What happens if you drill through rebar?

Drilling through reinforcement bars in concrete is one of the most common avoidable mistakes on a UK construction site. The consequences range from minor to severe depending on the structure, the bar type, and where the strike happens.

Conventional reinforcement

For ordinary rebar (mild steel reinforcement bars in conventional reinforced concrete):

• The bar is locally damaged at the strike.
• The drill bit may bind or break.
• The structural capacity at the cut location is reduced.
• The bar is no longer continuous, which affects load distribution in the surrounding concrete.

Whether this matters structurally depends on:

• The role of the specific bar — main reinforcement, distribution bar, link, etc.
• The number of bars cut.
• The location of the cut along the bar’s length.
• The structural redundancy at the affected section.

A single distribution bar nick on a heavily reinforced slab is usually inconsequential. A main flexural bar cut at midspan of a critical beam is a different conversation.

Post-tension tendons

For post-tension tendons, the consequences are much more serious:

• The tendon is highly stressed (loads of several hundred kilonewtons).
• A damaged tendon can shed its load suddenly.
• An unbonded tendon may fly out of the duct at speed when severed.
• The slab redistributes load onto remaining tendons and rebar.
• In the worst cases, partial collapse of slab sections.
• Costly remedial works including replacement-tendon installation, structural reanalysis, or local rebuilding.

A struck PT tendon is the most expensive avoidable mistake any drilling team can make on a slab. Pre-drill scanning is the standard mitigation.

What to do if you have struck rebar

If you suspect or know that drilling has hit reinforcement:

1. Stop drilling immediately. Do not try to “push through”.
2. Withdraw the bit carefully. A cleanly withdrawn bit minimises further damage.
3. Make the area safe. Particularly if the strike was on a service or PT element.
4. Photograph the location.
5. Notify the structural engineer. They assess the consequences and specify any remedial action.
6. Document the incident. It forms part of the project record.
7. Investigate why the strike happened. Was a pre-drill scan done? Was it followed? What needs to change?

What to do if you have struck a PT tendon

If a post-tension tendon has been damaged:

1. Stop work and isolate the area.
2. Clear personnel from the immediate vicinity.
3. Notify the engineer urgently.
4. Preserve evidence.
5. Follow the structural engineer’s specified remedial procedure.

A struck PT incident is a structural event, not a routine repair.

How to avoid it

The reliable way to avoid striking rebar is pre-drill GPR scanning:

• Mark the proposed drilling positions on the slab.
• Have a qualified surveyor scan the marked positions and a buffer.
• Reposition any drilling target that conflicts with a reflector.
• Sign off the cleared positions.
• Drill only at signed-off positions, within tolerance.

Pre-drill scanning is one of the cheapest forms of insurance available on a UK construction site. The arithmetic in favour of doing it is overwhelming.

For more, see GPR scanning before drilling — why it matters and Post-tension cables — the hidden danger.