paint, stucco, or plaster has hardened sufficiently not to
be damaged by the spray. Application of excess water to
the extent that it flows down the surface should be
avoided. Frequently enclosing the work area to maintain
high relative humidity will be sufficient.
3.4.6 Shell structures-Thin shells are unusually sus-
ceptible to shrinkage cracking if improperly cured. In hot
weather, preliminary fog spray curing, followed by wet
burlap or water curing is advisable. In cold weather,
special precautions against freezing such as protective
blankets are required. At moderate temperatures [40 to
70 F (4 to 21 C)], normal curing methods are usually
satisfactory.
3.4.7 Insulating concrete-The surface of insulating
concrete in which a dry unit weight of 50 (800
or less is attained, should normally be kept moist
for a period of not less than 3 days using whichever of
the procedures listed in Chapter 2 is most appropriate.
The insulating concrete should then be allowed to air dry
before application of supplementary covering. Ponding or
excessive water curing is not desirable since the concrete
may absorb considerably more water than is required for
hydration of the cement.
3.4.8 Concrete with colored or metallic surface-Such
concrete requires special curing procedures to avoid
staining. The manufacturer of the coloring or surfacing
materials should be consulted regarding methods for
avoiding such staining.
l “Refractory Concrete: Summary of State-of-the-Art” has been published. A
summary with a Chapter on “Curing, drying, firing,” appeared in Concrete
International, V. 1, No. 5, May 1979, pp. 62-77.