What is the minimum curing time for cement concrete?

What is the minimum curing time for cement concrete?
The period of curing of concrete is most important as it is very essential for keeping the hydration process of cement until concrete attains the maximum compressive strength, which increases but slowly after 28 days from initial placing time.

Compressive strength reduces to 50%, If it would be in air without curing.From the above figure, you can analyze that moist curing is essential for concrete to achieve the approximate maximum strength (Not 100%). Ideally the curing period depends upon the type of cement used, mixture proportions, required strength, size and shape of member, ambient weather, future exposure conditions, and method of curing.

The period should be as long as practical. Environmental temperature plays very big part in the process of curing as the hydration process is an exothermic reaction. Maintaining proper temperature is also very important and it affects the duration of curing. For most concrete structures, the curing period at temperatures above 5º C should be a minimum of 7 days or until 70% of the specified compressive or flexural strength is attained. The period could be reduced to 3 days if high early strength concrete is used and the temperature is above 10º C.

As the early strength of concrete is supportive towards the ultimate strength of concrete, we should do proper curing according to the environment condition, type of member etc.

So 7 days, in normal condition. But there is never a thing called normal. So, considering all uncertainties, 10 to 12 days of curing with proper method like ponding, gunny bag, sprinkling etc. has to be done to ensure almost 80% of the ultimate strength of concrete.