Do you in practice actually have to continuously wet or put water newly set concrete in mat foundation, rc columns, or beams, for how long? What happens if you don't?
Yes you do while it is green. That’s normally around 7 days every day. You can also use a curing compound to prevent major evaporation.
The reason is that concrete is a exothermic chemical reaction that takes place, which means that it releases energy as heat. But the chemical reaction requires water to react obviously we know when mixing to much water decreases the strength (water/cement ratio) but after the concrete is set the amount of water does not affect the water cement ratios anymore but promotes the increasing of strength through good curing.
Let’s say the concrete is still in its green phase and the concrete dries out the chemical reaction stops not really but it gets close to stopping.
The chemical reaction needs the water to continue strengthening to the required Mpa, so keeping the concrete wet you cool it down and prevent internal moister from drying up that would prevent the concrete from gaining strength.
That’s is the important part but also to prevent or minimize shrinkage cracks so water takes up space and if it is drastically minimized in a short time, the volume decrease and creating cracks that could expose the rebar which is now susceptible to rust.
Curing of concrete is very important but elements with large surface areas are the most vulnerable like a deck or raft foundation.
Also keep in mind this is only for hot or windy conditions. If it is cold the water again is susceptible to freezing which is a different story.
The reason for continuous wet curing is to prevent evaporation of water and the obvious loss of moisture in the mix near its surfaces. This allows hydration to continue and reduces the potential for surface cracking from drying shrinkage.
Wet curing should be done as long as practicable, preferably until the required strength is obtained, but this is often not practicable because of the speed of construction. Wet curing should also be done at any temperature above freezing, not just in hot or windy weather.
I would suggest that you obtain a copy of "Design and Control of Concrete Mixtures" provided by the Portland Cement Association and to obtain a copy of any of Neville's books on concrete.
