Core drilling and penetrations in existing slabs

Core drilling and penetrations in existing slabs

Every fitout, every services upgrade and every new bathroom in an existing building needs holes through concrete. Most of them are drilled without anyone checking what is inside the slab.

Usually nothing happens. When something does happen, it is expensive, and occasionally it is dangerous.

What is inside the slab

Reinforcement, first. Cutting a main bar in a region where it is doing the work reduces the capacity of the slab at that point, and slabs are usually reinforced most heavily exactly where a services riser wants to go.

Post-tensioning tendons, second, and this is the serious one. A post-tensioned slab has high tensile steel stressed along it and anchored at the edges. Cutting a tendon releases that force suddenly. The immediate hazard is real, and the structural consequence is that the slab loses the prestress across that whole strip, not just at the hole.

Conduits and services, third. Electrical conduits cast into a slab are common in older buildings and they are not on any drawing.

Scanning is the whole answer

Before any core, the location is scanned. Ground penetrating radar or an electromagnetic cover meter maps the reinforcement, the tendons and any conduits close to the surface, and the hole is set out to miss them.

It takes an hour. It costs a small fraction of what a cut tendon costs, and it produces a record that the location was cleared.

The mistake is scanning after choosing the location rather than before. Scan the zone, then place the hole where the slab allows it, and the services design follows the structure rather than fighting it.

Size and grouping matter more than a single hole

One small penetration in a slab is rarely a structural problem on its own.

Several close together is a different case, because collectively they behave like a larger opening. A row of holes through a beam, or a cluster in a slab near a column, can remove enough section to matter even when no single hole would.

Location matters as much as size. A penetration near a column, where shear forces are highest, is far more significant than the same hole at mid-span. A hole through the compression zone of a beam is different to one through the middle.

When it needs designing rather than clearing

A hole large enough to interrupt the reinforcement, or one in a high-shear zone, needs a designed solution. That usually means trimming the opening with additional reinforcement, a steel frame around it, or in some cases carbon fibre strengthening applied to restore what the opening removed.

That is ordinary work. It just has to be designed before the hole is cut rather than after.

Post-tensioned slabs need a different process

If the building is post-tensioned, and a lot of Sydney commercial and residential towers built since the 1980s are, no hole gets drilled without scanning and without an engineer nominating the location.

The original tendon layout drawings are worth finding, because they tell you where the bands run. Scanning still happens, because as-built and as-drawn diverge, but starting from the drawing makes the scan quicker and more reliable.

What to have in place

A scan report for each location. An engineer's sign off for anything above a nominated size or in a sensitive zone. And a record of what was drilled and where, because the next trade will want to drill in the same area.

We assess and design penetrations as part of structural engineering on fitouts and services upgrades, and the assessment is usually short. The repair, when someone skips it, is not.