The Strike: How to Know It Was Rebar
A rebar strike announces itself in three ways at once: a sharp metallic ring instead of the dull hammer drone, penetration that stops dead within a few millimetres, and bright steel glitter mixed into the grey drilling dust. Stop within the first second and you usually save the bit; lean harder and you usually lose the carbide.
Hard aggregate feels different. A flint or granite pebble bounces the bit and slows progress, but the note stays dull, there is no metal swarf in the dust, and the bit normally works past it within ten to fifteen seconds. If the hole stops at the same depth no matter what you do and the dust sparkles, you are sitting on steel — see also our general guide when a bit will not go in.
What a Strike Does to a Carbide Tip
A masonry tip is tungsten carbide in the HV 1300–1500 hardness class, ground for one job: crushing a brittle material. Concrete fails by fracture — the carbide hammers it to powder and never has to be tough. Reinforcing bar is the opposite material: ductile steel around 500 MPa yield that does not crack under the blow. Instead it grabs the cutting edge.
Three failure modes follow, in rising order of severity:
- Corner chipping. The hammer blow lands the brittle carbide corner on steel that will not give way, and the corner flakes off. The bit still drills, but it wanders and cuts oversize.
- Braze-line shear. On a 2-cutter bit the long chisel edge lands half on bar, half on concrete, and levers the insert sideways. The braze joint that holds the carbide plate takes the full shear load — a hairline crack here is the beginning of tip detachment.
- Heat damage. An operator who keeps pushing turns the strike point into a friction brake. Braze alloys begin to weaken from roughly 600–700 °C; a blued bit body just behind the head means the joint has been cooked.
The 60-Second Bit Inspection
After any suspected strike, back the bit out and run this check before drilling another hole:
| Check | How | Pass | Retire the bit |
|---|---|---|---|
| Carbide corners | Look at the tip head-on under a light | Both corners present, edge line continuous | A corner is missing or the edge is visibly stepped |
| Braze line | Inspect the seam where carbide meets steel body | Clean, unbroken seam | Any hairline crack — detached tips jam in the hole |
| Heat color | Check the body just behind the head | Original finish or light straw tint | Blue or purple band — the braze has been overheated |
| Runout | Roll the bit on a flat surface or spin it slowly | Tip stays centred | Visible wobble — the bit is bent |
A bit with one chipped corner but a sound braze can finish non-critical concrete holes — expect wander and slightly oversize diameters, which rules it out for anchor holes. A bit with a cracked braze line is done: if the tip lets go mid-hole, you spend longer fishing carbide out of the bore than a new bit costs.
Three Ways Forward — and One You Must Not Take
Option 1: relocate the hole. The default answer. Move at least 50 mm from the strike point, and when the hole is for an anchor, check the anchor approval's minimum spacing and edge distance before you shift — relocating a hole into a violation of the fixing's ETA spacing rules trades a tooling problem for a structural one. Our anchor drilling guide covers the spacing logic.
Option 2: dedicated rebar cutter. When the hole position is fixed — a baseplate template, a through-hole on a layout grid — the correct tool is a rotary-only rebar cutter: a crown-shaped head with carbide geometry made for ductile steel, run with hammer action OFF. Hilti, B&A Manufacturing and others build them precisely because a hammered masonry bit is the wrong physics for steel. Drill the concrete to the bar with the masonry bit, switch to the rebar cutter through the bar, then return to the masonry bit.
Option 3: thin mesh only — grind through. Light welded mesh (wire up to about 6 mm) is the one case a quality full-head 4-cutter bit can pass on its own: ease off the feed pressure, let the four edges nibble, and accept thirty slow seconds. This does not extend to structural bar.
The way you must not take: hammering through a structural bar. Reinforcement is in the concrete to carry tension. Severing a 10 mm+ bar without the engineer's sign-off is not a tooling decision — on slab soffits and beam edges it can be a structural one, and on post-tensioned slabs striking a tendon is a life-safety event. If the bar is bigger than mesh wire and the hole cannot move: stop and escalate. No drill bit is the answer to that problem.
Drilling Around Rebar in the First Place
- Know the cover. In most structures the first steel layer sits 20–50 mm below the surface, set by the exposure class. Shallow fixings that stay inside the cover zone — cable clips, conduit saddles — almost never meet steel.
- Expect a grid. Slab mesh typically runs on a 100–200 mm grid; beam stirrups follow regular centres. One strike tells you where the neighbours are: offset half the grid spacing diagonally and you land mid-bay.
- Scan when it matters. For anchor groups and core positions, a cover meter or radar scan costs minutes; a strike on hole three of a four-bolt pattern costs the layout.
- Start gently. The first 20 mm drilled with reduced feed gives you a chance to feel a shallow bar and stop before the full hammer energy lands on it.
Why Full-Carbide 4-Cutter Heads Survive Strikes
Strike tolerance is geometry plus joint design, and it is the main reason premium concrete bits moved to full-head 4-cutter designs:
- Four edges share the blow. A cross head lands the impact on four short edges instead of one long chisel edge, so no single corner takes the full energy of a steel contact.
- No lever arm. The 2-cutter failure mode — half the chisel edge on bar, the insert levered against its braze — does not exist on a centric cross head.
- More braze area, deeper seat. A monoblock carbide head sits in a machined pocket with several times the brazed surface of a flat 2-cutter insert plate, so the same shear load meets far more joint.
This is exactly the architecture of our full-carbide-head SDS-Plus line: one-piece cross heads joined to the steel body by resistance friction welding (RFW), with the head-to-body joint verified against the approved cross-section and drilling plan. It is a separate construction from our controlled-atmosphere mesh-belt-brazed insert lines. For the selection side (which bit class for which reinforced job), see best drill bits for reinforced concrete; distributors sourcing a rebar-tolerant program direct from the factory should start at the SDS OEM & wholesale page.
