Cold weather concreting is the process of placement, finishing, curing, and protection of concrete during cold weather, as per ACI 306.1. The cold weather occurs when the average outdoor temperature is below 4.4C (40F) for more than three successive days. If the temperature of fresh concrete is 12.7C (55F) or greater, and if the concrete is maintained at this temperature, then winter concreting should be free from problems.
Approximate set time for concrete at 21C (70F) is six hours. Set time jumps to just over 14 hours if the concrete temperature drops to 4.4C (40F). If it drops below this point and the concrete freezes early in the process, loss of strength, up to 50 percent, increased permeability and a lower resistance to weather may result.
The key is to start with warm concrete and keep it warm. The internal heat of the concrete mix may be raised by heating the materials, using extra or special cements, or by the addition of accelerators. The environment may be also 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
- Plan before cold weather hits.
- Prepare required equipment and materials, such as heaters; insulating materials; and enclosures, on construction site before concreting process begins.
- Consider air-entraining of concrete for members exposed to a series of freezing and thawing such as slab and other flatworks.
- Clean surfaces and reinforcements from snow, ice, and frost.
- Keep job condition records. Record, at least twice daily: weather conditions, temperatures of the air and the concrete surface.
2. Concrete Pouring
- Concrete shall not be poured around large embedment except if its temperature is above freezing temperature.
- For durability, the fresh concrete needs to be maintained based on Table 1. Consider using high-early strength concrete.
- Do not use "antifreeze" compounds to lower the freezing point of concrete.
- The use of calcium chloride or admixtures containing soluble chlorides is not recommended under certain conditions:
- In concrete containing aluminum or prestressing strand because of corrosion.
- Where discoloration of troweled surfaces cannot be tolerated.
- Where galvanized steel will remain in permanent contact with the concrete.
- In concrete 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
- Protection mechanism may include windbreaks with 1.8m height, enclosures, supplementary heat, or adjustment of concrete mixture constituents for the effect of ambient temperature on setting time.
- Minimum concrete temperature during protection period is required to be kept based on specifications provided in Table 1.
- Concrete temperature during protection is measured at the surface of the element.
- Protect concrete against damage from freezing for a minimum period of 3 days.
- If the concrete strength is required for structural safety, extend the duration of the protection period to ensure the necessary strength development.
- Protect concrete surfaces against freezing for the first 24 hr after placing during periods which are not defined as cold weather, but freezing temperatures may occur.
- Protections can be removed if concrete surface temperature is within -6.6 C (20 F) of surrounding temperature.
- Be careful in protecting cylinders for strength tests.
4. Concrete Curing
- Cure concrete to prevent the loss of moisture.
- When heated enclosures are used, provide extra moisture by sprinkling or use steam for heating. Vent salamanders and other fuel-burning heaters. Concrete should be allowed to cool slowly to prevent thermal cracking.
- Leave the forms in place if the job schedules permit.
- Reshoring is necessary until concrete reaches required design strength.
- If the concrete is to cure below 15.5 (60F), water reducers or retarders may prolong the set. Concrete placed in late fall or winter should not be exposed to salts applied as deicers or salts which drip from parked vehicles.