PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 273

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7.3.6 Forms Left in Place

Loss of moisture can be reduced by leaving the forms in place as long as
practical, provided that the top concrete exposed surface is kept wet. If wood
forms are used, the forms should also be kept wet. After removing the forms,
another curing method can be used.

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7.3.7 Steam Curing

Steam curing is used when early strength gain in concrete is required or ad-
ditional heat is needed during cold weather. Steam curing can be attained
either with or without pressure. Steam at atmospheric pressure is used for
enclosed cast-in-place structures and large precast members. High-pressure
steam in autoclaves can be used at small manufactured plants.

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7.3.8 Insulating Blankets or Covers

When the temperature falls below freezing, concrete should be insulated
using layers of dry, porous material such as hay or straw. Insulating blankets
manufactured of fiberglass, cellulose fibers, sponge rubber, mineral wool, vinyl
foam, or open-cell polyurethane foam can be used to insulate formwork.
Moisture proof commercial blankets can also be used.

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7.3.9 Electrical, Hot Oil, and Infrared Curing

Precast concrete sections can be cured using electrical, oil, or infrared cur-
ing techniques. Electrical curing includes electrically heated steel forms,
and electrically heated blankets. Reinforcing steel can be used as a heat-
ing element, and concrete can be used as the electrical conductor. Steel
forms can also be heated by circulating hot oil around the outside of the
structure. Infrared rays have been used for concrete curing on a limited
basis.

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7.3.10 Curing Period

The curing period should be as long as is practical. The minimum time de-
pends on several factors, such as type of cement, mixture proportions, re-
quired strength, ambient weather, size and shape of the structure, future
exposure conditions, and method of curing. For most concrete structures the
curing period at temperatures above 5

F) should be a minimum of
seven days or until 70% of specified compressive or flexure strength is at-
tained. The curing period can be reduced to three days if high early strength
concrete is used and the temperature is above 10

° C (40 °

° C (50 ° F).