NOTE: This discussion pertains to non-reinforced and conventionally reinforced concrete applications. It is not totally applicable to prestressed concrete, concrete for cover of permanent metal roof decking, or applications where dissimilar metals are embedded in concrete.
Cold weather concrete, as defined by ACI 306, is a period when, for more than three (3) days the mean daily temperature drops below 40 degrees F. Normal concreting practices can be resumed once the temperature is above 50 degrees F for more than half a day.
One of the most important problems confronting the construction industry in cold weather, is the production of satisfactory floor slabs. With exposure of fresh concrete floor slabs to cold, two important problems are posed: 1)The difficulty of securing a satisfactory, wear-resistant finish due to bleeding, followed by drying of a slab surface which has not sufficiently hydrated, and 2) The high cost of finishing, due to the slow setting and hardening of the slab.
The setting and hydration of Portland Cement in concrete mixes is a chemical function, commonly called setting and hardening. The rapidity with which this chemical change takes place, depends largely upon the temperature of the mixture. An increase in curing temperature, within limits, causes more rapid hydration and correspondingly higher rate of gain in early strength. (up to 3 months)
Concrete placed in wooden forms, or in larger masses, at temperatures higher than surrounding air temperatures, will hydrate and gain strength faster than the same concrete placed in thin sections, such as exposed slabs.
Liberation of heat due to hydration of Portand Cement at 70 degrees F, does not become pronounced until after about three hours. At a temperature of 50 degrees F, heat of hydration does not become pronounced for approximately 12 hours or longer. Therefore, at low temperatures, the heat of hydration of cement cannot be depended upon to accelerate the chemical reaction of hydration.
At low temperatures, concrete sets slowly, and development of strength is delayed. Therefore, job planning for cold weather concreting should include one or more of the following protective measures:
- Heating the water and concrete materials.
- Heating the area in which the concrete is placed.
- Use of additional cement or high-early strength cement.
- Addition of an accelerating admixture to the mix.
- Special provisions for curing.
High early admixtures accelerate setting time while producing higher earlier and higher ultimate strengths … and is highly recommended for cold weather concreting with these measures.
Temperature of all surfaces to be in contact with the new concrete, should be raised as close as practical to the temperature of the new concrete. Avoid the presence of ice or the possibility of its formation during concreting. High early, calcium chloride, non-chloride accelerating admixtures, extra cement or Type III cement are used to speed the set of concrete and develop high early strength when moderately low temperatures are expected. Up to 2% calcium chloride by weight of cement is permitted. High early strength cement provides high early concrete strength performance at low temperatures, but may have approximately the same setting time characteristics as Type I cement.
Slabs lose moisture and/or heat rapidly in cold weather atmospheres. Protect from rapid moisture loss, provide heated enclosure if necessary, and protect from wind. Avoid overworking of cooled slabs having delayed stiffening; don’t get on slabs too soon.
Provide insulation or heated enclosure to maintain concrete temperatures for minimum periods. Curing and protection from start to finish should be continuous and uninterrupted until concrete develops its designed strength. Concrete, both in its fresh and hardened states loses moisture and/or heat rapidly to cold weather air. This easy drying out of cold weather concrete can stop strength gain . AVOID IT! Enclosures, windbreaks, portable heaters, insulated forms and blankets should be ready to maintain concrete temperature.
Proper storage facilities and monitoring (temp.) of “Test Cylinders” during the curing process should be available to verify that precautions are adequate.