How Does GPR Concrete Scanning Work Before You Drill or Cut?

GPR concrete scanning sends radar pulses into concrete and maps the reflections from embedded rebar, post-tension cables, conduits, and voids — locating hidden elements so drilling, coring, and cutting avoid severing structural steel or releasing tensioned cables. Every renovation, tenant improvement, and MEP retrofit eventually needs to penetrate existing concrete. The drawings may show what was supposed to be in the slab, but as-builts are unreliable and older buildings may have no drawings at all. The scan replaces guessing with a map.

The technician reviews the structural drawings first — especially the post-tension tendon layouts — then lays out a scan grid over the work area and runs the antenna in perpendicular passes. The software processes the reflections into plan-view maps showing linear targets (rebar, tendons, conduits) with estimated depths, and the technician marks the findings directly on the slab with paint or chalk: rebar locations, tendon paths, and clear zones for drilling. The deliverable is a marked-up slab and a report with the scan images, the interpreted findings, and the limitations. The structural engineer then reviews the proposed penetrations against the scan: small cores through rebar-free zones are routine, while anything near a tendon gets relocated or gets a detailed engineering review.

The disasters all share a pattern: someone drilled without scanning, or scanned and ignored the results. Post-tensioned slabs are unforgiving — a severed tendon can whip, spall concrete explosively, and remove structural capacity that cannot be easily restored. My rules for any concrete penetration work.

• Scan every post-tensioned slab before any drilling, coring, or cutting — no exceptions, no matter how small the hole
• Review tendon layout drawings alongside the scan — the two together beat either one alone
• Mark findings directly on the slab and keep the scan report with the penetration plan
• Relocate penetrations away from tendons rather than engineering around them whenever possible
• Have the structural engineer review any penetration that cannot avoid embedded structural elements