Problem
In the planned construction of the Forsmark plant, the brackets for piping were to be fixed through expander bolts in the existing concrete. In order to avoid damage to the reinforcement, a method had to be developed for the precise location of the reinforcement bars, even through the sheet metal covering the concrete surface. Particularly demanding was the tolerance requirement of 70 × 70 mm for the location of the drill holes.
Results
The investigation showed that it was possible to localize both reinforcement layers with the combination of ground penetrating radar (GPR) and ultrasound tomography (UPE), both without plate and through plate, provided that there was good adhesion between plate and concrete. However, in the case of boom sections, the ultrasound signal was lost. A total of 14 holes were drilled for the attachment of expander bolts, and no reinforcement was found. This verifies the reliability of the method.
Solution
The methodology included:
- Visual inspection and beam tapping to identify any air pockets between sheet metal and concrete.
- Georadar (GPR) for locating rebar in test blocks without plates.
- Ultrasound tomography (UPE) to locate reinforcement through sheet metal, where both layers could be imaged when the sheet metal had good adhesion.
- Drill point marking based on the combination of GPR and ultrasound.
- Verification by drilling 14 holes, which confirmed the accuracy.
The combined methodology proved to be well suited for live application at Forsmark.
Equipment
- Georadar (GPR)
- Ultrasound tomography (UPE)
- Visual inspection equipment (boom tapping, marking)
Standards
- EN 1990: Eurocode – Basic rules
- EN 1992-1-1: Eurocode 2 – Concrete structures
- ISO 16311-1/2 – Condition assessment of concrete structures
- Non-destructive testing practices (GPR, ultrasound)
