Knowing structures is just part of being a GPR Operator.  At DCI we try to inform our clients and their subs of potential dangers when working in certain buildings such as post tension.  Many contractors and their employees either don't know how buildings are built, or haven't had any close calls to make them aware of the dangers lurking in the slab.

The following pictures are real life examples of what can happen in post tension buildings. 

EVEN IF YOU DON'T CORE DRILL, THE DANGERS OF HITTING CABLES ANY TIME YOU PENETRATE THE CONCRETE IS THERE!!

The cables were broken when penetrating the ceiling for metal framing track.  Typically, post tension will drape as low as 3/4" from the bottom of the slab.  Paint typically marks low points of the cables during construction of a slab.  Notice the orange paint mark to the upper left.  In this case, the marks were not accurate.  The cables blew a hole in the slab and left 3 broken and 3 more exposed.  This has to be fixed immediately.  However, in this case, the contractor decided to put insulation spray over it to hide it. 

The damage does not stop there.  Since this was not repaired right away, the weight of the concrete has severely crippled the supporting beam with the help of the cables causing a major crack in the slab.  This will push the repair cost through the roof and a potential lawsuit.

LESSON:  SCAN YOUR CONCRETE PRIOR TO ANY PENETRATIONS!!!