Concrete, Rebar & Hard Aggregate: Detect, Avoid or Cut Through
Concrete is not a uniform material. Inside it is hard aggregate, harder bar — and the approvals assume you missed both.
Why the approvals assume you scanned
Post-installed anchor approvals qualify performance in concrete without reinforcement at the location of the hole. A bar struck, cut or displaced by drilling is not covered by that qualification.
Structural drawings show intended bar positions; on-site bar positions vary. Cover meters and GPR give the as-built reality. For critical fastenings — baseplates, safety barriers, seismic restraints — scanning is a design expectation, not a courtesy.
Scanning also protects the tooling: a carbide bit striking a bar at full hammer energy can bind, snap the carbide tip, or transmit a shock that breaks the seal. Avoiding the bar is cheaper than cutting through it.
How to scan proportionately
- For a single MEP hanger point, a hand-held cover meter sweep to locate and mark nearby bars may suffice. For a pattern of structural anchors, step to a GPR grid and mark exclusion zones.
- Calibrate expectations: a cover meter locates bar centre and depth approximately; GPR adds depth resolution and distinguishes bar from mesh and post-tensioning ducts (which must not be struck).
- Where uncertainty remains, proof by light percussion at low feed to detect steel contact — short of full drilling — can confirm a location before committing the production hole.
Drilling near and through steel
If a bar is detected exactly where the hole must be, stop and escalate. Moving the hole within tolerance, re-designing the plate, or selecting an anchor qualified for the new position are structural decisions, not site improvisations.
A carbide rotary-hammer bit can, in many cases, penetrate light reinforcement, but the approval no longer applies and the bar's contribution to the element is compromised. Diamond coring can also cut bar, but slowly and at high crown wear.
Hard aggregate presents a similar decision. A four-cutter cross-head bit resists jamming on flint or granite aggregate better than a two-cutter; sharpness matters more than pressure. Do not lean on the machine — feed at the column's natural rate.