While concreting work can be done year-round, there are special considerations to make in extreme temperatures, both hot and cold. Any placement, finishing, curing, and protection of concrete in weather below 40 degrees Fahrenheit for longer than three days is considered cold weather concreting. Generally, cold weather concreting can be done effectively with the use of proper methods and tools like concrete formwork ties, but it is important to take extra care to ensure the integrity of the concrete.
The most important thing to keep in mind is the temperature of the fresh concrete itself. Concrete should remain at 55 degrees Fahrenheit or greater throughout use. If this is done, there should not be any further problems concreting in the winter.
The approximate set time is also impacted by temperature. At 70 degrees Fahrenheit, the set time is usually about six hours. If the temperature drops below 40 degrees, that set time jumps to 14 hours. If the concrete freezes early in the process, up to a 50% loss of strength can occur, leading to increased permeability and lower resistance to weather.
The general key is to start with warm concrete and maintain its warmth. The internal heat of concrete can be raised by heating the material, using extra or special cement, or adding accelerators. The environment can also be controlled to help through the use of enclosures and moist heat, applying insulated blankets, polystyrene sheets, or hay, and leaving concrete formwork in place for longer periods.
Preparation for Cold Weather
If you may be concreting in cold weather, it is important to take all the proper precautions and develop a plan before the cold weather begins. Ensure that you have any additional equipment or materials you may need, such as heaters, insulated blankets, and enclosures, before any construction begins.
You may consider air-entraining of concrete for anything exposed to a series of freezing and thawing, such as slab and other flatwork. All surfaces should always be cleaned from snow, ice, or frost that may develop.
It can also be helpful to keep detailed records of the job condition. At least twice a day, you should record weather conditions, the temperature of the air, and the temperature of the concrete surface.
Pouring Concrete in Winter
Concrete should never be poured around a large embedment unless it is a temperature is above freezing. To prevent freezing of concrete, you may use high-early strength concrete and choose the freezing point based on the size of your piece. Never use “antifreeze” compounds to lower the freezing point of concrete.
While calcium chloride and admixtures with soluble chlorides can sometimes be useful, certain conditions are contraindications, such as:
•Concrete that contains aluminum or prestressing strand because of erosion
•When discoloration of troweled surfaces cannot be tolerated
•Where galvanized steel will remain in permanent contact with the concrete
•Concrete that is subjected to alkali-aggregate reaction or exposed to soils or water that contains sulfates.
Concrete Protection in Cold Weather
Protecting concrete will involve calculating the proper minimum temperature for your concrete based on size. This temperature is measured at the surface of the element.
Protection mechanisms may include windbreaks with a 1.8m height, enclosures, supplementary heat, or adjustment of concrete mixture constituents for the effect of air temperature on setting time.
A minimum of three days is needed to properly protect concrete. If safety necessitates concrete strength, this duration should be extended appropriately. Even when periods are not defined as cold weather, if there is a potential for freezing temperatures, concrete should be protected for 24 hours after placement. If the concrete surface temperature is within 20 degrees of the ambient temperature, protections can be removed.
Curing Concrete in Cold Weather
Concrete should be cured to prevent loss of moisture. When heat enclosures are used, you can provide extra moisture by sprinkling or using steam for heating. Salamanders and fuel-burning heaters should be vented, and concrete should be allowed to cool slowly in order to prevent thermal cracking.
If the job schedule permits, it can help to leave forms in place. Reshoring may be necessary until the concrete reaches the required design strength.
When concrete needs to cure below 60 degrees Fahrenheit, water reducers or retarders may prolong the setting period. Any concrete placed in cold periods should not be exposed to salts applied as de-icers or salts that have dripped from parked vehicles.