Post-Tensioned Slabs: When Drawings Are Not Enough for Penetration Approval

Post-tensioned concrete carries stored energy. A single tendon under full lock-off stress holds somewhere between 100 kN and 200 kN of force depending on strand diameter and system design. Sever that tendon during coring or drilling and the released energy is not theoretical; it is immediate, violent, and potentially fatal. The slab also loses a load path it was designed to rely on.

The question contractors and facility managers often ask is straightforward: can we use the structural drawings to locate tendons before we drill? The answer is that drawings are a starting point, not a clearance document. Understanding why requires a look at how post-tensioned slabs are actually built and how tendons behave in three dimensions.

What the Drawings Show and What They Do Not

Structural drawings for a post-tensioned slab typically show tendon layout in plan, banded and distributed tendon zones, general spacing, and anchorage locations at slab edges. That information is useful for understanding the structural system. It is not sufficient for locating individual tendons before penetration.

Several factors explain the gap between drawing intent and field reality.

Construction tolerances. Tendons are placed by hand in a live pour environment. AS 3600 permits positional tolerances for reinforcement and tendons that, in practice, can shift a tendon 25 mm to 50 mm from its drawn centreline. Over a span of several metres, that drift accumulates. A tendon shown at 300 mm centres on a drawing may be sitting at 260 mm or 340 mm in the actual slab.

Vertical profile. Post-tensioned tendons are not flat. They follow a parabolic or harped profile through the slab depth, rising toward supports and draping toward midspan. Drawings show this in section, but the section cuts are typically at grid lines. Between grid lines, the tendon's vertical position at your proposed penetration point is an interpolation, not a confirmed measurement.

As-built deviations. Tendons may have been rerouted around penetrations, plumbing, or formwork obstructions during construction. These field changes are not always captured in as-built documentation, particularly in older buildings or projects where documentation discipline was inconsistent.

Drawing revision history. Multi-stage projects, tenant fit-outs, and building modifications can result in multiple drawing revisions. Confirming that the drawing set in hand reflects the actual constructed condition is not always straightforward.

For buildings constructed before the mid-1980s, original structural drawings may not exist at all, or may be held by a firm that no longer operates. In those cases, the drawing question is moot from the outset.

The Role of GPR in Tendon Location

Ground penetrating radar is the standard first-line method for locating post-tensioning tendons in concrete slabs before penetration. The technique works by transmitting a radar pulse into the concrete and measuring the time for reflections to return from embedded objects. Tendons, ducts, reinforcing bars, and voids all produce reflections at characteristic depths.

For post-tensioned slabs, GPR is particularly effective because the tendon system presents a strong reflective target. Bonded systems use grouted metal ducts that produce a clear hyperbolic reflection signature. Unbonded systems, which are common in Australian commercial construction, use individual strands in a greased plastic sheath; these are smaller targets but still detectable with appropriate antenna frequency and scan density.

A properly executed GPR scan over a proposed penetration zone will typically show:

• Tendon positions in plan at the scan surface
• Approximate depth to the tendon or duct
• Spacing confirmation against the drawing set
• Anomalies such as unexpected tendon positions, congested zones, or areas where reflection patterns suggest the layout does not match drawings

The output is a marked-up scan grid or CAD overlay showing tendon positions relative to the proposed penetration. Penetrations can then be positioned in clear zones between tendons, with a safety margin applied.

Scan resolution matters. A single line scan across a proposed core location is not adequate. The scan should cover a grid around the penetration zone, with line spacing tight enough to confirm tendon positions on both axes. For most slab penetrations, a 100 mm to 150 mm scan grid over a 1 m to 2 m area around the proposed location is a reasonable minimum.

When GPR Alone Is Not Sufficient

GPR is a powerful tool but it has physical limits. Concrete with high chloride content, carbonation, or moisture saturation attenuates the radar signal and reduces detection depth and resolution. Heavily reinforced slabs produce multiple overlapping reflections that can obscure tendon positions. Slabs with multiple layers of reinforcement above the tendon profile can make depth interpretation less certain.

In these conditions, GPR results should be treated as indicative rather than definitive. Additional verification methods or tighter safety margins are warranted.

There are also situations where the nature of the penetration itself requires consultant review regardless of scan quality:

Large diameter penetrations. A core over 100 mm diameter in a post-tensioned slab warrants structural engineer review before proceeding. The clear zone between tendons may be adequate for a 50 mm service penetration but not for a 150 mm mechanical sleeve.

Penetrations near supports or anchorage zones. Tendons converge toward column heads and slab edges. The tendon density in these zones is higher, spacing is tighter, and the structural consequences of a strike are more severe. Scan data in these areas should be reviewed by the structural engineer of record or a suitably qualified engineer before any penetration is approved.

Penetrations in transfer slabs or post-tensioned beams. Transfer elements carry concentrated loads and the tendon forces are proportionally higher. These are not standard slab penetration decisions.

Any situation where scan data is ambiguous. If the GPR operator cannot confirm clear tendon positions with confidence, that uncertainty should be escalated, not resolved by proceeding anyway.

The structural engineer's role in these reviews is to assess whether the proposed penetration location is structurally acceptable given the tendon layout, the slab geometry, and the load case. That assessment cannot be made by the scanning contractor alone. It requires engineering judgement applied to the specific structural system.

Practical Workflow for Penetration Approval

A defensible penetration approval process for a post-tensioned slab typically follows this sequence.

First, obtain and review all available structural drawings. Confirm the drawing revision, check for as-built annotations, and identify the tendon layout system (bonded or unbonded, banded or distributed). Note any areas where documentation is incomplete or uncertain.

Second, engage a qualified NDT contractor to conduct GPR scanning over the proposed penetration zones. The scan scope should be agreed in advance and should include sufficient coverage to confirm tendon positions on both axes. The scan report should include depth estimates, a plan showing tendon positions, and a clear statement of the operator's confidence level.

Third, compare scan results against drawings. Where results align with drawings, confidence in the tendon positions is higher. Where results deviate, the deviation should be documented and the cause investigated before proceeding.

Fourth, for penetrations that fall within the parameters requiring engineer review, submit the scan data and proposed penetration location to the structural engineer for assessment. Do not proceed until written approval is received.

Fifth, apply a minimum clearance margin to the approved penetration location. A 75 mm clearance from the nearest tendon edge is a commonly referenced minimum, though the structural engineer may specify a different margin based on the specific system.

Finally, mark the approved location on the slab surface and confirm with the drilling or coring crew before work begins. The mark should be checked against the scan overlay, not just the drawing.

Documentation and Hold Points

Post-tensioned slab penetrations should be treated as hold points in the construction programme. The penetration should not proceed until scan results are reviewed, any required engineer approval is received, and the approved location is confirmed on the slab surface.

This documentation trail matters beyond the immediate safety question. If a tendon is struck and a dispute arises, the record of the approval process determines whether the contractor followed a reasonable standard of care. A verbal clearance or a quick look at a drawing is not a defensible position when the consequences of a strike are as severe as they are in post-tensioned construction.

SiteOps conducts GPR scanning for post-tensioned slab penetrations across commercial, industrial, and residential projects. Scan reports include tendon position overlays, depth estimates, and a clear statement of findings to support engineer review and penetration approval. For projects where the structural documentation is incomplete or the penetration scope is complex, we can coordinate with the structural engineer of record to ensure the approval process is complete before work begins.

For more information on GPR scanning for post-tensioned slabs, visit siteops.au.