Winter is quickly approaching, with temperatures dipping and snowfall beginning many additional issues may present themselves on a construction site. Particularly when tight construction schedules need to be maintained the pressure to keep moving can cause colossal problems if no precautions are made. Cement hydration, like all chemical reactions, slows when the temperature drops (set time doubles when the temperature drops from 60°F to 40°F). In extreme cases, the concrete may not even set up if the water freezes too quickly for concrete strength development.
When constructing in cold weather with concrete, the guidelines in the American Concrete Institute (ACI) 306R-16, Guide to Cold Weather Concreting, should be used to aid in the placement of durable concrete. ACI considers concreting in cold weather to be when the air temperature has fallen to, or is expected to fall below, 40°F before the concrete has gained adequate resistance to not be adversely affected by the cold, what’s called the protection period.
In this article, we will outline some things to consider before deciding whether to continue concreting in cold weather or to wait for more favorable weather. The goal is to prevent damage to the concrete from early-age freezing and ensure the concrete gains the required strength through a combination of proper preparation, protection, and mix design modifications.
Preparation
Everyone involved with the handling of concrete, from the batch plant and delivery drivers to workers making preparations on-site, needs to be working together to produce concrete and establish on-site conditions suitable for cold weather. A preconstruction meeting to work out the details is essential. Plans for protection and keeping materials up to specified temperatures need to made well before the concrete arrives.
The batch plant needs to factor in aggregate conditions in the yard for the mixing temperature of the concrete. Considerations in delivery times are essential; temperature loss during transportation can negatively affect the concrete. Site preparation for keeping materials that come into contact with fresh concrete (such as formwork, reinforcement, or large metallic embedments) need to be completed before concreting can begin. Subgrades need to be thawed and have standing water removed. All the equipment must be ready to properly protect the concrete once it has been placed.
Protection
Protection of the concrete is critical to meeting compressive strength requirements. Concrete left unprotected can freeze, especially on exposed corners, and will gain strength at a considerably slower rate. The temperature of the concrete needs to be maintained at the minimums recommended in ACI 306-16, Table 5.1. For example, for air temperature between 0°F and 30°F, and for concrete sections less than 12 inches thick, the concrete temperature as mixed should be no colder than 65°F, and as placed and maintained, no colder than 55°F. This temperature must be maintained throughout the protection period, until a compressive strength of 500 psi has been reached. At that point the concrete, if air-entrained, can withstand one freeze-thaw cycle. If repeated freezing and thawing is anticipated, the strength must get up to 3500 psi before protection is removed. Maturity methods are one way to estimate the in-place compressive strength.
Insulation, heating systems, and enclosures are used to protect the concrete until it develops enough strength to be exposed to the cold weather. Polystyrene foam sheets, urethane foam, insulation blankets, and polyethylene sheets are used to insulate the concrete, allowing the heat produced from cement hydration or heating systems to keep the concrete above the minimum recommended temperature.
Corners and edges of the concrete are more susceptible to heat loss and need approximately three times the insulation. Do not, insulate more than required as chances of cracking from thermal shock and thermal shrinkage are increased by the higher internal temperature. The heating systems used should not dry out the surface of the concrete and should not expose the fresh concrete to excessive carbon dioxide emissions (as from an unvented heater) which will severely weaken the concrete surface.
Enclosures are the most effective method to protect the concrete during cold weather, although also the most costly. A combination of insulation and heating systems will typically suffice.
The period of protection required, roughly 1 to 7 or more days, will depend on the type of loads the segment of concrete will incur in service and what environmental conditions it will be exposed to when the protection is removed. Generally the greater the load, and the more exposed the concrete is, the longer the concrete will need to be protected before it can be exposed to cold weather. The process of removing the protection material also needs to be done gradually, large temperature differentials in the concrete will increase the likelihood of cracking.
During the protection period, the concrete should also be adequately cured. Water curing is not recommended when freezing temperatures are possible unless additional protection and precautions are taken. Curing compounds or impervious plastic sheets for concrete slabs should be used instead to prevent surface desiccation of the concrete.
Mix Design
Concrete with low water content is ideal in cold weather concreting since it reduces the chances of excessive bleeding leading to problems in finishing or freezing of the bleed water. Precautions need to be taken to protect the concrete if using accelerating admixtures, high-early-strength cement, or extra cement to achieve sufficient strength. Large thermal gradients can occur from accelerating the rate of hydration since additional heat is produced, increasing the likelihood of cracking. It is also important to confirm the type of accelerator to be used. Calcium chloride accelerators are typically disallowed for reinforced concrete leading to the specification of nonchloride accelerators.
With all these considerations and more in ACI 306R-16, Guide to Cold Weather Concreting, is it economical and possible to abide by the guidelines and place concrete in cold weather? Or can the construction schedule wait or be delayed? These are questions to ask before rushing into constructing in cold weather only to end up with concrete that does not meet specifications.
