Tenant Fit-Out in a PT Deck: What to Scan Before Anchors and Cores
# Post-Tensioned Deck Fit-Out: What to Commission Before Anchors and Large Cores
Post-tensioned concrete slabs are common in Australian commercial construction, particularly in multi-storey office buildings, retail centres, and mixed-use developments built from the 1980s onward. The structural efficiency of PT slabs comes at a cost to fit-out flexibility: unbonded monostrand tendons run through the slab in patterns that are not visible from the surface and are not always documented accurately in as-built drawings. Cutting or coring through a tendon causes immediate loss of prestress across the affected span, with consequences ranging from localised cracking to progressive structural failure depending on the slab geometry and tendon layout.
The risk is not theoretical. In a commercial office tower in Brisbane's inner south, a services contractor drilling a 150mm core for a mechanical penetration struck an unbonded tendon during a Level 4 fit-out. The tendon was located 80mm from the marked safe zone on a drawing set that had not been updated to reflect a design change during original construction. The outcome was a structural hold, emergency investigation, and a six-week programme delay while the engineer of record assessed the damage and specified a remediation approach. The penetration was ultimately sleeved and the tendon end anchored, but the cost and delay were entirely avoidable.
The sequence of investigation work required before fit-out penetrations in a PT slab is well understood by structural engineers. It is less consistently applied by fit-out project managers and services consultants, who often treat concrete scanning as a formality rather than a technical prerequisite. This article sets out what investigation is required, why, and how to interpret the outputs.
Why Post-Tensioned Slabs Demand a Different Approach
In a conventional reinforced concrete slab, striking a single bar during coring is a manageable event. The bar can be cut, the penetration completed, and the structural impact is typically minor if the bar is not in a critical location. In a PT slab, the situation is categorically different. Each tendon carries a continuous prestress force, typically 100-200 kN per strand for commercial slabs, and that force is anchored at the slab perimeter. Severing a tendon releases that force suddenly and permanently.
Unbonded monostrand tendons, the most common type in Australian commercial construction, are encased in a plastic sheath with grease and run in a profile that varies in depth across the span. At midspan, the tendon may be near the soffit. At the support, it rises toward the top of the slab. This variation in depth means that even a shallow core can intersect a tendon depending on where along the span the penetration is located.
As-built drawings for PT slabs frequently contain errors or omissions. Tendon spacing, profile, and anchorage locations may differ from design intent due to site adjustments during construction. Drawings produced 20 or 30 years ago may have been scanned from paper originals and lack dimensional accuracy. Relying on drawings alone to locate tendons before coring is not acceptable practice.
What GPR Scanning Detects in a PT Slab
Ground-penetrating radar is the primary tool for locating post-tensioning tendons, conventional reinforcement, and embedded services in concrete slabs prior to fit-out work. GPR operates by transmitting a pulsed electromagnetic signal into the concrete and recording reflections from interfaces between materials of different dielectric properties. Steel tendons, conduits, and voids all produce characteristic reflections that a trained operator can interpret.
In a PT slab, GPR scanning identifies tendon positions in plan and provides an indication of tendon depth at the scan location. The method is non-destructive, does not require access to both faces of the slab, and can be completed without disrupting tenancy operations in most cases. SiteOps GPR scanning services cover both grid scanning for full-area mapping and targeted line scanning for specific penetration locations.
GPR has limitations that must be understood before relying on the outputs. In slabs with dense reinforcement, the signal from upper layers of steel can attenuate the return from deeper elements, reducing confidence in tendon depth readings. Slabs with high chloride content or significant moisture can also reduce signal penetration. In these conditions, GPR results should be supplemented with Ferroscan or, where access to the soffit is available, physical probing to confirm depth.
Scan Coverage and Grid Spacing
For fit-out work involving multiple penetrations across a floor plate, a full-area GPR scan on a 200-300mm grid provides a plan-view map of tendon positions that can be overlaid on the services layout. This allows the services consultant to identify conflicts before penetration locations are fixed. For isolated penetrations, targeted scanning on a 100mm grid centred on the proposed location is the minimum requirement. Scanning a single line through a proposed penetration point is not sufficient to characterise the tendon layout in that zone.
The Penetration Register: What It Is and Why It Matters
A penetration register is a controlled document that records every proposed and completed penetration through a structural slab, including the location, size, depth, method, and structural clearance confirmation. In a PT slab fit-out, the register serves as the primary coordination tool between the scanning results, the structural engineer's approval, and the construction team executing the work.
Each entry in the register should include the penetration coordinates referenced to a fixed datum, the scan results for that location, the minimum clearance to the nearest tendon, the structural engineer's sign-off, and the as-drilled confirmation once the penetration is complete. Where a proposed penetration falls within the exclusion zone around a tendon, the register records the relocation or the engineering review required before proceeding.
The penetration register is not a bureaucratic exercise. It is the mechanism that prevents the scenario described in the introduction. On a 3,000 square metre floor plate with 40 or 50 service penetrations, the register is the only reliable way to ensure that every penetration has been assessed and cleared before drilling begins.
Services Versus Tendons: Resolving Conflicts in the Layout
The conflict between services routing and tendon positions is the central coordination problem in a PT slab fit-out. Services consultants design penetration layouts based on mechanical, electrical, and hydraulic requirements, often without detailed knowledge of the tendon pattern. When the GPR scan results are overlaid on the services layout, conflicts are common, particularly around columns where tendons band together and at slab edges where anchorages are located.
Resolving these conflicts requires a structured process. The services consultant reviews the scan overlay and identifies penetrations that fall within tendon exclusion zones. For each conflict, the options are relocation of the penetration, reduction in penetration size to pass between tendons, or referral to the structural engineer for assessment of whether the penetration can proceed with engineering controls. The structural engineer's role is to assess the impact of the proposed penetration on the slab's structural behaviour, not simply to approve or reject based on proximity to a tendon.
Exclusion zones around tendons are not standardised across all projects. A common rule of thumb is 75-100mm clear of the tendon centreline for small cores up to 50mm diameter, but this must be confirmed by the structural engineer based on the specific slab design. For large cores above 100mm diameter, the structural engineer should review the penetration regardless of proximity to tendons, as the opening may affect the slab's shear capacity or the load path around the penetration.
When LiDAR Adds Value to the Investigation
On complex fit-out projects involving significant services coordination, 3D LiDAR scanning of the existing floor plate provides a spatial model that integrates with the GPR scan results and the services design. LiDAR 3D scanning captures the as-built geometry of the slab soffit, existing penetrations, structural elements, and services, producing a point cloud that can be imported into BIM or CAD environments.
The value of LiDAR in this context is coordination accuracy. Services consultants working from a 3D model of the actual space, rather than drawings that may not reflect as-built conditions, can route services with confidence and identify spatial conflicts before they become site problems. When combined with GPR tendon mapping, the model provides a complete picture of what is in the slab and what is below it.
Case Reference: SouthPoint A PT Scan
The SouthPoint A investigation involved full-floor GPR scanning of a post-tensioned commercial office slab prior to a major tenancy fit-out. Scanning identified tendon positions across the entire floor plate and flagged 11 proposed penetration locations that fell within exclusion zones. Eight were relocated by the services consultant without structural review. Three required structural engineer assessment due to constraints in the services layout. In all three cases, the engineer confirmed that the penetrations could proceed with modified sizing or with supplementary reinforcement around the opening. No tendons were struck during the fit-out. The scanning programme added less than two days to the pre-construction programme and was completed without disrupting the existing tenancy on the floor below.
Commissioning the Right Investigation Programme
The investigation programme for a PT slab fit-out should be commissioned at the design development stage, not after penetration locations have been fixed in the services drawings. The sequence is: obtain and review as-built drawings, commission full-area GPR scanning, produce a tendon map overlaid on the services layout, identify conflicts, resolve conflicts through relocation or structural review, establish the penetration register, and confirm structural engineer sign-off before drilling begins.
For buildings constructed before the mid-1990s, as-built drawings should be treated as indicative only. GPR scanning is mandatory regardless of drawing quality. For buildings where slab condition is uncertain, half-cell potential testing or carbonation depth assessment may be warranted to confirm that the concrete and tendon sheaths are in adequate condition before fit-out loads are applied.
Conclusion
Post-tensioned slab fit-out is a technically controlled activity, not a standard construction process. The consequences of striking a tendon are serious enough that the investigation programme must be treated as a structural requirement, not an optional preliminary. GPR scanning, a properly maintained penetration register, and structural engineer involvement at the conflict resolution stage are the minimum controls for any PT slab fit-out involving anchors, large cores, or multiple service penetrations. Commissioning this work early, before services layouts are finalised, is the most cost-effective approach and the one most likely to keep the programme on track.