Protecting Structural Integrity: Post-Tension Tendon and Sub-Slab Void Detection Before Coring or Cutting

A core drill through an unmarked post-tension tendon releases forces that no building designer intended to concentrate in a single point — and the consequences are immediate and irreversible. Post-tensioned concrete is found throughout the built environment: suspended floor slabs in commercial office buildings, multi-storey car park decks, podium transfer plates in mixed-use towers, bridge decks, and hospital floor structures. In each of these applications, the continuous tension in the tendon system is integral to the structural performance of the slab. Severing a tendon does not merely damage the concrete — it locally collapses the structural design of that section, with load redistribution consequences that may propagate to adjacent bays.

The industry assumption that as-built drawings provide sufficient information for safe drilling has been demonstrated to be incorrect by incident after incident across every continent. Construction tolerances in post-tensioned concrete are wide by necessity: tendons must navigate around column heads, penetrations, and geometric transitions, and the actual routing installed by operatives in the field routinely deviates from design intent. Surveys of completed post-tensioned structures have documented tendon position deviations of 50 to 150 millimeters from drawing coordinates — deviations that exceed the positional accuracy required to drill safely between adjacent tendons in a standard flat plate slab configuration^[1]. Last-minute design changes, hand-drafted as-builts, incomplete documentation handover, and subsequent building modifications that added penetrations without structural engineering input have further degraded the reliability of existing records.

MCGPR-HDR pre-coring concrete scanning maps every tendon duct, rebar element, and sub-slab void in hours, before any cutting or coring begins. The survey produces a precise, site-marked tendon and void map that is handed to the drill operator before any equipment contacts the slab. The technology applies electromagnetic geophysical methods^[2] through GPR pavement and concrete scanning protocols^[3] refined for the specific reflection characteristics of grouted and ungrouted post-tension tendon ducts in reinforced concrete matrices.

The Post-Tension Risk Landscape

Post-tensioned concrete structures are ubiquitous in modern commercial construction. Suspended flat plate slabs in office buildings, parking structures, and residential towers are frequently post-tensioned to achieve longer spans with thinner slab profiles than conventionally reinforced concrete allows. Podium transfer plates — the large-span slabs that carry column loads over open ground floors in mixed-use developments — are almost always post-tensioned. Bridge decks, particularly those using segmental construction or balanced cantilever methods, rely on post-tensioning as a primary structural system component. Airport terminal aprons, industrial floor slabs designed for heavy racking loads, and hospital buildings where vibration-sensitive equipment requires stiff floor designs all frequently use post-tensioning.

In every one of these structures, the tendon routing as designed reflects the structural engineer’s intent at the time of construction documentation. What is installed in the field is something different. Post-tensioning operatives route tendons by hand through the rebar cage, negotiating around penetrations, support chairs, and geometric constraints that may not be fully represented in the structural drawings. Column heads and slab edges create zones where tendons must be deviated from their design trajectories. In thick transfer plates, multiple tendon layers at different depths require careful three-dimensional routing that cannot be perfectly replicated from two-dimensional plan drawings after the concrete has been poured.

The failure mode when a tendon is cut is not gradual. The tendon is under a permanent stress — typically 70 to 80 percent of its ultimate tensile strength. Severing it releases this energy instantaneously at the cut point, producing an explosive failure of the duct and surrounding concrete. Load previously carried by that tendon is instantaneously transferred to adjacent elements. In thin flat plate slabs, this redistribution can trigger punching shear failure at column heads. In thicker transfer structures, the consequences depend on the redundancy of the tendon layout and the extent to which adjacent tendons can accommodate the load redistribution. This is precisely why retail fit-out slab scanning and deep foundation clearance programs both incorporate mandatory pre-coring scanning for post-tensioned structures.

Why As-Built Drawings Are Not Enough

The limitations of as-built drawings for post-tensioned structures arise from multiple overlapping sources, each individually significant and collectively decisive. Construction-phase drawing management in post-tensioned structures is inherently difficult: design changes during the construction period may be issued as sketches or verbal instructions that are not formally incorporated into the as-built record. Where formal revisions are issued, they may not be systematically distributed to the post-tensioning subcontractor, resulting in tendons installed to an earlier design iteration.

Even where the design intent is accurately communicated, physical installation tolerances introduce deviation. The British Cement Association’s structural concrete post-tensioning guidance acknowledges that tendon position tolerances of plus or minus 5 to 10 millimeters are achievable under controlled conditions — but these tolerances apply to individual tendon segments between support points, and the cumulative deviation across a long tendon from anchorage to jack can be substantially larger. In structures with complex geometry, the deviation between design and as-installed position is not a constant offset but a varying quantity that must be measured individually at each proposed core location.

Building lifecycle changes compound the problem further. A structure that received a comprehensive post-tensioning survey at handover may have been modified over subsequent decades by multiple tenants, refurbishment programs, and fit-out contractors, each of whom may have drilled penetrations using drawing information that was already inaccurate and further degraded by successive modifications. The as-built drawings available to a contractor today may reflect the original design, the first major refurbishment, or some intermediate state — with no reliable way to distinguish which from document review alone.

The consequence of this degradation is that drawings should be treated as approximate guidance for survey planning, not as a source of tendon positions accurate enough to determine safe core locations. This position is increasingly reflected in structural engineering guidance and in the liability frameworks of contractors and structural engineers involved in post-tensioned slab drilling.

What MCGPR-HDR Detects in Concrete Structures

MCGPR-HDR concrete scanning detects features by identifying electromagnetic reflections at material boundaries within the concrete matrix. The MCGPR-HDR technology platform applies high dynamic range antenna technology to maximize signal sensitivity across a wide depth range, enabling simultaneous detection of shallow features — rebar cover depth, mesh position, electrical conduits — and deeper features such as post-tension tendon ducts at mid-slab depth in thick transfer structures.

Tendon ducts — whether grouted metallic ducts, ungrouted bonded tendons, or unbonded greased-sheath systems — all produce detectable GPR reflections because their electromagnetic properties differ from the surrounding concrete. Metallic duct systems produce strong reflections; non-metallic or greased-sheath systems produce weaker but still distinguishable reflections that can be identified by experienced analysts. Non-destructive concrete mapping using MCGPR-HDR can resolve individual tendon ducts in slabs as thin as 150mm, with lateral positional accuracy sufficient to mark safe drilling corridors between adjacent tendons.

Rebar grids are detected simultaneously with tendon mapping, providing cover depth measurements and bar spacing data that inform concrete cutting and anchoring operations in addition to drilling safety. Electrical conduits and MEP service routes cast into the slab — a common feature in commercial office construction — are identified and mapped alongside the structural elements. Sub-slab voids, debonding between the slab soffit and any structural topping, and delamination within the concrete body are all detectable in the same survey pass.

Scanning Protocol for Pre-Coring Operations

A pre-coring MCGPR-HDR survey follows a structured protocol designed to deliver a marked, site-safe drilling plan to the contractor before any drilling commences. The survey area is defined in consultation with the structural engineer and contractor to cover the full plan extent of proposed core locations plus a safety margin around each. Grid resolution is specified based on tendon spacing — for slabs with tendons at 300mm centres in two directions, a scan grid at 100mm intervals in both directions ensures that every tendon is crossed by multiple scan lines, providing the data density required for confident position interpolation between lines.

Survey outputs are delivered as processed GPR profiles, interpreted plan views showing tendon routing and depth at each proposed core location, and physically marked slab surfaces indicating safe drilling corridors. The marking protocol — typically using chalk lines or spray paint in an agreed colour-coding system — is carried out by the survey team immediately after data interpretation, providing a direct, unambiguous instruction to the drill operator that does not require them to interpret technical data. This AI-assisted subsurface analysis capability allows large slab areas to be processed rapidly, reducing the time between survey completion and drilling clearance to a minimum.

For complex structures with multiple tendon layers or unusual geometry — such as stadium foundation structures where transfer slabs carry concentrated point loads — the structural engineer reviews the interpreted tendon maps before drilling clearance is issued, providing an additional verification layer that confirms the interpretation is consistent with the design documentation. This collaborative protocol — MCGPR-HDR survey, AI-assisted interpretation, structural engineer review, on-slab marking — represents current best practice for pre-coring safety in post-tensioned concrete structures.

Sub-Slab Void Detection as Part of the Same Survey

While the pre-coring scan is being conducted for tendon detection, MCGPR-HDR simultaneously profiles the condition of the substrate beneath the structural slab. Sub-slab voids — air gaps between the underside of a ground-bearing slab and the supporting formation — are a common and frequently unsuspected structural risk, particularly in structures that have experienced post-construction ground settlement, erosion of fine-grained fill, or subsidence induced by nearby construction activity or leaking buried services. Detecting these voids before coring removes the risk that a drill creating a penetration in a slab already partially unsupported accelerates the structural failure of that slab under subsequent loading.

The same MCGPR-HDR technology that detects post-tension tendon positions in the slab body also identifies air-filled voids beneath the slab soffit with high sensitivity — air produces a strong electromagnetic reflection at the soil-slab interface. The prevention of urban sinkholes and subsidence through early void detection is an established application of GPR technology, and the pre-coring survey context provides an opportunity to deliver this structural intelligence at no additional field mobilization cost.

For complex structures — bridge piers with pier cap slabs, runway aprons, and elevated structures where the inspection of the slab soffit is physically difficult — the GPR profile through the full slab and sub-slab zone provides structural intelligence that visual inspection cannot deliver. The complete output: tendon map, rebar layout, embedded services, delamination, and sub-slab void condition characterizes the full structural and geotechnical state of the slab as a single underground infrastructure survey deliverable.