Here is the attachment about curing time

Curing is a required process for all concrete, not just precast concrete.

The idea of curing is to maintain the water within the concrete as much as possible to optimize the hydration process of cement with the water inside the mix.

Also temperature is important, since concrete hydration is a process that generates heat, heating could accelerate the hydration process as well.

Usually curing is done by giving the surroundings of the concrete a moisture environment and also maintaining the temperature, so the conditions for concrete to hardening are the best.

Lack of curing could produce low strength and cracking mainly .

So then the best curing conditions will be those that maintain a saturated environment with water or steam or sprayed water, and also keeping the temperature around 23°C .

At construction sites, curing conditions are quite variable, and this is one of the major problems that you will find in hardened concrete, the lack of curing.

Curing can also be done by applying to the surface of the concrete a curing compound that seals the surface of the concrete, avoiding water to go out of the concrete. This is also an effective procedure, but you have to be very careful when using those compounds to apply them at the right time and always following the supplier recommendations to guarantee a complete coverage, and avoiding leaving areas without covering. Dilution of the compound is not recommended.

In cold weather, heating is also recommended to avoid the hardening process to stop.

There are a lot of ways for concrete curing. The main part is to keep moisture and temperature of the concrete for at least the first 3 to 7 days, depending on the type of cement used in the mix.

construction on site: about 7 days in southern america is a classic time, but it depends on the size of the beams and other structural elements (see attachment) . precast: about 5 hours in case of tunnel ring elements (using steam chambers). the larger the curing time, the better the mechanical resistance (see attachment). concrete in Insulated Concrete Forms (ICF) , in a manner of speaking, is curing forever, since Expanded Polystyrene blocks cover this material as a part of the construction itself. some literature is available and describes compression resistance vs time depending on curing conditions.

Here is the attachment about curing time

Thanks, Jose Lozano and Thierry Poirier. i must appreciate all the knowledge that shared by both of you

I think the answers will be specific and clear if you mention the exact conditions of the structures and the sites.

Curing Conditions depend upon the temperature and Moisture conditions of the

area where the concrete is placed.

The objective is to keep the concrete moist and at a fairly uniform temperature

during the first 5 to seven days.

Keeping it moist keeps the concrete hydrating so it continues to gain strength.

keeping it a fairly uniform temperature keeps it from cracking while it

is gaining strength.

Flat slabs can have berms and flooding. sloping slabs can have burlap and

a sprinkler system. Cloudy Days help.

Remember, control the temperature, and control the moisture.

Uniformity, ambient air temperatures, normal ground temperatures,

no temperature spikes until it has gained strength.

ASTM C 31 allows only two curing regimes, standard and field cured.  

Under standard curing initally the samples are made, protected from disturbance and vibration,  weather, and kept between 16 and 27 degrees C.  After no more than 48 hours samples are move to storage where they are kept under lime saturated water or at 100 % RH at 23 degrees C until testing.

These procedures are needed to assure that all samples are kept under, as close as possible, the same temperature and humidity conditions.  If the fabrication, curing, and testing  has all been well managed, the cylinders should break at pretty much the same strength. 

For field curing, samples are kept with the structure to experience the same conditions as the structure.  ASTM C 31 allows these samples to represent the strength of the concrete in the structure for certain purposes.