Cracks at the slab bottom Cracks at the slab bottom concreteworld (Civil/Environmental) (OP) 28 Mar 17 10:09 Hello Everyone, Please see the attached photo of slab soffit where random cracks have developed. Also there are plastic shrinkage cracks at the slab surface. Kindly explain reasons http://files.engineering.com/getfile.aspx?folder=0ea2551d-3047-4446-a61a-66 RE: Cracks at the slab bottom dik (Structural) 28 Mar 17 14:03 Can you provide a sketch showing supports and the locations of the cracks on the soffit? Dik RE: Cracks at the slab bottom hokie66 (Structural) 28 Mar 17 23:32 You can't definitely say from the photos, but plastic shrinkage cracks at the top often go all the way through. Some could also be drying shrinkage cracks. Why did the plastic shrinkage cracks occurs? Inadequate/no compaction of the concrete, I assume.