GPR case study: We used GPR survey methods for a tendon location survey to detect and mark up tendons in the floor slab of a working warehouse.

Typically, the scan areas are small when instructed to undertake an on-site mark-up survey. They are usually scanned to locate rebar, tendons, or obstructions prior to drilling fixing holes or cutting penetrations for services. But there are exceptions. Sometimes, the survey can cover large areas, such as in this case, where large areas of warehouse floor slab were to be scanned and the results marked up in situ.


The survey’s purpose was to establish the location of post-tensioned tendons within areas of the ground floor slab. The position of all tendons was to be marked up in situ and plotted in CAD drawings.

Survey methods

A GSSI SIR-4000 was used with a 2.6 GHz antenna for this survey.

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. A lower-frequency antenna provides better penetration depth, albeit at a lower resolution. For this survey, a 2.6 GHz antenna was used, giving a penetration depth of up to 300 mm.

The floor slab was scanned by collecting GPR scan as and where required. The GPR data was analysed in real-time using the built-in screen on the control unit. Tendon positions were picked and marked up in situ. No off-site data analysis was necessary.

Survey findings

GPR detected two different forms of post-tensioned slab construction, with the joint between them visible on site. The thickness of the slab construction varied from 170 to 250 mm, and the depth of the tendons was variable.

The position of the tendons was also recorded by a topographic survey and plotted in CAD.

Find out more about GPR floor slab surveys and on-site mark-up surveys.