Cold weather concreting is a process that involves placing, finishing, curing, and protecting concrete during cold weather conditions. This process is guided by the American Concrete Institute’s standards, as stated in ACI 306.1. Cold weather is defined as when the average outdoor temperature is below 4.4C (40F) for more than three consecutive days. To ensure that winter concreting is free from problems, the temperature of fresh concrete must be 12.7C (55F) or higher and maintained at this temperature.
Concrete’s set time at a temperature of 21C (70F) is approximately six hours. However, if the concrete temperature drops to 4.4C (40F), the set time jumps to just over 14 hours. If the temperature drops below this point and the concrete freezes early in the process, it may result in a loss of strength, up to 50 percent, increased permeability, and lower resistance to weather.
To ensure a successful cold weather concreting process, it is essential to start with warm concrete and maintain its warmth. The internal heat of the concrete mix can be raised by heating the materials, using extra or special cements, or by adding accelerators. The environment can also be altered by using enclosures and moist heat, applying insulating blankets, polystyrene sheets, or hay, and leaving the forms in place.
Guidance and Strategies for Cold Concreting
1. Preparations
To prepare for cold weather on a construction site, it is important to plan ahead and gather the necessary equipment and materials, such as heaters, insulation, and enclosures, before the concrete pouring process begins. Additionally, it is advisable to consider air-entraining the concrete for flatwork and other exposed members that may undergo freezing and thawing.
It is essential to clean all surfaces and reinforcements of snow, ice, and frost before beginning any construction work during the cold weather. This ensures that the concrete sets correctly and maintains its structural integrity.
Keeping detailed job condition records is crucial during cold weather conditions. It is recommended to record the weather conditions and temperature of both the air and the concrete surface at least twice daily. This will provide useful data to track the progress of the construction project and identify any potential issues that may arise due to the cold weather.
2. Concrete Pouring
When pouring concrete around large embedded objects, it is important to ensure that the temperature of the concrete is above freezing. This is because pouring concrete in freezing temperatures can compromise the strength and durability of the concrete.
To ensure the durability of the concrete, it is important to follow maintenance guidelines outlined in Table 1. In cases where high-early strength concrete is used, this can be particularly important.
While there are compounds available that can lower the freezing point of concrete, it is not recommended to use them. Instead, it is better to rely on other methods for ensuring the temperature of the concrete is suitable for pouring.
In certain situations, it is not recommended to use calcium chloride or admixtures containing soluble chlorides. These situations include when the concrete contains aluminum or prestressing strand, as corrosion can occur. Additionally, soluble chlorides should not be used when discoloration of troweled surfaces cannot be tolerated, when galvanized steel will remain in permanent contact with the concrete, or when the concrete is subjected to alkali-aggregate reaction or exposed to soils or water containing sulfates.
Table 1 Minimum Concrete Temperature Immediately After Pouring and during the protection period
| Least dimension of section, cm (in) | Minimum temperature of concrete as placed and maintained during the protection period, C (F) |
| Less than 30.5 (12) | 12.7 (55) |
| 30.5 (12) to less than 91 (36) | 10 (50) |
| 91 (36) to less than 193 (76) | 7.2 (45) |
| Greater than 193 (76) | 4.4 (40) |
3. Concrete Protection
In order to ensure the proper setting and strength development of concrete, it is important to implement protection mechanisms. These may include windbreaks with a height of at least 1.8m, enclosures, supplementary heat, or adjusting the concrete mixture to account for the ambient temperature. During the protection period, the concrete temperature must be maintained according to the specifications outlined in Table 1, with measurements taken at the surface of the element.
Concrete must be protected against freezing for at least 3 days, and if the strength of the concrete is critical for structural safety, the protection period must be extended to ensure the necessary strength development. Additionally, during periods in which freezing temperatures may occur, but are not considered “cold weather”, the concrete surface must be protected against freezing for the first 24 hours after placing.
Once the concrete surface temperature is within -6.6°C (20°F) of the surrounding temperature, the protection mechanisms may be removed. It is important to exercise caution when protecting cylinders for strength tests, as they may require additional protection measures. By implementing these protection mechanisms, the proper development of concrete strength can be ensured, which is critical for the integrity of any structures built with concrete.
4. Concrete Curing
To prevent moisture loss in concrete, it is important to cure it properly. This can be done by using heated enclosures, sprinkling water, or using steam for heating. However, it is crucial to vent any fuel-burning heaters such as salamanders to prevent any hazards. Moreover, allowing concrete to cool slowly is necessary to prevent thermal cracking.
If job schedules permit, it is recommended to leave the forms in place to ensure proper curing. Reshoring is also essential until the concrete reaches the required design strength. In cases where the concrete needs to be cured below 15.5°C (60°F), water reducers or retarders may be used to prolong the set.
It is important to note that concrete placed during late fall or winter should not be exposed to salts applied as deicers or those that drip from parked vehicles. These salts can cause damage to the concrete and affect its curing process. Therefore, it is necessary to take extra precautions when curing concrete in cold weather conditions. Fig. 3 may illustrate some of the best practices for curing concrete in cold weather.
Fig. 3: Concrete Curing in Cold Weather