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Curing Concrete in Cold Weather Part II

Concrete in Cold Weather

In our last post, we started talking about curing concrete in cold weather. Sometimes covering your work area in blankets simply won’t cut it when you’re pouring concrete in cold weather. If your job requires more insulation in order for the cold weather concrete to set, there are more options.

If regular blankets alone aren’t enough to insulate your concrete, you can use hydronic heating pipes. You can lay the pipes over the concrete that you’re curing, and the heat will help add to the warmth within the covered work area. Additionally, you can use electric heating blankets instead of normal blankets, which will increase the temperatures surrounding the setting concrete.

Sometimes you may be involved in a job that requires placing concrete where it’s too cold to even pour it outside. In circumstances such as these, you need to enclose your workspace and heat the air within the space. Setting up a temporary enclosure is an expensive operation, but sometimes it’s the only option if a job must be completed at such a time.

Carbonation

Carbonation becomes a problem when you’re working with concrete in an enclosure, or even in a building that’s using a temporary heat source. Unvented heaters and gas-powered equipment can increase the levels of carbon dioxide in the workspace, which can create a chalky, carbonated layer on the surface of concrete. The layer created by this reaction is soft and an unacceptable result for the job. For this reason, it’s important to use heathers that can exhaust to the outside of your enclosed workspace. Always be sure to have someone monitoring the heater overnight, so that it remains fueled and functioning.

Another problem that occurs when curing concrete within an enclosure is that dry, hot air may sometimes cause the concrete to cure too quickly, which dries the concrete out and crusting may occur. If you’re using propane-powered heaters, you also run the risk of fire, which is something to be aware of at all times.

If the concrete stays at about 50°F, you can remove the protection you’ve put in place after two days. It’s best to wait about 4 weeks before using the area where the new concrete has been placed.




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Curing Concrete in Cold Weather

Method for Curing Concrete

After following our tips for pouring concrete in cold weather, your concrete has been placed and you’re ready to protect and cure it. What’s the next step? The traditional, tried-and-true method for curing concrete in cold weather is to cover it with blankets. This method is successful even if the air temperature falls below 20°F. Between the ground temperature, the heat generated by the concrete, and the insulation of the blankets, it’s enough to protect the concrete as it cures. Cold weather techniques are necessary whenever you’re working in a climate where the air temperature is below 40°F.

The Process

Regardless of the weather, part of the process when curing concrete is waiting for the bleed water to evaporate. As the concrete particles begin to settle, they push out any water that wasn’t absorbed during the hydration process. The bleed water is that excess that collects on the surface of the concrete. It’s important to let the bleed water evaporate, because the surface of your concrete will be weakened if any bleed water is finished into it. When it’s cold out, the concrete will set more slowly, which means the bleeding will start later and last longer. An alternative to waiting is trying to squeegee or vacuum some of the bleed water off, but more often than not you just have to wait.

If the concrete is kept at 50°F or warmer, you’ll only need to keep it covered with blankets for a couple of days. You can measure the temperature of your concrete by using an infrared temperature gun. The insulation that you’ll require to keep your concrete at 50°F will depend on how thick the concrete is, what the mix is made from, and the lowest you expect the air temperature to drop during the curing process.

You should wrap any rebars that are sticking out from the covered area, and secure the blankets so they won’t blow off overnight. Also, triple the layers of insulation at the corners or edges where freezing may occur.

Check back with us for more cold weather concrete curing tips.




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More Tips for Placing Concrete in Cold Weather

In our last post, we gave you some tips for pouring concrete in cold weather. Today we’ve got a few more tips to add to the list, in addition to some precautions to keep in mind before placing concrete in the cold.

More Tips for Placing Concrete in Cold Weather

  • In addition to the chloride-based accelerators we discussed on Monday, there are non-chloride accelerators that are both very popular and effective. Additionally, they won’t discolor the concrete, which becomes a problem when using chloride accelerators. The only problem with non-chloride accelerators is that they’re on the expensive side. Remember that accelerators do not prevent your mixture from freezing—they don’t have any anti-freeze properties at all—they just increase the rate of the hydration reaction, which helps the concrete set more quickly.
  • Avoid using fly ash or slag cement when doing jobs in cold weather. Both materials tend to set less quickly than other types of materials, and also produce less internal heat. When you’re trying to do what you can to make your concrete set as quickly as possible, to avoid the mixture from freezing, it’s just not a good idea to choose these kinds of materials.
  • You can try to make the reaction in your mixture a little hotter by having your ready mix producer add some extra cement (about 100 pounds extra per cubic yard), or you can use Type III cement, because it hydrates more rapidly.

It’s important to be aware of what you’re up against when it comes to placing concrete in cold weather. Here are some things to remember and some tips to follow as precautions before placing concrete in cold weather:
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  • Don’t ever place concrete on frozen ground, onto ice, or snow. Several problems can result if you place concrete in those conditions. If you place concrete onto frozen ground, it will crack once the ground thaws and settles. Also, cold ground will reduce the temperature of your concrete, causing it to set less quickly. In such a circumstance, you may wind up with a “crusting,” which is when the top part of the concrete sets first and the bottom remains soft.
  • Prepare any area where you plan to place concrete by removing all of the snow, ice, and any standing water that may get into the concrete mix.
  • Use hydronic heat pipes and blankets or electric blankets to thaw the ground if it is frozen.
  • Make sure nothing that will come into contact with the concrete is cold. You should warm up anything that will touch the concrete, including forms and anything that will be embedded into the concrete, to at least 32°F. You should keep your tools in your truck or trailer to keep them warm.
  • Even if you don’t think temperatures will drop very low, be prepared with blankets. Also remember that the sun sets fairly early in the winter, so consider bringing lights with you in case the concrete sets more slowly than you anticipated, and you wind up working through dusk.
  • Remember that your concrete mix will lose some heat from the time it leaves the plant until it reaches the job site. If the delivery takes one hour, the concrete temperature will drop approximately a quarter of the difference between the air temperature and the temperature of the concrete. This means that if the concrete leaves the plant at 65°F, and the air is 45°F, it will arrive at the job site at 60°F, after having lost 5°F over the hour-long trip.


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Tips for Pouring Concrete in Cold Weather

Winter is not the optimal time to be pouring concrete, but sometimes a job can’t wait for the weather to warm up. There are a number of problems that can arise when pouring concrete in cold weather. Two of the biggest problems that occur when pouring concrete in cold weather are that the concrete may freeze before it becomes strong enough, and that it sets much less quickly when it’s cold.

At temperatures below 50°F, the setting process is very slow, but below 40°F the concrete essentially stops strengthening because of the effects of the cold on the hydration reaction. Bear in mind that these temperatures are that of the actual concrete, not the air, so it’s most important to take great care in protecting the concrete before it can handle being exposed to the cold air temperatures.

A rule of thumb is that once the concrete has strengthened to about 500 psi, it’ll set just fine. Once the concrete has gained enough strength to measure 500 psi, the cement has absorbed enough of the water in the concrete mix through hydration that there isn’t enough water left in the pores to cause damage to the concrete if it were to begin to freeze. There are two things that contractors can do in cold weather to help their concrete reach 500 psi: change the mix so that it will set more quickly or protect the concrete from the cold. Usually both measures are required.

Some changes you could make to concrete in cold weather include:

Hot water: the producer of your ready mixed concrete will usually use hot water when it’s cold outside. When concrete leaves the plant during winter months, most producers try to have it measure at least 65°F, which is usually sufficient for work being done.

Accelerators: As we mentioned, cold weather slows the setting of the concrete, so when pouring concrete in cold weather, you can expect delayed set times. In order to keep your job on schedule, you can add calcium chloride to accelerate the hydration reaction. Determine the weight of your cement, and add 2% of that amount in calcium chloride. This method is not only very effective, but is also fairly inexpensive. HOWEVER, if you’re planning to embed steel into the concrete, like rebar, adding that much chloride could potentially lead to corrosion. It can also result in a mottled surface appearance in colored concrete.

Next time, we’ll continue looking at changes you can make to the mixtures when pouring concrete in cold weather, and also at how to protect the poured concrete while it sets!


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