The process of curing concrete properly is extremely important in ensuring that the final product has developed sufficient strength and the correct structural composition for durability. But curing concrete isn’t just leaving it out to dry, there are a series of steps and precautions to take to avoid compromising the integrity of the mixture. One of these major considerations is the ambient temperature around the newly poured concrete.
How cold is too cold to pour concrete?
Concrete should be protected from freezing in the first 24 hours or until it has reached a minimum strength of 500 pounds per square inch (psi) after it has been poured. If the concrete gets too cold while still curing, it can lose a lot of its overall strength. The American Concrete Institute (ACI) defines “too cold” as when, for more than three days successively, the average daytime temperature is below 40 degrees Fahrenheit. If this happens to be the case, then special techniques and precautions need to be applied.
What happens if it freezes after pouring concrete?
If it freezes after the concrete has been freshly poured, the mixture is at risk of losing much of its strength. In fact, concrete can lose up to 50% of its final strength if it becomes too cold too quickly. This happens because of the expansion resulting from the freezing moisture pockets, much like when a water bottle expands as it freezes. However, if the proper precautions and preparation tips are taken, concrete that cures in colder weather is actually preferred over concrete cured in warmer weather as the slower drying time helps increase its strength and durability.
Tips to Keeping Concrete at the Right Temperature
In order to pour and cure concrete successfully during colder periods, you must take certain precautions and apply specific techniques before, during, and after the curing process.
- Before Pouring. It is recommended to preheat the constituent material (the aggregate and the water) prior to pouring to ensure that the concrete is at the right temperature. It is important to heat the water and the gravel or sand aggregate, and not the concrete itself as it could impact its natural properties.
If it is too cold on-site, it is best to slowly warm the concrete mix by working in batches, heating only the amount of water and aggregate that is needed at the time as opposed to the entire amount.
Additionally, it is best to avoid using slag cement or fly ash during the colder periods. These components take longer to set and generate less internal heat resulting in too cold of a mixture.
- During Pouring. Because concrete takes longer to cure during the colder months, the process of pouring will take more time as crews need to wait for the mixture to reach its initial set. To ensure that the poured concrete stays at a consistent temperature during this initial period, workers can set up barriers to protect against heavy winds, called windbreaks, to prevent premature chilling.
If the temperatures start to really drop, heat enclosures may need to be set up. Generally, these enclosures are made from canvas, tarps, wood, or polyethylene sheets. Some teams opt for rigid-plastic enclosures paired with electric heaters. If you choose to use fuel-burning heaters, then it is best to use indirect fire heaters. This means the warm air is directed into the enclosure from an outside source. Keeping the fuel source outside offers more control to the internal temperature of the enclosure and is also safer in the event of a fire.
- After Pouring. If using forms to better set up the concrete, leave them in place for as long as you can. They can help retain heat and prevent premature drying. This is specifically applicable to the edges and corners as they are particularly vulnerable. Additionally, some teams use concrete curing blankets or insulated blankets to maintain the right heat level.
While the concrete is curing, you can check the internal temperature using an infrared temperature sensor. The optimal temperature for the concrete is at least 40 degrees Fahrenheit.
Before finishing the process, wait until all the bleed water has evaporated. Bleed water is the liquid that comes up to the surface as a result of excess moisture being brought up to the top of the slab due to expansive forces associated with the curing process. Because concrete poured in cold weather has a much slower curing time, the bleeding will also begin slower. Workers should be prepared to handle more bleed water than usual. It is best to not water your concrete during especially cold temperatures as excess water will freeze and damage the slab.
If you’re worried about the concrete drying out too quickly, you can pump live steam around the concrete to keep the moisture levels consistent. This helps prevent the concrete from drying out too quickly and cracking. Some teams also protect their concrete by applying a sealant to protect it from water getting inside, which could greatly reduce its overall lifespan.
Recommended Curing Temperatures
The American Concrete Institute (ACI) recommends specific minimum curing periods based on the type of concrete.
- ASTM C 150 Type I cement: 7 days
- ASTM C 150 Type II cement: 10 days
- ASTM C 150 Type III cement: 3 days
- ASTM C 150 Type IV or V cement: 14 days
- ASTM C 595, C 845, C 1157 cements: varies
Working in Cold Conditions
Construction teams who work in these freezing temperatures should consider their personal safety as well. Gloves, hats, and helmets are encouraged as body heat is lost primarily through the head and the extremities. Additionally, windbreakers or heavy jackets should be worn to protect against the brisk, strong winds that may occur. When heading in to warm up, it is recommended to heat the body’s temperature up slowly through layering warm and dry clothing. Any exposure to extreme heat, like dipping cold feet into hot water or getting in too warm of a bath can shock the body and result in pain and spasms.
Call A-Core Concrete Specialists
A-Core Concrete Specialists have been serving the Utah region since 1947. They are the premier team of trained concrete specialists committed to providing the best customer service and quality of work. We are skilled in a variety of concrete services including concrete cutting, core drilling, concrete pouring, demolition, GPR scanning, and grinding and grooving. With licenses to service over 12 states, we are here to help you in all your concrete needs. Call us today to make an appointment.