Why Does Concrete Crack in Cold Weather?

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Concrete’s use in everything from sidewalks to skyscrapers is a testament to its strength and flexibility. But when the temperature drops, it seems to break all on its own, leaving you to wonder: why does concrete crack in cold weather?

Concrete cracks in cold weather when the moisture inside it begins to freeze and expand. This expansion creates tiny pockets of pressure inside the material and eventually results in damage. Concrete damaged by cold weather can be repaired with epoxy or replaced with a low-temperature mix.

When water freezes, its volume increases by 9%. Keep reading to discover how this can damage your concrete and what you can do to prevent or restore it.

How Cold Weather Damages Concrete

Concrete is porous, like a sponge, and always maintains some level of hydration. As water gets absorbed into the concrete, it accumulates into tiny compartments called gel pores, capillary pores, and air voids. These pores and voids play a significant role in concrete’s freeze-thaw durability—a material’s ability to resist frequent temperature changes.

Freeze-Thaw Mechanics in Concrete

Gel pores, capillary pores, and air voids vary in size and exist throughout concrete in a kind of extensive, messy network. Gel and capillary pores are smaller than air voids, so the water they contain has higher surface tension. Water with high surface tension is less likely to freeze at low temperatures.

Air voids are larger than pores, so the water inside them freezes first. When the water in the voids starts to freeze, it expands, and the resulting pressure pushes any leftover water into nearby pores. This process relieves some of the pressure trapped in the air voids by distributing it to other parts of the concrete.

However, if the capillary and gel pores become too saturated (i.e., filled with water), the water in the air voids will have nowhere to go. The moisture in the voids will continue to freeze, and the pressure will build until the material around the voids eventually cracks.

Saturation and Freeze-Thaw Durability

The voids and pores inside concrete need space to move excess water and pressure when the temperature drops. If a concrete block absorbs too much water before it freezes, it can’t effectively redistribute pressure, and it will eventually break.

The Degree of Saturation is the amount of water that a given volume of material has absorbed, measured by a percentage. A “critical degree of saturation” in concrete occurs when both the capillary and gel pores are full (~88% saturation). At this point, the air voids have nowhere to release pressure, and the risk of thermal cracking significantly increases.

You can reduce the risk of oversaturating concrete by keeping it dry and free of puddles. The longer you allow your concrete to absorb standing water, the more susceptible it will be to damage when the temperature drops.

Can You Repair Concrete Cracks in Winter?

Unfortunately, you can’t control the seasons. But if a particularly brutal winter damages your walls or patio, there are ways to effectively repair them – even while it’s still cold outside.

Repairing Small Concrete Cracks

PC-Xtreme Polyurea Elastomeric Crack and Joint Filler is the easiest and most effective way to fix cracked concrete, especially on patios, driveways, and sidewalks. Used in conjunction with a dual-caulk gun, you can apply PC-Xtreme in temperatures as low as -40°F (-40°C). To apply the epoxy:

Use a dual-caulk gun to run the tip of the two tubes along the damaged area. The epoxy solution should automatically mix in the tip.
While the solution settles, you can use a small shovel or hand tool to spread it and push it further down into the crack. Doing this will ensure the epoxy bonds to as many points as possible, thereby strengthening the repair.
Take some tiny rocks and dirt that approximately match the color of your concrete and spread them loosely across the surface of the curing epoxy. Adding loose gravel will strengthen the repair and give it a natural look.

PC-Xtreme dries very quickly, so you will only have a few minutes to do the repair.

Alternatively, you can use an epoxy mix kit like PC Masonry Epoxy Adhesive. Masonry adhesive helps repair cracked, dented, or chipped concrete, and you can cover a wide area when you use it with a painter’s stick or putty knife.

One disadvantage to using this epoxy during wintertime is that you should only apply it in temperatures higher than 35°F (1.67°C).

To use the masonry epoxy, mix equal parts from each container and then apply the resulting adhesive to the damaged area. Like with the caulk gun fix, adding a mix of similarly colored rocks and sand to the surface of the curing adhesive can make the repair look more natural.

Repairing Large Concrete Cracks

Sometimes, the damage is too significant, and the concrete needs replacing outright. Sadly, this doesn’t always happen during the ideal time of year.

Pouring new concrete in the winter presents a host of unique challenges. For one, concrete takes longer to dry and mature to its full strength than epoxy, especially in colder temperatures. This lag means your mixture stays exposed and vulnerable to unpleasant weather conditions for longer.

That said, proper preparation and protection during winter projects can lead to potentially better outcomes than projects conducted in even warmer conditions.

Winterize Your Concrete Project

You should never allow your concrete to freeze within the first 24 hours of being poured. Fresh concrete that has frozen prematurely can lose up to half of its potential strength (measured in pounds-per-square-inch). Concrete should be kept between 40 and 50°F (4.44 to 10°C) while curing and protected from dramatic temperature fluctuations until it has matured to a strength of at least 500 psi.

There are several things you can do to winterize your concrete project:

Expect the project to take longer than usual. Concrete takes longer to prepare, pour, and cure in cold weather, so you and your crew should plan accordingly.
Install windbreaks to protect the drying concrete from sudden temperature changes and excessive evaporation.
In some situations, you can even build a climate-controlled enclosure around the construction area. Ensure the temperature and humidity in any enclosure stays consistent to prevent thermal shock (i.e., damage resulting from rapid temperature fluctuation).
Use low-temperature concrete mix, which is more resistant to thermal cracking than other concrete when poured at colder temperatures. Avoid using slag or fly ash mix.
Use additives, like Akona Liquid Air Entraining Admixture, in the concrete mix. They create air pockets in the mixture that will increase its long-term freeze-thaw resistance.
Heat the material in transit. You can bring the concrete mixture to an appropriate temperature by heating the water and aggregate material before mixing them. However, doing this usually requires special equipment.
Avoid rapidly heating or cooling forms. Forms help evenly distribute heat across the concrete surface and should be kept at about the same temperature as the mixture being poured into them. Additionally, forms should be left in place for as long as possible and removed cautiously to prevent thermal shock.
Do not pour concrete onto ice or snow. The concrete will break when the ground eventually thaws and settles. You can thaw the project area before you start by laying heated blankets across the ground for a few days. Remove unevaporated water with a broom or vacuum to prevent it from mixing with the new concrete.
Use insulation covers or blankets to protect the concrete while it cures. Leave the covers on for at least 7 hours, and remove them carefully to ensure the concrete’s surface temperature doesn’t drop significantly lower than its internal temperature.
Keep fossil fuel equipment away from the concrete while it cures. Heaters and generators produce fumes that stain concrete. If you’re using equipment in an enclosure, make sure it stays properly ventilated to prevent carbon monoxide poisoning.
Monitor internal temperature with an infrared thermometer gun. Concrete generates heat while it cures. If you’re using climate-control equipment or insulation covers, you’ll need to check your concrete’s temperature periodically to ensure it hasn’t exceeded 50°F (10°C).

It might not be necessary to use all of these tools and techniques for your replacement project. The important thing is that the temperature throughout the concrete mixture stays as close to the appropriate range as possible while it cures and that temperature changes are introduced gradually.

Final Thoughts

Concrete is a porous material that can crack when the moisture inside it freezes and expands. As concrete absorbs more water, it becomes more susceptible to thermal cracking. So, keep your surfaces dry and free of puddles.

If the cold damages your concrete, you can easily repair it with an epoxy solution or replace it with a low-temperature mixture. New concrete can even be future-proofed by adding an air-entraining admixture, which creates air compartments inside the mixture to increase its resistance to cold weather and thermal shock.