GPR is an effective, reliable and cost effective utility survey method for the detection and location of services in floor slabs in offices, shops, warehouses, etc. Surveys can be carried out in occupied premises, with minimal disruption to the workforce.

Sandberg was instructed to undertake a non-intrusive GPR floor slab survey at London office premises to detect and map services and reinforcement within nominated areas of the floor slab. Utility surveys to locate services in building floor slabs are a common request, often associated with shop, warehouse or office fit-out works.

In this case, the client was installing revolving doors at six locations on the Ground, First and Second floors. The installation works could not proceed for safety reasons until a survey to detect services and reinforcement within the floor slab had been completed.

Survey objectives

The brief and objectives specified by the client were as follows:

  • Detect services, conduits, etc., in the floor slab and map their location
  • Locate and map reinforcement
  • Check for post-tensioned tendons
  • Determine screed and reinforced slab thickness

All survey works were to be carried out in a working office environment with minimal disruption to the office staff.

Survey methods

A GSSI SIR 3000 Ground Penetrating Radar system with 1.5 GHz and 2.6 GHz antennas was used.

Ground Penetrating Radar (GPR) scans can be conducted using antennas with different central frequencies. A higher-frequency antenna provides better resolution and more detail; however, the penetration depth is limited. Lower frequencies provide greater depth penetration but at a lower resolution.

The nominated areas were scanned on a regular orthogonal grid with scan lines at 0.2 m centres.

Data processing included surface position correction, gain adjustment and migration. The GPR data files were then visually inspected and analysed.

Survey findings

A highly complex pattern of services was detected; these turned out to be underfloor heating pipes. The location of the detected services was plotted in CAD.

The screed and floor slab thickness were successfully determined using GPR at all locations except on the Ground floor slab. On the Ground floor slab, a layer of insulation was detected at the screed/concrete interface. This blocked the GPR signal, and we were unable to detect any features below this. Typically the screed thickness was 150 mm thick, with a 220-270 mm reinforced concrete slab below.

Reinforcement was successfully detected and mapped at all locations except on the Ground floor, where the GPR signal was blocked by an insulation layer.

Conclusions

The GPR survey was successful in meeting the objectives specified. The one exception was on the ground floor slab, where we were unable to map the reinforcement due to the presence of a layer of insulation, which blocked the GPR signal.

The survey also detected underfloor heating pipes, which were mapped in CAD. Underfloor heating pipes are notoriously difficult to detect. Being plastic and water-filled, most survey methods will not detect them. GPR is probably the only efficient means of locating them.

The survey was completed in a working office environment with minimal disruption. No intrusive work was required.

The revolving doors were successfully and safely installed without damage to the services.