What Is GPR Scanning and When Do You Need It?
What Is GPR Scanning?
Ground penetrating radar scanning is a non-destructive subsurface imaging technique. It sends short electromagnetic pulses into a material and records the reflections that bounce back from internal features. These reflections reveal rebar, post-tension cables, conduits, voids, pipes, and changes in material density, all without cutting, drilling, or damaging the structure.
GPR works on concrete slabs, walls, columns, bridge decks, pavements, and natural ground. The equipment is portable, battery-powered, and safe to use in occupied buildings. Unlike X-ray methods, GPR does not require evacuation or radiation safety zones.
How Does GPR Work?
A GPR antenna transmits a pulse of radio-frequency energy into the surface. When that pulse encounters a boundary between two materials with different electrical properties (for example, concrete meeting steel rebar, or concrete meeting an air void), part of the energy reflects back to the antenna.
The antenna records the timing, strength, and polarity of each reflection. Specialised software converts these signals into cross-section images called radargrams. A skilled operator reads radargrams in real time and marks up findings directly on the structure.
Modern GPR systems like the Proceq GP8000 use array antennas that capture hundreds of parallel scan lines simultaneously, producing 3D subsurface maps rather than single cross-sections.
When Do You Need GPR Scanning?
Before Cutting or Coring Concrete
This is the most common application. Before any saw-cutting, core drilling, or penetration through a concrete slab or wall, GPR scanning locates rebar, post-tension cables, electrical conduits, and plumbing. Hitting a post-tension cable during coring can cause catastrophic failure. Hitting a live electrical conduit creates an immediate safety risk.
As-Built Documentation
When original construction drawings are missing, unreliable, or simply do not exist, GPR creates a measured record of what is actually inside the structure. This is common on refurbishment projects, adaptive reuse conversions, and heritage buildings.
Structural Investigation
GPR maps rebar spacing, depth, and distribution across structural elements. This data feeds into structural capacity calculations, particularly when verifying whether an existing slab or beam can support new loads, equipment, or a change of use.
Utility and Service Locating
GPR locates underground pipes, cables, and services before excavation. In slabs, it identifies embedded services that may not appear on drawings.
Quality Assurance
GPR verifies that rebar has been placed at the correct spacing and depth during construction. This non-destructive verification can replace or supplement destructive methods like cover meter surveys and concrete breakout testing.
What Are the Limitations?
GPR cannot determine the grade or strength of concrete. It cannot identify the specific type of metal (rebar vs. copper pipe) without additional context. Depth penetration depends on the antenna frequency and the material: high-frequency antennas (1600 MHz+) provide detailed images to about 400mm in concrete, while lower frequencies penetrate deeper but with less detail.
Congested rebar areas can create overlapping reflections that are harder to interpret. This is where operator experience and multi-technology approaches (combining GPR with Ferroscan or radiographic methods) become valuable.
How SiteOps Uses GPR
SiteOps deploys GPR as part of integrated investigation programmes. We combine GPR with LiDAR, Ferroscan, and targeted verification to produce as-built documentation packages. Our structural scientists interpret GPR data in the context of structural behaviour, not just subsurface mapping, delivering reports that engineers can use directly for design decisions.
Every GPR survey includes a methodology statement, annotated scan images, marked-up plans, and confidence zoning to indicate where data reliability is high versus where further verification may be needed.