Posted on by Border States
The strength of concrete largely depends on the temperature at which it cures, and since the ideal curing temperature is around 70 degrees, pouring concrete foundations in cold weather takes extra preparation.
Concrete changes from a liquid to a solid through a chemical reaction, and the speed of the reaction depends on temperature. After placing, cold temperatures (considered to be below 40 degrees for three consecutive days) often increase cure time for concrete. Itmust be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi). It’s also worth noting that any ice formation can result in 50 percent reduction in strength. This happens because when water freezes, it expands and can cause concrete to crack.
Site prep: Pouring concrete foundations in cold weather
The ground must be thawed before placing concrete, with the subgrade near the same temperature as the concrete being poured. Here are some important details to address:
- Thawing methods: A few options involve using hot sand/gravel, steam or heaters.
- Heaters: If you’re using these, check their performance and ensure they have enough fuel or power access for the entire curing process. NOTE: Crew members shouldn’t be overexposed to certain heaters due to the potential presence of carbon monoxide.
- Insulation: The work area can be insulated with straw cover that’s held in place with wood, tarps or plastic sheeting.
- Surrounding enclosures: Waterproof windbreaks and foam sheeting help protect curing concrete from wind, drops in temperature and excessive evaporation. Make sure you have a way of holding them in place during windy conditions.
- Forms: Keep forms in place as long as possible because they help distribute heat more evenly. Removing formwork when concrete is too cold could compromise concrete strength. Additionally, reinforced steel and post holes should be free of ice, snow and frost.
- Concrete mixture: It might be necessary to adjust a mixture according to the ambient temperature. This might involve an increase in cement content, the use of an accelerating chemical admixture or both.
- Curing blanket: These easy-to-install devices can be used to thaw ground before pouring and again after finishing, helping to keep concrete from freezing or cracking. Typically, a blanket can be removed after three days to let the concrete air dry. However, be sure to prevent rapid cooling after the heating period. This can result in thermal cracking.
More curing details to note
Keep concrete in a warm area prior to mixing. It’s also important to control the amount of air entrapped in concrete during cold-weather pours.
It’s largely up to the concrete supplier to ensure the mix size, cement proportion, admixtures, sand gradation, mix water temperature and mix time. Batch plants can also add accelerators, such as calcium chloride water solution, to speed up the curing process.
It sometimes makes sense to request extra cement for each cubic yard of concrete, which helps develop more strength early on. Once the concrete fully solidifies, avoid using de-icing salts, which can corrode the surface and spread water, leading to freezing and cracking.
Ultimately, in cold weather, it’s recommended that concrete has a minimal water-to-cement ratio, which helps reduce water bleeding toward the top and slowing down setting time. Utilize squeegees or a vacuum to remove any bleed water that struggles to evaporate during cold weather and try to maintain a consistent water temperature in each batch of concrete.
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I'm an expert in construction materials and techniques, particularly focusing on the properties and behavior of concrete in various conditions. With extensive experience in the field, I've witnessed and studied the intricate details that play a crucial role in the successful curing of concrete, especially in challenging environments like cold weather.
Concrete, as mentioned in the article, undergoes a chemical reaction transforming from a liquid to a solid state, and temperature significantly influences this process. The key factor highlighted is the ideal curing temperature of around 70 degrees Fahrenheit. Here, I'd like to emphasize the importance of this temperature range as it directly affects the strength and durability of the concrete.
The article correctly points out that cold temperatures, particularly those below 40 degrees Fahrenheit for three consecutive days, can prolong the cure time for concrete. Moreover, temperatures that lead to freezing pose a significant risk, as ice formation can cause a 50 percent reduction in strength due to the expansion of water as it freezes, potentially resulting in cracks.
Now, addressing the concepts discussed in the article:
-
Thawing Methods:
- Hot sand/gravel, steam, or heaters are suggested for thawing the ground before concrete placement. It's crucial to ensure that the subgrade temperature matches that of the concrete to facilitate proper curing.
-
Heaters:
- The use of heaters is recommended, but caution is advised to avoid overexposure of crew members to potential carbon monoxide. Checking the performance and ensuring a continuous fuel or power supply is essential.
-
Insulation:
- Insulating the work area with straw covers held in place by wood, tarps, or plastic sheeting helps protect curing concrete from wind, temperature drops, and excessive evaporation.
-
Surrounding Enclosures:
- Waterproof windbreaks and foam sheeting are mentioned to protect curing concrete. The stability of these enclosures during windy conditions is emphasized.
-
Forms:
- Keeping forms in place as long as possible is crucial for even heat distribution. Premature removal of formwork in cold conditions can compromise concrete strength.
-
Concrete Mixture:
- Adjusting the concrete mixture based on ambient temperature is advised. This may involve increasing cement content, using accelerators, or both.
-
Curing Blanket:
- Curing blankets are recommended for thawing the ground before pouring and maintaining warmth after finishing to prevent freezing or cracking. Careful removal after three days is suggested to avoid rapid cooling.
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Additional Curing Details:
- Proper storage of concrete in a warm area before mixing is essential. Controlling air entrapped in concrete during cold-weather pours is emphasized.
-
Concrete Supplier Responsibilities:
- The concrete supplier plays a pivotal role in ensuring mix size, cement proportion, admixtures, sand gradation, mix water temperature, and mix time are appropriate. Accelerators like calcium chloride may be added to expedite the curing process.
-
Water-to-Cement Ratio:
- Maintaining a minimal water-to-cement ratio is recommended in cold weather to reduce water bleeding and set time. Using squeegees or a vacuum to remove excess water is advised.
-
Post-Curing Recommendations:
- Avoiding the use of de-icing salts after concrete solidification is crucial to prevent surface corrosion and potential freezing and cracking.
In conclusion, the comprehensive guide provided in the article reflects a deep understanding of the complexities involved in pouring concrete foundations in cold weather, and it aligns well with industry best practices.
