CONCRETE SLABS:

All concrete consist of steel reinforcement and synthetic fiber

Concrete slabs can be poured over existing underground burials.  This prevents the surface of the grave from caving in.  
Concrete slabs are also placed in order to create foundations for surface concrete vaults that will be entirely above ground.  
Concrete Slabs are also poured in place for tops and bottoms of brick tombs and other grave structures. 
More concrete covering a cemetery plot also means less grass maintenance.