Every building constructed using concrete in any shape or form has cracks. It is the size, frequency, and reason for their existence that matters, not that they happen to occur.
EXPECT SOME CRACKING
Cracks occurring in unexpected places should be thoroughly investigated, but not all cracks are structurally significant. In fact, the vast majority are caused by shrinkage or other tension stresses that develop as the structure supports itself and have no influence on strength or durability.
CAUSES OF CRACKING
Assuming that the concrete itself is not at fault, cracks in concrete structures develop from several causes: shrinkage cracking; plastic cracking; tension cracking; or rust cracking.
WHAT CRACKS ARE HARMFUL?
Cracks can be induced by shock loading or the removal of forms or shoring before the concrete has achieved adequate strength. Whatever the cause, the main thing is to recognize whether cracking is of a size and type that is harmful and act accordingly. Harmful cracking develops as a result of fundamental defects in the structure. The main causes are: chemical reaction resulting in carbonation of the concrete; structural overstressing caused by inadequate reinforcement, excessive deflections in slender members, or severe overloading; and lack of adequate cover. Remember, cracks don't cause structural deterioration, structural deterioration causes cracks.