How is concrete affected by improper or neglected curing?
Improper or neglected curing can severely reduce the quality of concrete even though it was properly proportioned, mixed and placed under the same conditions. Prolonged, thorough curing is necessary to obtain watertight concrete. Resistance to wear is improved by curing. Concrete cured 7 days has about 20 percent greater resistance to abrasion than concrete cured only 3 days. The 28-day compressive strength of concrete cured 28 days will be 20 percent higher than the 28-day strength of concrete that is cured 3 days and may be the same as the 28-day strength of concrete that is cured 7 days. But in 90 days the concrete cured 3 days will have only about 66 percent of the strength of the concrete cured 28 days, and the concrete cured 7 days will have only 80 percent of the strength of concrete cured 28 days. Forms left in place do not provide adequate curing unless they are kept continuously wet. When paper or plastic film is used, the joints and laps must be airtight. The concrete surface should be damp. While most of the cement hydrates in the first few days, hydration will continue indefinitely but at a progressively slower rate as long as enough moisture is available and some unhydrated cement remains.