Method for Scanning Post Tensioning

(Written by Reuben Barnes 20/09/2019)

One of the prime targets for Ground Penetrating Radar (GPR) scanning of concrete are Post Tensioning (PT). Post Tensioning (PT) is a type of reinforcing in concrete, where tensioning is applied to steel bars or wire after the concrete has strengthened. Usually these wires are places into a metal (or plastic) duct in bundles. Cutting PT can be dangerous for the sawing operator as well as damaging for the structure and is generally avoided. GPR is very effective for this application as it is less affected by other reinforcing present. A method by which GPR can be employed for this application follows in order to give guidance to users.

1. Start by visually surveying the structure and consult any available drawings. Key visual signals that PT is present can be staples or grouting tubes.

2. PT is placed in the tension zones of an element. So, for slabs they will be in the top of the slab where it is supported by a beam or close to columns.

3. PT is spaced quite widely in most instances (1-2m apart). Scans should be long to ensure you cross more than one. This is particularly important where there is conventional reinforcing.

4. Moving away from the initial scan continue scanning parallel to the first scan. I usually scan at 1m spacings. The PT should get deeper as you scan away from the supporting column or beam. This is a good confirmation that you are dealing with PT as that is fairly unique.

5. Another method to confirm the presence of PT is to scan parallel to it. If you manage to stay over it you will see a target dipping deeper along the scan.