If concrete is cured at colder ambient temperatures (32°F to 50°F) with constant moisture, strength increase will be sluggish, but the concrete will finally attain a high strength. During the curing time, concrete should not be permitted to grow hotter than 90 degrees Fahrenheit or to dry out. Curing concrete at warmer temperatures (50 degrees F to 70 degrees F) will result in faster strength development. However, if the temperature reaches 120 degrees F, the concrete begins to suffer damage due to thermal expansion and contraction.
Concrete must be allowed to cure for a period of time after it has been mixed to allow any air bubbles that may have formed during mixing to rise to the surface. If you cut into the fresh concrete before it has fully cured, some or all of the air will come out of the concrete when you cut it, making the job even harder the next time around. Curing concrete for too long will also cause it to harden up too much, which could prevent it from absorbing more water later on. If this happens, you'll need to add more water to the mix as needed to keep the concrete soft enough for workable days.
Concrete cures over time. The amount of time it takes for concrete to reach its maximum potential strength depends on several factors such as the type of cement used, the ratio of water to cement, and the temperature.
When the ambient temperature is too low in cold-weather concreting, the hydration of the cement is dramatically slowed, if not totally stopped, until the temperature rises again. According to standard rules, the concrete curing temperature should be kept above 5 °C (40 °F) for 48 hours (ACI 306). However, modern methods allow curing concrete at lower temperatures over a longer period. For example, a concrete floor can be cured at 20 °C (68 °F) for 30 days while still maintaining its strength.
As long as water is available to hydrate the cement, the concrete will cure. Concrete that has been thoroughly mixed and placed in the right type of mold will usually dry out within 24 hours if there is no rain or other source of moisture. The only time this does not happen is if the mixture contains more than 15 percent coarse aggregate, such as gravel, which provides enough space for the water to drain away.
Concrete that isn't exposed to air can be used after only 12 hours if it's stored in a relatively dry place with no more than 60 percent humidity. If the concrete is going to be exposed to wet conditions, such as a walkway along a lake, then it should be allowed to cure for at least 3 days before it is used.
Curing concrete at low temperatures may cause problems for people who suffer from hypothermia.
Temperature can have a negative impact on the development of concrete strength. However, effective cold-weather concrete drying will improve the development of concrete strength. Any combination of high ambient temperature, high concrete temperature, low relative humidity, and wind velocity is considered hot weather. These conditions limit the amount of water that can be absorbed into the concrete during its curing process.
Concrete that is cast in warm temperatures has less opportunity to dry out than concrete that is cast in cool temperatures. The reduced water content results in lower-strength concrete that takes longer to reach its ultimate strength. Concrete that is cast in cold temperatures can benefit from forced air ventilation during winter if it remains wet for several days after casting. This allows more time for the hydration reactions to occur which increases the rate at which the concrete hardens.
The effect of heat on the setting process depends on how long it takes for the concrete to reach its optimum mixing temperature. If the concrete is cooled too quickly, it will not have enough time to fully react with the cement and aggregate and will therefore have less strength.
Setting retarders can be used to prevent or slow down the hydration reaction so the concrete has time to set up before it reaches its peak temperature. Retarders are substances that delay the start of the hydration reaction and thereby allow more time for the concrete to cure.