Survey objectives

The purpose of the survey was to determine the general slab construction details of a warehouse floor slab. In particular, confirmation was required if pile caps or ground beams were present.

Survey method

GPR was used for the survey. It is particularly good at determining slab construction details, rebar distribution, slab thickness, and pile cap location.

The area was scanned using three different antenna frequencies. This provides a range of resolutions and penetration depths:

  • 1.5 GHz antenna. This provides high-resolution data to a penetration depth of up to 450 mm in dry reinforced concrete.
  • A digital dual-frequency antenna with 300 MHz and 800 MHz central frequencies. The higher frequency (800 MHz) provides high-resolution data to approximately 0.9 m depth. The lower frequency antenna (300 MHz) provides greater depth penetration (generally limited by site conditions), albeit at a lower resolution.

The warehouse slab was scanned on a regular orthogonal grid with scan lines at 0.5 m centres.

The survey was totally non-destructive. No intrusive survey methods were used.

Floor slab survey results

GPR detected a 200-270 mm thick reinforced slab construction with two reinforcement layers:

  • Top reinforcement layer at 60-120 mm depth with bars at 150 mm centres.
  • Bottom reinforcement layer at 150-200 mm depth with bars at 160-190 mm centres.
GPR radargram showing floor slab construction and detected pile caps
Radargram cross-section through floor slab showing rebar, slab underside and pile caps

Pile cap locations

GPR also detected evidence of pile cap foundations. Although the location of pile caps was not required, the information was readily available; therefore, we did a simple plot of the pile cap locations in CAD. Just because we could 😊

Extract from CAD drawing showing detected pile cap locations
Extract from CAD drawing showing the position of detected pile caps