Hot-Weather Concreting - Design and Control of Concrete Mixtures - page 232

with a soil-soaker hose—and allowed to run down inside
the forms. On hardened concrete and on flat concrete sur-
faces in particular, curing water should not be more than
about 11°C (20°F) cooler than the concrete. This will mini-
mize cracking caused by thermal stresses due to tempera-
ture differentials between the concrete and curing water.
The need for moist curing is greatest during the first
few hours after finishing. To prevent the drying of exposed
concrete surfaces, moist curing should commence as soon
as the surfaces are finished and continue for at least 24
hours. In hot weather, continuous moist curing for the
entire curing period is preferred. However, if moist curing
cannot be continued beyond 24 hours, while the surfaces
are still damp, the concrete should be protected from
drying with curing paper, heat-reflecting plastic sheets, or
membrane-forming curing compounds.
White-pigmented curing compounds can be used on
horizontal surfaces. Application of a curing compound
during hot weather should be preceded by 24 hours of
moist curing. If this is not practical, the compound should
be applied immediately after final finishing. The concrete
surfaces should be moist.
Moist-cured surfaces should dry out slowly after the
curing period to reduce the possibility of surface crazing
and cracking. Crazing, a network pattern of fine cracks
that do not penetrate much below the surface, is caused by
minor surface shrinkage. Crazing cracks are very fine and
barely visible except when the concrete is drying after the
surface has been wet. The cracks encompass small con-
crete areas less than 50 mm (2 in.) in dimension, forming a
chicken-wire like pattern.

the concrete. As a general rule a 5°C to 9°C (10°F to 15°F)
temperature rise per 45 kg (100 lb) of portland cement can
be expected from the heat of hydration ( ACI Committee
211 1997 ). There may be instances in hot-weather-concrete
work and massive concrete placements when measures
must be taken to cope with the generation of heat from
cement hydration and attendant thermal volume changes
to control cracking (see Chapters 15 and 18 ).

REFERENCES

ACI Committee 211, Standard Practice for Selecting
Proportions for Normal, Heavyweight, and Mass Concrete, ACI
211.1-91, reapproved 1997, American Concrete Institute,
Farmington Hills, Michigan, 1997, 38 pages.

ACI Committee 305, Hot-Weather Concreting, ACI 305R-99,
American Concrete Institute, Farmington Hills, Michigan,
1999, 17 pages.

ACI Committee 308, Standard Specification for Curing
Concrete, ACI 308.1-98, American Concrete Institute,
Farmington Hills, Michigan, 1998, 9 pages.

Burg, Ronald G., The Influence of Casting and Curing
Temperature on the Properties of Fresh and Hardened Concrete,
Research and Development Bulletin RD113 , Portland
Cement Association, 1996, 13 pages.

Bureau of Reclamation, Concrete Manual, 8th ed., Denver,
revised 1981.

Gaynor, Richard D.; Meininger, Richard C.; and Khan,
Tarek S., Effect of Temperature and Delivery Time on Concrete
Proportions, NRMCA Publication No. 171, National Ready
Mixed Concrete Association, Silver Spring, Maryland,
June 1985.

ADMIXTURES

For unusual cases in hot weather and where careful in-
spection is maintained, a retarding admixture may be ben-
eficial in delaying the setting time, despite the somewhat
increased rate of slump loss resulting from their use. A
hydration control admixture can be used to stop cement
hydration and setting. Hydration is resumed, when de-
sired, with the addition of a special accelerator (reactivator).
Retarding admixtures should conform to the require-
ments of ASTM C 494 (AASHTO M 194) Type B. Ad-
mixtures should be tested with job materials under job
conditions before construction begins; this will determine
their compatibility with the basic concrete ingredients and
their ability under the particular conditions to produce the
desired results.

Klieger, Paul, Effect of Mixing and Curing Temperature on
Concrete Strength, Research Department Bulletin RX103 ,
Portland Cement Association, http://www.portcement.
org/pdf_files/RX103.pdf , 1958.

Lerch, William, Hot Cement and Hot Weather Concrete Tests,
IS015 , Portland Cement Association, http://www.port
cement.org/pdf_files/IS015.pdf , 1955.

Menzel, Carl A., “Causes and Prevention of Crack
Development in Plastic Concrete,” Proceedings of the
Portland Cement Association, 1954, pages 130 to 136.

Mindess, Sidney, and Young, J. Francis, Concrete, Prentice
Hall, Englewood Cliffs, New Jersey, 1981.

NRMCA, Cooling Ready Mixed Concrete, NRMCA
Publication No. 106, National Ready Mixed Concrete
Association, Silver Spring, Maryland, 1962.

HEAT OF HYDRATION

Heat generated during cement hydration raises the tem-
perature of concrete to a greater or lesser extent de-
pending on the size of the concrete placement, its
surrounding environment, and the amount of cement in