Finer aggregates may need thawing if there is ice present in the aggregate. Care needs to be taken if the aggregate is thawed in the mixer so that excessive amounts of water are not present due to ice melting.
Controlling the amount of air entrapped and entrained in the concrete is critical during cold weather pours. When your batch arrives at the work site, it should contain no more than the specified air for the project. The concrete supplier needs to pay close attention to the slump of the mix, aggregate size, the proportion of cement, admixtures, gradation of sand, mix water temperature and mixing time to ensure the concrete is within specification.
Another way of modifying concrete for cold weather use is to add more cement to the mix. The additional cement content will provide additional heat during hydration. While Type 10 (1) cement will achieve this goal, the use of Type 30 (III) will achieve even higher early strengths.
Cold weather will cause little damage to fresh concrete once it has achieved a compressive strength of 500 PSI (3.5Mpa). The sooner this compressive strength can be attained the less likeliness that the pour will fail. To ensure early compressive strength, batch plants can modify the mix by adding accelerators to speed up the curing process. The most commonly used accelerator is a calcium chloride water solution or a calcium chloride and a water-reducing admixture. Calcium chloride is not antifreeze, and protection after the pour is still required for a successful result. Calcium chloride helps speed up the hydration process of concrete by acting like a catalyst to help free calcium oxide within the cement, allowing faster initiation of the hydration process and quicker achievement of the critical 500 psi (3.5 Mpa) compressive strength.
After the pour
Here’s where all your careful preparations pay off. Set your protective materials in place as soon as possible. Pay special attention to corners, edges and narrow areas, as these will tend to dry and freeze first. Check the effectiveness of your insulation by taking the temperature of the concrete. Placing a thermometer between the insulation and the concrete will allow you to determine the effectiveness of the insulating layer. If the temperature begins to drop, use additional material or material with a higher “R” value.
In some cases you will be able to construct heated shelters for your pour area. In these cases you can set up properly vented heaters to keep the pour area warm while avoiding direct contact of fresh concrete with carbon dioxide associated with non-vented heaters or open flame. Carbon dioxide combines with calcium hydroxide in fresh concrete and forms a layer of calcium carbonate, which will result in dusting on the pour surface.
The American Concrete Institute recommends a maintained temperature of 70 degrees F for three days or 50 degrees F for five days to gain sufficient strength for a cold pour to perform up to specification. With careful planning of a work site and the right mix from your supplier these parameters should be attainable for most contractors from late fall to early spring on the construction calendar.
John Kulczycki is a London, Ontario-based freelance writer.