Post-tension scanning is the most common reason we are called to a UAE site. Most high-rise floor slabs and podium decks built here in recent decades are post-tensioned, and coring, anchoring or saw cutting through one without knowing where the tendons run is the single most expensive mistake on a fit-out. This guide explains what a tendon looks like to ground penetrating radar, how we tell it apart from ordinary reinforcement, and where the method has limits.
What is actually in a post-tensioned slab
A post-tensioned slab contains steel strands, usually several to a tendon, running through ducts or plastic sheathing. After the concrete has gained strength the strands are stressed with a jack and locked off at anchorages at the slab edge. That stored force is what lets the slab span further and sit thinner than a conventionally reinforced one. The slab still contains ordinary reinforcement as well, typically top and bottom mesh and extra bars over the columns.
Two layout facts matter for scanning. First, tendons are rarely spread evenly. A common arrangement bunches them into tight bands along the column lines in one direction and spreads them more widely in the other. Second, tendons are draped: they sit high over the supports and low near mid-span, so their depth changes along their length. A tendon found 60 mm below the surface near a column may be much deeper halfway across the bay.
How GPR sees a tendon
The Proceq GP8000 sends short radio pulses into the concrete and records reflections from anything with different electrical properties. Steel reflects strongly. As the antenna crosses a tendon at right angles, the reflection traces a characteristic arch on the screen, and the top of the arch marks the position and approximate depth. Moving the antenna along a grid in both directions shows which way each object runs.
Because results appear live, the operator can mark each tendon on the surface as it is found, then join the marks up across the grid into runs. On a typical suspended slab the tendons and bars sit within the first 300 mm, well inside the GP8000's range in concrete.
Telling tendons from rebar
On the screen, a single bar and a tendon can look similar. They are told apart by pattern and context rather than by one reflection:
- Spacing. Mesh bars repeat at a regular pitch. Tendons appear in groups, with bands close together near column lines.
- Depth change. Mesh stays at roughly constant depth. A tendon's depth changes steadily as it drapes between supports.
- Direction and continuity. Tendons run long and continuous between anchorages; extra bars over columns stop short.
- Position relative to the structure. Bands follow column lines; knowing where the columns are helps interpretation.
- Drawings. Even an inaccurate PT layout tells us roughly what to expect and where.
Where the pattern is clear, we mark tendons as tendons. Where it is not, for example where a tendon band crosses heavy top reinforcement over a column, we mark the object as an obstruction to avoid and say in the report that it could not be classified with confidence. For coring purposes the practical answer is the same: do not cut there.
Treat every slab in a UAE tower or podium as post-tensioned until a scan shows otherwise. You cannot tell from the floor finish, and anchor pockets at the slab edge are often hidden by cladding or finishes.
Where post-tension detection gets difficult
Radar is very good at this job, but it has limits and it is better to know them before you rely on a scan.
- Congestion over columns. Dense top bars can hide a tendon directly beneath them, because the upper layer dominates the reflection.
- Objects stacked vertically. Two items directly above one another are hard to separate.
- Metal decking or metal-backed finishes. Radar does not pass through metal sheet, so a composite slab on steel deck cannot be scanned from the deck side.
- Thick toppings and screeds. They push the tendons deeper from the scanning surface and reduce clarity.
- Young or wet concrete. Moisture slows and weakens the signal.
When one of these affects your slab, we say so on site and in writing. A report that reads more confident than the data is worse than no report. For a fuller explanation of what affects results, see how accurate is GPR concrete scanning.
Scan every penetration, not a sample
Because tendons bunch, drape and wander, a scan of one position tells you very little about the next one. On a post-tensioned slab we scan the zone around every planned penetration. The extra cost of scanning the next position once the operator is already on the floor is small, and it is far less than the cost of the one position that was skipped.
When a tendon is in the way
If a tendon sits where you planned to cut, we stop and tell you. Usually the core or anchor can move a short distance into clear concrete, and we mark the nearest clear position on the slab. Remember that clearance depends on the core diameter: a gap between two tendons may fit a 50 mm core but not a 150 mm one.
Where the position cannot move, the decision goes to the structural engineer. They may accept a revised position, specify a different fixing, or in rare cases design for the tendon to be de-tensioned and dealt with properly. That is their call, and the scan gives them what they need to make it.
Anchors and saw cuts, not only cores
Post-tension scanning is not only for cores. Anchors for ceiling hangers, fixings for machinery, saw cuts for new openings and chases for conduits can all reach a tendon, especially where it drapes high near a support. Shallow fixings are not automatically safe: near the columns, tendons can be close to the top surface.
Getting a post-tension scan
We carry out GPR concrete scanning across all seven emirates with the Proceq GP8000, and in Dubai most jobs are attended the same day or next morning; see concrete scanning in Dubai. Marks go on the slab during the visit and the PDF report follows within 24 hours. If you are preparing for coring, what to prepare before a scan covers the practical side.