Concrete slabs and floors can develop cracks for a variety of reasons. One of the most common reasons is a shrinkage crack. Water is one of the key components of concrete. As the concrete cures, the water is eliminated. This reduces the volume of the concrete, it shrinks in size. The shrinkage causes stress to develop in the concrete and this stress must relieve itself. So the concrete cracks. This is a normal condition, especially in residential basement floors.
Another reason that concrete cracks is from the substrate under the concrete heaving. When this happens, the concrete is broken and it cracks. Usually, the concrete slab on one side of the crack is elevated a little compared to the other side.
Finally, it is possible for a settlement crack to occur. This is the opposite of a heaving crack. The soil under a concrete slab or concrete floor is not properly prepared prior to the concrete placement. Eventually, the area under the slab settles and is no longer in contact and supporting the concrete slab. The concrete slab then gives way and cracks.
