GPR is a great survey method for the location of services in floor slabs in buildings. Surveys can be carried out in working offices and other premises, with minimal disruption to the workforce.
Sandberg was instructed to undertake a non-intrusive survey at London office premises, to detect and map services and reinforcement within nominated areas of the floor slab. This is a common request, most often associated with shop or office fit-out works. In this case, the client was installing revolving doors at six locations on the Ground, First and Second floors. For safety reasons, the installation works could not proceed, until a survey to detect services and reinforcement within the floor slab had been completed.
The brief and objectives specified by the client were:
- 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
A GSSI SIR 3000 Ground Penetrating Radar system was used with 1.5 GHz and 2.6GHz antennas.
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 0.20m centres.
Data processing included surface position correction, gain adjustment and migration. The GPR data files were then visually inspected and analysed.
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; an example is included below.
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 150mm thick, with a 220-270mm reinforced concrete slab below.
Reinforcement was successfully detected and mapped at all locations (sample included below), except on the Ground floor, for the reasons described above.
The GPR survey was successful in meeting the objectives specified. The one exception being 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 detected underfloor heating pipes. Their location was 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 works were required for the survey.
The revolving doors were successfully and safely installed with no damage to the services present.