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In parts of the world that experience extremely cold weather, builders find it challenging to lay concrete in freezing temperatures. Projects in cold areas have to be completed during warmer temperatures. You can’t postpone construction in cold regions indefinitely. Here you are: it’s cold and you must finish a project you undertook. What do you need to do if the concrete you pour freezes? Well, we’ve dug through different researches and got some answers for you.
When concrete freezes, it becomes weak and doesn't attain its properties to the fullest. Water in the mixture freezes. The concrete will:
- become porous and will not hold well,
- crack if laid on the cold ground,
- deform and flake, especially at the corners.
This usually happens at the stage when the mixture is in a paste form. Curing after the damage has happened won't restore the concrete properties.
Before going into panic mode and discarding your project, continue reading to see how you could bypass the above-mentioned tragedies.
What Happens When Concrete Freezes
Concrete is made up of water, cement, and an aggregate like sand or gravel. This water should remain warm enough temperature for hydration to take place. Hydration is when the amount of water decreases gradually as the cement hardens.
If the water freezes, it becomes less and turns to ice. In turn, the ice expands and discontinues hydration. The ice crystals that form in the concrete make it porous.
The concrete should cure gradually. What does this mean? Curing is when concrete has been provided for adequate moisture, time, and temperature to get to the desired state for utilization. When these implementations are not in place, you’ll have a sub-standard product.
The hardened concrete is weak due to the air pockets trapped in the paste. The strength of the concrete will be reduced by 50 percent. This happens in the first few hours when the concrete has been placed. This could also happen if the concrete has not cured to a strength of 500 psi and there's a sudden temperature drop. When the concrete mixture is well proportioned, it can attain 500psi strength between 24 to 48 hours.
When placing concrete, ensure that the ground has thawed thoroughly. When you lay concrete on the cold ground, it will crack. The temperature change will cause cracks in the concrete. The concrete should cure gradually but not too slowly either.
The slower concrete cures the more it will be prone to crack. The temperature shock will alter its properties. Concrete should not come into contact with cold before it gets to the required strength to resist expansion. The result is a structure that can't hold any weight or stand.
Deformation and Flaking
Concrete is not frost-resistant when in the paste form. Frost deforms concrete very fast. The ice crystals that have formed could cause the concrete to be bumpy instead of smooth and even. Trying to even out hardened concrete will not give the desired results.
The concrete deforms and starts flaking especially at the corners that are more exposed. The concrete won't withstand the weight it is expected to. The structure could slant or crumble altogether. This shows that the strength of the concrete has not been achieved. Experts have clear instructions on how to pour concrete in extreme temperatures.
How can you tell if concrete is frozen?
A sure sign that concrete froze before it set is when you see ice crystal imprints on it. Corners are the most vulnerable because they have two sides exposed. Break a piece off and examine the fracture for crystals in the coarse aggregated sockets. If ice crystals are absent then, you should know that the concrete is alright.
You could also wet the concrete surface and the imprints will be clearly visible. If you need a thorough check on the strength loss of the concrete, you could have a laboratory test conducted.
What do you add to concrete to keep it from freezing?
There are substances you could add to the concrete besides water and aggregates, to stop it from freezing before it completely hardens. These substances are called admixtures. They come in liquid or powder form. You must take extra steps to strengthen concrete and speed up the cure time. Contractors use admixtures to achieve better results.
- Superplasticizers: These are agents that reduce the water in the concrete mixture by 10 to 30 percent. The concrete mixture is still workable and needs to place fast. The effect lasts for 45 minutes.
- Accelerators: Add calcium chloride flakes to the concrete mixture. Calcium chloride hastens the hardening of concrete without allowing it to freeze. Alternatively, you could opt for a non-chloride accelerator. It comes in liquid form and works fast too.
- Air Entraining Agents: These agents create air bubbles in the cement mixture. They accommodate the expansion of ice when cement is poured in cold temperatures.
How long does concrete need to cure before freezing?
The duration concrete takes to cure before freezing primarily depends on where it is being placed, the type of concrete, and its intended use. Concrete should be poured and placed at 40 degrees Fahrenheit and should be left to cure for a day. Temperatures should stay at 40 degrees Fahrenheit for at least 24 hours.
Concrete that carries a lot of weight, particularly at the foundation, should be left to cure for at least 20 days at 50 degrees Fahrenheit. In such cases, you will need to trap the heat. The heat will ensure that the concrete is well cured before it is exposed to freeze-thaw cycles.
How does temperature affect concrete curing?
Experts have come to a consensus that curing concrete should be at a temperature of 40 degrees Fahrenheit to 140 degrees Fahrenheit. Low temperatures affect the strength of concrete. But in other case studies, experts say that concrete can be laid at 25–27 degrees Fahrenheit.
How do you get concrete to set in cold weather?
When it comes to making sure that concretes sets despite the cold and damp weather, you need to be vigilant. Some of the things that will help immensely are:
- Thawing the ground for a couple of days with heaters before placing concrete. The ground has to be warm enough so that the laid concrete won’t crack or get deformed.
- Using admixtures to hasten the curing process.
- Ensuring workers on-site remove the bleed water and moisture from the surface. During hydration, the bleed water may be a lot, and it will slow the curing process.
- Covering the concrete with electric concrete blankets or heat pipes.
- Erecting a temporary structure to keep away any extra moisture falling onto the concrete like snow or rain.
- Covering the corners and protrusions thoroughly.
How do you melt ice without damaging concrete?
Concrete can be easily damaged. Be extra careful when scraping ice off it. Some chemicals to use that are safe for the environment and the concrete are Magnesium Chloride, Calcium Chloride, and Calcium Magnesium Acetate just to mention a few.
You could also use stuff that can be found easily at home such as white vinegar. Add water to the vinegar and pour it on your driveway or the sidewalk. It works just as well. Gently scrape the ice off the concrete using a plastic shovel. Metal shovels tend to scratch the surface of the concrete and damage it.
Salt can be used, but it will eventually damage the concrete. It is very effective though. The concrete laid won’t serve for long if salt is often used to melt the ice on it.
What time of year is best to pour concrete?
The best time of the year to pour concrete is when temperatures are not too warm and it’s not too humid. Summer is ideal. The warmer the temperature the stronger the cured concrete will be. Ensure that it isn’t too hot either. You don't need the poured concrete to hydrate too fast either. Avoid pouring the concrete when the temperatures are more than 140 degrees Fahrenheit.
We now know that freezing temperatures aren’t ideal for laying or curing concrete. It's also clear that when concrete is damaged, there’s no way to rectify it. We have also seen ways to bypass the cold temperatures and get good results. With this newfound knowledge, you can boldly embark on your project.