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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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