Posted on November 07, 2019
In a perfect world we would place concrete in temperatures between 50 F- 70 F but we don’t live in a perfect world. The demands of today’s construction schedules means our projects are under construction when it is hot and when it is cold. Recognizing that, we also understand that with the proper precautions and practices concrete can be placed virtually year round.
The American Concrete Institute (ACI) defines cold weather concreting as when: “the air temperature has fallen to or is expected to fall below 40 degrees F during the protection period.”
If you are going to be placing concrete during such a period there are a few things to understand.
- Hydration is the process from which concrete gains its strength
- It begins as soon as water is introduced to cement
- The reaction exothermic (it gives off heat)
- It ceases at 14 degrees F
- Frozen concrete
- Up to 50% reduction of ultimate strength can occur if frozen within a few hours and before reaching a strength of 500 psi
- If frozen once, curing can help the ultimate strength gain but the concrete will be less permeable and less durable
What follows is a brief listing of objectives, guidelines, steps in the planning process, protection and curing methods for cold weather concrete construction.
- Prevent damage to concrete due to early age freezing
- Assure strength of the concrete is adequate for form removal
- Maintain curing conditions that foster strength development without excessive heat
- Limit rapid temperature changes (particularly before the concrete can withstand thermal stresses)
- Short term construction economy on protection should not be obtained at the expense of long term durability
- Use air entrained concrete
- Keep surfaces coming into contact with the concrete free from ice and snow, and at a temperature above freezing before placing
- Place concrete at the lowest possible slump
- Protect plastic concrete from freezing or drying
- Protect plastic concrete from early age freeze-thaw cycles
- Limit rapid temperature changes after removing protection
- Pre-con meeting should clearly define what and how concreting methods will be used
- Contractor should determine the specific methods to be used to satisfy the minimum requirements
- Equipment and materials should be in place well in advance of the occurrence of the cold weather
- Insulating Materials
- Blankets may be used on unformed surfaces, or insulating forms may be used
- Must be kept in contact with the surface to be effective
- Some types that may be used:
- Polystyrene foam sheets
- Urethane foam
- Foamed vinyl blankets
- Blanket or batt insulation
- Selection based on the amount of protection required
- Protect the concrete from drying so adequate hydration occurs
- Not typically an issue during cold weather
- If dry heating is used, the concrete should be covered with an impervious material or a curing compound
- If concrete has been wet or steamed cured, it should be allowed to dry prior to being exposed to freezing temperatures
As has been noted, concreting in cold weather, while posing some challenges, can be accomplished with the proper preparation and knowledge.
Additionally we have a webinar on Placing and Protecting Concrete in Cold Weather available for viewing and download.