Concrete cures best at temperatures above 50°F. When ambient temperatures are not warm enough, extra steps must be taken to ensure concrete reaches its full strength. Often, concrete admixtures that accelerate hydration are used.
Cold Weather and Concrete
The American Concrete Institute defines cold-weather concrete as “a period when the average daily ambient temperature is below 40°F (5°C) for more than 3 successive days.”
As temperatures fall, exothermic reactions that hydrate cement and transform it into hard, durable concrete slow significantly. This can mean long delays in projects as you wait for concrete to set. Additionally, when cement freezes, it will expand and create harmful pressure on the mixture. This can significantly weaken the final cured concrete. When placing concrete in cold weather, it’s important to take the necessary steps that will speed cure time and prevent the harmful effects of freezing. Admixtures can be an effective solution.
What are Admixtures?
Admixtures are special ingredients added to concrete besides cement, water, and aggregate. They are used to modify and enhance the properties of concrete. These additives are often used to ensure concrete quality is met when unideal conditions exist (i.e. cold weather)
The majority of admixtures come in ready-to-use liquid form and are added to the concrete at the plant or at the jobsite. Some admixtures like pigments, expansive agents, and pumping aids are used only in extremely small amounts. These admixtures are usually batched by hand from pre-measured containers.
Admixture effectiveness depends on a few different factors such as: type and amount of cement, water content, mixing time, slump, concrete temperature, and ambient temperatures.
Concrete Admixtures for Cold Weather
The following are admixtures that help concrete cure quickly and reach adequate strength when cold weather is present:
- Accelerators– Calcium chloride admixtures are the most common type of accelerators and are used to speed the curing process by increasing the rate of cement hydration. The amount added to cement mixture will depend on ambient conditions. Non Chloride accelerators are used in situations where calcium chloride admixtures are prohibited.
- Air Entraining Agents- Entrained air improves concrete’s resistance to damage from freezing and thawing. Air Entraining agents create millions of tiny bubbles within the cement mixture. These bubbles help combat the added pressure that freezing temperatures create within cement paste; they create extra volume to accommodate the expanding nature of ice.
- Superplasticizers- These are high range water reducers. Superplasticizers can reduce water content of a concrete mix by 10% – 30%. Because this additive allows cement to maintain it’s workability, it’s a great option when easy-to-place concrete is still preferred. The effects, however, only lasts for approximately 45 minutes, so projects must be scheduled accordingly.
Other Cold-Weather Concrete Precautions
Besides adding cold weather admixtures, here are a few other things you can do to help maintain concrete strength when cement is placed in cold temperatures:
- Thaw the ground- Use heaters or heated blankets to thaw surfaces where concrete will be poured. Pouring concrete on frozen ground will quickly cool concrete well below ideal temperatures. Using a heater to prep surfaces will prevent too-quick cooling or freezing and help keep the necessary reactions going.
- Use hot water in your cement mix- Typically, you should aim to have your concrete mix at 65°F or higher during winter months. Hot water can help you reach this temperature.
- Use Extra Cement- Adding extra cement to your mix ( (typically an extra 100 pounds per cubic yard) will cause your concrete to hydrate more rapidly.
- Use Type III Cement- Type II cement mixes produce a high early strength and hydrate more rapidly.
- Remove Bleed Water- Utilize squeegees or a vacuum to remove bleed water that has a difficult time evaporating during colder weather.
- Use concrete blankets to keep curing cement warm- insulated and heated concrete blanket will keep cement at ideal temperatures as it cures. This will prevent any delays due to extended cure time and strength issues with cured concrete.