By: Sean McPartland
There are many factors that affect the performance of concrete that is poured-in-place. These include, but are not limited to, the quality of the raw materials, water to cement ratio, air temperature, humidity, site preparation, placement methods, and curing. Pouring concrete in cold weather can be challenging. Without proper planning, problems that affect the overall quality and longevity of the finished product might occur. The strength of concrete that freezes can be reduced by more than 50% if it freezes prior to achieving a compressive strength of 500 psi. The American Concrete Institute prepared the “Guide to Cold Weather Concreting” (ACI 306R-10) that is used by many engineers, architects, and various agencies as a reference for developing project specifications.
Most concrete mixes are designed to achieve their minimum compressive strength within 28 days of placement. Typically, concrete reaches approximately 40% of its compressive strength after 3 days, 65% after 7 days, and 90% after 14 days. The time when concrete continues to gain strength following placement is referred to as the curing period, and the temperature of the concrete during this period is referred to as the curing temperature.
Concrete should not be placed on frozen subgrade. The subgrade temperature must be brought up to above freezing and protected. Thermal blankets will help prevent the subgrade from freezing; however, external heat is typically necessary if the subgrade is already frozen.
During the cold weather months, concrete suppliers use hot water to increase the temperature of concrete delivered to job sites to approximately 60°F. Once the concrete is on-site, the Contractor must ensure that they have heated and maintained the temperature of the subgrade, formwork, and reinforcement to a minimum of 40°F. It is recommended that the temperature of the subgrade is as close as possible to the temperature of the concrete being delivered to the site. When the air temperature is less than 32°F, a heated enclosure is required.
As concrete cures, it has a tendency to crack due to temperature changes and shrinkage. Rather than allowing concrete to crack at random undesirable locations, control joints are used to force concrete to crack at planned locations in a straight line. For a typical sidewalk replacement project, control joints may be tooled into the concrete at the time of placement or saw cut after the concrete has sufficiently cured. The timing of the saw cutting is also critical; if the sawing is performed too early and the concrete has not reached the proper compressive strength, the edges of the pavement on the sides of the cut will chip. If saw cutting is performed too late, however, uncontrolled cracking may occur.
While concrete mix design and the types of admixtures used are factors, it’s critical that air temperature dictate the timing of control joint cutting. In hot weather conditions, saw-cutting may start as early as four hours after placement, while in colder weather, it can be performed twelve hours after the pour has been completed.
Concrete generates heat during the initial stages of the curing process. This is referred to as the heat of hydration. The amount of heat that is generated is dependent upon the size of the placement, and more specifically, its minimum dimension. For most placements, concrete will continue to generate heat for up to 24 hours after placement. The heat of hydration plays a crucial role in successful concrete placement during cold weather. Thermal insulating blankets are placed over fresh concrete immediately after placement. A thermal blanket traps warm air that is generated from the concrete between the surface of the concrete and the thermal blanket. This prevents concrete from freezing when the air temperature drops below 32°F. It’s imperative that thermal blankets are properly secured to prevent wind or rain from shifting them and exposing the concrete surface during the curing period.
As a means of quality assurance, engineers and architects must monitor the curing temperature of concrete to ensure that it remains above 40°F until it has attained a minimum of 500 psi. Surface thermometers must be placed between thermal insulating blankets and the surface of the concrete to monitor the high and low temperatures of the concrete surface during the curing period. Since colder air temperatures are more likely to occur during the nighttime hours when work is rarely being performed, the ability for thermometers to record high/low temperatures is critical. Low temperatures that are recorded below 40°F is an indication that the contractor’s means and methods for maintaining curing temperatures are not effective. In this case, the contractor must consider alternate ways of adding heat to raise the curing temperature of the concrete.
Good cold weather concrete placement practices begin with proper job site preparation. All necessary materials and equipment must be available and on-site prior to scheduling the concrete pour. The project team must review the extended forecast and determine if the expected whether is acceptable for concrete placement, and whether any protective measures are required. At a minimum, concrete must be protected from freezing from the time it’s mixed, while it’s in transit, during placement, and until it has achieved a minimum compressive strength of 500 psi.