14 Questions to Ask a Commercial Concrete Contractor Before Hiring
To help ensure you feel confident that you’ve chosen the right concrete contractor, we’ve compiled a list of important questions [...]
Although concrete is a highly versatile building material, it does have its limits.
One is temperature, affecting the strength and durability of the finished product.
A chemical reaction occurs when cement is hydrated to form concrete. The reaction needs to maintain a temperature threshold to complete the process.
This can pose a problem for concrete projects in colder environments. Some might even say building with concrete in the wintertime is impossible.
However, although it will take more forethought, planning, and preventative measures, concrete projects can be built in cold weather.
Keep reading to learn how.
Table of Contents
1. Understanding the Impact of Cold Weather on Concrete
2. Best Practices for Cold Weather Concrete Curing
3. Recap
To understand why some may be apprehensive about placing concrete in cold weather conditions, you must know how cold temperatures affect fresh concrete:
Low temperatures slow the chemical reaction between water and Portland cement. This reaction is what causes strength gain and hardening. Slowing this process down will require a more extended curing period to reach the necessary strength.
When the temperature drops, water in the mix can freeze and expand, causing cracks. Early freezing weakens the surface layer, resulting in scaling and reduced durability.
Curing concrete in cold weather may also cause it to develop a lower initial strength. The long-term impact of a reduced initial strength is less overall strength and integrity of the concrete.
Thermal stress from rapid cooling can cause cracking. Cold, dry air speeds up evaporation, causing plastic shrinkage cracks, which are cracks in the surface before the concrete sets.
Cold weather can also extend the setting period for concrete pours, which can lead to construction delays until the concrete is set.
This slower setting time complicates the finishing process. Workers may need to stay on the job site longer to finish, increasing labor costs overall.
Colder air temperatures make the concrete mix stiff and more challenging to work with.
Here are all the recommended procedures for cold-weather concrete placement.
First and foremost, you need to plan well, and it all comes down to the weather.
A week before your planned pouring day, check the upcoming weather temperatures. Look for days with higher temperatures, preferably above 40℉.
Prepare for below-40℉ days with preventative measures, which we will cover in more depth later.
On the day of the pour, begin at the hottest time of the day (usually early afternoon).
Of course, the weather may not be as predicted, so be willing to adjust your timing according to temperature changes.
You need to prepare the site before pouring the concrete. The first step is to remove any snow or ice from the area.
Next, ensure the ground where the concrete will be poured isn’t frozen. You can heat the subgrade with a mix of:
Heat all your formwork, including the reinforcements, to between 15 degrees colder and 10 degrees hotter than the temperature of the concrete itself.
Insulation is a critical material in this process. It should be laid on the ground, prepared for the pour, along the heated formwork.
Even with all these precautions, you still need additional heating. To create a controlled environment, erect a heating enclosure with heaters inside. You may also need to place additional portable heaters in critical areas.
Learn more: The Full Guide to Concrete Forming: 7 Questions Answered
A very effective way to protect your concrete project from the disastrous effects of a freeze is to use admixtures that manipulate the characteristics of your concrete mix to be more resistant.
Accelerating admixtures creates faster setting and curing times. Both of these benefits provide early concrete strength development.
Common types of accelerators include:
To create air-entrained concrete — which improves freeze-thaw resistance and enhances durability — you must add an agent to the mix. These admixtures create microscopic air bubbles in the concrete.
Common types of air-entraining agents include:
These ingredients lower the water content of the concrete, enhancing its compressive strength without reducing its workability. They prevent shrinkage and cracks and can be transported further since they increase the setting time of the concrete.
Some common types include:
These admixtures lower concrete’s freezing point, giving you more wiggle room in your temperature window. They also provide extended workability, lessening the chance of error. Common antifreeze admixtures usually consist of calcium chloride, sodium chloride, nitrates, sulfates, and carbonates.
Adding silica fume to the cement content increases the strength of the concrete and reduces its permeability. Microsilica is a common form of silica fume used in concrete.
To increase the heat of the concrete, you can add more cement to the mix, increasing the exothermal reaction. Another option is to use Type III cement, a high-early strength cement. It hydrates faster, leaving less time for freezing.
We’ve already discussed how important it is to pour during the hottest part of the day. The lower the ambient temperatures, the more protection the concrete needs.
Timing is critical in cold weather because concrete has a 48-hour protection period. This is the amount of time that needs to pass with the concrete kept at about 50℉ to achieve 500 psi compressive strength.
Of course, this protection period is for concrete slabs that won’t be supporting loads during construction. Beams, columns, or elevated slabs will require a more extended protection period to gain more compressive strength and withstand the loads they need to support.
Another important part of timing is formwork removal. Don’t remove forms too early; the concrete needs time to gain strength.
Maturity testing can be done to determine when it is safe to strip off the forms. This test can be performed through sensors embedded in the concrete that send data to a device that calculates the temperature and timing to determine the concrete’s maturity level.
Keep reading: Concrete Forms and When to Remove Them
During the protection period, the concrete must be constantly kept warm (around 50℉). To ensure this happens, you need to monitor the concrete temperature diligently.
Warm the aggregates before mixing them into your concrete, and use hot water. This will make your concrete warm to begin with.
Place temperature sensors into the cement to keep an eye on any significant change. Adjust the amount of heat accordingly.
If you can successfully place the concrete at the proper ambient temperature for the hydration reaction to occur, the concrete will keep itself warm since this reaction emits heat.
There are several different methods of heating your concrete to add more protection and keep it warm.
Insulation is a simple but effective way to prevent concrete from cooling. The insulation thickness depends on the weather; colder weather requires thicker insulation.
Three different types of outdoor area heaters can be used inside or outside of an enclosure:
A direct-fired heater is the most efficient, but it has drawbacks. These heaters release combustion byproducts, including carbon dioxide, which damages the surface of the concrete, and carbon monoxide, which is a health hazard to workers.
Because of this, direct-fired heaters are only used in areas with ample ventilation.
Indirect-fired heaters release no byproducts, but they aren’t as efficient and are more expensive.
Hydronic heaters protect a much larger area and do not require an enclosure. However, they take longer to set up and heat up.
Setting up heating enclosures is the most effective but expensive option in temperature control. These are usually reserved for critical areas and are understandably challenging for larger pours.
These enclosures can be made from plastic sheeting or canvas tarps, but the more sturdy options are to construct a temporary wooden shelter or rigid plastic enclosure.
Gradual cooling of the concrete is required to reduce cracking due to thermal stress.
The curing process in cold weather requires a different approach. Water curing could cause early-age freezing and add more problems down the road.
Luckily, there are alternative curing techniques available.
Liquid membrane formation holds the liquid in so the curing compound won’t freeze. This technique is beneficial for its ease of application, cost-effectiveness, and extended curing action.
Another option is a moisture-retaining cover, such as polyethylene plastic sheeting or a curing insulation blanket. These covers seal in water from evaporation, protect from UV damage, and maintain a 100% relative humidity condition.
Lastly, add a cure and seal product after the water and sheen on the concrete surface have disappeared. This last process enhances durability, protects against deicers, provides long-term aesthetics, and locks out external moisture. Then, apply again a month after pouring.
The ability to pour concrete in cold weather has its limits. However, with the proper precautions in place, the pour can be successful.
