Hairline cracks are common in concrete.
This shrinkage is the major cause of cracks in concrete. Most of these cracks are small. The cracks may not extend through the floor slab, or wall. If the size, width, and length of cracks is substantial, rather than hairline, then there may be some cause for concern. Whether there will be a water problem depends not only on the crack, but also on the amount of water in the soil pressing against it, on the type of drainage that was installed when the house was built, and on the grade of the land around the house.