Hot Weather Concrete Curing
Curing is critical at any time, but it’s especially important in hot, dry weather, and it should begin as soon as your finishing processes are finished. Use a white pigmented curing compound with plain gray concrete that help reflect sunlight. If you’re going to use a curing blanket, go for the white ones, either single-use or multiple-use.
If you’re going to use a curing compound, make sure it’s capable of doing the job. “Place a 3-foot-square piece of plastic on the slab and sand around the edges,” Kozeliski recommends. If there remains moisture under the plastic after a few minutes, the curing chemical isn’t doing its job. When contractors see cracks in the concrete, they often blame the concrete when the problem is that the curing compound didn’t work correctly.
How to Deal With Hot Weather on the Job Site
If you start with concrete below 80 degrees, your goal is to keep it cool and prevent it from drying out. Place the concrete early in the morning or later in the evening if the weather is too hot. Prepare yourself! When the concrete arrives, make sure your team and all equipment are ready to go so you can get the concrete out of the truck—it may get quite hot just sitting in the mixer waiting to be poured. During the mixing process, friction within the concrete can generate enough heat to elevate the temperature of the concrete by 5 degrees in 30 minutes.
If slump loss becomes an issue, instead of adding more water, use a superplasticizer (high-range water reducer). These admixtures can increase slump without impacting the final strength or look of the concrete.
If at all possible, keep the sun off the concrete surface; for interior slabs, position them after the structure has been closed in; for exterior slabs, use sun shades. Also, keep all of your tools and equipment out of the sun, especially anything that will come into direct contact with the concrete, such as pump hoses.
Before pouring external concrete on a subgrade, wet down everything, including the subgrade and forms, with cool water to prevent moisture from being absorbed by the concrete, which can lead to cracking. “Wet the subgrade so that the moisture penetrates all 4 inches,” explains Kozeliski. “Too frequently, the contractor simply wets the top 1/16 inch, and it sucks all the water out.”
Use a monomolecular film, or evaporation retarder, as soon as the concrete is down and bull floated. These materials evaporate within a few hours and have little effect on the concrete, but they prevent surface water from evaporating. Plastic shrinkage cracking and surface crusting are prevented by using a monomolecular layer.
Cold Weather Concrete Curing
To protect fresh concrete from freezing conditions, use concrete blankets, enclosures, and heaters.
How to Cure Concrete in Cold Weather
Here is advice for curing concrete in freezing temperatures:
- Remember the definition: Use cold weather procedures if the air temperature is below or forecast to drop below 40°F.
- Concrete must be cured in cold weather because, if the concrete is warmer than the air, the surface can dry up even faster than in warm weather.
- Even though it’s cold weather, you must wait for all of the bleed water to evaporate before finishing concrete. Bleed water is the result of concrete particles settling (much like dirt in a stirred-up pond) and squeezing out any excess water. If the water is finished into the surface, the water-cement ratio rises, resulting in weak surface concrete. Because the concrete takes longer to set in the cold, bleeding takes longer and you can receive more bleed water. You can use squeegees or vacuums to remove it, or you can wait.
- Cure concrete without adding water in cold weather; adding water will keep the concrete saturated, causing it to freeze even after it has reached 500 psi compressive strength.
- In cold climates, concrete additives such as a non-chloride accelerator may decrease or eliminate the requirement for concrete protection and heating.
How to Use Concrete Curing Blankets
Covering concrete with blankets after it’s been finished is the traditional, and still the best, approach to protect it from the cold for flatwork. Blankets will keep it warm even if the temperature drops below 20°F because the ground is somewhat warmer and the concrete generates its own heat.
- If the concrete is warmer than 50°F, you only need to leave the blankets on for a few of days.
- If you want to be sure, use an infrared temperature gun or maturity methods to verify the concrete temperature. Maturity is a method of determining whether concrete has grown sufficient strength to stand on its own, and it is based on a combination of time and temperature.
- Place three layers of insulating blankets at the corners and edges of the area that might freeze. Wrap any rebars that protrude. Make sure the blankets aren’t going to blow away in the middle of the night.
- If blankets alone are insufficient to keep the slab warm (or the walls in the case of formed concrete), hydronic heating pipes or electric heating blankets set on top of the slab and insulated can be used.
- Protection can usually be removed after two days if the concrete is kept at roughly 50°F. If the concrete remains at 50°F, depending on the type of cement and the amount of accelerator used, you should wait a few weeks—preferably four weeks—before bringing it into operation. If you’re unsure about the strength, you can always test it.
- In cold weather, abruptly removing the blankets might cause a temperature differential to form between the concrete’s outside and inner surfaces. The heat differential can produce cracking, although usually only in thicker members.
How to Use Enclosures and Concrete Heaters
If that isn’t enough, or if it’s too cold to even start pouring concrete, you’ll need to enclose the work and heat the air. Temporary enclosures are costly, but they are sometimes the only alternative if the construction must continue.
- You must consider the possibility of carbonation in an enclosure or even a building heated by temporary heat. Carbon dioxide levels can rise with unvented heaters (salamanders) or even gas-powered equipment. As the carbon reacts with the concrete, a chalky carbonated coating forms on the surface. This layer will be soft and unacceptably.
- Heaters that vent to the outside of an enclosure or structure and only blow warm air in are available. This takes care of the carbonation issue. Assign someone to ensure that the heaters are properly fueled and will remain operational throughout the night.
- The concrete surface can quickly dry out when using hot, dry air in an enclosure, resulting in crusting or plastic shrinkage cracking. Also, with propane heaters, be cautious of fire.
