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.

Method

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

  • 1.5GHz antenna. This provides high resolution data to a penetration depth of up to 450mm in dry reinforced concrete.
  • A digital dual frequency antenna with central frequencies of 300MHz and 800MHz. The higher frequency (800MHz) provides high resolution data to approximately 0.9m depth. The lower frequency antenna (300MHz) 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 0.5m centres.

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

Floor slab survey results

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

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

Pile cap location results

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 position of detected pile caps.
Extract from CAD drawing showing position of detected pile caps.

Find out more about the floor slab surveys we offer. Sandberg can undertake a wide range of GPR surveys. Find out more about all the different GPR site surveys we offer.

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