With the approach of winter, it’s time to remember cold weather concrete practices. According to the American Concrete Institute (ACI)’s publication, “Guide to Cold Weather Concreting,” (ACI 306R) cold weather is defined as any time the air temperature falls or is expected to fall below 40°F during the protection period (the time required to prevent concrete from being affected by cold weather).
Cold weather affects concrete in many ways. Hydration in concrete, the process of gaining strength, is a chemical reaction. When concrete is placed at cold temperatures, hydration can be slowed - even stopped - which impacts finishing and curing times. Permanent damage can also occur if concrete freezes at an early age.
The American Concrete Institute (ACI) has developed the following chart, which provides minimum concrete temperatures at time of placement as a function of thickness:
|Section Size, Minimum Dimension|
Temperature as Placed
Carefully monitor the temperature of concrete, formwork, reinforcing steel, subbase, subgrade and any other items associated with placement. Surfaces upon which concrete will be placed should be above 32°F to prevent the concrete from freezing at the interface. Once normal-set concrete has been placed, it must be maintained at the above temperature and kept moist for at least 72 hours to allow adequate initial curing. Insulating blankets, heated mats, and/or heated enclosures may be necessary.
Proper handling of field-cured cylinders is critical during cold weather. When used to confirm suitable field strength for formwork removal, post-tensioning, or steel erection, every reasonable effort must be made to ensure these cylinders receive the same temperature and moisture treatment as the concrete they represent.
Mix Design Considerations
Modifying concrete mix designs during the winter may be worth considering. Using higher-strength mixes can lead to early strength gain, thereby allowing faster formwork removal. Other suggestions include lower water/cementitious material ratios; adding additional cement; using a non-chloride accelerating admixture; or using a Type III cement .
Using slag or fly ash mineral admixtures should be approached cautiously during cold weather. Mineral ad mixtures can retard initial strength gain, which can be more pronounced in cold weather. An accelerating admixture may be required if the delayed strength gain impacts finishing or formwork removal.
To help achieve good concrete results, ECS recommends that you review the guidelines presented in ACI 360R before beginning a cold weather concrete pour.