As the tile loses moisture during curing, it gets a little smaller. As the concrete shrinks, the slab may crack to relieve stress.
Shrinkage cracksare common and can occur as soon as a few hours after the tile has been poured and finished. They are generally not a threat to the structure. It's natural to worry about cracks in freshly poured concrete.
The truth is that some cracks are inevitable due to the structure of the surface. Let's look at the reason why fresh concrete can crack. There are other reasons for cracking, but one explanation takes much longer; and from your description I think you are witnessing shrinkage cracking.Concrete naturally cracks on its own, unless it is given a place to crack, such as a control joint or expansion joint. As concrete cures, heats up and expands.
In summer, I saw the slab crack before I could walk on it. There are a few reasons why concrete cracks, but the most common reason by far is shrinkage cracking.When your patio or concrete driveway is poured, it is in liquid form, which has to be so that it can form properly. As the concrete begins to dry, the water inside will evaporate and, as it does, the concrete will beg to shrink. A concrete slab will shrink approximately ¼ inch per 100 square feet of floor space.
When concrete dries and shrinks, cracks can occur hours after pouring. Any 1/8 inch or smaller crack in your yard or driveway is considered standard. Some fine cracks may form and disappear after a month or two. These cosmetic repairs aren't that hard to do, but things can get really ugly quickly with concrete and replacing a slab requires a lot of physical effort and a lot of extra money, so my advice is to follow the correct procedure to concrete and cure it and its new entry or whatever the slab is used has no imperfections and looks good for a long time. In general, cracks wider than a credit card and going through the depth of concrete are structural in nature and could be a sign of more serious problems (see Evaluating Concrete Crack Repair).
How to Prevent Concrete CrackingWhile shrinkage cracks can appear on the surface within hours of pouring concrete, it takes a full month for new concrete to fully settle. As soon as the concrete begins to harden, or in the plastic or semi-hard stage, in this semi-hard stage, the concrete is difficult to work with, especially if it is hot. Good, strong, high-quality concrete is achieved by using the right amount of water in the mix and an experienced team that has the right amount of labor to be able to use a slightly drier, stiffer concrete mix.
After the concrete has fully cured, you may also consider using a concrete sealing compound to improve appearance and reduce cracking. But it's important for concrete contractors to follow well-established guidelines regarding concrete placement. It may be tempting to add more water to concrete to make it easier to work with, but this is a mistake and cracks could form and greatly reduce the strength of the concrete. As the concrete cures, excess water evaporates and the volume of the concrete at the inlet is reduced. Wetting the concrete and covering it with plastic or a tarp to reduce evaporation works well, but the problem is that you will have to re-wet the slab daily and could damage the concrete by peeling off and replacing the cover. However, a large majority of concrete used in residential work has too much water added to the concrete on the job site. Read here about methods to cure concrete and understand how your contractor will cure concrete.
The ideal way to cure any concrete is to build barriers around the slab and flood the surface with water and, if possible, leave the slab flooded for 7 days to ensure a really strong concrete slab.