Post-Tension Slab Penetrations: Scan Scope and Hold Points

Post-tensioned (PT) concrete slabs are among the highest-risk substrates for unplanned penetrations. Unlike conventionally reinforced slabs, PT systems carry permanent tensile loads within the tendons - typically monostrand or multistrand cables stressed to 70 to 80% of their ultimate tensile strength. Severing a tendon during coring or saw-cutting releases that stored energy instantaneously, with consequences ranging from localised slab delamination to catastrophic structural failure. In Southeast Queensland, where PT flat-plate construction has been the dominant commercial and residential high-rise slab system since the 1980s, this risk is present across a substantial proportion of the existing building stock.

The challenge for project engineers and contractors is that as-built tendon layouts are frequently unavailable, inaccurate, or reflect design intent rather than actual installed positions. Construction tolerances, last-minute design changes, and decades of missing documentation mean that even when drawings exist, field verification is mandatory before any penetration is made. GPR scanning for post-tension mapping is the primary tool for this work, but the scope, hold points, and handoff process between scanning technician and structural engineer must be clearly defined before mobilisation.

Understanding how to structure a scanning programme for PT slabs - and where the boundaries of NDT interpretation sit - is essential for anyone managing penetrations in Southeast Queensland's commercial, residential, or mixed-use building stock.

Why PT Slab Penetrations Require a Dedicated Scanning Scope

Standard GPR scanning scopes written for rebar detection are not appropriate for PT tendon mapping. The two tasks differ in target geometry, scan line density, and the level of engineering interpretation required. PT tendons in flat-plate slabs typically run in two orthogonal banding patterns, with high-stress zones concentrated at column heads. Tendons may also be draped - varying in depth across the span - which means a single scan line may show the tendon at different depths at midspan versus support zones.

A PT-specific scanning scope must define the full penetration zone, not just the immediate core location. Tendons can deviate laterally from their design position by 50 to 100 mm or more in older construction, and the scan area needs to account for this. At SiteOps, our standard PT scanning scope for a single penetration includes a minimum 600 mm radius around the proposed core location, with orthogonal scan lines at 50 mm centres across the full zone. This density is necessary to resolve tendon spacing accurately and identify any anomalous deviations from the expected banding pattern.

For projects involving multiple penetrations - mechanical risers, hydraulic penetrations, structural openings - a systematic grid scan of the affected floor plate is more efficient and provides a spatial dataset that can be referenced across the construction programme. See our GPR scanning services for scope options applicable to PT slab investigations.

GPR for Post-Tension Tendon Mapping

Ground-penetrating radar is the primary NDT method for PT tendon detection. High-frequency antennas (typically 1.6 GHz or 2.0 GHz) provide the resolution needed to distinguish individual tendons in slabs with 150 to 250 mm cover depths. The GPR signal reflects off the metallic tendon sheath or bare strand, producing a characteristic hyperbolic reflection in the radargram that can be located in plan and depth.

The method works reliably in slabs with low reinforcement congestion above the tendon layer. In slabs with dense top mesh or multiple reinforcement layers, signal attenuation and reflection interference can reduce confidence in tendon identification. Wet concrete, carbonated concrete with high moisture content, and slabs with embedded metallic services all present additional interpretation challenges. These limitations are not reasons to avoid GPR - they are reasons to document confidence levels explicitly in the scan report and flag zones requiring additional verification.

Ferroscan (magnetic flux-based detection) is sometimes used as a complementary method for near-surface rebar mapping, but it does not reliably detect PT tendons at depth in the same way GPR does. The two technologies serve different functions in a PT investigation. Learn more about ground-penetrating radar and Ferroscan and how they are applied in combination on complex slab investigations.

As-Built Tendon Layout: Drawing Verification and Field Reconciliation

Where original structural drawings are available, the scan programme should include a drawing reconciliation step. The design tendon layout is overlaid on the GPR scan data to identify agreement, deviation, or missing tendons. This process has practical value: it confirms whether the as-built condition matches the design, and it identifies zones where field deviations require engineering review before penetration proceeds.

In a 2019 investigation of a 12-storey residential tower in inner Brisbane, GPR mapping of a PT flat-plate slab revealed tendon banding deviating up to 120 mm from the structural drawings in two bays adjacent to a post-construction mechanical penetration. The deviation was consistent with tendons having been repositioned during construction to accommodate a formwork change. The structural engineer used the scan data to reposition three proposed hydraulic penetrations, avoiding what would have been direct tendon strikes on two of the original core locations.

Where drawings are unavailable - common in buildings constructed before the mid-1990s - the GPR dataset becomes the primary spatial reference. In these cases, the scan report must clearly state that no drawing verification was possible and that the tendon layout is based solely on field data interpretation.

Defining Hold Points in the Penetration Programme

Hold points are mandatory stops in the construction sequence where work cannot proceed without sign-off from a nominated authority - typically the structural engineer of record or a specialist structural consultant. For PT slab penetrations, hold points should be defined at three stages.

• Pre-scan hold point:: No coring, saw-cutting, or drilling commences until the GPR scan is complete and the scan report has been issued.
• Pre-penetration hold point:: The structural engineer reviews the scan data and confirms the proposed penetration location is clear of tendons, or issues a revised location. This sign-off must be documented in writing.
• Post-penetration hold point:: Where the penetration passes through or near a zone of reduced confidence in the scan data, a visual inspection of the core sample and hole walls is required before the penetration is used or made permanent.

These hold points should be written into the project's Inspection and Test Plan (ITP) before work commences. Verbal approvals are not sufficient. The consequences of a tendon strike - structural, legal, and financial - make written documentation of each hold point non-negotiable.

Scan Report Requirements for Structural Engineer Handoff

The GPR scan report is the primary document transferred to the structural engineer for engineering review. A report that is adequate for rebar detection is not adequate for PT tendon mapping. The PT scan report must include the following as a minimum.

• Scaled plan drawings: showing detected tendon positions, scan line coverage, and the proposed penetration location marked clearly.
• Depth data: for each detected tendon at the penetration zone, reported in millimetres from the slab soffit or top surface with the datum clearly stated.
• Confidence ratings: for each zone, distinguishing between areas of high signal clarity and areas where interference or attenuation reduced detection confidence.
• Anomaly notation: for any detected features inconsistent with the expected tendon layout, including potential voids, delaminations, or embedded services.
• Scan parameters: including antenna frequency, scan line spacing, time window, and equipment used.
• Technician qualifications: in Australia, GPR operators working on structural investigations should hold or be working towards PCN or ASNT certification in the relevant method.

The structural engineer receiving this report is making an engineering judgement about penetration safety based on the scan data. Ambiguous or incomplete reports shift risk back to the contractor and scanning provider. Clear, complete documentation is a professional obligation.

Limitations and When Engineering Review is Mandatory

GPR cannot confirm tendon stress condition, detect broken or de-bonded tendons, or assess the structural capacity of the slab in the penetration zone. These are engineering questions that require analysis beyond what NDT can provide. Any penetration that falls within 150 mm of a detected tendon, or within a zone flagged as low-confidence in the scan report, requires structural engineering review before proceeding - regardless of programme pressure.

Slabs with post-installed anchors, previous penetrations, or evidence of prior repair work require additional scrutiny. Existing penetrations alter the local tendon layout and may have introduced stress redistribution that is not visible in the GPR data. In these cases, the scanning scope should be expanded and the structural engineer engaged earlier in the process.

For penetrations in column head zones - where tendon banding is densest and structural demand is highest - engineering review is mandatory regardless of scan confidence level. These zones carry the highest consequence for tendon damage and require explicit structural sign-off on penetration feasibility, not just location clearance.

Contractor and Site Manager Responsibilities

Site managers and contractors bear direct responsibility for ensuring that the scanning programme is completed before any penetration work commences. This includes confirming that the scan scope matches the penetration programme, that hold points are documented in the ITP, and that the scan report has been formally reviewed by the structural engineer before work proceeds.

Common failures in this process include scanning only the immediate core location rather than the full penetration zone, proceeding on verbal clearance rather than written sign-off, and using scan reports from previous investigations on the same building without confirming that the scope covered the current penetration locations. Each of these shortcuts has resulted in tendon strikes on Southeast Queensland projects.

Engaging the scanning provider and structural engineer together at the scope definition stage - before mobilisation - eliminates most of these failures. The scanning scope, hold points, and handoff process should be agreed in writing before any site work begins.

Conclusion

Post-tension slab penetrations in Southeast Queensland require a structured, documented approach that begins with a PT-specific GPR scanning scope and ends with written structural engineering sign-off at each hold point. The GPR data is a critical input to the engineering review process, not a substitute for it. Scan reports must be complete, clearly documented, and explicitly rated for confidence across the penetration zone before handoff to the structural engineer.

The combination of accurate field scanning, clear reporting, and defined hold points is what separates a managed penetration programme from an uncontrolled risk. In PT slab construction, the cost of getting this process right is negligible compared to the structural and financial consequences of a tendon strike.