Crazing in concrete is the development of a network of fine random cracks or fissures on the surface of concrete caused by shrinkage of the surface layer. These cracks are rarely more than 3mm deep, and are more noticeable on over floated or steel-troweled surfaces.  Image on the left illustrates finished surface showing shrinkage cracks and trowel chatter (lines at the bottom of the image).  Image on the right is a photomicrograph showing detail of finished surface. Green arrows indicate groove; yellow arrows highlight crazing cracks.