Placing Concrete During Cold Weather
If the temperature of fresh concrete is 55ºF., or greater — and if the concrete is maintained at a temperature of 55ºF. or higher — then winter concreting should be trouble-free. Approximate set time for concrete at 70ºF. is six hours. Set time jumps to just over 14 hours if the concrete temperature drops to 40ºF. If it drops below this point and the concrete actually 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.
Basic Guides for Winter Concreting
- Plan in advance. Have equipment and materials ready before cold weather hits. Be set with heaters, insulating materials and enclosures.
- Use air-entrained concrete.
- Don’t place concrete on a frozen subbase. Be sure that all ice, snow and frost are removed from surfaces the concrete will touch.
- For durability, the fresh concrete should be kept at 55ºF. or higher for thin sections. Consider using high-early strength concrete.
- Cure concrete to prevent 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.
- Do not use "antifreeze" compounds in an attempt to lower the freezing point of concrete.
- Leave the forms in place as long as the job schedules permit. Reshoring is necessary until concrete reaches required design strength.
- Keep job condition records. Record, at least twice daily: weather conditions, temperatures of the air and the concrete surface.
- If the concrete is to cure below 60ºF., water reducers or retarders may prolong the set.
- 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.
11. Be especially careful in protecting cylinders for strength tests.
12. Concrete placed in late fall or winter should not be exposed to salts applied as deicers or salts which drip from parked vehicles.