In cold weather, curing concrete becomes more challenging. To achieve its maximum strength and proper setting, newly poured concrete should be protected from freezing for at least 24 hours or until it reaches 500 pounds per square inch (psi). Concrete that freezes early on can lose a significant amount of strength. However, with the right precautions, you can pour concrete in cold weather.
According to the American Concrete Institute (ACI), cold-weather concreting is when, for three consecutive days, the average daily temperature drops below 40 degrees Fahrenheit, maintained for at least half of a 24-hour period.
Concrete poured in cold weather requires careful consideration. When done correctly, concrete set in cool weather can actually be stronger than concrete poured in hot weather due to the slower curing time.
It’s crucial to prevent concrete from freezing immediately after pouring in cold weather. Additionally, the concrete must develop the strength needed for safe form removal while minimizing the need for excessive heat to aid in its strength development.
If the outside temperature is below 20 degrees Fahrenheit, it’s advisable to avoid pouring concrete outdoors. At that temperature, the hydration process, essential for concrete setting, stops. Even with insulating blankets and enclosures, maintaining a sufficiently high temperature at the work site becomes challenging in very low outdoor temperatures.
CONCRETE PATIO SERVICES
If Concrete Freezes Too Soon
Concrete that freezes before it has a chance to cure and become strong enough to withstand water expansion will permanently lose its strength. The final strength of concrete can be reduced by as much as 50% if it freezes too early.
Concrete contractors should follow cold-weather concreting practices if the temperature is expected to drop below freezing within 24 hours, and the air temperature is less than 40 degrees Fahrenheit at the time of pouring or placing.
Certain preparations can enhance the curing of concrete during cold weather.
- Preheating the water or aggregate before pouring can help ensure the concrete is the right temperature. This is achieved by heating water, sand, or gravel instead of the Portland Cement, a capability some ready-mix companies have. Concrete leaves the plant on trucks at around 65 degrees Fahrenheit. When mixing small quantities of concrete on-site, hot water can be used, or the aggregate may need to be stored in a warm location.
- Adjusting the mixture components may be necessary, usually involving adding more cement to the mix or using a chemical additive like calcium chloride at a ratio as high as 2 percent.
- Using Portland cement type III is recommended, as it helps the concrete set without affecting its quality. It’s crucial to avoid high moisture levels in concrete reinforcement.
- In cold weather, consider using slag or fly ash cement instead of fly ash. These materials are more heat-resistant and set up slower.
Pouring and Placing Tips
Concrete can be set up and cured properly in cold weather by using additional techniques during pouring and placement.
- Be sure to plan for the crews to stay on site for an extended period. Concrete exposed to low temperatures takes longer to set up, so the crew may need to stay on site for a greater period of time.
- Windbreaks protect concrete and workers from wind that can cause rapid temperature drops or evaporation. In general, windbreaks up to 6 feet tall are adequate.
- You may need heated enclosures. You can make these enclosures out of wood, canvas, polyethylene sheets, or commercial rigid plastic enclosures. Electric heaters are the best way to heat an enclosure. Indirect-fired heaters work best if fuel-burning heaters will be used. Warm air is blown into the enclosure by a burner placed outside. Hydronic systems are another option. A warm mixture of water and glycol is circulated throughout the enclosure using pipes or hoses.
ACI 306 Guidelines
ACI 306 mandates and establishes temperatures for placing and protecting concrete during cold weather. ACI 306 aims to keep concrete warm for 48 hours at temperatures above 5 degrees Celsius. This is crucial in developing concrete strength. Fresh concrete that freezes during the first 24-hour period can lose up to 50% of its strength potential for 28 days. 3.
For concrete with high early strength, a day at temperatures over 40 degrees will suffice. Concrete foundations or structures that are expected to carry heavy loads early on require 20 days at temperatures of at least 50 degrees.
Remember that the cement curing process is an exothermic one that generates heat. It is often enough to cover the concrete in polyethylene sheets or blankets that trap heat. Concrete should not freeze in the first 24 hours following pouring.
There are techniques you can use during the curing phase to ensure the concrete is as strong as possible:
1. Leave forms in place for as long as possible: Forms retain heat and prevent concrete from drying too quickly. They help release heat from corners and edges.
2. Use live steam in low humidity conditions: In winter months, when humidity is low, pumping live steam around the concrete enclosure prevents it from drying too quickly.
3. Wait for complete evaporation of bleed water: In cold weather, concrete sets and cures slower, so bleeding may start later. Be prepared to manage more bleed water than usual during placement.
4. Verify concrete temperature with an infrared thermometer: As the concrete cures, it should maintain a temperature of at least 40 degrees Fahrenheit.
5. Avoid rapid cooling of concrete: If active heating is stopped, lower temperatures gradually within an enclosure or cover the concrete with insulating blankets. For large structures, gradually cool over several days or weeks to prevent temperature differences that can lead to cracking.
6. Follow ACI Committee 308 minimum curing periods:
- ASTM C150 Type I cement: 7 days
- ASTM C150 Type II cement: 10 days
- ASTM C150 Type III cement: 3 days
- ASTM C150 Type IV or V cement: 14 days
- ASTM C595 C845 C1157 cement: variable
7. Seal freshly cured concrete: Use a sealant to prevent water from leaking into the concrete. Concrete sealants extend the life of concrete and reduce the risk of curing failure. Choose a sealant that allows moisture to evaporate in extremely cold areas.
Ready to Start Your Cold-Weather Concrete Projects?
As you prepare for your upcoming construction projects in colder weather, it’s crucial to approach concrete work with precision and foresight. This guide has given you with essential insights for successfully pouring, placing and curing concrete even in low temperatures.