Best Practices for Cold Weather Concreting
If the average temperature is above 40°F, or if it only drops below this temperature for short periods, you do not need to account for the cold weather when placing and curing concrete.
Understanding the definition of cold weather concreting helps you determine when to make adjustments to the process and when you can proceed as usual.
How Does Cold Weather Impact Concrete?Low temperatures affect concrete in several ways. Once you know the challenges of cold weather concreting, you can take steps to account for them.
Formation of Ice CrystalsIce crystals can form and stop the hydration process when concrete is placed in cold temperatures. This is a significant problem because, under ordinary circumstances, the creation of calcium silicate hydrate during hydration strengthens the concrete. If ice crystals slow the process before it reaches this crucial stage, the cured concrete can have reduced strength.
The effect of ice crystals is more pronounced during the first day of curing. If the concrete freezes in the first 24 hours after being placed, it can lose up to half its 28-day strength. In cold weather concreting, concrete cannot withstand a freeze cycle until it has reached a strength of at least 500 psi.
Increased Set TimesConcrete set times are longer when the temperature is low, so you must account for this situation when planning projects.
Concrete placed on frozen ground can cause cracking. When the temperatures increase, the ground underneath the set concrete thaws and settles. If it shifts, it can create surface cracks. Placing concrete on frozen ground also means that the bottom layer sets slower than the top layer, leading to surface defects.
Ultimately, the concrete is not as strong and has more surface defects when placed in cold temperatures without the proper precautions.
What Are Best Practices for Cold Weather Concreting?Best practices for cold weather concreting can help you achieve desired results.
Making some adjustments to the mix can offset the impact of the cold weather and ensure that concrete cures properly without surface defects.
- Use Type III cement - Type III cement is finer, so it reacts faster, speeding up the hydration process and magnifying the early strength increases. Therefore, the strength losses from ice crystals can be somewhat counteracted.
- Reduce supplementary cementitious materials (SCMs) - SCMs slow the setting time of concrete, so you should reduce them in the mix when placing concrete in cold weather. However, don't forget the importance of permeability and strength and how the SCMs impact those factors. Reducing SCMs too much could cause more damage than cold temperatures in some cases.
- Use accelerating admixtures - Accelerating admixtures can speed up the drying process to counteract the impact of the weather. Calcium chloride, sodium nitrite, and calcium nitrite are all good options, but you shouldn't use calcium chloride in reinforced concrete. Bear in mind that accelerating admixtures do not prevent freezing. Additional precautions are essential in these situations.
Check Subgrade Temperature
Before placing concrete, check the subgrade temperature. If the subgrade is frozen, wait until it thaws and the temperature increases before moving forward.
Heating your materials can help you counteract the low temperatures in cold weather concreting. In the right conditions, it is possible to achieve regular setting times and strength using this method. However, it would be best if you took other measures because the conditions need to be highly specific.
You can heat water to between 140°F and 180°F. In frigid temperatures (below 25°F), you can heat the aggregates to prevent ice and frozen lumps from forming, but you don't need to heat the aggregates to more than 60°F if you also heat the water.
It's not advisable to heat aggregates with boiling water as any residual water can impact the final mix. Instead, you can use circulating steam to heat aggregates to the required temperature.
All de-icing substances create an adverse reaction with freshly placed concrete and ruin the end product. If they come into contact with the concrete during setting, the de-icers cause flaking and scaling on the surface. Sometimes, sodium chloride and calcium chloride can be used on high-quality, dense concrete. However, it's best to avoid de-icers altogether because alternative methods are more than adequate.
After placing the concrete, you must ensure it maintains the correct temperature while it sets. You can use heated enclosures and combustion heaters, but you need to keep the heaters well-vented to avoid directly heating the concrete. Enclosures should be weather and windproof to ensure a stable temperature for the entire setting period. Insulating blankets can also help create steady temperatures.
While the concrete sets, constant temperature monitoring is key to avoiding rapid fluctuations that may impact the hydration process and cause cracks. You can also place temperature sensors into the concrete and set them to send updates wirelessly.
The set times need to be extended based on the temperature. The following table gives you a general indication of set times in different temperatures:
Temperature Average setting time (hours)
|20°F||Concrete will not set|
Once the curing process finishes, you must slowly remove the heated enclosures, blankets, and combustion heaters. Taking them away all at once causes rapid cooling, so start by turning the temperature off but leaving the enclosure up. Continue monitoring the temperature until it has stabilized and you can safely remove the enclosure.
Cold weather concreting can be challenging, but you can achieve good results by following this important advice.
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