Cold Weather Concreting: Tips for Achieving a Successful, High-Quality Placement
While most of society “hunkers down” when winter weather begins, for players in the construction industry, the project must go on. There are no snow days or slowing down of schedules when the days get shorter and colder—which creates challenges for even the most experienced of contractors.
Placing and curing concrete during the winter months—even those considered relatively “mild” for winter—presents obstacles that need to be overcome at the jobsite. As ambient temperatures plummet, concrete temperatures fall as well, resulting in a slower rate of cement hydration, set times, and strength development. Dealing with frozen subgrades, preventing early-age freezing, ensuring concrete develops the desired strength, and avoiding cracks or other surface defects are just some of the potential problems that need to be considered in upfront planning.
According to the American Concrete Institute (ACI 306), cold weather exists when the air temperature has fallen to or is expected to fall below 40 °F during the protection period. Effective protection allows the concrete to gain strength at a normal rate and prevents the concrete from early-age damage by freezing. For most concrete maintained at 50 °F, the protection period lasts until the concrete reaches a saturation level below 92% and attains a minimum strength of 500 psi, which is about 48 hours after placement.
When working in severe cold temperatures during winter season, producers and contractors can mitigate risks and obtain a high-quality finished product by making modifications to their concrete mix designs and implementing a variety of preventive measures and best practices.
Adjust the Concrete Mixture
A key first step in a successful cold-weather concrete placement is the selection and proportioning of the materials needed for achieving the best mix in controlling performance properties at low temperatures.
Concrete sets more slowly when it is cold—and very slowly below 50 °F. If the concrete temperature falls below 40 °F, the hydration reaction basically comes to a halt and strength gain stops. For every 20 °F drop in temperature, set time will double and early strength gain will be much lower, which will slow down finishing and form-removal operations. When expedited turnover is required, it may be appropriate to adjust the concrete mixture to speed up setting and strength development. This may require an increase in cement content, the use of an accelerating chemical admixture, or both.
The fact that hydration is an exothermic (heat-producing) reaction helps offset the impact of low air temperatures, but only slightly. To make the reaction a bit hotter, some concrete producers will add 100 pounds per cubic yard of Type I/II cement. Others will add Type III high-early strength cement, which hydrates more rapidly and achieves a faster setting time. Increasing cement content too much, however, will cause more water demand and heat, and the potential for thermal and plastic shrinkage cracking.
If concrete temperature falls below 50 degrees, consider adding accelerating admixtures to the mix to increase the hydration reaction and set up the concrete more quickly. By accelerating the set rate, the mixture does not remain plastic for an extended period, making the concrete less prone to freezing. Chloride-based accelerators work best but are limited due to the corrosion potential of embedded rebar reinforcement. Non-chloride accelerators work well if the manufacturer's recommendations for dosages are followed.
Reducing the percentage of slag, fly ash, and other pozzolans in the mix is a common practice in the wintertime since they slow the set and generate less internal heat. In many situations, this may not be possible if the concrete has been designed to achieve specific durability properties and/or sustainable construction goals of project owners. This is where the use of accelerators is especially beneficial because they help low-carbon mixes attain specified performance properties in cold-weather applications.
Keep Everything Warm
Maintaining concrete within an ideal temperature range is important for achieving a high-quality finished product. The recommended concrete temperature at the time of placement for most applications is around 50 °F and should not exceed 70 °F to avoid increased water demand and cracking. Concrete temperature also should not fall below 40 °F since cement hydration and strength gain stop at that point. If fresh concrete temperature reaches 25 °F, it will freeze and compromise strength and durability.
When the initial temperature of the concrete is sufficient and moisture levels are appropriate, it is possible to achieve normal set times and required strength gain. At the time of mixing, the temperature of concrete can be increased by heating one or more of the concrete constituents. Water has the largest impact on increasing concrete temperature and may be heated up to 180 °F to offset the effects of cold weather. Increasing the water temperature by 4 °F will increase the concrete temperature by 1 °F.
Heating aggregates, which account for 60 to 75 percent of the volume of concrete, is also an option when there are extremely cold temperatures at the jobsite. As a rule of thumb, a 1 °F rise in the aggregate will result in a 1 °F rise in the concrete temperature. Due to quantities and heat capacity, hot cement is not an effective method for raising the initial concrete temperature.
The temperature of the subgrade also must be taken into consideration. Do not place concrete on frozen subgrades and keep the concrete temperature 20 degrees higher than ground temperature. This can be accomplished with heated formwork, supplementary heaters, and enclosures. Avoid sprinkling calcium chloride on the ground as this can lead to spotty setting. Depending on the ambient temperatures, anticipated wind velocities, relative humidity, and concrete placement temperatures, protection of a concrete placement may require the use of windbreaks.
Maintain Proper Curing Practices
Cold-weather plans must include active protection and curing strategies to provide satisfactory and uninterrupted moisture content in the concrete for at least seven days. The longer the concrete is cured, the better the durability will be. Drying, even intermittently, can produce shrinkage cracks, thermal cracks, and a considerable loss in the strength of the surface layer.
Concrete exposed to cold weather is normally unlikely to dry at an undesirable rate. Since little or no added external moisture is needed, water curing is not recommended, especially in freezing weather. Instead, rely on one of the two other options for curing the concrete in cold weather. The first method involves wrapping a moisture-retaining covering, such as polyethylene plastic sheeting or a curing insulation blanket, over the concrete so that it seals the evaporation of water. Polystyrene insulation blankets provide protection against UV degradation and maintain a 100% relative humidity condition.
The second option is the most common curing method—the application of a liquid membrane-forming compound to lessen moisture loss from the concrete surface. This is the best approach to take in cold climates because the curing compound will not freeze. Advantages include ease of application, cost effectiveness, and the extended curing action over 30 days. To prevent the application of the cure from marring the surface, apply it immediately after finishing, when the sheen has disappeared from the concrete.
Concrete is tough, but it is not invincible. The application of a combination cure and seal product (ASTM 1319) will enhance durability, resistance to deicing salts, and long-term aesthetics. It will also lock out moisture intrusion, thereby lessening damage that comes from freezing and thawing. Wait at least 30 days before applying a sealing product to ensure the concrete has achieved its desired 28-day strength.
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