Tips for Pouring Concrete in Cold Weather

Pouring concrete in cold weather can be challenging since concrete needs to be protected from freezing for 24 hours to reach its minimum strength of 500 pounds per square inch.

Concrete that freezes early can lose much of its overall strength. However, concrete can be successfully poured in cold weather if the precautions are taken to eliminate the problems associated with low temperatures.

Minimum Temperatures for Pouring Concrete

Cold weather, in terms of concrete pours, is more than three successive days when the average daily air temperature drops below 40°F and stays below 50°F for more than one-half of any 24-hour period. Under these cold weather conditions, special techniques need to be applied.

• Longer work period: Arrange for crews to be on-site longer. Concrete exposed to cold temperatures will take longer to reach an initial set, which may mean that the finishing crew needs to be present longer.
• Windbreaks: Windbreaks can help protect the concrete (and workers) from winds that can cause quick temperature drops and evaporation that is too swift. Generally, windbreaks about 6 feet high are sufficient.
• Heated enclosures. Enclosures can be made of wood, canvas tarps, polyethylene sheets, or commercial rigid-plastic enclosures. If possible, use electric heaters. If fuel-burning heaters are used, use indirect-fired heaters. Another option is a hydronic system in which a warm mixture of glycol and water is circulated through the enclosure through pipes or hoses.

Preparation Tips

Proper curing of concrete in cold weather can be enhanced through certain preparation practices:

• Heat the water, not the cement: During cold weather, rather than heating the Portland cement, you can heat the water before mixing. Alternatively, you can heat the sand or gravel by storing the aggregate in warm indoor locations before mixing.
• Adjust the components: This usually involves increasing the cement content in the concrete or using an accelerating chemical admixture, such as calcium chloride, in ratios as high as 2-percent.
• Use Portland cement Type III: This type of cement helps set the concrete without reducing its quality. This is important because high moisture content can induce corrosion problems in steel reinforcement.
• Avoid using fly ash or slag cement: These materials set up more slowly and generate less internal heat.

Pouring and Placing Tips

Additional practices during the pouring and placing can also help concrete set up and cure properly during cold weather. The objective of the ACI 306:

• Prevent freezing: Concrete should not be allowed to freeze within the first 24 hours after being poured and placed.
• Minimum temperatures: Keep concrete warm, over 40°F or 5 degrees Celsius, for the first 48 hours, where concrete strength development is critical. Fresh concrete frozen during the first 24 hours can lose 50 percent of its potential 28-day strength.
• High-load concrete: A concrete foundation or other structure that will carry high loads at an early age requires 20 days or more at a minimum temperature of 50 degrees.
• Use concrete’s own heat: Concrete curing is an exothermic reaction that produces some heat on its own. Often, covering the concrete with polyethylene sheeting or insulating blankets is sufficient to trap the heat.

Curing Tips

There are techniques you can use during the curing process to ensure the concrete reaches maximum strength:

• Leave forms in place longer: Where forms are used, leave them in place as long as possible since they will retain heat and help prevent the concrete from drying too quickly. Corners and edges are most vulnerable, and forms will help during the heat release process.
• Adding live stream: Live steam can be pumped into the enclosure around the concrete to prevent it from drying out too quickly in low-humidity levels expected during cold weather.
• Handling bleed water: Wait until all bleed water has evaporated before finishing. Concrete poured and placed in cold weather has a much slower setting and curing time, so bleeding will start later than expected. Be prepared to handle more bleed water than is typical in regular concrete placement.
• Check temperatures: While the concrete is being cured, verify the temperature using an infrared temperature gun. Make sure the concrete maintains a temperature of at least 40°F during its curing period.
• Guard against concrete cooling too rapidly: This can be done by gradually lowering the temperatures within an enclosure or by covering the concrete with insulating blankets. For large structures, several days or even weeks of gradual cooling is advised. Removing the blankets suddenly in cold weather can cause a temperature differential to build up between the outside of the concrete and its middle, resulting in cracking.

How Long to Cure Concrete in Cold Weather

Make sure to cure the concrete for the time period recommended for the type of cement being used. ACI Committee 308 recommends the following minimum curing periods: