Concreting in Cold Weather What You Should Know
Concrete and cold weather are not the best of friends. If concrete freezes before it gains strength, then it can be damaged. And when concrete sets in cold weather, the hydration reaction can be halted, which results in concrete that does not have sufficient strength.Keep in mind, however, that when we talk about temperature, we are talking about concrete temperature, not air temperature. It is therefore vital that the concrete stay protected until it can handle the cold on its own.
Generally speaking, concrete can handle the cold when it reaches 500 psi. Most concrete can reach this compression point at about the second day. To reach the magical 500 psi number, you can either change the mix of the concrete to allow it to set more quickly or protect the concrete from the cold. In most situations, a concrete foreman will likely do both.
Your ready mix producer should have a supply of hot water on hand as soon as the weather begins to turn colder. A general goal is to have the concrete reach at least a temperature of 65 degrees before it’s on its way to the job site.
Air-entrained concrete should also be used to reduce bleeding during times of cold weather.
Using Accelerators to Combat Weather Accelerators, such as calcium chloride, are the easiest (and cheapest) way to accelerate the hydration reaction, but caution must be exercised because too much chloride can lead to the corrosion of any steel that is embedded in the concrete and a mottled surface appearance (on colored concrete) can then appear.
Nonchloride accelerators, on the other hand, will not damage the steel and discolor the concrete, but are more expensive than calcium chloride.
Fly ash and slag should be avoided during the colder months, as they typically set up at a lower rate and generate less internal heat.
Many ready-mix producers may also add a bit more cement or use Type III cement to make the reaction a bit hotter and to more rapidly hydrate the concrete.
It is best to avoid water reducers during colder weather, as they typically slow down the concrete’s set time.
If you are using admixtures on the job site, do not use any that have frozen, as they are less likely to work because the chemicals may have separated.
Cold Weather Tips:
- Never place concrete onto frozen ground, snow or ice. Frozen ground will settle when it thaws, which will immediately crack the concrete.
- Use electric blankets or hydronic heat pipes to thaw frozen ground.
- Remove any standing water at the concrete site, as it could become mixed in with the concrete.
- Warm up any tools and forms that will come in contact with the concrete. Cover the materials and supplies with tarps or, if possible, keep them in your truck or trailer.
- Factor in the heat loss as the concrete travels from the ready mix plant to the job site.
- Be prepared to lose daylight much earlier during the winter months.