Ground penetrating radar inspection of a large concrete spillway: A case-study using SFCW GPR at a hydroelectric dam

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Hydroelectric dams are subject to aging and degradation. The concrete structure under investigation here had visible signs of spalling, cracking, and erosion, but it was unclear how damaged the spillway was below the surface. The asset owners wanted to identify delamination and voids 60 cm or larger so they could mitigate the effects. While most ground penetrating radar (GPR) inspections conducted on dams have been completedusing low frequency ranges between 16 MHz and 500 MHz, these frequencies may overlook small, but critical defects. Our study is the first to use a small hand-held sized stepped frequency continuous wave (SFCW) GPR to acquire information at frequencies ranging up to 4 GHz to evaluate its effectiveness on such a large asset. The two main questions we aimed to answer are: (1) how will a small GPR unit perform on a large, irregular concrete surface, and (2) could the higher frequencies provide more detailed images of small cracks and defects. The study provided useful results in real-time which supported a repair and refurbishment program. The high frequency system allowed for detailed visualization and measurement of reflections events consistent with expectations for delamination and voids. The investigation also identified other non-critical defects that can be targeted for ongoing monitoring programs.