Fine cracks in concrete, thinner than a credit card thickness, are considered hairline cracks. here are a number of reasons concrete structures could form hairline cracks.
Common causes of hairline cracks in concrete structures and foundations
Shrinkage
Concrete shrinks in volume as it dries. During this process of contraction, hairline cracks may develop.
Thermal Movement
Concrete expands and contracts during temperature changes. This process can cause hairline cracks to form in the structure.
Settlement Foundation settlement can cause stress in a structure and lead to cracking. Voids varying in size from hairline cracks to more severe fractures and fissures can be observed.