GPR case study: How 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, when we are instructed to undertake an on-site mark-up survey, the scan areas are small. Usually, they are 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 purpose of the survey 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.
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 with GPR scan lines collected 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.
GPR detected two different forms of post-tensioned slab construction; the joint between them was visible on site. The thickness of the slab construction varied from 170-250 mm; the depth of the tendons was variable.
The position of the tendons was also recorded by a topographic survey and plotted in CAD.