severe crack in a concrete slab – notice the large (around an inch) vertical displacement
Concrete cracks & separations
Every material has its good and bad characteristics. Concrete cracks, steel rusts, and wood rots and is food for termites and other creatures. Concrete also will separate as it cures under some conditions.
The term “cracked slab” is an unfortunate one. Plus, it is misleading. Most people view a crack or fracture as a clear and unambiguous sign of failure. The truth is that cracks in concrete are normal and expected. They are rarely a sign of structural failure.
I like to compare concrete cracks to grain in wood framing material. No one seems to be bothered by studs that have grain. On the other hand, no one I know likes cracks in concrete. Grain is an imperfection. It makes wood weaker compared to clear material. Many people like the way grain looks. One of the advantages of staining wood for cosmetic reasons is that the stain makes the grain more visible as shown in the photo below.
Most of the time concrete cracks will be visible only in an attached garage and in the exposed portion of the grade beam. The visible garage cracks can easily be checked for any significant vertical displacement. Just take your index finger and move it across the crack. If there is no or minimal vertical displacement, that is a strong indication that they crack is not due to an overload situation or that the aggregate interlock has been breached. (The term “aggregate interlock” refers to the aggregate rocks overlaying each other so that the concrete can carry a shear load.
Location & orientation
Based on many studies on how slab-on-ground foundations react to vertical loads from the house sitting on top of the foundation and from heaving soils pushing upward against the foundation, we know roughly where bending moments in the slab will be the greatest. This is important because structural cracks only occur where the bending moment is high.