There are many reasons for initial cracks in concrete. The major reason is shrinkage. If the cracks are spread on a wider area, hairline widths and haphazard in direction then shrinkage is the reason. Structural cracks do not occur because there is still shuttering beneath the slab. The other reason is non-compatibility of admixture with cement.
These cracks can be treated cosmetically. Usually, the concrete supplier is made responsible for these cracks and they treat the cracks at their cost
PLASTIC SHRINKAGE CRACKING
As the concrete’s heavier particles settle due to gravity, they push the water and lighter particles toward the surface. This is called bleeding. If you fail to monitor the temperature, wind, and humidity conditions properly the evaporation rate of the surface water may exceed the bleed rate, drying out the concrete’s superficial layer and therefore shrinking it due to dehydration. The concrete beneath the surface layer is still well hydrated, however, and maintains its volume. This applies to oppose tensile forces to the lower part of the drying concrete on the surface, causing a cracked concrete profile.
These plastic shrinkage cracks are usually shallow and only from 1 to 2 mm in width, which means you cannot repair them with the injection method. They may, however, self-heal through continual cement hydration or by the precipitation of calcium carbonate from the concrete.
If the cracks are wider than 2 mm and do not self-heal, it is important that you repair them with a suitable coating or flood-grouting product to stop them from penetrating the full depth of the concrete slab. If they do become active their
reaction to stresses may result in further cracking that weakens the structure either directly or by exposing its reinforcement steel to contaminants that will in time corrode it.
These are causes of initial cracks in concrete, such as plastic shrinkage cracks.