GPR provides a real-time cross-section of what is embedded in concrete, making it well suited to building renovation, drilling holes for services, and inspecting existing structures with no construction drawings. It is one type of non-destructive structural evaluation (NDE). This article explains how it works, what GPR can detect, how deep it can see, and its limitations.

SPN engineer moving a GPR unit to scan for rebar on a concrete floor
GPR concrete scanning in the field: the antenna is moved along the concrete surface to locate reinforcement and embedded objects before coring

How Does GPR Work?

GPR works on the principle of radar wave reflection. The antenna sends high-frequency electromagnetic waves into the concrete. When the waves meet a material with different electrical properties, such as steel, an air void or a conduit, part of the wave is reflected back to the receiver.

The unit records the travel time and strength of the reflected signal and converts them into a cross-sectional image (profile) showing the position and depth of embedded objects. Rebar appears as an inverted arc (hyperbola): when the antenna is directly above the bar the wave path is shortest, and it lengthens as the antenna moves to either side, so the apex of the arc marks the bar location.

2D diagram of how GPR concrete scanning works: a slab section on top and the radar profile shown by the unit below, explaining where the hyperbola comes from The upper part is a section through a reinforced concrete slab on a sub-base, with a row of top reinforcing bars, bottom bars, an embedded plastic conduit and an air void. The GPR antenna is moved along the concrete surface, shown at three positions around one bar. When the antenna is directly above the bar, the two-way wave travel path is shortest; when it is offset to the left or right, the path is longer, so the reflection returns later. The lower part is a conceptual radar profile on the same horizontal axis as the section; the vertical axis is wave travel time, converted to approximate depth. Each bar appears as an inverted arc whose apex lies at the bar location, and the apex depth gives the approximate concrete cover. The three points on the orange arc match the three antenna positions in the section. A non-metallic conduit gives a fainter arc, and a void gives a reflection that differs from its surroundings. The bottom bars and slab soffit beneath closely spaced bars appear unclear because they are shadowed by the top layer. The bottom strip summarizes that the apex locates objects well, depth is approximate, and the object type must be interpreted by a specialist together with site information. The radar image in the diagram is conceptual, not real data. GPR principle: from reflection to hyperbola (2D) Not to scale A · Slab section Move antenna along concrete surface Scan direction PVC pipe Void Bottom bars Top rebar Cover 1 Send, receive echo Waves reflect at materials with different elec. props. e.g. steel, conduit, void 2 Above bar: shortest path Offset: longer path so echo returns later (orange dashed line) B · Display (Radargram) Conceptual, not real data ← Depth (wave travel time) Position along scan (same axis as A) 3 Inverted arc (hyperbola) Apex = bar location Apex depth ≈ cover 4 Other objects differ Non-metal pipe: fainter arc Void: atypical reflection 5 Dense bars shadow below Bottom bars and soffit below dense bars unclear ✓ Position: reliable Read from apex ≈ Depth: approximate Depends on concrete ? Object type: interpret with site information
2D diagram: the slab section on top and the GPR display below share the same horizontal axis. Each bar produces an inverted arc (hyperbola) with its apex at the bar location, while non-metallic conduits and voids look different. The radar image is conceptual, not real data.
GPR profile showing hyperbolas from reinforcing bars at different depths
Example GPR scan result: objects embedded in concrete appear as arcs (hyperbolas), allowing the engineer to identify their position and depth
GPR results showing survey lines on an aerial photo paired with the subsurface profile along the same line
Reporting: scan lines referenced on an aerial photo, paired with the profile along the same line, so embedded objects can be located back on site
Color-processed GPR image showing the likely path of a buried pipe below the surface
Additional signal processing helps highlight reflection anomalies, such as the likely path of a buried pipe, before conclusions are drawn together with site information

What Can GPR Detect?

GPR detects both metallic and non-metallic objects embedded in concrete. There are 5 common applications

  • Locating reinforcement and bar spacing before drilling or cutting concrete
  • Measuring approximate concrete cover
  • Detecting voids, delamination and gaps beneath slabs
  • Locating electrical conduits, water pipes or post-tension tendons embedded in concrete
  • Estimating slab or wall thickness when only one side is accessible
💡 Why scan before drilling

Before drilling service holes or coring concrete, a GPR scan identifies the main bars and post-tension tendons that must not be hit. Drilling into a post-tension tendon can cut the strand and affect the load capacity of the whole slab span.

Scanning before drilling follows a sequence: set out a reference grid on the slab surface, scan in two perpendicular directions, mark the bars and embedded objects on the surface, then choose drilling locations in the gaps that avoid bars and conduits. For how to read scans from real jobs, see How to read GPR rebar scan results

3D model of GPR scanning of a concrete slab before coring, with a corner cut away to show two layers of reinforcement, a conduit and a void, plus the scan grid and surface markings Conceptual 3D view of a concrete slab with the front corner cut away to reveal what is embedded inside: a top reinforcement mesh, a bottom reinforcement mesh, a non-metallic electrical conduit between the two layers, and a void in the concrete at the cut face. On the slab surface is a blue two-way scan grid. An engineer pushes a cart-type GPR unit along the grid lines. The scanned part of the surface has orange lines marking the reinforcement detected in both directions and a purple dashed line marking the conduit. Green circles are drilling locations chosen in the gaps between bars, clear of bar and conduit lines. The right panel summarizes the four-step procedure: set out a reference grid on the surface, scan in two perpendicular directions, mark bars and embedded objects on the surface, and choose drilling locations that avoid main bars and conduits, together with three limitations: wet concrete or high chloride reduces wave penetration, closely spaced bars shadow deeper objects, and the scan image does not identify the object type directly, so it must be interpreted by a specialist. 3D model: GPR scan before drilling concrete Not to scale GPR unit 2-way scan grid Marked bar lines Chosen drill points A B C D A Top bars Clearest B Conduit Non-metal C Bottom bars May be shadowed D Void in concrete Interpret Corner cut away to show embedded items · Conceptual, not actual layout Scan-before-drill steps 1 Set out reference grid Ties scan results to real location 2 Scan 2 perpendicular ways Each bar set shows clearly when antenna crosses the bars 3 Mark on surface Bars, conduits, objects 4 Choose drill points Avoid main bars and conduits Key limitations Wet concrete / high chloride Less penetration Dense rebar Shadows deeper objects No object type Needs expert interpretation No concrete strength Use Core / UPV too
3D model: a corner of the slab is cut away to show two layers of reinforcement, a conduit and a void. On the surface are the scan grid, the marked bar lines and the drilling locations chosen in the gaps between bars (conceptual image, not an actual layout).

How Deep Can GPR Scan?

GPR depth depends on the antenna frequency, which trades off against resolution: the higher the frequency, the finer the detail but the shallower the depth.

← Scroll to see the full table →

Antenna frequencyScan depthResolution
High (e.g. 1.5–2.6 GHz)ShallowHigh resolution; resolves small, closely spaced bars
LowDeeperLower resolution

For scanning rebar in typical slabs or walls, a high-frequency antenna gives a clear image of the top bars. Where greater depth is needed, a lower frequency is chosen at the cost of resolution. The engineer selects the frequency to suit the purpose and the thickness of the structure.

GPR Concrete Scanning vs GPR Subsurface Survey: What Is the Difference?

They use the same principle and can use the same equipment, but the reference standards and working depths differ. Scanning of reinforced concrete structures follows DPT 1507-51 (issued by the Department of Public Works and Town & Country Planning) together with ACI 228.2R, while subsurface investigation follows ASTM D6432, the Standard Guide for using GPR in geotechnical investigation.

← Scroll to see the full table →

TopicDPT 1507-51ASTM D6432
Type of workInspection of reinforced concrete structuresSubsurface investigation
Used to findRebar position, concrete cover, voids, embedded conduitsBuried pipes, soil layers, underground cavities, pavement layer thickness, geotechnical investigation
Typical working depthSeveral tens of centimeters, depending on antenna frequency and concrete conditionAbout 2–3 m or more in dry sand, depending on the antenna and soil conditions (much less in moist clay)
Source of standardDepartment of Public Works and Town & Country Planning (used with ACI 228.2R)ASTM International: an international Standard Guide

Both are part of SPN's non-destructive structural evaluation (NDE) services. The engineer selects the antenna frequency and reference standard to match the purpose of each site.

📷 Site Photos: GPR Scanning by the SPN Team

Examples from real SPN GPR jobs, covering both concrete structure scanning and surveys for buried pipes or objects, in a variety of site conditions: from open ground and old buildings to urban construction sites and concrete floors inside buildings.

GNSS receiver on a tripod with traffic cones and a marker recording the GPR survey line on a concrete yard
Recording scan-line coordinates with GNSS so the scan results are referenced to real positions on site and can be mapped
SPN staff moving a GPR unit to survey buried objects around an old timber building
Scanning for buried pipes and objects around an old building, where construction drawings or utility plans are often unavailable
SPN staff GPR scanning on a construction site in central Bangkok with high-rise buildings in the background
Scanning on an urban construction site: checking for pipes and buried objects before excavation to reduce the risk of striking utilities
SPN staff in a hard hat moving a GPR unit to scan a concrete floor inside a building, with markings on the floor
Scanning a concrete floor inside a building and marking reinforcement and embedded objects on the surface before coring

What Are the Limitations of GPR?

GPR is good at showing "where", but it does not directly tell you the "object type" or the concrete "compressive strength". There are 4 limitations to understand before interpreting results

  • Very wet concrete or high chloride content: reduces wave penetration
  • Very closely spaced reinforcement: can shadow deeper objects so the lower layer is not visible
  • Raw images do not identify the object type: interpreting hyperbolas requires a specialist
  • It does not indicate concrete quality or compressive strength: strength assessment relies mainly on core testing, with rebound hammer and UPV as supporting screening methods. For an overview, see NDT guide for concrete structures

For quick spot measurements of cover and top-bar position, a Cover Meter / Rebar Scanner is a good complementary tool.

⚠️ Raw images are not the final answer

GPR shows where waves are reflected but does not directly identify the "type" of object. Concluding whether it is rebar, a conduit or a void requires specialist interpretation together with site information and construction drawings (if available).

SPN Soil Engineering provides GPR scanning across Thailand, both concrete scanning before coring and surveys of buried objects below ground. Reports are certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number. Contact us for a free quotation within 24 hours.