Plastic shrinkage cracking
Short, random, often parallel cracks that open in a slab surface within hours of placement, caused by the surface drying faster than bleed water can replace it.
What does plastic shrinkage cracking mean?
These cracks form while the concrete is still plastic and has almost no tensile strength, so they can appear while the crew is still finishing. The driver is evaporation rate, meaning wind and low humidity matter as much as temperature. They are usually shallow and cosmetic, but they open a path for water and can widen with drying shrinkage later.
In practice
A slab placed on a dry windy day that starts showing scattered short cracks before anyone has swept it is showing this failure.