Concrete Curing - The Facts
- Cold fog gets absorbed into the warmer surrounding air more efficiently and evenly compared to steam. Steam will condensate a lot of its moisture as it enters the cooler surrounding air, in addition to increasing heat !
- An increase in heat will work against attempts to increase humidity levels. As air heats up, it expands and will decrease its RH %
- The amount of energy necessary to convert 1 lb. water to steam is 970.4 (Btu/lb)
- The amount of energy necessary to atomize 1 lb. water using the Aquafog system is 25.9 (Btu/lb)
Reprinted from Design and Control of Concrete Mixtures, EB001.12T, Chapter 10.
©Portland Cement Association, 1980
Curing has a strong influence on properties of hardened concrete such as durability, strength, watertightness, wear resistance, volume stability and resistance to freezing and thawing.
When portland cement is mixed with water, a chemical reaction called hydration takes place. The extent to which this reaction is completed determines the strength, durability and density of the concrete. Most fresh concrete contains considerably more than enough water for complete hydration of the cement; however, any appreciable loss of water by evaporation or otherwise will delay or prevent complete hydration. Since hydration is relatively rapid the first few days after fresh concrete is placed, it is important for the water to be retained during this period, that is, for evaporation to be prevented or at least reduced. The objects of curing, therefore, are:
2. To control the concrete temperature for a definite time.
With proper curing, the concrete will become stronger and more resistant to stress, abrasion and frost. The improvement is rapid at early ages but continues more slowly for an indefinite period. For example, Fig. 1 charts the strength gain of concrete with age.
The most effective method for curing concrete depends upon the circumstances. For most jobs, normal curing is adequate but in some cases, such as in hot and cold weather, special care is needed.
When moist curing is interrupted, the development of strength continues for a short period and then stops. However, if moist curing is resumed, strength development will be reactivated. Although it can be done in a laboratory, it is difficult to resaturate concrete in the field. Thus, it is best to moist-cure the concrete continuously from the time it is placed until it has sufficient strength, impermeability and resistance to abrasion, freezing and thawing and chemical attack.
Loss of water will also cause the concrete to shrink, thus creating tensile stresses at the drying surface. If these stresses develop before the concrete has attained adequate tensile strength, surface cracking can result. All exposed surfaces, including exposed edges and joints, must be protected against moisture evaporation.
Hydration proceeds at a much slower rate when the concrete temperature is low. Temperatures below 59°F (10°C) are unfavorable for the development of early strength; below 40°F (4.5°C) the development of early strength is greatly retarded; and at or below freezing temperatures, down to 14°F (-10°C), little or no strength develops. In recent years, a maturity concept has been introduced to evaluate the development of strength when there is variation in the curing temperature of the concrete. "Maturity" is defined as the product of the age of the concrete and it's average curing temperature. The method is fully described in American Concrete Institute Committee 306 Report, Cold-Weather Concreting. It follows that concrete should be protected so that it's temperature is kept favorable for hydration, and moisture is not lost during the early hardening period.