The four cases where scanning is mandatory
Post-tensioned slabs, always and without exception. Large-diameter cores, because the probability of hitting steel grows with area. Moment zones or locations near supports, where reinforcement works hardest. And any element with no reliable reinforcement drawing.
Outside those cases, on a small non-structural hole, scanning may be unnecessary. Inside them, skipping it is gambling.
What is detected and what is not
Electromagnetic locators find near-surface rebar well and give approximate depth. Ground-penetrating radar gives better depth and distinguishes layers, but needs interpretation by someone who can read it.
Neither sees everything. Empty plastic conduits, for example, are hard. That is why scanning is always combined with the drawings and the judgement of someone who knows the structural system.
Cutting a post-tensioning strand
A strand is stressed along the whole slab. Cutting it releases that force instantly: there is immediate risk to the operator and the slab loses part of the capacity it was designed for.
Repair is possible, but it means design, special materials and time. No site deadline justifies that risk when scanning is a matter of hours.
How the record is kept
Floor marking of the located reinforcement, a photograph of the marking referenced to gridlines, and a note of the estimated depth. That record serves today's core and the next one anybody has to drill in the same slab.
