Cold Weather Concrete and Frosty Soils
Placing and Curing Concrete in Cold Weather
Placing and curing concrete in cold weather requires planning and preparation. Concrete is a mixture of aggregates (sand and gravel), Portland cement, and water. The mixture gains strength from hydration - a chemical reaction between the water and cement. For hydration to take place, concrete mixtures need moisture and the temperature must be near or above 50°F. The American Concrete Institute (ACI) defines "cold weather" as any time the average daily temperature is less than 40°F or the air temperature is less than 50°F for more than one-half of any 24-hour period. If concrete is placed under these conditions, precautions must be taken to ensure that the concrete is maintained at or above 50°F for as long as three days following placement.
When the temperature is below freezing, concrete must be protected following placement, or it will be at risk of freezing. If concrete freezes during the first 8 to 36 hours following placement, significant and irreversible damage will occur. The damage will be characterized by a sharp reduction in strength, poor durability, or a complete failure of the concrete. Concrete must reach a compressive strength of 500 psi to 1,000 psi to withstand the first one or two freezing cycles without experiencing permanent damage. Proper curing will prevent the possibility of early concrete freezing and damage.
What are some of the things you can do to meet these conditions?
- Never place concrete on a frozen sub-grade. In very cold weather, the concrete forms must be covered with insulated blankets or heated enclosures must be built around the structure to protect the foundation soils.
- When concrete is placed within an existing building or in a partially completed structure, the ambient air temperature must be maintained above 50°F during placement and throughout the duration of curing.
- The temperature of the concrete delivered to the jobsite should be between 50°F and 90°F. It may be necessary to heat the mixing water as well as the aggregates during cold weather to accomplish this.
- The use of set accelerating admixtures or high early strength cement may be useful in helping the concrete gain strength more quickly. However, admixtures or special cements cannot completely eliminate the need for heating during concrete curing.
- It is important to provide proper curing for both temperature protection and moisture retention throughout the entire curing process.
| Curing Time for Concrete | 70° F | 50° F |
|---|---|---|
| Plain Concrete | 3 days | 7 days |
| Plain Concrete with Calcium Chloride | 2 days | 3 days |
| High Early Strength Concrete | 2 days | 3 days |
Additionally, the concrete should be kept above 40°F for at least four days beyond the times listed in the table. The concrete should not be allowed to dry out during this period. After the curing period has been reached, the concrete temperature should not be allowed to drop more than 40°F in any 24-hour period.
Observing these guidelines makes it possible to properly place and cure concrete throughout the winter months in Michigan. Field-cured concrete cylinders are helpful in evaluating in-place concrete strength and measuring the effectiveness of the curing process.