Concrete scanning with ground-penetrating radar

How concrete scanning works

An antenna moves across the surface and records electromagnetic reflections at material boundaries. Interpretation helps estimate the position and depth of detectable items. GPR can collect data from one accessible face.

A scan complements available drawings. It does not guarantee that every object will be found or authorize cutting reinforcement.

Slab scanning and rebar detection

Openings for plumbing, ventilation or anchors can intersect rebar, conduits or post-tensioning tendons. Scanning helps the project team review the proposed opening locations.

Rebar orientation, spacing and estimated depth can inform further assessment. Structural capacity and conformity require appropriate interpretation.

Detecting voids beneath slabs

A gap between a ground-supported slab and its support may produce a detectable reflection. Detectability depends on size, depth and whether the bottom of the slab is readable.

An anomaly alone does not identify its cause or the required repair. Additional investigation may be needed before filling or reconstructing the affected area.

A void under the slab and a delamination within its thickness both sound hollow, yet they do not call for the same repair. The distinction is developed on the voids beneath slabs and concrete delamination page.

What can be detected

Detection depends on site conditions.

Use cases by project type

Concrete scanning is most useful when work must pass through or anchor into existing concrete. The situations below are common. The exact survey scope still depends on each site.

A heated slab raises one more question, since the heating occupies the same thickness as the reinforcement. What that changes is developed on the radiant floor heating and heating cables page.

• Commercial renovation

  Locate rebar and conduits before opening a slab for plumbing, electrical or ventilation work in a refitted space.

• Industrial building

  Identify embedded items at the planned locations for machine, racking or guardrail anchors.

• Parking and structural slabs

  Prepare coring, cutting or local repairs while accounting for rebar and, where present, post-tensioning tendons.

• Structures without current drawings

  Document detectable elements when drawings are missing, incomplete or no longer reflect existing conditions.

GPR survey methodology

GPR sends a short electromagnetic pulse into the concrete. When it meets a material with different properties, such as steel, a conduit or air, part of the energy returns to the antenna. On the recorded profile, a bar crossed at a right angle usually appears as a hyperbola whose apex marks its estimated position.

Depth is calculated from the signal’s travel time and the wave velocity in the material. Velocity varies with moisture and concrete composition, so depths remain estimates that can be refined using a known thickness or an existing opening.

Antenna selection involves a trade-off: a higher frequency generally provides better resolution near the surface, while a lower frequency reads deeper with less detail.

1. 01

   Scoping

   Define planned openings, the survey area, available drawings and the expected deliverable.

2. 02

   Data collection

   Scan the cleared surface in parallel passes, in one or two directions depending on the question.

3. 03

   Interpretation

   Read the profiles to estimate the position and depth of detectable items.

4. 04

   Reporting

   Deliver results in the agreed format: marking, photographs, sketches or a report.

When to request a scan

Describe the planned work to define the survey area.

• Before coring for plumbing, ventilation, electrical work or sampling
• Before cutting an opening (door, window, hatch, stairway)
• Before anchoring heavy equipment, racking or guardrails
• Before drilling post-tensioned slabs
• Before partial demolition of a structure
• To locate radiant floor heating or a pipe before repairs

Preparing the survey and using the findings

Provide the location, drawings, planned openings and access conditions. Agree on the required marking, photographs, sketches and report in the scope of work.

Post-tensioned slabs require particular care. A scan alone does not authorize drilling; the work must be reviewed by the parties responsible for the site and structure. Prestressing, strands and ducts are set out on the post-tension, prestressing strands and ducts page.

GPR limitations

Dense reinforcement, metal surfaces and wet concrete may reduce readability. Depths are estimates, and no detected indication does not prove an area is clear.

Read all findings together with their assumptions and limitations. Antenna selection and complementary tests depend on the investigation objective.

Frequently asked questions