The American Concrete Institute’s (ACI) definition of cold-weather concreting under ACI 306 is a period when for more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees Fahrenheit for more than one-half of any 24-hour period. Under these conditions, special techniques need to be applied. There is no reason to avoid pouring concrete in cold weather, provided you can take the proper precautions. Done properly, concrete poured in cool weather is actually stronger than concrete poured in hot weather, thanks to the slow curing period. When concrete is being managed under cold weather, it must be protected from freezing shortly after being poured. Also, concrete must be able to develop the required strength for the safe removal of forms while reducing the circumstances where excessive heat must be applied to help concrete develop the required strength.
If Concrete Freezes Too Soon
Concrete that freezes when it is fresh, or before it has cured to a strength that can resist the expansion associated with the freezing water, will suffer a permanent loss of strength. Early freezing can reduce the final strength of the concrete by up to 50 percent. If the air temperature at the time of pouring and placing is below 40 degrees Fahrenheit and if below-freezing temperatures are expected within the first 24 hours, contractors should follow a number of recommended practices for cold-weather concreting.
Preparation Tips
Proper curing of concrete in cold weather can be enhanced through certain preparation practices:
During cold weather, preheating one or more of the constituent materials (the water and the aggregate) can ensure that the proper concrete temperature is present during the pour. Rather than heating the Portland cement, this is done by heating the water and/or sand and gravel before mixing. Ready-mix companies may have this capability; generally, the concrete leaves the plant in trucks at about 65 degrees Fahrenheit. On-site mixing of small amounts of concrete may involve using hot water or keeping the aggregate stored in warm indoor locations before mixing. The mixture components may need to be adjusted. Most commonly, this involves increasing the ratio of cement content in the concrete, or using an accelerating chemical admixture, such as calcium chloride in ratios as high as two percent. Use Portland cement Type III, cement that helps in setting without reducing the concrete’s quality. This is important because high moisture content can induce corrosion problems in steel reinforcement. Avoid using fly ash or slag cement in cold weather. These materials set up more slowly and generate less internal heat.
Pouring and Placing Tips
Additional practices during the pouring and placing can also help concrete set up and cure properly during cold weather:
Make sure to arrange for crews to be on site for a longer period. Concrete exposed to cold temperatures will take more time to reach an initial set, which may mean that the finishing crew needs to be present for a longer period of time.Windbreaks can help protect the concrete (and workers) from winds that can cause quick temperature drops and evaporation that is too swift. Generally, windbreaks about 6 feet high are sufficient.Heated enclosures may be necessary. These can be made of wood, canvas tarps, or polyethylene sheets—or you can use commercial rigid-plastic enclosures. Heating within the enclosure is best accomplished with electric heaters. If fuel-burning heaters are used, the best option is to use indirect-fired heaters, in which warm air is funneled into the enclosure from a burner unit placed outside. Another option is a hydronic system in which a warm mixture of glycol and water is circulated through the enclosure through pipes or hoses.
Temperatures for placement and protecting concrete in cold weather are established and mandated under ACI 306. The objective of the ACI 306 is to keep concrete warm, over 5 degrees Celsius, for the first 48 hours, where concrete strength development is critical. (Fresh concrete frozen during the first 24 hours can lose 50 percent of its potential 28-day strength.) For high-early strength concrete that won’t be exposed to freeze-thaw cycles, one day at temperatures above 40 degrees is sufficient. But a concrete foundation or other structure that will carry high loads at an early age requires 20 days or more at a minimum temperature of 50 degrees. Remember that cement curing is an exothermic reaction that produces some heat on its own, and often it is sufficient to cover the concrete with polyethylene sheeting or insulating blankets to trap the heat. Under no circumstances should concrete be allowed to freeze within the first 24 hours after pouring and placing.
Curing Tips
Finally, there are techniques you can use during the curing process to ensure the concrete reaches maximum strength:
Where forms are used, leave them in place as long as possible since they will retain heat and help prevent the concrete from drying too quickly. Corners and edges are most vulnerable and forms will help during the heat release process.Live steam can be pumped into the enclosure around the concrete to prevent it from drying out too quickly in the low-humidity levels that are common during cold weather.Wait until all bleed water has evaporated before finishing. Concrete poured and placed in cold weather has a much slower setting and curing time, so bleeding will also start later than expected. Be prepared to handle more bleed water than is typical in regular concrete placement.While the concrete is being cured, verify the concrete temperature using an infrared temperature gun. Make sure the concrete is maintaining a temperature of at least 40 degrees Fahrenheit for its curing period.As active heating is discontinued, guard against the concrete cooling too rapidly. This can be done by gradually lowering the temperatures within an enclosure or by covering the concrete with insulating blankets. With large structures, several days or even weeks of gradual cooling is advised. Removing the blankets suddenly in cold weather can cause a temperature differential to build up between the outside of the concrete and its middle, resulting in cracking.Make sure to cure the concrete for the time period recommended for the type of cement being used. ACI Committee 308 recommends the following minimum curing periods:ASTM C 150 Type I cement, 7 daysASTM C 150 Type II cement, 10 daysASTM C 150 Type III cement, 3 daysASTM C 150 Type IV or V cement, 14 daysASTM C 595, C 845, C 1157 cements, variableSeal newly cured concrete by applying sealant to prevent outside water from seeping into the concrete. Concrete sealants will extend concrete’s life and will reduce the chance of curing failure. In extremely cold regions, use only a breathable concrete sealant that allows the evaporation of moisture.
